In this article we will discuss about:- 1. Introduction to Pteridophytes 2. Origin of Pteridophytes 3. Sporophytic Generation 4. Gametophytic Generation 5. Classification 6. Heterospory and Seed Habit 7. Economic Importance 8. Sexuality 9. Indian Work.
Introduction to Pteridophytes:
Pteridophytes constitute a significant and important group in the plant kingdom. As the first true land plants, they offer a very favourable material for the study of various adaptations that have made the colonization of land possible for the plants.
Pteridophytes have a long geological history on our planet. They are known from as far back as 380 million years. Fossils of pteridophytes have been obtained from rock strata belonging to Silurian and Devonian periods of the Palaeozoic era.
The life cycle of a typical member comprises a regular heteromorphic alternation of generations in which both the gametophyte and the sporophyte exist as independent individuals. Nevertheless the sporophyte is the predominant generation. In terms of duration in the life cycle the gametophyte is insignificant.
In this respect pteridophytes are totally different from bryophytes. The reasons for the predominance of the sporophyte in the life cycle are not far to seek. Being a more robust body a sporophyte can effectively meet the challenges of a terrestrial environment better than the gametophyte. As a group, the pteridophytes lie between bryophytes and spermatophytes. They share the characters of both these groups in addition to their own unique features.
Origin of Pteridophytes:
Problems concerning the origin and evolution of any plant group have not been solved to date, to the satisfaction of all. Problems of origin and evolution by their very nature give ample scope for speculation and imaginative interpretation. Hence, it should surprise none, if the views concerning the origin and evolution of a group are as varied as the morphologists themselves.
The problem of the origin and evolution of vascular cryptogams is no exception to this general rule. There have been various interpretations and perhaps many more are yet to come. A detailed analysis of all these theories is beyond the scope of this book. Basic outlines of a few of the more important theories concerning the origin of pteridophytes is given here.
There are mainly two schools of thought. According to one, pteridophytic ancestry is to be sought among bryophytes. According to the other, pteridophytes arose from some algal stock and evolved parallel with bryophytes. While the latter experimented with the gametophyte on a terrestrial habitat with little success, the former experimented with the sporophyte and achieved phenomenal success paving way for other terrestrial plants.
Even among the proponents of bryophytic ancestry there is no unanimity as to the precise group which constituted the ancestral stock or the mode of origin. There are four theories.
Campbell (1895) and Smith (1938) are the chief supporters of this theory. They believe that the sporophyte of Anthoceros has all the attributes of becoming the ancestor of land plants. Of all the members in bryophytes, according to Campbell (1895), Anthoceros is the most advanced, in having a mechanism for indefinite growth, limited tissue for spore production etc.
Campbell (1895), has reported that in one of the Californian species; (A. fusiformis), the sporophyte almost reached the stage of an independent plant body. Campbell (1939) compares the sporophyte of Anthoceros to the plant body of Rhynia and points out many similarities between the two.
The Green naked plant body, possession of a meristem assuring indefinite growth, presence of a columella remotely resembling the vasculature of land plants are some of the characters in the sporophyte of Anthoceros which support its claim of being the ancestral stock of land plants.
Campbell (1939) states, considering the extraordinary close resemblance between the structure of these ancient pteridophytes (Rhynia etc.) and the sporophyte of the Anthocerotaceae, it is a fair assumption that the Rhyniaceae were derived either directly from some Anthocerotaceae or from forms very much like them.
Smith (1938) has further elaborated the anthocerotean hypothesis. He traces the origin of Rhynia, from an Anthocerotean type stock, with some modifications (Fig.203). He suggests if an anthocerotean type of sporophyte had a shifting of the meristematic region from the base to apex, this would make possible an initiation of dichotomous (Fig.203c) branching by the meristem and the restriction of spore formation to branch apices.
He believes that if the columella in the sporophyte of Anthoceros were to evolve into a vasculature in the course of evolution, a land plant of the type of Rhynia could be visualised. Smith (1938) extended his observations to the sex organs and opined that the sex organs of pteridophytic gametophytes have much in common with those of Anthoceroies.
The points of resemblance in the sex organs are as follows:
These are embedded in both Anthoceros and pteridophytes. The primitive type of antheridium seen in eusporangiate forms like Lycopodium, Equisetum, etc., consists of a large number of fertile cells embedded in the gametophytic tissue and surrounded by a single layer of jacket.
Smith (1538) compares the fertile cells of this eusporangiate type antheridium to the entire antheridium of Anthoceros and observes that the jacket of the antheridium is comparable to the roof of the antheridial chamber in Anthoceros.
(Fig 205) The archegonium of Anthocerotes is more reduced than in Hepaticae; (Fig.205b). Evolution of the pteridophytic archegonium is the result of further reduction in antogeny. Since the archegonial initial in pteridophytes (Fig ,205c) directly forms the axial cell, the entire archegonium is comparable to the axial row and cover cells of the anthocerotean archegonium.
Most of the botanists however do not favour an anthocerotean origin for land plants.
This theory has been advocated by, Bower (1894) favouring a bryophytic origin for a sporophyte, from the zygote with a great amount of elaboration.
According to him in a life cycle without alternation of generations, the zygote immediately undergoes reduction division producing haploid cells (spores, zoospores, etc.) which develops into a new plant body. Hence the entire zygote is fertile i.e., it produce fertile cells. Bower postulated the origin of a sporophyte (antithetic origin) from such a zygote by the postponement of reduction division to a later stage.
Meanwhile the zygote divides mitotically to produce a diploid tissue which forms the sporophyte. Hence originally in a sporophyte all the tissues are devoted to spore production i.e., fertile. Subsequently by the progressive sterilization of the potentially sporogenetic tissue, a well differentiated sporophyte arose.
Such sterilization is necessary, because for a free living sporophyte reproduction is not the only function. It has to sustain itself first and only then comes the perpetuation of the race. Bower (1894) gave a series of examples from both liverworts and early pteridophytes to prove his point.
While Bower’s theory seems to be quite appealing and logical it has also been discarded as the broyophytic origin itself is in dispute. Further, some of the fossils of early pteridophytes were quite complex and not simple strobiloid plants as Bower liked them to be.
According to this theory the sporophyte was originally leafy i.e., the fundamental part of the plant body was a leaf and not the axis (stem). The axis arose later due to the fusion of leaf bases.
The phyton theory proposed by Celekovsky (1901) is related to the strobilar theory in that at the stage where the sporophyte is like a strobilus (a cluster of leaves – sporophylls, attached to a central core) the plant is a cluster of leaves and there is no axis as such. Subsequently the leaf bases fuse resulting in the formation of an axis. The pericaulome and leaf-skin theories are related to the phyton theory in that both these theories assume that a considerable portion of the axis is built up by the leaf bases.
According to Eames (1964), in the face of overwhelming evidence in support of the axial nature of the sporophyte, the phyton theory seems to be irrelevant. He states that the cauloid theory, an old theory which assumes the derivation of the modern sporophyte from a primeval cauloid, (an axis) seems best to fit the facts of history.
This theory was put forward by Treub (1890) who regarded that basically a primitive pteridophytic sporophyte was an undifferentiated mass very much like a gametophyte. He cites the example of the protocorm in some species of Lycopodium as an evidence to the protocormous origin of the sporophyte.
He regards the protocorm to be a vestige of an archaic sporophyte and a transitional stage recalling an earlier step in the evolution of the sporophyte.
Another evidence cited by Treub (1890), is the similarity between the protocorm and the adult plant body of Phylloglossum, which according to him is a permanent protocorm. But it has to be conceded that the so called protocorm is present not in the primitive species, but in the more advanced species of Lycopodium.
Many morphologists opine that protocorm is nothing but an opportunistic growth, evidently being an adaptation to certain special environmental conditions when it is not possible to organise a definite sporophyte. Detailed investigations conducted on Phylloglossum have revealed that its simplicity is not due to primitiveness but due to reduction.
The current trend seems to favour a direct algal origin for vascular plants. But even here, there is no consensus as to which group among algae could be regarded as the ancestral stock. Below is given a few hypotheses of some prominent botanists.
Church (1919), has enunciated his theory of the origin of land plants in his famous essay entitled Thallasiophyta and the sub-aerial transmigration. He believed in a polyphyletic origin. According to him a hypothetical group of marine algae called thallassiophyta (brown algae) formed the ancestral stock for all land plants.
The main points of his theory are as follows:
1. The surface of the earth in the remote past was completely enveloped by a common ocean.
2. There were many kinds of marine plants which were mostly planktonic (free floating).
3. Due to geological changes there was upheaval of the ocean bottom.
4. Due to the emergence of the land, planktonic forms changed to benthic (fixed) forms.
5. The new environment (terrestrial) gradually introduced all the adaptations like roots, leaves, vasculature etc.
The observed geological phenomena in the earth’s history do not envisage an all pervading ocean. There was in fact the land first and only later oceans came into existence. Another demerit of this theory is the non-homology of marine algae and early land plants in their pigmentation.
