In this article we will discuss about the vegetative and sexual modes of reproduction in lycopodium with the help of diagrams.
Vegetative means of propagation is quite common in the sporophyte of Lycopodium which take place by the following methods:
(i) By the Formation of Gemmae or Bulbils:
These are modified lateral branches which develop on the stem apex in the axils of leaves (Fig. 7.26A, B). Each bulbil consists of a short axis where several thick and fleshy leaves are arranged spirally and compactly.
These leaves store food material. These bulbils fall on the ground and grow into new sporophytic plants, e.g., many Urostachya members like L. selago; L. phlegmaria and L. lucidulum.
(ii) By Fragmentation and Decay:
In this method, the progressive death and decay of older parts reach the region of branching; as a result the two branches separate and each branch develops into a new plant e.g., L. incondatum and other creeping species.
(iii) By the Formation of Adventitious Buds:
The adventitious buds are formed near the base of the main stem and on separation from the main axis they are capable of forming new plants e.g., L. phlegmaria, L. reflexum.
(iv) By the Formation of Root Tubercles:
Bud-like tubercles are formed in the apical region of the adventitious root by the proliferation of the parenchymatous cells. These tubercles on germination produce new sporophyte e.g., L. ramulosum, L. cernuum.
B. Reproduction by Spores:
The plant body is a sporophyte i.e., it bears spores. The spores are developed in the sporangia.
Sporangia — the Spore-Producing Organ:
Position of the sporangium and organisation of the strobilus.
Sporangia always occur singly on the adaxial surface of the specialised leaves called sporophylls or fertile leaves (Fig. 7.27A, B). The sporophyll forms a protective covering around the sporangium. The sporophylls may be aggregated into a definite strobilus (aggregation of sporophylls is called strobilus).
The sporophylls of such strobili are different from vegetative leaves in size, shape, and colour. These types of strobili may occur on leafy stems or may be erected on lateral branches having very small, scale-like leaves (e.g., L. clavatum; L. digitatum; L. obscurum).
In certain species of Lycopodium (e.g., L. lucidulum and L. selago) the sporophylls are similar to vegetative leaves. In these species no definite strobili are formed and the “fertile* areas alternate with “sterile” regions along the stem. Epiphytic species, viz., L. phlegmaria, has dichotomously branched pendulous strobilus.
Structure of the Sporangium:
Mature sporangia of most species are unilocular, sub-spherical or reniform (kidney- shaped) in shape (Fig. 7.27B, C). The size of the sporangium varies from 1.0 to 2.5 mm in diameter and colour ranges from yellow to orange. The sporangium has a short and massive stalk.
Development of the Sporangium:
Mode of sporangial development is of the eusporangiate type (originating from a group of superficial cells). A single transverse row of superficial cells at the adaxial (upper) surface of the sporophyll divides periclinally (Fig. 7.28A, B).
The outer cells form the multilayered wall and the sporogenous tissue/cells derived from the inner cells of such divisions (Fig. 7.28C-E). The innermost layer of the sporangial wall functions as tapetal layer (i.e., nourishing tissue).
The sporogenous tissue undergoes repeated mitotic divisions and ultimately gives rise to spore mother cells. As the sporangium matures, spore mother cells separate from each other and undergo meiotic division (reductional division) to form numerous spore tetrads (i.e., group of four cells produced by a meiotic division of a spore mother cell).
Spores are triangular in shape with triradiate ridge, present on the inner (proximal) face (Fig. 7.29).
Lycopodium is a homosporous pteridophyte i.e., it produces spores of equal size and shapes. The mature spores are yellow in colour. The spore wall is divisible into two layers viz., the inner wall, called the intine, and an outer layer, the exine. The exine displays ornamentation that varies with the species (Fig. 7.29).
Dehiscence of Sporangium:
The mature sporangium splits along a vertical line of weakness (stomium) chiefly because of the stress and strain caused by drying of the sporangial cells. The spore mass projects out of the open slit and eventually disseminate by air currents.
Lycopodium is homosporous, therefore, spore germinates exosporically to produce gametophytic prothallus, which bears both male and female sex organs (i.e., monoecious and homothalic). The germination of the spores may be immediate in some species (e.g., Lycopodium cernuum, L. inundatum) or after a delay of several years (L. clavatum, L. complanatum).
