In this article we will discuss about:- 1. Occurrence and Distribution of Lycopodium 2. Sporophyte of Lycopodium 3. Gametophyte 4. Phylogeny.
Occurrence and Distribution of Lycopodium:
Lycopodium is popularly called the club moss. Among the genera of the family it enjoys widest distribution though its preference is to tropical and sub-tropical forests. It is not uncommon to find Lycopodium growing in arctic and temperate regions.
The genus consists of about 180 species of which 33 are found in India. Some of the common Indian species are L. phtegmaria, (Fig.31). L. wightianum, (Fig.32) L. hamiltonii, L. lucidulum etc. Most of the species of Lycopodium prefer to grow in cool moist humus rich soil. Sorrie of them like L. phlegmaria are epiphytes growing in the humus packets of trees.
Sporophyte of Lycopodium:
The plant body of adult sporophyte is herbaceous and may reach a height of one or two feet. They usually grow either prostrate or semi-erect or pendant. Some species like L. volubile are stragling climbers.
Classification of the Genus:
The various species of Lycopodium exhibit such an amount of diversity in their form, growth, habit, anatomy etc., that many people regard it as impossible to put these under a single genus. As early as 1944, Rothmaler suggested that the Linnaean genus Lycopodium should be divided into four independent genera namely Lycopodium, Huperzia, Lepidotis and Diphasium.
Parihar (1965) reports that there is cytological dissimilarity between these genera thus supporting the contention of Rothmaler (1944). Earlier, Pritzel (1900) had suggested that the genus Lycopodium be divided into two sub- genera viz., Urostachya and Rhopalostachya.
Some of the important differences between the two sub-genera are:
Chemosystematics of Lycopodium:
On the basis of flavonoid glycoside distribution, Lycopodium is divided into three groups. These are:
Flavonoid C glycosides present eg. L. cernuum.
5-0 glycosides present eg. L. scariosum.
Mixture of first and second groups based on the presence or absence of 4-0 glycosides.
Markham and Moore (1980) have reported the presence of flavone C glycosides in Lycopodium cernuum.
The works of Pedersen and Ollgard (1982) have revealed that phenolic acids like chlorogenic acid, protocatechuic acid etc. are present in several species of Lycopodium.
Morphology of the plant:
The plant body is differentiated into stem, roots and leaves. In the sub-genus Urostachya the stem is either erect or pendant (L. phlegmaria). The stem may be branched or un-branched. Branching is always dichotomous. In Rhopalostachya the stem is prostrate or semi-erect. The branching is dichotomous in the beginning but later becomes apparently monopodial (L. olavatum).
Arranged on the stem are found the leaves. The leaves are small, eligulate, sessile with a broad base. In some species (L. cernuum) the leaves are extremely small and vary in size from 2 to 10 mm; in members like L. phlegmaria they may be 2-3 cm., long.
The arrangement of the leaves on the stem varies. It may be spiral (L. olavatum), whorled (L. verticellatum) or in pairs (L. alpinum). The leaves are microphyllous and have a central un-branched vein. All the leaves on the stem are of the same size except in L. complanatum and L. volubile where the lateral branches have heterophyllous leaves.
The stem is anchored to the substratum with the help of roots. The primary root is short lived and the adult sporophyte has only adventitious roots. In Urostachya the roots are produced only at the base of the stem. But some species like L. pithyoides and L. selago produce certain peculiar roots called cortical roots.
These roots arise endogenously in usual manner but instead of coming out of the cortex at the point of their origin grow down the cortex and come out only at the base of the stem. Consequently in these species, a transverse section of the stem at the basal region reveals several cortical roots.
In Rhopalostachya also, the roots are adventitious but arise all along the length of the prostrate stem. The main reproductive organs of the sporophyte viz., the sporangia are produced on the adaxial surface of the leaves. In the organisation of the spore producing organs the two sub- genera show many differences.
Here, in some species like L. hamiltonii there is no difference between the leaves that bear the sporangia (sporophylls) and the sterile leaves. Consequently in these species there is no organisation of a separate strobilus.