If land plants indeed arose from brown algae they should have at-least had some traces of the brown pigments. Scott (1924), supports Church (1919) and states the discovery of Rhynie plants goes to show that they evolved from a fairly highly evolved group of algae.
Based mainly on the branching similarities, Greguss (1955) derived bryophytes and pteridophytes from three groups of algae viz., Chlorophyceae, Phaeophyceae, and Rhodophyceae.
He derived mosses from chlorophyceae and liverworts from the phaeophyceae. Similarly Rhynia and Horneophyton were derived from chlorophyceae while Psilotum and Temesepteris were derived from phaeophyceae. All these based only on the branching pattern without any regard to the phylogenetic relationships.
Andrews (1956, 1959) also is a believer of polyphyletic origin for vascular plants. His observations are based on the discovery of fossils of certain marine algae (Nemotothallus, Crocelophyton and Protosalvinia) which had several adaptations for a terrestrial life.
Occurrence of such plants made Andrews to believe that several algal groups independently attempted the invasion of the land. These groups gave rise to different groups of vascular plants. He believes that the morphological diversity exhibited by psilophyta, lycophyta etc., is due to their independent origin.
Based on the paleopalynological studies, Leclercq (1954, 1956) proposed a polyphyletic origin for vascular plants. According to her, land plants must have had their origin somewhere during the Precambrian era. This assumption is supported by the discovery of fossil spores probably of land plants in the rock strata belonging to ordovician and cambrian periods.
She considers the simple psilophytes like Rhynia as the descendants of complex race that existed prior to the Devonian period. Axelrod (1959) supported the polyphyletic origin of Leclercq and elaborated it by his own palaeopalynological discoveries.
Merker (1961) also believes in the algal origin.
He recognises five main evolutionary lines:
(1) Rhyniaceae, Psilotaceae and ophioglossaceae,
(4) Lycopsida and
(5) Pteropsida (including pteridosperms).
He also agrees with Leclercq that Rhyniaceae are simple due to reduction and not due to primitiveness.
In his essay on the Phytogeny of Carmophyta, Lam (1955) has suggested a di-phyletic origin for vascular plants. The two independent lines are psilopsida and lycopsida which arose from Thallophyta independently somewhere during the Cambrian period.
Psilopsida gave rise to three groups viz., spenopsida, ptcrospida and cycadopside. The last mentioned group gave rise to protoangiosperms and angiosperms. The lycopsid stock gave rise to coniferopsida.
There are mainly two objections to this hypothesis:
(1) It is stretching the imagination a little too far if one has to believe a lycopsid origin for confideropsida and
(2) Inclusion of Casuarina under gymnosperms is untenable.
According to P.N. Mehra (1968) the ancestors of land plants are to be found among the green algae. He opposed the polyphyletic origin of vascular plants. While agreeing that the different groups of pteridophytes diverged from the very beginning, he argued that all of them have come from a common group.
Mehra (1968), envisages the origin of a hypothetical group Protoarchegoniatae from the chaetrophoraceous ancestors. From these protoarchcgoniates two lines viz., Psilophytaceous line and lycopodiaceous line, evolved.
In-spite of the innumerable theories proposed, it may be said that the problem remains as before i.e., without a solution. However, it seems quite possible that bryophytes and Pteridophytes evolved parallely from some algal ancestor which in all probability could be a green algal stock.
Sporophytic Generation of Pteridophytes:
It is the sporophytic generation which constitutes the main plant body in pteridophytes. In any description whenever we refer to the plant body of a ptcridophyte, we mean the sporophyte.
There is a great variety in the nature and organisation of the sporophytic plant body in pteridophytes. The simplest sporophyte in an extant member is to be found in Psilotum. The plant body here is a naked, branched axis with no evidence of roots.
The axis is distinguishable into an underground prostrate system and an erect aerial system. Rhizoids help in anchoring the plant to the substratum. There are no leaves. The stem itself takes up the function of photosynthesis.
From such a simple plant body further evolution (mainly necessitated by the environment) resulted in the complex and diverse types of sporophytes that we see today. The first step is the differentiation of photosynthetic laterals (leaves) and roots.
The leaves of pteridophytes are basically of two types viz., microphylls and megaphylls (macrophylls).
Microphylls have a single un-branched mid vein. Further, when a leaf trace departs from the main vascular cylinder to provide the leaf, no gap is left.
In megaphylls there is a branched venation. The branching may be dichotomous or reticulate. Further, there will invariably be a leaf gap in the main vascular cylinder above the leaf trace.
Microphylls are seen in members like Lycopodium, Selaginella etc. Megaphylls are found in members like Adiantum, Pteris, etc.
These are found in the leaves and stems of all members.
Mehra and Soni (1983) have studied the stomatal structure and ontogcmy in a number of pteridophytes.
They have classified the pteridophyte stomata into the following four types:
iii. Marattiaceous and
Mehra and Soni (1983) believe that the psilophytaceous stoma is the basic type and the others are derived.
Origin of Leaves:
According to general opinion, the fundamental nature of the sporophyte was mainly axial. The leaves and branches arose later as a modification of the axis.
Bower (1908) first suggested that originally the plant was wholly axial in nature and that the leaves arose later in the course of evolution. Lignier (1908) proposed that a primitive pteridophytic shoot was basically a leafless dichotomously branched structure and some of the branches got themselves modified into leaves.
Mainly there are two theories to account for the origin of leaves. These are the telome theory and the enation theory. While telome theory accounts for the origin of both microphylls and megaphylls, enation theory explains only the origin of microphylls.
Origin of Megaphyll:
According to telome theory first proposed by Zimmermann (1930, 1938), primitive pteridophytic shoot was naked and dichotomously branched. The plant body resembled the fossil member Rhynia. The ultimate dichotomies are called telomes and the internodes below the telomes are known as the mesomes. The telomes and mesomes together constitute a telome truss.
According to telome theory, a megaphyllous leaf originates as per the following steps:
In this, the original dichotomous branching system changes to unequal dichotomy resulting in the formation of short and long branches. This leads to sympodium and ultimately to a monopodium with a main stem and lateral branches (Fig. 4a,4b).
The branching of the telome trusses which were originally in all directions orient themselves in a single plane and come closer (Fig.4c).
3. Syngenesis or Webbing:
The telomes which have come closer, laterally fuse by the development of parenchymatous tissues between them (Fig.4d). This results in a leaf blade possessing a number of free ending veins. Evidence for this theory is obtained from many fossil ferns where the leaves have an open venation. A further evidence is in the ontogeny of the fem leaves where the adult leaves have a closed venation while the first formed ones have open veins.
Origin of Microphyll:
The origin of a single veined leaf is explained by both the telome theory and the enation theory. Followers of the telome theory argue that the leaf arose from a surviving telome. After overtopping and planation, only one of the telomes survived (reduction), the remaining degenerated. The surviving telome developed parenchymatous pads which formed the lamina.
There are however some objections to the telome theory accounting for the origin of the microphyll. These are:
1. If indeed a telome has transformed itself into the central vein of the microphyll, then in all instances the microphylls should possess a complete vein running to the tip. But there are any number of examples of microphylls having no veins or incompletely developed veins. This is indeed difficult to comprehend as per the telome theory.
2. It is one of the morphological principles that an organ, develops first and only later the vasculature travels into it. If we accept the telome theory for the origin of microphyll we will be arguing for the emergence of vasculature first and then to justify the vasculature, the development of the leaf.
According to Enation theory proposed by Bower (1908), the leaves are not the modifications of telomes (branches), but new developments or outgrowths from the shoot. According to Bower the outgrowths which he called enations were first spine like, later became flattened and leaf like.
In the beginning these were only emergences and lacked a vasculature. Subsequently the main vasculature of the stem gave out a branch which ran up to the base of the leaf only. From this stage by further evolution a vascular strand grew up to the leaf-tip. Indeed there are quite a good number of fossil evidences depicting these stages.
In the evolution of the vasculature, Psilophyton (a fossil member of the order psilophytales) is quoted as the first step (no vasculature), Asteroxylon, the second step (vasculature only up to the base of the leaf), and Arthrostigma, the third step (vasculature traversing half way up the leaf apex). In the present day Lycopodium and Selaginella the vasculature runs up to the leaf apex. Regarding the origin of the microphyllous leaves enation theory seems to be more convincing than the telome theory.
Origin of Equisetaceous Leaf:
It is difficult to account for the origin of the equisetaceous leaves because of their whorled condition. Neither telome theory nor enation theory can explain convincingly the origin of whorled leaves. Members like Sphenophyllum (a fossil member of sphenopsida) had sessile wedge shaped leaves with dichotomous venation. Such leaves may be modifications of a telome. The single veined leaves found in Equisetum and others, may be a reductional form.
Origin of Roots:
According to Zimmermann (1930) some of the branches of the fundamental axis grew downwards thus forming the root system. In some primitive members, in the type of branching and mode of growth, the root and the aerial shoot are similar indicating that they are modifications of the same axis. Zimmermann (1930) believes that the roots differentiated before the origin of leaves.
