In this article we will discuss about:- 1. Meaning of Pteridophytes 2. Origin of Pteridophytes 3. Adaptive Features 4. Distribution/Habitat 5. Diagnostic Characteristics 6. Reproduction 7. Life Cycle 8. Relationship/Affinities 9. Stelar System 10. Economic Importance.
Meaning of Pteridophytes:
Pteridophytes (pteron — feather, phyton — plants) are the non-flowering vascular plants. Hence they may be defined as ‘vascular cryptogams’. They are represented by about 400 genera and about 10,500 species including both the living and fossil plants.
They are the earliest known vascular plants which originated in the Silurian period (400 million years ago) of Palaeozoic Era and subsequently diversified and formed the dominant vegetation on earth during Devonian to Permian period.
The pteridophytes are the successful colonisers on land habit. They had acquired certain characteristic features in the early geologic period that helped them to make their successful adaptation on land.
Origin of Pteridophytes:
The pteridophytes evolved in the Silurian period and subsequently got diversified in the Lower Devonian. There are controversies regarding their origin and evolution. There are two broad theories about their origin: according to one, pteridophytes have originated from algal ancestor, while the other school supported the bryophytic origin hypothesis of pteridophytes.
I. Algal Origin:
Many scientists believe that pteridophytes have originated from algae, though they are not unanimous about the type of ancestral algae. The concept of algal origin of pteridophytes is based on the similarity between algae (specially chloro- phyceae) and pteridophytes.
The common characteristics for both the groups are:
1. Thailoid gametophytes,
2. Similar photosynthetic pigments (chlorophyll a, b; carotenoids a, (5),
3. Cell wall made up of cellulose,
4. Starch as reserve food,
5. Flagellated sperms,
6. Water essential for fertilisation.
7. Cell plate formation during cytokinesis, cell division features a complex network of microtubules and membrane vesicles (the “phragmoplast”).
a. Lignier’s Hypothesis:
Lignier (1908) supported the algal origin of land plant. He postulated that the pteridophytes arose from the Chlorophyta with dichotomising parenchymatous thallus. For the transmigration from water to land, the basal part entered the soil for anchorage and absorption purposes.
The erect parts retained the photosynthetic function and the aerial portion with terminal sporangia became the primitive three-dimensional dichotomous branching system (e.g., Rhynia).
b. Church’s Hypothesis:
Church (1919) is believer of polyphyletic origin of pteridophytes and proposed the theory in his essay “Thallasiophyta and the sub-aerial Transmigration”. According to Church, a hypothetical group of advanced marine seaweeds called Thallasiophyta formed the ancestral stock for land plants (both bryophytes and pteridophytes).
This transmigrant algae had metabolic efficiency of Chlorophyceae, somatic equipment and reproductive scheme of the Phaeophyceae.
The principal points of this theory are:
1. The surface of the earth in ancient times was covered by a common ocean.
2. The marine algae were planktonic in nature.
3. There was upheaval of the ocean bottom due to geological plate-tectonic revolution.
4. Subsequently, there was emergence of land and those planktonic forms became benthic in the shallow seas.
5. Further transmigration of those benthic forms to the sub-aerial conditions led to the establishment of a large advanced group of extinct seaweeds called Thallasiophyta.
6. Both the bryophytes and pteridophytes originated from this hypothetical Thallasiophyta group. The new terrestrial environment gradually allowed introduction of the various adaptive features like roots, leaves and vasculature in land plants.
The geological phenomena do not support Church’s hypothesis. The difference in pigmentation between ancestral marine algae and early land plants also is a demerit of this hypothesis.
c. Fritsch’s Hypothesis:
Fritsch (1916, 1945) suggested that Chaetophoraceous (green algae) ancestor gave rise both to the bryophytes and the pteridophytes. These algae had heterotrichous habit, comprised of prostrate and upright systems.
An apical growing point was established in upright branches, thus the heterotrichous thallus- gave rise to erect land plants (early vascular plants) by elaborating the erect portion and by diminishing the prostrate part.
d. Andrews’ Hypothesis:
Based on the fossil marine algae such as Nematothallus, Protosalvinia, Crocelophyton, Andrews (1950, 1959) hypothesised that these algal groups independently gave rise to different groups of early vascular plants. The morphological diversities in Psilopsida, Lycopsida, Sphenopsida and Pteropsida support the above view.
e. Mehra’s Hypothesis:
Mehra (1969) proposed a polyphyletic origin of pteridophytes. He opined that both the bryophytes and pteridophytes have originated from the same hypothetical Proto-archegoniate group’. This hypothetic group arose earlier from its ‘Chaetophoraceous ancestors’. The Proto- archegoniate group gave rise to ‘Psilophytaceous line’ on one hand and to the ‘Lycopodiaceous line’ on the other hand.