In L. selago also, there is no morphological difference between the sporophylls and the sterile leaves; however the basal region of the branch is sterile and this is followed by alternate fertile and sterile regions.
This type of distribution is generally referred to as the Selago condition, In L. phlegmaria, there is not only a clear cut difference between foliage leaves and sporophylls but the latter accumulate at the apices of branches to form a compact strobilus. In this species the strobilus is usually branched. In some cases the lip of the strobilus may resume growth and produce a short vegetative shoot.
In all the species of this sub- genus there is an organisation of the strobilus. The strobili usually occupy the apices of the branches. The sporophylls are distinct from the foliage leaves in being smaller and paler in colour. The strobili may be produced at the tips of ordinary branches (L. innundatum, L. alpinum etc.) or may be produced on special branches which have sparse distribution of leaves (L. clavatum, L. complanatum etc.)
The various species of Lycopodium clearly illustrate the phenomenon of evolution of the sporophyll and strobilus:
The evolution of strobilus has the following steps:
1. In some species all the leaves bear the sporangia on their adaxial surface.
2. Some of the leaves here and there become sterile.
3. The sporangia bearing leaves are distributed only towards the apex; yet there is no difference between fertile and sterile leaves.
4. The sporangia bearing leaves which show sparse distribution in the beginning like the foliage leaves, gradually accumulates and produce a compact strobilus.
The different species of Lycopodium exhibit a wide range of variation in the stelar organisation while the cortex remains almost uniform.
A transverse section of the stem exhibits three distinct regions viz., epidermis, cortex and stele.
This is single layered. The outer walls of the cells are cutinised. The continuity is interrupted by the presence of stomata.
It may be homogenous (L. phlegmaria) or heterogenous (L. cernuum etc.). When homogenous, the cortex is made up of only parenchyma or only sclerenchyma. In heterogenous cortex there are usually three distinct regions namely outer sclerenchyma, middle parenchyma and inner sclerenchyma.
In L. cernuum the inner and outer zones are parenchymatous while the middle zone is sclerotic. Delimiting the cortex from the stele is the endodermis with its characteristic Casparian thickenings. In some cases the endodermis is not clearly distinguishable.
Internal to the endoderm is the pericycle made up of one or two layers of cells. The stele is protostelic in all the species. However, there is a good deal of variation in the architecture of the xylem.
In L. phlegmaria the xylem is stellate having a number of rays. The protoxylcm is situated at the tips of the rays (exarch). Phloem completely surrounds the xylem. This is called an Actinostele.
In L. serratum also the stele is actinostelic, but the xylem rays are expanded outwards giving a fan like outline (Fig.38a).
In L. wightianum, the xylem breaks up in to a number of plate like lobes. Phloem however not only surrounds the xylem but also is distributed between the plates of the xylem. Such a type of stele is called ‘Plectostele’. This type of stele is also present in L. volubile (Fig.38d), L. densum etc. Occasionally in L. phlegmaria also plectostcle is seen.
In L. cernuum (Fig.38c), L. laterale, L. drummondii etc., the xylem and phloem are intermixed, the former forming scattered masses. This type of protostele is called a mixed protostele.
The xylem is exarch and consists only of tracheids with annular, reticulate or scalariform thickenings. Phloem consists of seive cells and phloem parenchyma.
A transection of a leaf shows epidermis, mesophyll and a concentric vein. The upper and lower epidermis are made up of a single layer of cells each.
Leaves may be amphistomatic (Stomata present on both the surface) or hypostomatic (stomata present only on the lower surface).
The mesophyll is usually undifferentiated with angular or rounded cells.
The vein is concentric with xylem surrounded by phloem. The vascular bundle may or may not have an endodermal sheath. In the apices of the leaf however the xylem is not surrounded by phloem.
A transection of a root shows an epidermis, cortex and stele (Fig.40).
Epidermis, is single layered and is composed of thin walled cells. Unicellular root hairs arise from some of the epidermal cells. Cortex consists of many layers of parenchyma cells. In mature roots the outer cortex may have thick walled cells.