Development of Root and Shoot Apex:
The stem and roots usually develop with the help of a single two or three sided apical cell. Bhambie and Puri (1963) have worked on the shoot apex organisation in lycopodiales, while Bhambie and Rao (1973) have worked on the root apical organisation in ferns. According to them the single apical cell found in the roots of leptosporangiate ferns is simpler than the group of initials found in the root apex of eusporangiate ferns.
Vasculature in Pteridophytes:
All the pteridophytes possess a vasculature in the center of the axis traversing from one and to the other and branching with all the branches. The two main vascular elements are the xylem and phloem. The xylem consists of mostly, tracheitis and rarely, vessels (Selaginella, Pteris etc.,).
The tracheids have different types of thickenings like scalariform, pitted, annular etc. The tracheitis or vessels are of two types viz., protoxylem and metaxylem. The former matures early and has a narrow lumen while the latter matures later and has a wide lumen.
The relative distribution of protoxylem and metaxylem elements forms the basis for the classification of xylem group into three types viz., exarch (centripetal-protoxylem pointing towards the periphery), endarch (centrifugal- protoxylem pointing towards the center), and mesarch (protoxylem having metaxylem on either side).
The phloem is composed of sieve tubes and phloem parenchyma. A sieve tube consists of a series of long, living cells with sieve plates. Sometimes a distinction could be made between proto- and metaphloem. But there is no pronounced morphological difference between the two.
Types of Vasculature (Stele):
A transverse section of the stem or root of the plant body shows an outer epidermis, middle cortex and central stele. The cortex is either parenchymatous or may have sclerenchyma also. The central region consists of the concentration of the vasculature known as the stele. A stele may be defined as a vasculature surrounded by pericycle and endodermis.
Originally plant anatomists thought that only the vascular bundles (xylem and phloem) formed the fundamental unit of the vasculature. Stele as the fundamental unit of the vasculature was first proposed by Van Tieghem and Douliot (1886). According to them, the cortex and the stele are separated by the endodermis. (The endodermis consists of barrel shaped cells and possesses thickenings known as casparian thickenings on the radial and tangential walls).
There are mainly three types of steles in pteridophytes. These are:
(2) Siphonostele and
A protostele has the vasculature (xylem) occupying the central region of the stele. There is no pith. In a siphonostele the vasculature is like a tube or siphon. Here the central region consists of non vasculated cells (parenchyma) forming the pith. In a dictyostele the siphon breaks up into a number of individual bits called meristcles.
Basically all protosteles are alike in not having a pith.
They may be classified into the following types based mainly on the configuration of the xylem:
This is the simplest type of stele that could be visualized for a vascular plant. The central region of the stele consists of a smooth, core of xylem surrounded by phloem (Fig.7a). This type of stele is seen in Rhynia.
The xylem is star shaped and the phloem completely surrounds the xylem. The contour of the stele is smooth (Fig.7b), e.g., Psilotum, Lycopodium phlegmaria etc.
The xylem breaks up into a number of bands or plates arranged parallelly. The phloem not only surrounds the xylem but is distributed between the xylem plates (Fig.7c), eg., Lycopodium wightianum.
In this type, the xylem breaks up into a number of small masses. The phloem is intermixed with the xylem (Fig.7d). eg. Lycopodium cernuum.
Generally the stem is traversed by a single stele. But in some cases as in some species of Selaginella, several independent steles run parallelly in the stem. Each stele has its own endodermis. The individual steles are haplostelic (Fig.8f).
The steles having a pith in the centre are referred to as siphonosteles. These are further classified into two types viz., simple siphonostele and solenostele.
Here the central region of the stele consists of a parenchymatous or sclerotic pith. Leaf traces that depart from the main vascular cylinder do not leave any leaf gaps (a parenchymatous cavity found immediately above the leaf trace in the vascular cylinder). This type of siphonostele is also called medullated protostele or ‘cladosiphonic’ stele. For example, Psilotum sp.
Siphonosteles with leaf gaps are called solenosteles. In solenosteles, the leaf gaps are successive so that there is only one break in the vascular cylinder at any one given point. Solenosteles are also known as phyllosiphonic steles.
Depending on the pattern of distribution of phloem, solenosteles may be classified into two – Amphiphloic solenostele in which the phloem lines the xylem both on its inner and outer face (Fig.8a) eg., Adiantum, Marsilea) and Ectophloic solenostele in which phloem lines the xylem only on its outer face, eg. Osmunda).
There is also a third category of solenostele called polycyclic solenostele in which there will be more than one concentric ring of vasculature (Fig.8e).
Basically dictyosteles are similar to solenosteles in having a pith and leaf gap. In dictyosteles, however, the leaf gaps overlap (many occur at a point), as a result the vascular ring breaks up into many arcs. Each arc is known as a ‘meristele’. All the meristeles are surrounded by a common endodermis.
Based on the number of vascular rings dictyosteles may be classified into simple dictyosteles in which there is only one ring and polycyclic dictyosteles in which there will be at least two concentric rings of vasculature.
Origin of the Vasculature:
The origin of the vascular tissue is one of the unsolved problems of plant morphology. It is closely linked with the problem of origin of land plants, which also is eluding a solution. If the algal ancestry for a land plant is proposed, then, there are no steps indicating the origin of vasculature.
If one believes in the bryophytic ancestry of land plants, the columella in the sporophyte of Anthoceros seems to be a good starting point for the origin of vasculature. Indeed in its ideal location and to some extent in its function, (through not in structure) the columella of the sporophyte of Anthoceros tempts any one to consider it as a precursor of the vasculature.
Evolution of the Vasculature:
Among the different types of steles, it is generally believed that a solid core of vasculature is more primitive than a cylinder of vasculature. From this standpoint, protostclcs are admittedly primitive in not having a pith.
Among the different types of protosteles, haplostele is regarded as the most primitive and the mixed protostele as the most advanced. The tendency seems to be towards the breaking up of a single xylem group into several bits.
If we regard that a siphonostele is more advanced then a protostele and that the former evolved from latter, we have to account for the origin of the pith. The method by which pith originated is debatable. There are mainly two theories viz., medullation theory and cortical intrusion theory.
According to medullation theory, the pith originated in situ i.e., by metamorphosis of the vascular elements into parenchyma (Boodle 1901, Gwynne – Vaughan 1903). The presence of ‘mixed pith’ (tracheids intermixed with parenchyma in the pith) is used as an evidence to show the medullation of the central vasculature. Steles with mixed piths are regarded as transitional stages between protosteles and siphonosteles.
According to the cortical intrusion theory, the cortical parenchyma cells intruded into the stele and produced the parenchymatous pith (Jeffrey 1902). Similarity of tissues (parenchyma) in the pith and cortex is an evidence for cortical intrusion.
An additional evidence for cortical intrusion is the presence of an inner endodermis. Jeffrey (1902) regards endodermis to be a part of the cortex. In such a case presence of an inner endodermis lining the pith proves the cortical origin.
Supporters of this theory believe that steles with inner endodermis are more primitive than those without an inner endodermis. It is held that steles of the latter type arose during the course of evolution by obliteration of inner endodermis. This theory is mainly based on the cortical origin of the endodermis. But the origin of the endodermis whether cortical or stelar is an open question.
Whatever may be the course pith took in its origin, siphonosteles are no doubt more complex and more evolved than the protosteles. The overlapping of the leaf gaps in the siphonosteles seems to have given rise to a dictyostele.
A dictyostelic condition may also result due to lacunae in the vascular cylinder which are not associated with the leaf. Such lacunae are called ‘perforations’. Polycyclic dictyostele is the most complex type of vascular organisation seen in pteridophytes.
The sporophyte of pteridophytes generally reproduces by two methods viz., vegetative propagation and spore production. Vegetative propagation takes place by a variety of methods such as fragmentation, bulbil formation, resting bud formation, persistent apices etc. (The details of these are given with the life histories of individual forms).
The characteristic method of reproduction is by the formation of spores. Spores are haploid and are produced in sporangia. The sporangia are generally borne on the adaxial surface of a leafy appendage called the sporophyll. There are mainly three lines of evolution seen in sporophylls.
Evolution of Sporophylls of Lycopsida:
The sporophylls of lycopsids always have a single sporangium borne on their adaxial surface (surface close to the axis). According to telome theory, the sporophylls originated by the modification of telomes in a telome truss.
Followers of the telome theory believe that in a telome truss, of the two telomes the upper was fertile and the lower sterile. The sterile gradually flattened while the fertile telome had a gradual reduction of the basal sterile portion, with the result, the sporangium came to lie on the leafy appendage below (Fig.9a,9b,9c).
According to enation theory, the lycopodian sporophylls were evolved on a naked shoot bearing a number of sporangia. Below each sporangium an emergence evolved which ultimately grew into the sporophyll. Thus according to this theory the sporophyll and the sporangia are fundamentally different.