II. Bryophytic Origin:
Many hypotheses have been put forward in support of bryophytic origin of pteridophytes, although there is no unanimity regarding the ancestral stock as well as the mode of origin. The bases of these hypotheses lies on the similarity between the early vascular plants and the sporophytes of certain mosses and hornworts.
The characteristics common for both the groups are:
1. Heteromorphic life cycle,
2. Multicellular sex organs,
3. Motile and flagellated sperms,
4. Thalloid gametophytes,
5. Water necessary for fertilisation,
6. Retention of zygote within the female sex organs and embryo develops from it,
7. Plant and spore surfaces are covered with cutin (cuticle).
Some of the theories in support of bryophytic origin of pteridophytes are described:
a. Anthocerotean Theory:
This theory was advocated by Campbell (1895). According to this theory, the sporophyte of Anthoceros shows many characteristics comparable to the sporophytes of early vascular plants. These include:
(i) Sporophytes bear rhizoids instead of roots,
(ii) Mechanism for indefinite growth through meristematic tissue,
(iii) Presence of columella (e.g. Horneophyton),
(iv) Limited tissue for spore production,
(v) Capsules comprises of photosynthetic tissue and provided with stomata.
Thus, the early vascular plants like Rhynia are derived directly from Anthoceros-like ancestor.
Smith (1955) supported the anthocerotean hypothesis as proposed by Campbell with some modifications. He postulated that the anthocerotean sporophyte had shifted the meristematic region from the base to the apex (Fig. 7.131A-D). Thus, initiation of dichotomy was established by the apical meristem and the spore formation became restricted at the branch apices.
The columella eventually evolved to form the vasculature of the land plant (e.g., Rhynia). So, in the course of evolution, the Sporogonites (an early hornwort or moss?) evolved. The Sporogonites consist of many parallel oriented sporangial stalks that terminate in elongate capsules containing a central columella.
Later, Horneophyton evolved from Sporogonites through the distal dichotomy. The lower parenchymatous corm and columellate capsule of Horneophyton are interpreted as a transitional form between Anthoceros and Rhynia type vascular plants. The further elaboration of basal part of Horneophyton eventually gave rise to the rhizomatous stem of Rhynia.
b. Strobilar Theory:
Bower (1894, 1908) was the propounder of this theory. According to Bower, the pteridophytes have evolved from Anthoceros-Xype ancestor. He suggested that a well-differentiated sporophyte evolved by progressive sterilisation of the potentially sporogenous tissue (Fig. 7.132A-E).
In this theory, Lycopodium selago (in which sporophylls are arranged singly throughout the body surface without organising into a definite stobilus) arose from Anthoceros ancestor through the following steps:
(i) Formation of continuous sporogenous tissue just outside the central columella (e.g., Anthoceros).
(ii) The sporogenous tissue became superficial.
(iii) The sporogenous tissue was segmented into separate masses by intercalated sterile tissues. This has been evidenced in some members of Anthocerotales.
(iv) The isolated, segmented sporogenous masses became more superficial, thus alternating masses of sporogenous tissue and green sterile tissue were formed.
(v) The green tissue expanded laterally to form leaf, thus sporogenous tissue (sporangium) came adaxially to the base of the leaf (e.g., Lycopodium selago).
c. Phyton Theory:
This theory was put forward by Celekovsky (1901). According to this theory, the fundamental part of the sporophyte was a leaf and not the axis (stem) and the axis formed later by the fusion of the leaf bases.
d. Protocorm Theory:
This theory was proposed by Treub (1890). According to this theory, the sporophyte of primitive pteridophytes was an undifferentiated mass, very much like a gametophyte. This has been substantiated by the occurrence of protocorm in some species of Lycopodium.
Treub hypothesised protocorm to be an archaic sporophyte and an early transitional stage in the evolution of the sporophyte. The presence of permanent protocorm in Phylloglossum supports the protocorm theory.
Bower (1907, 1935) discarded the protocorm theory and concluded that the protocorm is merely an adaptive structure to certain special environmental conditions.
Modern studies of cell ultrastructure, biochemical nature and molecular studies (5S/16S- rRNA sequence, molecular sequence data from plastid, nuclear and mitochondrial encoded genes etc.) suggest that bryophytes are not the ancestor of vascular plants.