The stele is surrounded by endodermis and pericycle. It is protostelic with monarch to polyarch xylem. In some of the creeping species (L. clavatum) the xylem is polyarch, in L. pithyoides it is diarch. When the xylem is diarch it is usually crescent shaped with the protoxylem situated at the tips. In L. selago stele is diarch in one part of the root and tetrarch in the other part.
Anatomically both cortical roots and normal roots are essentially similar. In the cortical roots however the epidermis is poorly developed, so also the root hairs.
The sporophyte reproduces by two methods viz., vegetative propagation as well as by spore production.
1. Vegetative Propagation:
This takes place by the following methods:
These are also called gemma and are produced on stem tips. The morphology of these is still obscure. These occur in place of leaves. According to Smith (1930), gemmae are modifications of leaves.
Each gemmae or bulbil consists of a short reduced axis surrounded by a number of fleshy leaves rich in reserve food. On falling upon the ground the gemma develops into a new plant. In L. phlegmaria and L. phyllanthum, Chowdhury (1933) has reported the formation of bulbils near the base of the main stem.
By the progressive death and decay of the older parts of the stem, the branches get separated and develop into a new plant. This is seen in species with creeping stems.
3. Persistent bud:
In certain species like L. innundatum, the tip of the stem survives while the entire plant gets killed during the unfavourable season. At the return of the favourable season the surviving tip functions like a bud and gives rise to a new plant.
4. Root tubercles:
In L. cernuum and L. ramulosum, Holloway (1917) has reported the formation of small tubercles on the adventitious roots.
2. Spore Bearing Organs:
As has already been pointed out only in some species there is an organisation of the strobilus. A strobilus usually has a central axis on which are spirally arranged sporophylls. The sporophylls on their adaxial surface bear a single sporangium. Sporangia are 1-2.5 mm., in size Kidney shaped, orange to yellow coloured and have a short massive stalk.
Development and Structure of the Sporangium:
The development is of the eusporangiate type. There is a great deal of variation in the place of origin of these cells. Though there are a number of sporangial initials only one cell is seen in a radial section (Fig.48a, 48b). The sporangial initials divide and produce three rows of superposed cells (Fig.45d).
Of these three rows, the outermost forms the jacket initials, the middle one constitutes the archesporium and the lowermost is called the sub- archesporial layer. The jacket initials divide by anticlinal and periclinal divisions to contribute to the multilayered wall of the sporangium.
The innermost wall layer distinguishes itself into tapetum (Fig.45). The function of the tapetum is to provide nutrition to the spore mother cells. The tapetum at the base of the sporangium is derived from the sub- archesporial layer. A unique feature of the tapetum in Lycopodium is its retention even in the mature sporangium.
The archesporial cells meanwhile divide in all the planes and increase their number. They separate from one another, round off and form the spore mother cells (Fig.45i). The spore mother cells undergo reduction division to form tetrads of haploid spores. The above account of sporangial development is based on the work of Bower (1894).
The sporangial development is essentially similar in all the species of Lycopodium with minor variations here and there. In L. clavatum and L. alpinum, the archesporium consists of three tangential rows of cells and consequently there is an increase in the number of spores.
A mature sporangium consists of a short bulky stalk and an expanded capsular region. The capsular region is reniform and has a multilayered wall, the innermost layer of which forms the tapetum. Lying in the sporangial cavity are found a number of spores of the same type.
Dehiscence of the Sporangium and Liberation of Spores:
In a mature sporangium there is a transverse strip of cells called the stomium. This marks the line of dehiscence. Due to the hygroscopic reaction of the wall cells the sporangium breaks open along the stomium into two clam shell like valves. The spores are exposed and are disseminated by wind.
Gametophyte of Lycopodium:
Structure and Germination of the Spore:
The spores of Lycopodium are minute ranging in diameter from 0.03 to 0.05 mm. Soon after liberation from the sporangium they are tetrahedral in shape. The spore wall is two layered. The architecture of the outer layer is highly variable.
Based on this, Lustner (1898) classified the spores into three types:
(1) Netzsporen – the wall is covered by reticulate ridges,
(2) Tupfelsporen – the wall has knob like projections and
(3) A transitional type with a fine ridged pattern.