Evolution of Sporangiophores in Sphenopsida:
The fertile appendages of sphenopsida are strikingly different from the usual sporophylls of other pteridophytes. Hence they are referred to as sporangiophores. According to telome theory, after overtopping and planation, the fertile telomes bearing sporangia at their tips recurved and the bent arms fused resulting in a sporangiophore with sporangia on the lower surface (Fig.9g-9i). The recurved appendages of Calamophyton, Hyeriia etc. give credence to this view.
Evolution of Sporophylls in Pteropsida:
According to telome theory, in a fertile telome truss the usual processes of overtopping, planation and syngenesis resulted in the formation of a web like sporophyll with the sporangia borne at the tips of veins in the margin of the lamina (Fig.9a-9c).
In Lycopsida, as also in Sphenopsida there is a tendency for the sporophylls or the sporangiophores to aggregate at the apices of branches to form a compact strobilus or cone. The organisation of strobilus or cone takes place after the sporophylls are clearly differentiated from the sterile foliage leaves.
In ferns of the present day, generally there is no difference between sporophylls and foliage leaves. A foliage leaf besides helping in photosynthesis also bears the sporangia.
While in lycopods and sphenopsids the tendency is for the accumulation of sporophylls to form strobili, in ferns the tendency seems to be towards the aggregation of sporangia into a sorus. A special case is that of Marsilea where the spore bearing organs form a compact bean shaped structure called the Sporocarp.
Pteridophytes are homosphorous (Isosporous) or heterosporous. Homosporous members produce only one type of a spore in the sporangium which develops into a gametophyte bearing both antheridia and archegonia. Heterosporous members produce two types of spores i.e., microspores and megaspores (macrospores).
The microspores are smaller in size and are produced in microsporangia which are borne on microsporophyll’s. Microspores develop into the male gametophyte (Micro-gametophyte). Megaspores are bigger in size and are produced in mega-sporangia borne on megasporophylls. On germination megaspores develop into the female gametophyte (mega or macro-gametophyte).
Structure and Development of a Sporangium: A sporangium usually has a stalk which bears the globular capsule region in which are produced the spores. The wall of the sporangium is one to many layered. Within the wall are found the spore mother cells which undergo reduction division to give rise to haploid spores. There are mainly two types of sporangial development i.e., Eusporangiate type and Leptosporangiate type.
In Eusporangiate type the sporangium is derived from a group of cells; further after the initial periclinal division the sporogenous tissue is derived from the inner daughter cell. In Leptosporangiate type, the sporangium is derived from a single initial cell; further the sporogenous tissue is derived from the outer daughter cell of the sporangial initial (The details of the development of Eu-or Leptosporangiate sporangia are discussed with the individual forms).
A mature sporangium dehisces either vertically or transversely. Special types of cells may or may not be present in the wall to help in the dehiscence. Usually in ferns the sporangia have a thick walled annulus and a thin walled stomium. The annulus may be vertical, shield shaped or cap shaped. By the differential hygroscopic response of these cells, the sporangium breaks open liberating the spores. The spores are usually wind disseminated.
Gametophytic Generation of Pteridophytes:
The haploid spore is the starting point in the development of the gametophyte. In heterosporous forms two types of spores are formed (mega and microspores) hence there will be two types of gametophytes (male and female). In homosporous forms there will be only one type of a gametophyte.
The spores rarely exceed a few mm in size. Their shape varies from triradiate to spherical. They have a two layered wall. The outer sculptured layer is known as exine (exospore) and the thin inner layer is known as intine (endospore). Sometimes there will be an outermost epispore and a middle mesospore in addition to exine and intine.
The germination and development of the gametophyte is different in homosporous and heterosporous forms.
Rashid (1976) recognises two stages in the germination of spores of pteridophytes. These are:
(i) A spore distension process and
(ii) A spore extension process.
During the first phase the spore absorbs moisture and becomes swollen, while during the latter germ tube is formed. Nayar and Kaur (1968) have classified spore germination in homosporous ptoridophytes into three categories.
(i) Bipolar e.g. Equiselum, Lycopodium, Osmunda etc.
(ii) Tripolar e.g. Trichomanes, Hymenophyllum, Macodium etc.
(iii) Amorphous e.g. Angiopteris
In heterosporous forms the gametophytcs are extremely reduced and their size is limited to the confines of the spore. The development of the gametophyte here is called endosporic. In homosporous forms the gametophytes are comparatively bigger and they break open the spore to grow independently on the soil.
This type of development is said to be exosporic. Homosporous gametophytcs are filamentous, cordate or tuberous in shape. Some of them exhibit dorsiventral symmetry. The gametophytes are composed mainly of parenchyma. The nutrition is either autotrophic (when chlorophyll is present) or saprophytic (when mycorrhiza is present).
Gametophytes also reproduce by two methods viz., Vegetative propagation and sexual reproduction. Vegetative propagation is uncommon and it takes place by the formation of gemmae or brood bodies (See Lycopodium).
Sexual reproduction takes place by the formation of antheridia (male) and archegonia (female). Both the sex organs are extremely reduced in comparison with their counterparts in bryophytes. An antheridium is a globose structure partially or completely embedded in the gamatophytic tissue. It has a single layered jacket enclosing the androgonial cells. The atherozoids are spirally coiled and may be bicilliate or inulticiliate.
An archegonium is a flask shaped structure having a basal venter and a short neck. Within the archegonium are found one or two neck canal cells one venter canal cell and egg cell (Details of the development of sex organs are given with the individual forms).
Even in pteridophytes fertilization is dependent upon external moisture. The antherozoids that come out of the antheridium are attracted chemotactically by the mucilaginous mass produced by the disintegrating cells (neck canal and venter canal cells) of the archegonium. Many antherozoids enter the archcgonium. Ultimately one succeeds in fusing with the egg resulting in the formation of a zygote.
The zygote represents the first cell of the sporophytic generation. The first division of the zygote is either transverse or vertical. When the division is transverse two superposed cells are formed, of which the one nearer to the archegonial neck is the cpibasal cell and the one away from the archegonial neck is the hypo-basal cell.
The embryo proper may be derived from either epibasal cell (exoscopic) or the hypo-basal cell (endoscopic). In the latter case the epibasal cell forms a tubular structure known as the suspensor. The function of the suspensor is to push the developing embryo deep into the gametophytic tissue so that it can easily absorb nutrition.
Further divisions in the embryonal cell are variable. After the first few divisions, the characteristic parts of the embryo like foot, root, cotyledons and stem apex are differentiated. Until the root establishes itself on the soil, the foot, behaving like a haustorium absorbs food from the gametophyte. After the root grows into the soil the sporeling separates itself from the gametophyte and grows independently.
Aberrations in the Life Cycle:
In the normal type of life cycle showing alternation of generations the sporophyte gives rise to a gametophyte and vice versa. Syngamy and reduction division occur at specific points in the life cycle so that the gametophyte is always haploid and sporophyte is always diploid.
In some cases there are deviations or aberrations in the life cycle disturbing the chromosome constitution of the sporophyte and the gametophyte. The aberrations are of three types viz., apogamy, apospory and parthenogenesis.
This was first discovered by Farlow (1874) in Pteris cretica. Winkler (1908) defines apogamy as follows; “It is the formation of sporophyte directly from the vegetative cells of the gametophyte without the act of syngamy or gametic union”.
In nature, apogamy has been reported in some twenty genera and fifty species of pteridophytes. Apogamy is of frequent occurrence in ferns. It is seen in Pteris, Adiantum, Osmunda etc.
Apogamy may also be experimentally induced as in Lycopodium,Equisetum etc.
Apogamous sporophytes may originate from one or more cells of the gametophyte.
Regarding the causes of apogamy several explanations have been offered. According to Lang (1902) starving the pro-thalli of water may induce spoiophytic buds. It is generally believed that failure of normal fertilization results in apogamy.
Cytologically apogamous sporophytes are haploid. But there are many instances of such apogamous sporophytes having 2 x number of chromosomes. This is possible when both generations have same chromosome numbers. A gametophyte can have diploid chromosomes if there is failure of reduction division during spore formation.
The phenomenon of production of a gametophyte directly from the vegetative cells of the sporophyte without reduction division (spore formation) is called apospory. Apospory was first discovered by Druery (1894) in a fern Athytiumfillix-fimina var. clarissima.
He observed the development of gametophytes from the sporangia. Subsequently apospory has been reported in several genera including Osmunda. Apospory may also be experimentally induced like apogamy. There are reports of induced apospory in Pteridium Cyclosorus etc.
In apospory, a filamentous or chordate gametophyte may be formed from one or more vegetative cells of the sporophyte. The aposporous structure may be an antheridium, a rhizoid, or even an antherozoid. Cytologically aposporous gametophytes are diploid because they are produced without reduction division. By experimental induction of apospory it is possible to produce polyploid gamctophytes.
The development of an unfertilized egg into an embryo is called parthenogenesis. This is different from apogamy because here only the egg is capable of developing into an embryo. Parthenogenctic embryos are haploid. But in some cases when the spores are unreduced, the resultant gametophytes are diploid.