The spores absorb water before germination. The first division of the spore is asymmetric to produce one small biconvex rhizoidal cell and a large cell. Soon after this division, the exine ruptures along the triradiate ridge. The rhizoidal cell disintegrates, while the large cell again divides by a vertical wall to form two cells.
Of these two cells, the one nearer to rhizoidal cell is called basal cell which does not divide further. The other cell, by further divisions, forms apical cell with two cutting faces. The further development of gametophyte does not proceed if there is no infection into the basal cell by the mycorrhizal fungus.
Three main types of mature prothalli (singular prothallus) may be distinguished in Lycopodium:
1. Cernuum Type (Fig. 7.30A):
These types of gametophytes are found in most of the tropical species (e.g. L. cernuum, L. innunda- tum). Here spore germinates immediately and the gametophyte completes its growth in one season. The prothalli are small, green and aerial with a lower conical basal region buried in the soil. Rhizoids occur in the colourless subterranean (basal) region.
The subterranean region always contains an endophytic fungus. The entire plant body may not be over 3 mm long and are annual in nature. The upper green part is exposed and has a number of irregular leaf-like lobes (photosynthetic) forming a crown. Nutritionally, the prothallus is both autotrophic and saprophytic. The sex organs (antheridium and archegonium) generally occur near the bases of the aerial lobes.
2. Clavatum Type (Fig. 7.30D-G):
In this type, the spore germination is delayed for a long time (one to many years), thus the prothallus has a longer lifespan. Here the prothalli are fleshy, non-green, totally saprophytic and completely subterranean and perennial in nature. Development takes place beneath the surface of the ground or within a layer of humus.
The prothalli are large and may be up to 2 centimeters in length. They may be top-shaped with a convolute margin (L. clavatum) (Fig. 7.30E, F), or carrot- shaped (L. complanatum and L. annotinum) (Fig. 7.30D, G).
The top of the prothallus are lobed and the sex organs and the growing embryos are located on these lobes. Although all the gametophytic cells are parenchymatous, the tissue differentiation is noted in the lower portion.
The central region constitutes storage tissue made up of vertically elongated cells. The radially elongated, closely packed chlorenchymatous cells constitute the palisade mycorrhizal layer. External to the palisade tissue is the cortical mycorrhizal region. The epidermis is present outside the cortical mycorrhizal region, some of the epidermal cells produce rhizoids.
3. Phlegmaria Type (Fig. 7.30C):
Here the prothalli are aerial but saprophytic in nature, grow on tree trunks below a coating of humus. This type is found in epiphytic species of Lycopodium (e.g., L. phlegmaria). Here the spore germination is immediate and the gametophyte grows for only one season.
The prothallus consists of a short, tuberous central part from which a number of colourless, slender and cylindrical branches develop in an irregular fashion. These branches bear sex organs and they are usually surrounded by glandular hairs called paraphysis.
There are also some intermediate types in between these forms. For example, the gametophyte of L. selago (Fig. 7.30B) is in-between the Cernuum and Clavatum types. Here spore germination and gametophyte development take place immediately like Cernuum type.
However, the spores germinate after a long resting period if the spores are deeply buried in the soil. As a result a subterranean saprophytic Clavatum type of gametophyte is formed. Hence more than one type of prothalli may occur in the same species.
The gametophytic prothallus of Lycopodium is monoecious (homothallic) i.e., male (antheridia) and female (archegonia) sex organs are developed on the same prothallus.
The antheridia and archegonia are generally intermingled near the bases of the upright lobes in those species where gametophytes are of the green annual type. In subterranean perennial forms, the sex organs are segregated into definite groups. In both types, antheridia generally appear first near the middle of the crown of the gametophyte.
The antheridium develops from a single superficial cell called antheridial initial of the prothallus (Fig. 7.31 A). The initial cell divides transversely forming an upper jacket initial and a lower primary androgonial cell (Fig. 7.31 B).
The jacket initial divides anticlinally to form one-layered jacket. The lower androgonial cell forms a mass of androcytes through many irregular divisions (Fig. 7.31 C-D). Each androcyte matures into a biflagellate sperm resembling the sperms of certain bryophytes or algae (Fig. 7.31 E).