Some amount of chlorophyll may be present in the spores occasionally. Spores also have oily reserve food. The different species of Lycopodium show a great deal of variation in the time taken for germination and also in the pattern of germination.
In L. cernuum, L. innundatum etc., the spores germinate within a few days after their liberation from the sporangium. As a result, the gametophytes are short lived completing their life-cycle in one season. In L. clavatum and other species the spores germinate only after three to eight years. This unusual delay in germination is due to their thick cutinised walls. In these members, the gametophyte has a life span of 6-15 years.
The first sign of spore germination is the rupturing of exospore. Before the rupture, two cells would have been formed within the spore. Of these, one is small and biconvex and the other one is large. Soon after the rupture the spore contents come out in the form of a conical mass.
The small biconvex cell is the rhizoidal cell. It does not take any further part in the development of the gametophyte. The larger cell divides by means of an oblique wall and produces two cells. Of the two cells so formed, the one nearer the rhizoidal cell is called the basal cell. It does not divide further. The other cell by further divisions differentiates an epical cell with two cutting faces.
The development does not proceed beyond the five celled stage if there is no infection by the mycorrihizal fungus.
In aerial prothalli if there is no infection, the gametophytes die. In subterranean prothalli there ensues a period of rest of nearly a year after the five celled stage during which mycorrhizal infection takes place. Only then the growth continues.
The precise nature of the triggering mechanism of growth provided by the fungus is not known though it is believed that the fungus supplies certain vital substances necessary for the growth of the gametophyte.
After the fungal infection, growth is rapid and very soon the apical cell is replaced by a group of meristematic cells. These divide and produce the pro-thallus.
According to Wetmore and Morel (1901), the spore germination is rather unique in L. cernuum, L. innundatum and a few’ others. Here the spores are achtorophyllous in the beginning but acquire it a little earlier to germination. The first division of the spore produces two cells, of which the smaller one does not develop further.
The larger cell produces an apical cell with two cutting faces. This cuts off cells alternately towards left and right. The segments cut off by the apical cell produce a mass of cells called the primary tubercle. According to Treub the growth of the pro-thallus stops at this stage unless it is invaded by the mycorrhizal fungus.
Structure of the Mature Pro-thallus:
The species of Lycopodium exhibit a great range of diversity in the types of gametophytes. Usually three main types of gametophytes are identified based on growth pattern, nutrition, life span etc. It should however, be pointed, that there: re many intergrading forms between these three types.
First type: (L. cernuum, L. innundatum etc.)
This is found mainly in species inhabiting tropical regions. The spore germination here is immediate and consequently the gametophyte completes its growth in one season. The gametophyte is extremely small, ranging from 2 to 3 mm, in size. It is above ground and has a basal portion buried in the soil. The upper green part is exposed and has a number of small finger like photosynthetic lobes forming a crown (Fig.46c).
The basal portion consists of the mycorrhizal fungus. Rhizoids are produced from the basal region. Sex organs are formed between the photosynthetic lobes. Anatomically pro-thallus consists of only parenchyma.
The nutrition of the pro-thallus is both autotrophic and saprophytic.
The pro-thallus of L. alakense belongs to the first type but it lacks the endophytic fungus, the nutrition being only autotrophic.
Second type: (L. clavatum (Fig 46b.) L. complanatum, L. annotinum, L. obscurum etc.)
In this type, the spore germination is delayed for a long time and consequently the prothalli have a longer life span. In size also, prothalli belonging to this type are the largest ranging from 1 to 2 cm.
Their shape varies, they may be top shaped, carrot shaped or disc like. These prothalli are brownish or yellowish in colour and are completely subterranean being present at 1-8 cm., below the surface of the soil. From the lower surface a number of rhizoids are produced.
Anatomically, the pro-thallus exhibits tissue diversification not encountered in the other types. It should be pointed out however, that all the cells in the gametophyte are parenchymatous. Further, tissue diversification is seen only in the lower cylindrical portion whereas the upper flattened portion has only polygonal parenchyma cells.