Consequently the eggs are also diploid and they develop into an embryo without fertilization. In these instances the parthenogenctic embryos are diploid. This type of regularization of parthenogenesis coupled with failure of reduction division during megaspore formation has been reported in Marsilea drummondii.
Attempts have been made to study pteridophytes from the point of view of chemosystcmatics. Czcczuga (1985) has studied the distribution of carotcnoids in about 66 representative members of pteridophytes. According to him β carotene, β cryptoxanthin, lutein epoxide and zcaxanthin are seen in Lycopods and horsetails, while ferns are characterized by β cryploxanthin, lutein epoxide, violaxanthin and rhodoxanthin.
Classification of Pteridophytes:
Since a long time vascular plants are customarily divided into pteridophyta and Spermatophyla. This classification was based on the assumption that the former lack the seeds while the latter produced them. But the discovery of Ptcridosperms (seed bearing ferns) broke down this artificial classification.
Sinnott (1935) introduced the term Trachcophyta to include all vascular plants. Trachcophyta are further divided into four main groups viz., Psilopsida, Lycopsida, Sphenopsida and Ptcropsida. But it is not certain whether these are divisions or classes. Haupt (1953) considers them as classes in the division Trachcophyta. While Zimmcrmann (1930) Arnold (1947) Wardlaw (1952) etc., regard them as divisions.
Reimers (1954) considers Pteridophyta as a division and divides it into live classes viz., Psilophytopsida, Lycopsida, Psilopsida, Articulatac and Filices.
Smith (1955) Bold (1957) and others divide vascular cryptogams into five divisions viz., Psilophyta, Lycophyta, Sphenophyta, Noeggerathiophyta and Pterophyta. Some people have changed the name Lycophyta to Lepidophyta, Sphenophyta to Arthrophyta or Calamophyta and Pterophyta to Filicophyta.
In this book the classification is mainly based on the one proposed by Reimers in the 1954 edition of Engler’s Syllabus der pflanzenfamilies.
This classification is simple and easy to follow. Below is given the outline classification and the members discussed in this book.
Heterospory and Seed Habit in Pteridophytes:
Pteridophytes have many unique characters in both structural and reproductive features. Being the first true land plants they include in their bodily features the results of many experiments that nature conducted with the sporophytic plant body on a primordial terrestrial habitat.
The existence of heterospory and its relation to seed habit has an important bearing on the origin and evolution of seed plants (spermatophytes). Most of the pteridophytes are homosporous while only a few are heterosporous. In the foregoing paragraphs we will discuss heterospory, its origin and its relation to seed habit.
What is heterospory? The phenomenon of the production of two types of spores in an individual is called heterospory. As opposed to homospory, in heterosporous members two types of spores viz., microspores and megaspores are produced.
Microspores are smaller in size and many of them are produced in a microsporangium. Megaspores are larger in size and a few of them are produced in a mega-sporangium. Heterospory would not have had any significance, if the difference between microspores and megaspores were to lie only in their sized. But as it happens, there is a difference in their sexual behaviour also.
While microspores develop into male gametophytes, megaspores develop into female gametophytes. Sex differentiation that is seen only in the gametophyte in homosporous members has reached the sporophyte also, in heterosporous members. Among the living pteridophytes heterospory is seen in Selaginellales, Isoetales, Marsileales and Salviniales.
Origin of Heterospory:
It has been accepted by all the botanists that homosporous condition is more ancient than heterospory. Then naturally we must find in some of the more enterprising homosporous members certain indication towards a heterosporous condition. Evidences for the origin of heterospory may be obtained from three different types of studies viz., paleobotanical studies, developmental studies and experimental studies.
As the fossil evidence indicates, heterospory is quite widespread among the ancient pteridophytes; in some of them heterospory is not well pronounced indicating a transitional stage. In Calamostachys, both homospory (C. binneyana) and heterospory (C. casheana) is seen.
In C. binneyana some of the sporangia show spores of unequal size. Based on this, Scott (1894) observed that spore degeneration that is seen in most of the sporangia is an important factor in the origin of heterospory. If the process of degeneration, stops when still many spores are surviving, leads to the formation of more number of spores.
Since the sporangial cavity has limited space and nutrition, the spores that are many in number cannot grow to a bigger size. They remain small. If on the other hand, as Scott (1894) observes, the process going on more freely in some sporangia than in others, may ultimately have rendered possible the excessive development of those spores that survived, at the expense of the others and may thus have led to the development of specialised megaspores.
Transitional stages leading to a heterosporous condition have been met with in members like Spenophyllum dawsonii, Stauropteris burntislandica etc.
Developmental studies conducted on living heterosporous members like Selaginella, Isoetes, Marsilea, etc., clearly point out that basically micro and mega-sporangia are alike and that the differentiation starts only at the spore mother cell stage. In a sporangium that is to become a microsporangium many spore mother cells survive whereas in a sporangium that is to become a mega-sporangium only few spore mother cells survive.
Some of the experimental studies indicate nutrition, temperature, light, etc., may also play a role in the manifestation of heterospory. According to Goebel (1891) plants of Selaginella grown under feeble illumination tend to produce only microsporangia in their strobili. Shattuck (1910) has demonstrated in Marsilea that variance of temperature may induce heterospory.
By suddenly lowering the temperature (by a cold water spray) it is possible to kill all the megaspores and a few of the microspores in Marsilea. The surviving microspores grow to a larger size simulating the megaspores. The growth of the microspores to a larger size is made possible by increased nutritional supply.
A solution to the problem of the origin of heterospory possibly has to be found by biochemical studies. It is quite possible that some biochemical compounds (possibly hormonal) determine the sexual behaviour of spores.
In homosporous spores the hormones that govern the male and the hormones that govern the female character are balanced. When the process of degeneration starts, if it goes beyond a certain limit, naturally, there will be imbalance and this leads to a change in the sexual behaviour.
Biological Advantages of Heterospory:
Heterospory has brought in many advantages to the individuals possessing two different types of sporangia. Since the female gametophyte develops within the confines of the megaspore it has an assured supply of nutrition. It need not be dependent on the external conditions, like a free living gametophyte.
This is a boon to a gametophyte on the terrestrial habitat and forms a good starting point for the development of embryo than an independent pro-thallus which has to meet successfully the challenges of a terrestrial environment for survival.
Heterospory introduces drastic reduction in the size of the gamclophytcs. As has been pointed out in the preceding paragraph, the female gametophyte is reduced to such an extent that it never outgrows the limits of the megaspore. The reduction in the male gametophyte is still extreme. In most cases it is nothing but an antheridium with but one or two cells representing the vegetative tissue.
Another advantage of heterospory is the extension of the sex differentiation from the gametophyte to the sporophyte. The culmination of such a tendency leads to production of two types of sporophytes. This, of course has not been achieved in pteridophytes.
Implications of Heterospory:
While heterospory brought in many advantages certain other problems also cropped up by its introduction. In homosporous gametophytes, the two compatible cells i.e., the egg and the sperm are on the same pro-thallus and have no difficulty in coming together.
But with the introduction of heterospory, the two are separated in time and space. There has to be some mechanism to bring together the two compatible gametes. This is made possible by the innovation of pollination.
Of course, in most of the pteridophytes, pollination does not seem to be necessary since both male and female gametophyte are released out. But with the retention of the female gametophyte in the mega-sporangium this becomes a necessity. Even in pteridophytes there are indications of an incipient pollination as in Selaginella where there is temporary retention of the female gametophyte within the mega-sporangium.
What is Seed Habit?
A seed may be defined as an integumented mega-sporangium. It represents a further elaboration or fortification of a mega-sporangium so as to enable it to be a good starting point for the development of a new plant. The seed combines within itself three generations viz., parent sporophytic tissue (integuments and nucellus), gametophytic tissue (female gametophyte) and offspring sporophytic tissue (embryo).
The formation of seed may be said to be an example of parental care in land plants. In a terrestrial habitat, gametophyte is a weak link in the life cycle and if it is independent there is every possibility that it may not survive. In-order to eliminate this hazard, gametophyte (female) is permanently retained in the mega-sporangium thus paving way for the seed habit.
Relation between Heterospory and Seed Habit:
While it is true that all cases of heterospory have not led to the seed habit (because heterospory has originated independently in several groups of lower vascular plants) it is also true that heterospory is the prerequisite for a seed habit. There is no example anywhere in the plant kingdom of a homosporous condition leading to a seed habit.
Origin of Seed Habit:
Mainly there are four steps leading to the formation of seed habit,
1. Heterosporous condition
2. Permanent retention of megaspore within the megasporangium.
3. Development of only one megaspore in the megasporangium, and
4. Development of protective layers (integuments) around the megasporangium.
It is difficult to say whether seed habit arose only once or originated along several independent lines. Any of the living or fossil plants exhibiting the above mentioned steps may be said to be the forerunner of seed plants.
One of the important features that begin the seed habit is the organic connection between the megasporangial wall and the megaspore. In non-seed bearing plants, the megaspores are isolated from the wall of the megasporangium by the deposition of callose. Such a callose deposition is absent in seed plants though it is present in vestigial form in pteridosperms and in some of the cycadales.