It has been indicated that both the bryophytes and pteridophytes have evolved from green algal ancestors, probably from Coleochaete, closely related to charophytes, and mosses appear to be a sister group to the tracheophytes.
Thus, Coleochaete is an excellent model for the algal ancestor of land plants and may be a modern representative of the algal group that gave rise to the land plants.
The evidences in support of the above view are:
(a) Coleochaete is a soil alga in the order Coleochaetales, subfamily Charophyceae, which produces a small vegetative thallus as the major, haploid part of its life cycle.
(b) This thallus produces egg cells toward its periphery.
(c) Once the eggs are fertilised, the zygotes are retained in this position on the parent thallus and supplied with nutrients through specialised transfer cells.
(d) Zygote undergoes cell divisions while still retained on the parent plant.
(e) There is greater number of meiotic products (8-32).
(f) Reproduction with specialised male gamete similar to males of Bryophytes.
(g) Phragmoplast cell division pattern like those of land plants.
(h) Sporopollenin in the inner wall of the zygote.
(i) Presence of lignin (a principal component of land plants).
(j) Group II introns in the chloroplast genome like land plants.
Adaptive Features of Pteridophytes for Land Habit:
Pteridophyte spores are bounded by two concentric wall layers, the outer thick acid resistant exine and the inner thin acid degradable intine. The spores have the capacity to protect themselves from adverse environmental conditions (desiccation or the acid treatment). This desiccation resistant spore was a fundamental event which allowed the spread of plant life over land surfaces.
b. Cuticle and Stomata:
A surface cuticle is a feature of the exposed parts of all land vascular plants. The principal function of cuticle is to prevent water loss from the body. In addition, it is also resistant to gas exchange, chemical substances, microbial attack, abrasion and mechanical injury.
As the cuticle inhibits gaseous exchange, land plants have evolved the apertures (stomata) that control the passage of gas and water depending on the requirement of the plants.
Stomata are not a prerequisite for a terrestrial habit, because functional stomata are not found in pre-Devonian vascular plants and in bryophyte gametophytes. For the adaptation of plants on the land surfaces, especially nearer to water bodies, the co-evolution of a cuticular coat and stomata became advantageous.
c. Tubes and Tracheids:
There was little requirement for a translocation system within the plant body that remained in an aquatic environment. But a conducting system becomes essential when the plant adopts an upright habit which grows away from the aquatic environment to the ground surface and exposes itself to the desiccating effect of the wind.
The main selective pressure that led to the evolution of an erect habit is in releasing spores into higher wind energies encountered at greater distances from the source. This facilitates the long distance transport of spores, thereby increasing the chance of colonising new areas.
The initial trend in this direction might have begun with the distal positioning of sporangia in erect land habits which has been evidenced in the earliest known (Silurian) land vascular plant, Cooksonia.
A variety of tubular structures are found in early land plants that have fulfilled the role of trachieds.
Hence it may be concluded that the trachieds arose de novo without passing through some kind of evolutionary grade. The tracheids have lignified walls where lignin, a metabolic by-product of photosynthesis, is a major constituent of most vascular plant cell walls and apparently absent in all aquatic plants.
Lignin is a secondary metabolite, it forms as a result of detoxification process and is stored in land plant tissue. As the land plants are unable to excrete waste products, the toxic compounds are chemically transformed into non-toxic products by secondary metabolism and stored in tissue.
Thus lignin, stored in this way in the tracheidal cells, proved to be advantageous to the erect land plants by resisting attack by microbes and increasing the efficiency of fluid conduction in tracheids by decreasing the permeability of their walls.
Distribution/Habitat of Pteridophyta:
The pteridophytes grow in diverse habitats. Mostly they show terrestrial habits growing in moist, cool and shady places. All pteridophytes require water for transfer of sperms to ovum to complete the process of sexual reproduction.
A few members are aquatic (e.g., Azolla, Isoetes, Marsilea, Salvinia) or xerophytic (Selaginella lepidophylla, S. rupestris, Equisetum arvense) and many are epiphytic (Lycopodium phlegmaria, Selaginella oregana, Ophioglossum vulgatum, ferns like Polypodium, Drynaria, Pleopeltis, etc.).
Pteridophytes range from small herbaceous annual (Azolla, Salvinia) to large perennials trees (Cyathea, Alsophila). Mostly, pteridophytes are herbaceous in nature.
i. The major plant body is a nutritionally independent sporophyte which is differentiated into roots, stem and leaves. Some primitive members do not have true roots or leaves (e.g. Rhynia, Cooksonia, Psilotum).
ii. The sporophyte develops from the diploid (2n) zygote. The primary roots are ephemeral and subsequently replaced by the adventitious roots.
iii. The stem is generally branched; either dichotomous or monopodial.
iv. The leaves may be simple, small and sessile (e.g., Lycopodium, Selaginella); scale like (e.g., Equisetum) or compound, large and petiolate as in ferns (e.g., Pteris, Marattia).