In the lower cylindrical portion the following tissues are seen. The central region consists of vertically elongated cells (Fig.47) constituting the storage tissue. The cells are radially elongated and closely packed. They constitute the palisade tissue. External to the palisade tissue is the hyphal tissue the cells of which have the fungus. Outside the hyphal tissue is the epidermis, some of the cells of which produce rhizoids.
Sex organs are borne on the upper surface of the pro-thallus. Antheridia develop first at the centre followed by archegonia at the periphery.
Third type: (L. phlegmaria (Fig.46d) and other epiphytic species).
To some extent this combines the characters of the first and second type. Like the first type here also the spore germination is immediate and consequently the gametophyte is small, growing for only one season. Here, like the second type, nutrition is always saprophytic but unlike the second type, anatomically the gametophyte is simple in having ordinary parenchyma cells infested with mycorrhizal fungus.
The prothalli grow on the trunks and rotting barks of trees close to the soil. The main body of pro-thallus is an irregularly shaped tuber. From this central tuber arise several slender cylindrical branches. The length of the branches range from 1 to 6 mm. Sex organs are borne on the upper surface of these branches.
It has already been pointed out that the above mentioned three types of gametophytes are not clear cut in their characteristics. There are many intergrading type combining the characters of one or more types of gametophytes making the categorization untenable. As Lang (1899) observes: the similarity in ground plan of the prothalli would appear rather to indicate that they are all more or less profound modifications of a type not unlike that of L. ocernuum.
In this connection it will be interesting to mention the observations of Bruch-mann (1898-1910). He has studied in detail the development of the pro-thallus in L. selago. Two types of prothalli are found in this species. If the spores germinate immediately, the resultant gametophyte would be chlorophyllous resembling that of L. cernuum.
If the spore germination is delayed (i.e. if they are deeply buried in the soil) the resultant gametophyte would be an achlorophyllous, subterranean one. The shape of the gametophyte depends on the type of soil. The prothalli may be either radially symmetrical or may show a bilateral symmetry. According to Lang (1902) the diverse nature of the prothalli of L. selago clearly point out that the so called different types are nothing but variations of a basic type.
The -gametophyte also reproduces by two methods, namely (1) Vegetative propagation and (2) Sexual reproduction
(1) Vegetative Propagation:
In L. phlegmaria a small gemma arises near the lips of the branches. It is usually club shaped with a stalk. On separation from the parent body gemma develops into a new gametophyte.
It has been noticed that in L. innundatum small buds arise from the injured parts of the lobes.
Decay of older parts:
In L. phlegmaria and L. billardieri progressive death and decay of the older parts result in the separation of the branches. These separated branches live as independent prothalli.
(2) Sexual Reproduction:
The prothalli are generally monoccious with the antheridia appearing first (Protandrous). Usually antheridia and archcgonia appear at distinct regions on the upper surface of the pro-thallus. But, occasionally as in L. lucidulum, sex organs may be intermingled.
Structure and Development of the Antheridium:
The antheridium develops from a superficial cell behind the apical meristem. The first division results in forming two superposed cells. Of these, the upper one called the jacket initial undergoes repeated anticlinal divisions to build up the single layered jacket of the antheridium (Fig.50d).
Meanwhile the lower cell (primary androgonial cell) divides in all the planes to form a large number of androgonial cells. The last generation of the androgonial cells are known as androcytes. These androcytes metamorphose into biflagellate antherozoids (Fig.50e). A mature antheridium may or may not be sunk in the gametophytic tissue. It has a single layered jacket enveloping a large number of antherozoids.
Dehiscence of the Antheridium:
When the antheridium is about to mature a triangular cell called the Opercular cell gets differentiated in the jacket This opens out allowing the antherozoids to escape.
Structure and Development of Archegonium:
A single superficial cell gets differentiated on the surface of the pro-thallus. This (archegonial initial) undergoes a transverse division to form an upper primary cover cell and a lower central cell. The primary cover cell divides twice where the second division is at right angles to the first one.
As a result, four quadrately arranged cells divide several times transversely to form a 3-4 celled high neck of the archegonium. Meanwhile the central cell elongates and divides transversely to form a primary canal cell and a primary venter cell. The primary canal cell in turn divides several times transversely to form a varying number of neck canal cells (Fig.51e).