Advantages of Seed Habit:
Accepting the fact that the gametophyte is a week link in the life cycle (though it is necessary to bring about sexual recombination) it is not conducive for the plant to allow the gametophyte to live independently.
In seed plants the female gametophyte is retained permanently in the ovule (megasporangium) so that it has assured nutrition. As has already been pointed out, the seed combines three generations and is a perfect example of parental care in land plants wherein the parent sporophyte assures the survival and growth of the embryo.
Implications of Seed Habit:
Implications may be said to be after effects of the introduction of a seed habit. With the introduction of heterospory and with the permanent retention of the female gametophyte in the megasporangium the two compatible cells i.e., the egg and the sperm are separated in times and space, hence there arises the necessity of pollination. With the advent of seed habit a fresh difficulty arises in the matter of fertilization.
The megasporangium develops protective layers (integument) around it to offer protection to the female gametophyte. But this impedes fertilization as there are barriers between the microspore and the egg.
In order to overcome this, siphonogamy is introduced to carry the male gamete to the vicinity of the female gamete. In siphonogamous fertilization a tube is formed from the microspore which grows through the wall layers of the megasporangium and releases the sperms in the vicinity of the egg.
Has any Living Pteridophyte reached the Seed Habit?
Of all the extant members Selaginella may be said to provide the closest approach to a seed habit. It satisfies all the prerequisites of a seed habit.
(a) It is heterosporous.
(b) The megaspore usually germinates within the megasporangium and is not shed for quite some time.
(c) In some species only one megaspore survives in the megasporangium.
(d) There is an indication 6f incipient pollination in species (S.monospora) where megaspore is retained upto fertilization in the megasporangium.
But Selaginella fails to qualify as a seed plant, for the megaspore is ultimately shed and there is no formation of integument in the megasporangium. It has no’ dormancy (resting period) in the embryo which is characteristic of seed plants.
As Parihar (1966) observes “Indeed if one could conceive of a hypothetical case of a single spored megasporangium of Selaginella whose egg had been fertilized and had developed into an embryo while still surrounded by a megasporangium with integument, we should have a what might be regarded as the morphological equivalent of the seed of spermatophytes”.
Though Selaginella had all the attributes of seed plants, the ultimate shedding of the megaspore pulled it down to the’ pteridophytes, otherwise its rightful place should have been with the seed plants.
Economic Importance of Pteridophytes:
Nothing much is known regarding the economic value of pteridophytes. This is not because pteridophytes lack economic value, but because enough attention has not been paid towards harnessing the potentialities of pteridophytes towards human welfare.
Since a long time pteridophytes have remained the exclusive domain of academicians, rarely heard outside the academic world. It is quite evident however that further investigations on the practical application of the study of pteridophytes would certainly yield very interesting results. A few uses of the pteridophytes are given below.
The dusty spores of Lycopodium have been used in pharmacy as water repellants, as protective dusting powder for tender skin, in the compounding of pills and in the preparation of suppositories.
The foliage of L. obscurum is used for decorations. Extracts of Lycopodium plant have been used in the past as kidney stimulants. The spores are highly inflammable (Vegetable Brimestone) and are used in the manufacture of fire-works and to produce stage lightening in the theatre.
Species of Selaginella (S.wiltdenovil and S.caesia) are cultivated for their metallic and many hued tints and are in great demand for their bronze and bluish colours. S. serpens has periodic change in the colour of the leaves from deep green (morning), paler (day) to deep green again (evening). S. lepidophylla is sold under the name resurrection plant.
The species of Equisetum were used in the past for polishing wood and scouring pewter dishes. E.arvense is used in German pharmaopoeia as a diuretic. It also has haemostatic and haemopoictic properties. E.arvense also consists of silica in the therapeutically active form. Some species act as mineral indicators of soil and are of value in ore prospecting. Actinopteris austrlys is an indicator of cobalt, while Asplenium adulterinum is a nickel indicator.
The roots and stem of Osmunda provide fibres which could be used for growing orchids.
The rhizomes and frond bases of Dryopteris are used as teanifuge in pharmacy under the name ‘Filix mas’.
Dry fronds of many ferns make good litter for livestock. They are also used as thatching material for cottages, as a source of potash (from the ash) and also as vegetables.
Most of the pteridophytes (specially ferns) are cultivated in green houses and gardens as ornamental plants. The delicate and attractive foliage of ferns is a joy to see as they beautify the garden or the green house.
Sporocarps of Marsilea (M.salvatrix) are used as a source of nutrition. The starchy sporocarp of M.drummondii, may be cooked into cakes,. It is used by the natives of Australia under the name nardoo.
Sexuality in Pteridophytes:
The determination of sex is pteridophytes has been a topic which has attracted the attention of plant morphologists since a very long time. There does not seem to be much to discuss about sexual differentiation in heterosporous forms, for in them the gametophyte arises from the microspore and the female gametophyte from the megaspore. Even here, a point of interest is the origin of two different types of spores in two seemingly different sporangia.
In many of the heterosporous pteridophyte, all the sporangia look alike until the spore mother cell stage. The differentiation begins after this. In some sporangia which are destined to produce megaspores, a large number of spore mother cells degenerate while the few surviving ones develop into megaspores.
In a microsporangium however, there is very less degeneration of spore mother cells, and consequently a majority of them develop into microspores. The question very pertinent here is what makes a microspore develop into the male gametophyte and a megaspore into the female gametophyte? An answer to this question certainly lies in the biochemical events that trigger the degree of degeneration of spore mother cells in the sporangium.
Unfortunately no study as yet seems to have been made to chemically analyse and study the physiological processes in the sporangium. Hence an answer to the question of sexual behaviour of the spore in heterosporous forms must await a biochemical study.
The study of sexuality in homosporous forms is a matter of great interest and has been studied in quite some detail. Because, here the spores are potentially capable of producing a bisexual gametophyte and at the genetic level there does not seem to be an evidence of gene segregation. Yet in many of the instances unisexual gametophytes have been found.
Sexuality in Equisetum:
From the morphological point of view, Equisetum is homosporous and produces spores which develop into only one type of gametophyte. This gametophyte produces both the sex organs, namely, antheredia and archegonia. But there seems to be conflicting reports regarding the sexual behaviour of gametophytes.
The gametophytes are dioecious, according to Campbell (1895), but Eames (1936) regards them as monoecious. Similarly, some of the later reports continue the conflict. According to Bold (1957), the gametophytes are heterothallic, while Fostser and Gifford (1959) clearly state that they are homothallic. From these reports it is obvious that there are two kinds of gametophytes in Equisetum. Those are the a) basic and b) modified types.
In the basic type (when all the factors are balanced) both the sex organs develop simultaneously. The modified types are of two kinds namely, male and female. It is obvious that the environmental factors have a profound influence in the suppression or expression of a particular sex.
As the experiments reveal, starved prothalli with limited nutrition tend to produce only antheridia, whereas better nutrition and space tend to encourage the production of archegonia. This clearly points out that the gametophytes of Equisetum are inherently bisexual and the occurrence of uni-sexuality is secondary and a consequence of environmental stresses and strains.
The experiments of Joyet Lavergne (1927) in connection with the sexuality of Equisetum spores is most interesting. According to him the spores of Equisetum are physiologically heterosporous. Those spores which have a lower oxidation reduction potential and have more osmic acid reducing fats, tend to produce female gametophytes.
The reports of Sehartz (1928) also confirm this. From this it is clear that there is a range among the spores in the oxidation reduction potential. While the two extremes of the range produce either male or female gametophytes, the intermediate condition produces bisexual gametophytes.
Sexuality in Homosporous Ferns:
Several studies have also been conducted with reference to the variable behaviour of prothalli in ferns. According to Atkinson and Stokey (1964), in homosporous ferns there is a sequence in the formation of gametangia. Antheridia are formed first followed by archegonia.
Favourable culture conditions tend to produce archegonia, while only antheridia are formed under starved conditions. Nagai (1914) indicated that culture solutions deficient in certain minerals tend to produce only male prothalli.
Light seems to play a regulatory role in the initiation of sex organs. Darkness suppresses both the sex organs while low intensity favours only antheridia and high intensity initiates both the sex organs. Exceptionally however, some forms like Allosorus, Onoclesa etc., are known to form antheredia even in darkness.
The quality of light whether green, red etc., is also known to influence the selective initiation of sex organs. According to Sabota and Partanen (1966) green light helps in the differentiation of antheridia in Pteridium aquilinum. In Microlepia speluncae, red light seems to promote antheridia while under white light it produces archegonia.
The discoveries of Dopp (1950) and Naf (1956) with reference to the hormonal control of sex differentiation in fern prothalli are most interesting. It was Dopp (1950) who first discovered an organ inducing substance in Pteridium aquilinum. In a culture of prothalli of this fern, he observed that the young gametophytes had only antheridia when they were in the company of old gametophytes.
He visualised that the substance produced by older gametophytes hastens the onset of antheridia. This substance was given the name ‘Pteridiuim factor and was later named ‘Antheridiogen’. Antheridiogen has been found in many families of homosprous ferns. Some of the recent studies have indicated that antheridiogen is a complex substance and it is of various types in different families of ferns.