Two types of leaves are found in pteridophytes:
(a) Microphylls or Microphyllous Leaves:
The leaves are simple with a single unbranched mid-vein; the leaf trace is not associated with any leaf gap. (Fig. 7.1 A) e.g., Lycopodium, Selaginella, Isoetes.
(b) Megaphylls or Megaphyllous Leaves:
The leaves are large, compound with dissected veins, the leaf trace is always associated with leaf gap (pig. 7.1 B), e.g., Pteris, Marattia, Marsilea.
i. The leaves and stems, in most of the cases, are provided with filiform trichomes.
ii. A well-developed vascular system, comprising of xylem and phloem, is present. Cambium is generally absent, thus secondary growth does not take place in majority of the pteridophytes except Botrychium Isoetes, and arborescent pteridophytes like Lepidodendron, Catamites.
iii. The nature of stele varies in different groups. It may be protostele (Psilotum, Lycopodium, Selaginella); Siphonostele (Equisetum, Marsilea, Botrychium); dictyostele (Pteris, Polypodium) or Polycyclic (Angiopteris, Marattia).
Diagnostic Characteristics of Pteridophytes:
1. Pteridophytes are non-flowering (seedless) vascular plats.
2. Sporophyte is the predominant plant body, differentiated into root, stem and leaves.
3. There is a regular heteromorphic alternation of generation where both the sporophytic and gametophytic generations are nutritionally independent.
4. The stem is generally branched either dichotomous or monopodial.
5. The primary roots are ephemeral and are soon replaced by adventitious roots.
6. Pteridophytes are polysporangiate, either homosporous or heterosporous.
7. Pteridophytes are free sporangiate where isospores or micro- and megaspores are released through the dehiscence of sporangia.
8. Presence of multicellular sex organs i.e., antheridia and archegonia.
9. Water is essential for fertilisation where flagellated sperms swim over a thin film of water and are attracted chemotactically towards the archegonium.
10. The zygote undergoes repeated mitotic divisions to form embryo. The first division of the zygote determines the polarity of the sporophyte.
Reproduction in Pteridophyta:
i. The sporophytic plant reproduces by means of spores produced in the sporangia (singular: sporangium).
i. The position of sporangia may vary in different groups;
ii. They may be borne on the stems i.e., cauline (e.g., Psilotum; Rhynia) or on the ventral (adaxial) surface of the leaves i.e., foliar (e.g., Lycopodium, Selaginella) or in the axil of the leaves (e.g., Ophioglossum).
iii. The sporangia containing leaves are called sporophylls.
iv. The sporophylls may be scattered (e.g., Lycopodium selago), uniformly distributed (e.g., Pteris, Adiantum and other ferns) or grouped in definite areas to form strobili (Selaginella, Equisetum).
v. In some aquatic pteridophytes the sporangia are present within a specialised structure, called sporocarps (e.g., Azolla, Salvinia, Marsilea).
On the basis of mode of development, the sporangia are of two types viz., the euspo- rangiate and the Leptosporangiate (Table 7.1). In the eusporangiate type, the sporangia develop from several sporangial initials (e.g., Psilotum, Lycopodium, Selaginella). In leptosporangiate, on the other hand, sporangia develop from a single initial cell (e.g., Salvinia, Pteris).
In some forms (e.g., ferns) the sporangia are aggregated in clusters termed sori (singular sorus).
On the basis of maturity of the sporangia, the sori are of three types:
(a) Simple Sorus:
A sorus in which all the sporangia originate, grow and mature at the same time (e.g., Botrychium, Ophioglossum). The forms showing such condition are grouped together as simplices (Fig. 7.2A).
(b) Gradate Sorus:
Here sporangia develop over a period of time, where the central part of the sorus has mature sporangia and the peripheral part has younger sporangia (e.g., Hymenophyllum, Marsilea, Cythea). The forms showing this condition is called Gradatae (Fig. 7.2B).