In L. cernuum neck canal cells are three in number; in L. selago seven and in L. complanatum there are as many as 14-16 neck canal cells. The archegonium of L. complanatum with its large number of neck canal cells is something of a rarity in pteridophytes, the like of which being encountered only in mosses. The primary venter cell divides to form a venter canal cell and an egg cell.
In a mature archegonium only a part of the neck protrudes out. All the cells in the archegonium except the egg dissolve to form a mucilaginous mass. This chemotactically attracts the antherozoids. Many antherozoids enter the archegonium, finally one succeeds in fusing with the egg, resulting in the formation of a diploid zygote.
Much of our knowledge regarding the embryo development is due to the discoveries of Treub (1884, 1886, 1890), Bruchmann (1910), Holloway (1909, 1915); Wigglesworth (1907), Bower (1935), Browne (1913) etc. Early stages of embryogeny seem to be uniform in all the species. In subsequent development however species of Lycopodium show a great variety.
The first division of the zygote is transverse resulting in the formation of an epibasal cell (cell nearer to the archegonial neck) and a hypo-basal cell (cell away from the archegonial neck. The epibasal cell does not contribute to any part of the embryo. Usually it does not divide further, instead elongates to form a suspensor. The suspensor however is not very long.
The entire embryo is derived from the lower hypo-basal cell. This type of embryogeny where only the lower cell contributes to embryo is called Endoscopic development. The divisions in the hypo-basal cell are extremely variable in different species.
In L. clavatum and L. annotinum the first two divisions are vertical resulting in the formation of a quadrant. A subsequent transverse division results in the formation of two tiers of four cells each. Of these, the lower tier (next to the suspensor) develops into foot.
The upper tier develops into stem, leaf and root. Gradually the stem and leaf come out of the gametophyte and the root grows downwards. This primary root is short lived. New adventitious roots arise from the sporeling. It will be several years before the sporophyte establishes itself on the soil.
The account of embryogeny given above is the one seen in subterranean saprophytic prothalli. In the photosynthetic prothalli (L. cernuum, L. innundatum etc.) embryogeny is slightly different. Up to the formation of suspensor and formation of two tiers of four cells, the embryogeny is similar to what is seen in subterranean prothalli. The quadrant cells of the lower tier form the foot.
In the upper tier the conventional parts of the embryo are not differentiated. The four cells constituting the upper tier divide rapidly and form a tuberous mass of tissue. This gets itself separated from the pro-thallus and lives independently. Treub (1890) calls such a structure as the protocorm.
Structure of a Protocorm:
The protocorm is a tuberous, lobed structure. The upper surface of the tuber produces many long cylindrical chlorophyllous out growths called protophylls (Fig.52). From the lower surface of the tuber, arise a number of rhizoids helping in anchorage and absorption. Rhizoids and protophylls assure an independent existence to the protocorm.
Internally a protocorm has only parenchyma infested with the fungus. There is no vascular tissue.
Eventually from the upper surface of the protocorm, the typical sporophytes with stem, root and cotyledons gets differentiated.
Mitsuta (1980) has studied the structure of the protocorm in Lycopodium cernuum. According to him prophylls are arranged alternately on the protocorm and the growing apex of protocorm directly transforms itself into the growing apex of stem.
Studies on the protocorm of Lycopodium corolinianum have shown that after the emergence of protophylls, the rhizome apex is formed laterally and the first root initials originate. Dorsal leaves characteristic of the rhizome of the adult plant emerge only after the second pair of leaves are formed.
Morphology of the Protocorm:
The formation of an independent, intermediate phase between the sporophyte and gametophyte has led to various types of interpretations on its morphological nature. The points of interest in the morphology of the protocorm lie in its autotrophic behaviour and gametophytic internal structure with a sporophytic cytology. Because of this it has been variously interpreted as a sporophyte, as a gametophyte and as a special phase intermediate between sporophyte and gametophyte.