Physiology of Spore Germination:
The spore of a pteriodophyte is a unique unit of reproductive ability. Even though it is a product of sporophyte, it does not seem to have inherited any sporophytic Character, for on germination it develops into an entirely new generation namely the gametophyte. It is quite likely that certain biochemical events that take place during spore maturation obliterate the sporophyte characters of spore and invest it with gametophytic characters.
Several physiological factors seems to be necessary for the initiation of germination. The first requirement is water. Germination begins on hydration of the spore. Some of the polypodiacious ferns seem to have spore dormancy and the viability ranges from few hours to few days.
The spores of some pteridophytes are capable of germinating in darkness. In some species of Lycopodium, spores germinate in darkness and produce gametophytes. Among the ferns spore germination in darkness has been noticed in species of Osmunda, Pteridium, Polypodium etc.
In certain species of Botrychium, light actually inhibits germination. As a contrast to the examples mentioned above, in some of the pteridophytes light seems to be necessary for germination. According to Mohr (1963) germination of spores in Dryopteria filix can be induced by alternate exposure to red and far red light, the last exposure should be red.
According to Raghavan (1971), in Asplenium nidus only five percent of spores kept in darkness produced rhizoids. There is sufficient evidence to indicate that phytochrome mediated response is an important factor in germination.
Biochemistry of Germination:
At the onset of germination the storage proteins are hydrolysed and there is high accumulation of RNA. A high rate of metabolic activity also has been noticed during germination.
Development of Gametophyte:
For the initiation of gametophyte from the mononucleate, single celled spore, establishment of polarity seems to be necessary. Because, the cells produced from the spore develop into different parts of the gametophyte. In many of the fern spores, the first division is unequal thereby establishing polarity. According to Mosebach (1943), application of light to a side makes it to become the apex.
The shaded area gives rise to rhizoids. The several divisions taking place in the spore ultimately lead to the formation of gametophyte. There are two distinct phases namely, the uni-dimensional phase (UO) and the bi-dimensional phase (B.D.) in the establishment of the gametophyte. U.D. phase is usually short lived and leads on to the B.D. phase.
Several physiological factors seem to control the shift from U.D. phase to B.D. phase. Of these factors light, particularly its quality and the attainment of sufficient cell number seems to be important. Biochemically the transition of B.D phase is attributable to a high rate of protein synthesis, and high RNA content.
Tissue Culture Experiments on Sporophyte and Gametophyte:
Several attempts have been made to culture the sporophytes and gametophytes on the culture medium. There have been reports of induction of callus and formation of individual plant body.
The cells of the gametophyte seem to have undergone little differentiation when compared with their counterparts of the sporophyte; as such the cells of the gametophyte are extremely totipotent. Complete gametophytes can be cultured from practically a single cell.
Regeneration of the gametophyte from fragments of prothalli has been studied extensively in ferns. Fragments lacking an apex develop adventitious sprout whereas those with apex develop into a full-fledged pro-thallus.
Under normal conditions prothalli lack the vascular tissue. A very rare example of a normal vasculated gametophyte is to be seen in tetraploid forms of Psilotum nudum. Under experimental conditions tracheids have been induced in fern prothalli by enriching the medium with sucrose and auxin.
The reports concerning the callus formation in gametophytes are very few. According to Morel and Wetmore, (1951), spontaneous callusing occurred in the gametophytes of Osmunda cinnamonia. Similar callusing has been reported in species of Lycopodium, Selaginella, Pteridium etc. In Pteridium aqualinum, callusing occurred on a medium containing salt, glucose and yeast extract. Normal prothalli may be obtained from this callus.
Callusing of gametophytic tissue has been reported by Kato (1964) in Pteris vittata.
According to him three types of tissues may be obtained. This are:
(a) Callus which occasionally develops into sporophytes.
(b) Compact callus which forms sporophytes in darkness and gametophytes in light.
(c) Friable callus capable of continuous proliferation.
Callus development from culture of sporophytic tissue has been reported in several pteridophytes. Culture of the stem and rhizophore segments in Selaginella produces callus. Similar callus formation has also been reported form the culture of stem segments in Equisetum. One of the reports indicates the callusing of shoot spices of Marsilea. To introduce the callus the medium has to be supplemented with Kinetin.
Peterson (1967) in Ophioglossum petiolatum and Bristow (1962) in Pteris critical have reported callusing. According to Bristow, the callus remains undifferentiated when the medium has 2.40 (0.1 ppm) or IAA (5 ppm). Differentiations in the callus may be obtained if auxin concentration reduced to 0.01 ppm. Higher concentrations (0.1% or more) of sucrose, glucose or fructose induce sporophyte formation.
Alternation of Generations and Aberrations (Experimental Studies):
The life cycles of all the plant groups from Bryophytes onwards exhibit an alternation of generations. Alternation of generations is defined as the alternation between a sexual, haploid gametophyte and a diploid asexual sporophyte.
In Pteridophytes as in other higher plants alternation of generations is always heteromorphic. Two phylogenetic theories have been put forward to explain the origin of sporophyte. These are the arthithetic theory and the homologous theory.
Antithetic theory proposed by Celakowsky and later improved by Bower is based on the concept that the sporophyte is a new generation interpolated between two gametophytic generations. According to Bower the origin of sporophyte is to be traced to the Zygote. This Zygote which was originally wholly sporogenic develops into the sporophyte by progressive sterilization of the potentially sporogenetic tissues.
The homologous theory of Pringscheim (1878) maintains that the sporophyte is not a new structure but is a modification of the gametophyte.
Evidences from ontogeny and comparative morphology have been cited in evidence of both the theories.
The discovery of apogany and apospory contributed to the controversy on the interpretation of alternation of generations.
The alternation of two phases, a sexual phase and an asexual phase is of very great interest from the point of view of this behaviour. To be precise, the differential behaviour of reproductive cells namely, the spores and gametes ultimately govern the alternation of generations. It is very interesting and at the same time perplexing to analyse as to why a cell from a particular plant body develops into a new type of individual.
For instance, in the sporophyte the spores which are haploid develop into the gametophyte. If one regards the halving of the chromosomes as responsible for the development of spore into a gametophyte, in reality the spore should have developed into merely a reduced sporophyte and not into a gametophyte with an entirely different reproductive behaviour.
Similarly in the gametophyte the egg or even the zygote should have merely developed into a giant sized gametophyte. The fact that this does not normally happen is because of certain biochemical events that take place during spore maturation and egg maturation. In an article ‘Archegoniate revolution’ (Bell, 1964) it is reported that lysosomes appear very prominently in the cytoplasm of maturing spore and egg.
According to Bell (1964) these lysosomes destroy in spore the parental characters (sporophytic characters in spore and gametophytic characters in egg) and invest them with characters of new generation. That is why a mature spore would be regarded not as the last generation of sporophytic cell, but as the first generation of gametophytic cell.
The reports of Blackman (1909) also indicate the extra chromosomal components of the cell being responsible for the differential behaviour of the spore and egg or zygote resulting in alternation of generations.
Some of the studies of Lang (1909) also give credence to non-chromosomal factors as being responsible for the differential behaviour of germ cells. According to him, while the free germination of a spore tends to make it a gametophyte, the fertilized egg developing within constraints tend to make it a sporophyte.
An experimental evidence was provided by Ward and Wetmore (1954) when they partially released the pressure on the zygote by eliminating the surrounding prothallial cells. This zygote with no pressure from the surrounding cells had disturbed embryogeny and developed into an irregular aggregation of cells instead of into a normal sporophyte.
Experimental Induction of Apospory and Apogamy:
Apospory and apogmy may be defined as aberrations in the normal course of alternation of generations. Apogamy deals with the formation of sporophytes without syngamy, while apospory deals with the formation of gametophyte without fertilization. Apogamous sporophytes are haploid while aposporous gametophytes are diploid.
A variety of factors, internal as well as environmental are known to help in the artificial induction of apospory and apogamy. In Doodia caudata, vertical growth of the gametophyte resulted in the induction of apogamy. Lang (1896) reported apogamy in fern prothalli by exposing them to direct illumination.
Some of the more recent experiments by Whittier and Steves (1960) in Pteridium aquilinum indicate that prothalli grown on medium with excess of glucose tend to be apogamous. By varying glucose concentration, apogamy has been induced in a number of ferns.
Sucrose seems to be more effective than glucose in inducing apogamy. Besides glucose, a number of other factors like carbohydrate and mineral nutrition and plant hormones are also known to be effective in inducing apogamy.
Experimental induction of apospory has been reported in a number of ferns. According to Bell and Richards (1958), injury and starvation of the sporophyte are helpful in bringing about apospory. Reduction in the rate of metabolism and absence of nutrition favour aposporous responses in a sporophyte.
Even sucrose seems to play an important role in the induction of apospory. Gametophytes are known to be produced on the roots of young plants of Pteridium. Elimination of carbohydrates from the medium hastenes the induction of apospory.