(c) Mixed Sorus:
Here the mature and immature sporangia of different ages are arranged in an irregular fashion (e.g., Pteridium, Pteris, Adiantum), and the condition is termed as Mixtae (Fig. 7.2C).
i. Meiotic (reduction) divisions of spore mother cells produce numerous haploid spores inside the sporangium. If all the spores produced are of equal sizes and shapes, then the plant is called homo- sporous (e.g., Lycopodium, Equisetum, Dryopteris), and if they are of two different sizes and shapes the plant is called hetero- sporous (Selaginella, Isoetes, Marsilea).
ii. In the heterosporous type, the two different types of spores are produced in separate sporangia. The smaller spores are called microspores or male spores and are produced in microsporangia. The microspores are produced in large numbers.
The larger spores which are produced in smaller numbers are termed megaspores and are developed in megasporangia. Microspores, after germination, produce male gametophyte, while megaspores produce female gametophyte.
iii. Sporophylls with megasporangia are called megasporophylls, while sporophylls with microsporangia are called microsporophylls.
i. The spores germinate to form haploid game- tophytes or prothalli. The gametophytes of pteridophytes are small and inconspicuous as compared to the sporophytes.
ii. The germination of spores in the homosporous forms are of three types.
a) Bipolar (e.g., Lycopodium, Equisetum)
b) Tripolar (e.g., Hymenophyllum)
c) Amorphous and irregular (e.g., Angiopteris).
iii. The gametophytes are of two types. In homosporous forms, the development of gametophyte is exosporic in which the pro- thallus develops outside the spore wall (e.g.-, Psilotum, Lycopodium, Ophioglossum).
Therefore, it is vulnerable to outside environment. Gametophytes that develop from heterospores are endosporic in which the development of prothallus is confined within the spore wall (e.g., Selaginella, Isoetes and Marsilea). The development of endosporic gametophyte is independent of external environment.
i. The gametophytes or prothalli (singular – prothallus) bear the sex organs viz., the male antheridia (singular antheridium) and female archegonia (singular archegonium). These sex organs are embedded/impregnated in the prothallus (e.g., Lycopodium, Selaginella, Equisetum) or are projected (e.g., Psilotum, Pteridium).
i. The antheridium is a sessile or shortly stalked globular structures surrounded by a well-defined jacket inside containing androcytes or antherozoid mother cells.
ii. Each androcyte gives rise to a single motile antherozoid.
iii. The number of antherozoids per antheridium varies from 4 (e.g., Isoetes) to few thousands (e.g., Ophioglossum).
iv. The antherozoids are unicellular, uninucleate and spirally coiled bearing apical flagella (e.g., Lycopodium, Selaginella) or are multiflagellate (e.g., Psilotum, Isoetes, Equisetum and ferns).
i. The archegonium is a flask-shaped structure consisting of a basal, swollen venter and a short neck, the venter is embedded in the prothallus while the neck is projected.
ii. The venter encloses an egg and a ventral canal cell. Neck is made up of vertical rows of neck cells with neck canal cells inside.
iii. At maturity the neck canal cells disintegrates to form a passage for the antherozoids to reach the egg.
i. The disintegration of neck canal cells also produces a mucilagenous substance which contains organic compounds like malic and fumaric acid. These substances act as sperm attractant.
ii. Water is essential for fertilisation and sperms swim over a thin film of water and attracted chemotactically towards the archegonium.
iii. The antherozoid and egg — of haploid chromosome number — fuse to form a diploid zygote, which is the mother cell of sporophytic generation.
i. The zygote undergoes repeated divisions to form embryo. Further development of embryo results into a well-developed sporophyte differentiated into roots, stem and leaves. The first division of the zygote determines the polarity of the sporophyte.
ii. According to the polarity, the embryo may be categorised into two types viz., Exoscopic embryo and Endoscopic embryo. In excoscopic embryo, the shoot-forming apical cell is directed towards the neck of the archegonium i.e., directed outward (e.g., Psitotum, Equisetum, Ophioglossum).
In endoscopic embryo the shoot-forrping apical pole is directed downward i.e., towards the base of the archegonium (e.g., Lycopodium, Selaginella, Isoetes).
iii. The basic life cycle pattern of pteridophytes shows a regular heteromorphic alternation of generations between a gametophyte (sexual) phase and a sporophyte (asexual) phase. The main plant body is sporophytic which forms a dominant phase in the life cycle. The gametophytic generation bears male and female sex organs viz., antheridia and archegonia.
The antherozoids (male gamates) produced in large numbers are motile, while the eggs (female gametes) are non-motile and are borne singly in archegonia. Fusion between an egg and an antherozoid results in the formation of a diploid (2n) zygote.
The zygote develops directly by mitotic divisions into sporophyte. The sporophyte plant develops sporangia which produce haploid spores through reductional division (i.e., meiosis). The life cycle is then completed when these spores germinate and grow into haploid gametophytes (n).