According to Treub (1890) the protocorm is a sporophyte retaining the primitive undifferentiated structure that was once possessed by the ancient sporophytes. He believes that in the course of evolution the primitive undifferentiated plant body gave rise to the differentiated plant body. Protocorm represents one such primitive stage.
Bower (1908, 1935) disagrees with Treub (1690) and believes that the establishment of the so called protocorm is nothing but an adaptation of the plant to meet a specific circumstance under which perhaps it would not be possible to establish the sporophyte.
Browne (1913) considers protocorm to be a reduced stem.
According to Holloway (1939), the protocorm is produced to meet certain physiological conditions such as perenneation during unfavourable season etc. He does not attach any phylogenetic significance to the protocorm.
Some of the experiments conducted by Wardlaw (1955) appear to indicate that certain genetic factors and the metabolic pattern govern the development of protocorm. According to him a higher C/N (Carbohydrate/Nitrogen) ratio would induce cell division and thus delay the organisation of the parts of the sporophyte.
It is possible to expect a higher C/N ratio in the prothalli of L. cernuum and others because they possess two types of nutrition, autotrophic as well as saprophytic. As Wardlaw (1955) believes, when C/N ratio comes back to normalcy a definite sporophyte is established. But the one point that has not been convincingly explained by Wardlaw (1955) is the autotrophic behaviour of the protocorm.
If indeed an abundance of food has resulted in the delay of differentiation and established the protocorm, the moment the excess reserve food is exhausted or comes to the normal state, the sporophyte should differentiate.
Instead the protocorm separates itself from the gametophyte and develops organs like protophyll and rhizoids necessary for the manufacture of food. This indeed is not expected of the protocorm because according to Wardlaw (1955) there is already a higher amount of nutrition. The protocorm is compared to the mature plant of Phylloglossum wherein the plant possesses a bulbil like structure.
Apogamy and Apospory:
This generally does not occur in nature, but some scientists have been able to induce these experimentally. Freeberg (1957) induced apogamy in L. complanatum, L. cernuum and L. selago. According to him insufficient water unfavourable for fertilisation induces the development of apogamous sporelings.
Apospory has also been observed by Freeberg (1957) in L. complanatum.
Chromosome numbers are known for various species of Lycopodium. The number ranges from n = 34 to n = 136. In L. clavatum, L. annotinum n = 34. In L. wightianum n = 48. In L. hamiltonii n = 136. Cytological studies conducted on three species of Lycopodium (L. complanatum, L.flabelliforme and L. iristachyum) by Hersey and ‘ Britton (1981) have shown that chromosome number is n = 23.
According to Foster and Gifford (1959) the chromosome number in various species of lycopodium ranges from n = 14 to n = 78. The variation in chromosome number suggests the heterogenous nature of the genus and supports the view that it could be divided into several genera.
Phylogeny of Lycopodium:
The Linnaean genus Lycopodium seems to be a convenient assemblage of several closely related genera. The support for this view comes not only from comparative morphology but also from cytological studies.
The dichotomous branching of the stem so characteristic in some species suggests a habit of lower plants rather than of vascular plants.
Lycopodium however is more advanced than Psilotum as evidenced by its more advanced morphology and anatomy. The leaves are microphyllous and the first leaves on the young sporophyte are apparently devoid of vasculature. This suggests the origin of these leaves as enations.
The stele (protostele) no doubt is of a primitive type but the xylem does considerable exercise to introduce some sort of complexity.
The different species of Lycopodium clearly show the different steps in the evolution and differentiation of sporophylls and strobilus.
The gametophyte also exhibits considerable diversity though it has developed on a basic plan. According to Eames (1964), the green cylindrical gametophytes are more primitive than the saprophytic ones. The nature of the sex organs particularly the long neck of the archegonium recalls the feature seen in bryophytes. In this respect Lycopodium is something unique among land plants.
These features suggest that Lycopodium is apparently a primitive plant which has become much specialized along various lines as an adaptation to a habitat. But one special feature is, while some characters have remained primitive others have advanced far in specialization. The recognition of this primitiveness and the presence of certain peculiarities have been the basis for the origin of sporophyte in land plants.