Aposporous production of gametophytes have also been seen in the very young leaves of Pteridium aquilinum. Similar production of aposporous gametophytes has been reported from the juvenile leaves of Adiantum pedatum.
Morphogenesis in the Sporophyte:
The differentiation of the various sporophytic organs and in some case of the entire plant body itself under culture conditions comes under the purview of morphogenesis. The various forms of Marsilea serve as a very good illustration of whole plant morphogenesis.
Under natural conditions, the land and water forms of Marsilea can be clearly distinguished. By culturing these in the laboratory Alsopp (1955) has shown that at different levels of concentration, carbohydrates determine the differentiation of sporophyte into either land or water forms.
According to them 5 percent glucose in the culture solution produced a land form even though the plant was completely submerged. At 2 percent glucose, only water forms developed.
Differentiation of Individual Sporophytic Organs:
In Equisetum arvens, the excised roots were cultured on White’s medium. Different types of auxins like IAA, IB A and NAA increase the number of lateral roots. Partanen and Partanen (1963) successfully cultured excised roots of Pteridium aquilinum on Knudson’s medium containing 1.5 percent sucrose. Application of Kinetins to roots induced proliferation of the cortical cells.
Some of the experiments of Wardlaw (1947) have indicated the autonomous behaviour of the apical meristem in Dryopteris. He was able to obtain a normal shoot from excised apical meristems. Culture of excised apical meristems has also been successful in Adiantum padatum. Such cultured species grow into normal plants.
In Selaginella there is a particular type of meristem at the region of branching. This is called ‘angle meristem’ and is responsible for rhizophore growth. Culture of the angle meristems have resulted their development into leafy shoots. Angle meristems in some cases may also be cultured into roots. According to Williams (1937) if the cultured meristems are treated with auxins, they develop into roots.
Excised leaves of many Pteridophytes have been cultured. Steeves and Sussex (1957) cultured the excised leaves of Osmunda cinnamomea on a salt agar-sugar medium. Increased concentration of sugar brought about the multiplication of pinnae. Addition of Kinetin to the medium also brought about increase of pinnae. Leaf primordia also can be excised and cultured. On culturing, they develop into normal pinna.
Sporangia have been induced on the leaves of Heptopteris by increasing the concentration of sucrose. Generally low concentration of sucrose tends to induce sterility (in the leaves) in the culture medium. In Todea Barbara leaves remained sterile when the medium had 6 percent sucrose and at concentration of 12 percent sucrose, 25 percent of the leaves produced sporangia.
Similar inductions of sporangia have also been reported in the excised leaves of Osmunda cinnamomea. In this case also concentration of sugar coupled with environmental conditions such as day length, light intensity and temperature induce the formation of sporangia.
Indian Work on Pteridophytes:
A number of Indian botanists have worked on various aspects of pteridology since a long time. While earlier contributions were mostly on morphological and anatomical studies, many of the recent contributions have been on experimental pteridological aspects like growth, morphogenesis etc. The foregoing account is a brief outline of the Indian contributions to Pteridology.
Mahabale and Deshpande (1942) reported the occurrence of Psilotum triquertum from Lonavala, near Bombay; similarly the presence of P. triquerium was reported from South India by Venkateswarulu (1944) and Dutt (1950).
Chaudhary (1937) reported the occurrence of 33 species of Lycopodium in India, providing a key to their identification. Ghosh (1942) described the presence of twin sporangiate sporophylls in Lycopodium phlegmaria. Mahabale (1948) studied the structure of the gametophyte of L.hamiltonii an epiphytic species from North Kanara and Kailganga valley in Gharwal (U.P). According to him, the sex organs were restricted to the central conical portion of the pro-thallus.
Mchra and Verma (1957) and Ninan (1958) conducted cytological studies and reported the haploid chromosome number for Lycopodium species to be n = 165, 170 and 136. Bhambie and Puri (1963) and Bhambie (1965) studied the organization of shoot apex, leaf initiation and phyllotaxy in a number of species of Lycopodium.
According to them slow growth rate is due to flat apex, while fast growth rate is seen in species with a conical apex. Bhambie’s (1965) work further showed that the leaf in Lycopodium is initiated by a number of cells rather than a single cell. Surange (1966) has reported that the Indian fossil lycopods are represented by Cyctodendron, Gondisporites, Lycopodites, LycoxyIon etc.
The earliest study is that of Majumdar (1942) who discussed the origin of medullation in the three species of Selaginella (S.inaquaelifolia, S.waliichi and S.canaliculata) and related it to the polystelic condition. Chowdhury (1943) studied the stelar structure in the xerophytic species – S. bryopteris from U.P and S. wightii from Nandi Hills, from South India (presently in Karnataka). The resurgent stem consists of a horse shoe shaped, flattened protostele with two protoxylem groups.
Mitra (1944) reported the occurrence of a pathogenic fungus (Melanopsamma ranjanii) on Selaginella chrysocaulos. Alston (1946) reported 46 species of Selaginella from India. Panigrahi and Chowdhury (1962) reported eight species of Selaginella from the eastern parts of our country.
Sharma (1966) has worked-on the dermal morphology of several species of Selaginella. As reported by him in contrast to the very high chromosome numbers of pteridophytes like Ophioglossum (n = 630), Selaginella has a very low basic number (n = 9, 10, 11). It is quite likely that taxa with very high chromosome number could be polyploids.
The work of Bhambie (1966) has shown that whenever the trabeculae are multicellular, only the cell adjacent to the stele is truly endodermal in origin while the rest or cortical cells.
Bir and Bhusri (1985) reported 6 species of Selaginella from Shimla Hills while Pasha (1985) has reported on the occurrence of six species from Bangladesh.
Ethnobolanical studies of Dixit (1982) have shown that in parts of Northern India S. bryopteris is sold in herbal markets under the name Sanjivani.
Rao (1944) has described a new species of lsoetes (I. sampatkumarini) from Bangalore, while Shende (1945) and Shende and Mahabale (1947) described another new species lsoetes dixtii from panchagani (now in Maharashtra).
Panigrahi (1981) has reported a species of lsoetes from India. Bhambie (1960) has worked on the ‘corm’ morphology and regards that it has an upper cone axis and a lower stigmarian axis.
Goswami (1985) has reported the occurrence of protruding hairs inside the lacunae in the leaves of lsoetes pantii.
Singh et al (1983) have reported that in certain specimens of lsoetes coromandaliana mixed sporangia occur possessing monolete microspores , trilete megaspores and alete sterile spores.
Mahabale (1945) described the structure of the gametophyte in Equisetum debile from Poona. Pant and Mehra (1964) studied the stomaial development in Equisetum and other genera. The stomatal development conformed to syndetocheilic type. Ninan (1955) has reported a haploid chromosome number of n = 108 in Equisetum.
The work of Mohan Ram and Chatcrjee (1969) on Equisetum ramosissium has shown that the gametophytes are potentially bisexual but the environmental conditions may make them unisexual.
Bir and Bhusri (1985) have reported 3 species of Equisetum from Shimla Hills. Sharma and Bohra (1979) have studied the vascular structure of the rhizome and aerial shoots in Equisetum ramosissimum s.sp ramosissimum.
Kashyap (1914) has worked on the development of archegonium in Equisetum debile.
Nayar and Kachroo (1952) studied the development of antheridium in higher leptosporangiatae. Wardlaw and Sharma (1961) studied the placenta/sorus in ferns and opined that it is a special kind of meristem. Nayar (1964) studied the types of spores in pteridophytes. Nayar and Kaur (1972) studied the detailed development of fern gametophytes.
Systematics of ferns have been specially studied by Chowdhury (1937), Alston (1945), Gupta and Bharadwaj (1957), Panigrahi and Chowdhury (1961) and Abraham et al (1963) etc. Morphology and anatomy of ferns have been studied by Puri and Garg (1953), Bhambie and Puri (1963) etc.
Nair (1964) has studied the spore morphology of ferns. Gopal (1968-72) has studied the ecology of ferns.
Mahabale and Kamble (1981) have studied the cytology of 51 species of ferns and noticed polyploidy in Lygodium, Pteris, Adiantum, Dryopteris and Asplenium.
Chandra and Kaur (1985) have reported that nearly 137 species of ferns are endemic to India.
Srivastava (1979) has studied the structure of the foliar epidermis of 20 species of ferns belonging to 15 genera. Mehra and Soni (1985) have studied the tracheary elements in 110 species of ferns belonging to 63 genera.
Kolhatkar (1937, 1940) has studied the life history of Marsilea doonensis. Majumdar (1944) has studied the adventitious root formation in Marsilea. Mahabale and Gorji (1948) have recorded various types of Juvenile leaves. Latex containing cells in the phloem and inner cortex have been reported in Regnellidium by Mahabale (1-948).
Gupta has reported a new species M. rajasthanensis from Rajasthan. Puri and Garg (1953) have studied the morphology of sporocarp in Marsilea. According to them the sporocarp is a modified pinna consisting of several pinnules.