In homosporous member, spores germinate to produce monoecious i.e., homothalic gametophyte bearing both male and female sex organs. In this case, the sex determination takes place at the time of formation of antheridium and archegonium (Fig. 7.3).
In heterosporous member, micro- and mega- spores, on germination, produce male and female gametophytes, respectively (i.e., het- erothallic or dioecious). Here, the sex determination takes place much earlier, at the stage of sporogenesis (Fig. 7.4).
Life Cycle of Pteridophytes:
The alternation of sporophyte (2n) and gametophyte (n) generations in the life cycle is normally coordinated with a periodic doubling (because of syngamy) followed by a halving (because of meiosis) of the chromosome number.
However the syngamy and meiosis are not always the essential processes in the production of sporophytic and gametophytic generations respectively, rather certain deviations from a “normal” reproductive cycle viz., Parthenogenesis, apospory and apogamy are observed in some members of the pteridophyte.
In some ferns, the embryo develops from an unfertilised egg, a phenomenon termed as parthenogenesis (i.e., virgin birth) (e.g., Marsilea; Cyathea; Athyrium). Apospory is the development of gametophytes, without a haploid spore stage, from vegetative cells of the sporophyte.
Cytological studies have shown that their gametes are diploid and that a sporophyte resulting from fusion of gametes is tetraploid (e.g., Osmunda regalis, Pteris vittata). A third deviation from the usual life cycle of pteridophyte is the phenomenon of apogamy.
In this case, sporophyte is formed without the act of fertilisation, from vegetative cells of the gametophyte. The chromosome number (haploid or diploid) of the apogamous sporophyte depends upon the chromosome number of gametophyte (i.e., whether normal haploid or aposporous diploid) e.g., Pteridium; Cheilanthes; Pteris.
Relationship/Affinities of Pteridophytes:
The Pteridophyta has been placed in between bryophytes and spermatophytes (gymnosperm and angiosperm) in the subdivisions of plant kingdom. The presence of true roots, stem, leaves and vascular tissues thus have distinguished them from the preceding group i.e., bryophytes.
The pteridophytes resemble the Spermatophyta in having well- developed vascular tissues but differ from them in lacking flowers, fruits or seeds. However, owing to its intermediate position between bryophytes and spermatophytes, the Pteridophyta shows certain similarities with both the groups.
Stelar System in Pteridophytes:
The name stele has been derived from the Greek word stele meaning column or pillar or set or stand. The concept of the stele as the fundamental unit of vascular system was put forward by van Tieghem and Douliot (1886).
According to them the fundamental parts of a shoot are the cortex and the stele (a central cylinder) where the endodermis is the anatomical boundary between these two fundamental units. The central cylinder or core of vascular tissue, consisting of xylem, phloem, pericycle, and, sometimes, medullary rays and pith, is designated as stele.
In lycopsids, like Lycopodium and Selaginella, the stele of the axis and the leaf traces develop independently. The primary vascular system of the stem is cauline. The leaf traces are very small and superficially connected to the vascular cylinder of the stem (Fig. 7.1 A).
But in ferns (e.g., Pteridium, Pteris), the stelar anatomy of the stem is largely affected by the large leaf traces and leaf gaps (Fig. 7.1 B). It is considered as a composite structure consisting of both cauline (stem) and foliar (leaf) vascular components.
Types of Steles in Pteridophytes:
Various types of vascular cylinder can be recognised in the roots and stems of pteridophytes. Most of the workers recognised two principal types of stelar organisation in pteridophytes — Protostele and Siphonostele.
It is characterised by the absence of central column of pith. Thus it is a non-meduMated stele, in its most simplest form it is merely composed of a central strand of primary xylem surrounded by a cylinder of phloem. From the phylogenetic as well as ontogenic standpoint the most primitive type of stele is the protostele.
All other types of steles have been derived from it in the course of evolution. Protosteles are found in primitive psilophytes like Rhynia, Horneophyton and also in many living primitive vascular plants e.g., Psilotum, Tmesipteris, Selaginella, Lycopodium.
In advanced vascular cryptogams (ferns), protostele is generally associated with the sporeling stage, but is also permanently maintained in the adult stems of some relatively primitive ferns (e.g., Lygodium, Gleichenia, etc.).
Different types of the protostele encountered in the pteridophytes are:
It is the simplest and most primitive type of protostele consisting of a central solid and smooth core of xylem surrounded by a ring of phloem (Fig. 7.133A) e.g., Rhynia, Horneophyton, Cooksonia (fossil pteridophytes); Tmesipteris, Selaginella (living pteridophytes).