A sperm is a blunt-ended, fusiform cell, 8-10 µm long and 4-5 µm wide. There are two flagella, each one is about 38 µm long. The antheridia are almost wholly embedded in the gametophytic tissue. The sperms are released by breaking down the operculum at top.
The archegonium develops from a superficial archegonial initial cell. The first periclinal division of archegonial intial gives rise to an upper primary cover cell and a lower central cell (Fig. 7.32A, B). The central cell divides by a transverse wall to form a lower primary venter cell and an upper primary canal cell.
The primary canal cell, by repeated divisions, forms 4-8 neck canal cells (exception, one neck canal cell in L. cernuum, 14 in L. complanatum). The venter cell forms the egg often after cutting off a ventral canal cell. The upper primary cover cells divide and redivide to form neck of archegonium. Venter is embedded in the gametophyte tissue and the neck of the archegonium protrudes out (Fig. 7.32C-E).
Fertilisation takes place in the usual way. The neck canal cells and the ventral canal cell disintegrate to form a passage for the entrance of the motile biflagellate sperms. The sperm reaches the archegonium by swimming through a film of water on the surface of the gametophyte.
Free citric acid or salts of citric acid, available in the canal as a by-product of disintegration of canal cells, may play a role in the attraction of sperms to the archegonia. Only one sperm eventually fertilises the egg that develops into the zygote.
Embryo (New Sporophyte):
The early stages in the development of zygote reveals a common basic plan in all the species, but later stages differ according to the species.
In species with subterranean gametophyte.
The first division of the zygote is transverse to right angle to the long axis of the archegonium which produces an outer suspensor cell and an inner embryonic cell (Fig. 7.33A). The outer cell may or may not enlarge but it does not undergo further divisions and becomes a suspensor.
Subsequent divisions of lower cell produces a multicellular embryo (Fig. 7.33B-C). The embryo in Lycopodium is endoscopic in nature (where the future shoot apex is directed away from the mouth of the archegonium). Further development of the embryo produces shoot apex and foot. The shoot apex grows laterally and upward Suspensor and the foot develops along the lower side of the embryo.
The roots generally come out from the areas between the first leaf and foot (Fig. 7.34A). The foot enlarges, and with the help of the foot the embryo remains embedded in the gametophyte. The foot acts as a haustorial structure until the sporophyte becomes physiologically independent e.g., L. clavatum; L. anno-tinum; L. phlegmaria; L. selago; L. complanatum.
In species with green surface-living gametophytes.
The early developmental stages, until the differentiation into specialised parts, are similar to that of the subterranean species. In this case, foot is formed as usual, but, instead of shoot apex, a spherical parenchymatous body termed protocorm (extraprothalial undifferentiated tuberous body) is developed (Fig. 7.34B).
No root is produced but rhizoids occur on the lower surface and leaf-like outgrowths, called Proto- phylls or prophylls, arise on the upper surface of the protocorm. The protocorm remains in this stage for some time and a shoot apical meristem becomes organised and a “normal” type of shoot is produced, e.g., L. cernuum; L. corolinianum; L. inundatum; L. laterale etc.
Morphological Nature of the Protocorm:
Treub (1890) considered protocorm to be an undifferentiated sporophytic structure of great antiquity and the phylogenetic precursor of the sporophyte. In the course of evolution the protocorm gave rise to sporophytic plant body that is differentiated.
According to Bower (1908, 1935) the proto- corm is an adaptive structure of the plant to escape the unfavourable condition for differentiation of sporophyte.
Goebel (1918) and Holloway (1939) considered the protocorm as merely a structural modification to meet certain physiological conditions like perreneation during unfavourable seasons.
According to Browne (1913) the protocorm is a reduced stem.
On the basis of experimental evidences, Wardlaw (1955) suggested that the development of protocorm is dependent- on certain genetic factors and metabolic patterns. The higher C/N (Carbon/Nitrogen) ratio induces protocorm formation and eventually delay in differentiation of sporophyte. As soon as the excess reserve food is exhausted or comes to the normal state, the differentiation of sporophyte starts.
The protocorm of Lycopodium may be compared with the mature plant of Phylloglossum which bears permanent protocorm.