A protostele in which the contour of the core of xylem is lobed or star- shaped in transectional view and is known as actinostele (Fig. 7.133B). The phloem, instead of forming a continuous ring, is situated in the form of small groups in between the radiating arms of the xylem e.g., Psilotum, Lycopodium serratum (living pteridophytes); Asteroxylon, Sphenophyllum (fossil pteridophytes).
In this type of protostele, the xylem cylinder breaks into several separate plates (Fig. 7.133C). The plates are more or less parallel and each xylem plate is surrounded by phloem, e.g., aerial shoots and cone axis of Lycopodium clavatum and L. volubile.
(d) Mixed Protostele:
In this type of protostele, the solid xylem core is broken into small irregular groups of tracheids that are embedded in the phloem (Fig. 7.133D). e.g., stem of Lycopodium cernuum.
A stele with a central column of parenchymatous pith or medulla is called a siphonostele or medullated stele. It is considered as an advancement in anatomical development over protostele. In siphonostele, the vascular tissues are arranged in the form of a cylinder, with a distinct pith in the centre. The siphonostele and its variations are found frequently in the sphenopsids and ferns.
On the basis of position of phloem, siphonostele may be divided into two types:
(i) Ectophloic Siphonostele:
In this type of siphonostele, the phloem occurs as a single ring only on the external side of the xylem core (Fig. 7.133E). The pith is central in position and the phloem is externally surrounded by the pericycle and endodermis. e.g., Equisetum, Osmunda, Schizaea.
(ii) Amphiphloic Siphonostele:
In this case, the vascular cylinder consists of xylem surrounded on both sides (external and internal) by phloem (Fig. 7.133F). This type of stele characteristically has two endodermal layers (outer endodermis and inner endodermis) e.g., Marsilea, Adiantum, Dryopteris.
Depending upon the presence or absence of leaf trace and branch trace, Jeffery (1910) divided siphonostele into two groups:
It is characterised by the absence of leaf traces e.g., lycopsids.
It is characterised by the presence of both leaf and branch traces, e.g., members of Filicales.
On the basis of non-overlapping or overlapping gaps, the following three types of siphonostele are categorised:
It is a type of siphonostele where the leaf gaps are successive so that there is only one break in the vascular cylinder at any one given point (non-overlapping). The solenostele may be ectophloic (Fig. 7.133G) or amphiphoic (Fig. 7.133H).
A siphonostele with overlapping leaf gaps is known as dictyostele (Fig. 7.1331). In this case, the leaf gaps are large and overlap each other (many occur at a point), and the vascular cylinder thus appears to be highly dissected.
It is usually found in ferns where rhizome is very small and crowded with leaves e.g., Dryopteris, Pteris, Ophioglossum. A dictyostele has many scattered vascular strands. Each strand is known as ‘meristele’. All the meristeles are surrounded by a common endodermis.
(iii) Polycyclic Stele:
This is a complex type of siphonostele, where vascular tissue is present in the form of two or more concentric cylinders (Fig. 7.133J). e.g., two rings of vascular tissue in Pteridium aquilinum, three in Montania peotinata and four in Pteris podophylla.
Polycyclic stele may be of two types: i.e., Polycyclic solenosteles and polycyclic dictyostele. A polycyclic stele having an outer cylinder of solenostelic type is called polycyclic solenostele, while the outer cylinder of dictyostelic type is called Polycyclic dictyostele.
Economic Importance of Pteridophytes:
The economic importance of pteridophytes is not well-documented, because due attention has not been given towards their use in human welfare. However, there are many reports on their uses, specially as food plants, medicinal plants and horticultural plants.
Some of the aspects of economic importance of pteridophytes are given:
i. Pteridophytes Used as Food:
The young leaf tips of ferns, the circinate ptyxis or the chroziers are used as vegetable. The young fronds of Ampelopteris prolifera are sold in the market as ‘dheki shaak’ in India and Bangladesh. The croziers of Matteuccia struthiopters as canned or frozen are served as spring vegetable in USA and Canada. Leaves of Marsilea, commonly called ‘shushni’, are used as vegetable.
The rhizome of many ferns such as Pteris, rich in starch, is used as food.
The corm (modified stem) of Isoetes is used as food by pigs, ducks and other animals.
ii. Pteridophytes Used as Fodder:
Dry fronds of many ferns form the livestock for catties. The quadrifid lamina of Marsilea resembles a clover (Trifolium) has been used as fodder for animals as a substitute for clover.
iii. Pteridophytes Used as Medicine:
The spores of Lycopodium have been widely used in pharmacy as protective dusting powder for tender skin and also as water-repellants. The foliages of Lycopodium are used as tincture, powder, ointment and cream as a stomachic and diuretic. The foliage decoction is used in homeopathy to treat diarrhoea, bladder irritability, eczema, rheumatism, constipation and inflammation of liver.
Equisetum is rich in silicic acid and silicates. Potassium, aluminium and manganese, along with fifteen types of flavonoid compounds, have been reported from Equisetum. The flavonoids and saponins are assumed to cause the diuretic effect. The silicon is believed to exert connective tissue-strengthening and anti-arthritic action.
Several ferns have been used as herbal medicine. An oil (5% Filmaron and 5-8% Filicic acid) extracted from the rhizome of Aspidium is used as a vermifuge, especially against tapeworm. The decoction of Asplenium is used for cough and a good hair wash. The expectorant of Polypodium is used as a mild laxative, while the tonic is used for dyspepsia, loss of appetite and hepatic problem.
The root decoction of Osmunda regalis is used for treatment of jaundice. The ointment made from its root is used for application to wound. The extraction of Osmanda vulgaris, commonly known as ‘Green oil charity’, is used as remedy for wounds. The chemically active principal ‘Marsiline’ isolated from Marsilea is found to be very effective against sedative and anti-convulsant principal.
The rhizome and frond bases of Dryopteris have been used to determine the origin and pathways of dispersed pathogenic insects like corn ear- worm. The preparation of Ophioglossum vulgatum as ‘Green oil charity’ is also used as remedy for wounds.
iv. Pteridophytes Used as Horticultural Plants:
Many species of pteridophytes are cultivated for their aesthetic value. Many variants and cultivars of Psilotum have been brought in cultivation in nurseries and greenhouses in the nickname of ‘whisk fern’.
Some epiphytic species of Lycopodium (e.g., L. phlegmaria, L. lucidulum) are aesthetically more valued and can be grown on hanging baskets.
Several species of Selaginella are used as a ground cover in an undisturbed area because of their decent foliage and colour. Salaginella willdenovii, S. uncinata, etc., are grown in gardens for their decent blue colour. 5. lepidophylla, S. bryopteris, etc., are sold as dried under the name ‘resurrection plants’ which rejuvenate on contact with water.
Several ferns such as Angiopteris, Asplenium, Marattia, Microsorium, Nephrolepis, Phymatodes, etc., have aesthetic values for their beautiful habit, graceful shape of the leaves, and beautiful soral arrangement. Thus, these characteristics make them horticulturally important plants.
v. Pteridophytes Used as Biofertiliser:
Azolla is a free-floating water fern which can multiply very quickly through vegetative propagation. There are hundreds of moss-like leaves harbouring live colonies of dinitrogen fixer Cyanobacterium — Anabaena azollae.
The relationship between the alga and Azolla is symbiotic where the alga provides nitrogen to the plant. Thus, Azolla in full bloom in the waterlogged rice fields may serve as a green manure. Rice farmers of our country are using Azolla as biofertiliser for the better production of their crops.
vi. Pteridophytes Used as Indicator Plants:
Like angiosperms, pteridophytes are being used as indicator plants.
Equisetum accumulates minerals, especially gold, in their stem. The rate of accumulation even reaches up to 4.5 ounce per ton. Equisetum may be referred to as gold indicator plants which help in searching a region for gold ore deposits. Similarly, Asplenium adulterinum is an indicator of nickel and Actinopteris australis is a cobalt indicator plant. Thus, these plants are found to be valuable in prospecting for new ore deposits.
vii. Pteridophytes Used for Various Purposes:
There are various applications of pteridophytes:
The stem of Equisetum was used for polishing wood in ancient times and to clean utensils.
The roots and stems of Osmunda are used to make beds for growing orchids. Water boiled with Lycopodium clavatum is used for dyeing the woollen clothes which becomes blue when dipped in a bath of Brazil wood.
The powder of Lycopodium is highly inflammable and is used in pyrotechny and for artificial lighting. Thus, Lycopodium powder finds its wide use in demonstration of artificial lighting on the stage, because it disperses easily in the air and only a small quantity is needed to produce an explosion.
Some of the pteridophyte members are considered to be the obnoxious weeds. Pteridium aquilinum is a carcinogenic plant which can rapidly invade the open forest lands, thus eliminating the other plants of the forest floor. The free-floating water fern, Salvinia, quickly propagates vegetatively, and thus occupy the entire water surface of lakes, ponds and irrigation reservoirs preventing free flow of water.