Brief account and life cycle of Salvinia
Salvinia:-
1. Systematics:-
Division:- Pterophyta
Class:- Leptosporangiopsida
Order:- Salviniales
Family:- Salviniaceae
Genus:- Salvinia
2. Habit and Habitat:-
> The genus Salvinia has 12 species most of which are found to be distributed in the African continent.
> The genus is represented in India by two species namely S. natans and S. auriculata. S. natans is found in Kashmir.
> Of the several species of Salvinia, S. natans is the only annual.
3. Study of Sporophyte:-
> The sporophytic plant body consists of a long slender floating rhizome which grows horizontally.
> The rhizome is completely covered by whorls of leaves.
> The plant body generally exhibits radial symmetry. But according to Loyal and Grewal (1966), in S. auriculata, the stem exhibits bilateral symmetry at intermodal region, while it is radial at the nodal region.
a. Stem:-
- The stem is divided into nodes and internodes and a branch initial is always produced at every node.
- But the stem is sparsely branched because of the non-functioning of many of the branch initials.
b. Root:-
- There are no roots in the sporophyte. Absorption is mainly carried out by the general surface.
- It is suggested that the hair like structures found on the submerged filiform leaves may help in the absorption of water and nutrients.
- These hairs however are different from root hairs in being multicellular.
c. Leaves:-
- Leaves are oblong or sometimes hemispherical.
- They usually arise in groups of three.
- Of the three leaves, two lateral ones float on the surface of water while the third one is submerged.
- There is a distinct morphological differences between the submerged and floating leaves.
- The submerged leaves are highly dissected, long and filiform. They very much resemble the roots. But is it not certain whether they also function like roots.
- It is very often suggested that the function of fill form leaves is:
i. To protect the sporocarp.
ii. To act as a ballast or a stabilizer in preventing undue drift of the plant.
- The floating leaves are green and ovate or oblong. They are covered with stiff hairs that prevent wetting. Besides the hairs, papillate projections are also found.
- The members of the leaf whorl alternate at successive nodes, so that there are actually six rows of leaves though the arrangement apparently suggests three rows.
4. Reproduction:-
> The sporophyte reproduces by means of sporangia.
> As in Marselia, the sporangia are aggregated into sporocarps.
> The details of reproduction have been studied extensively in S. natans.
> Sporocarp:-
- The sporocraps appear in clusters ranging in number from 4 to 20.
- The sporocarps are usually borne on the inner segments of submerged leaves.
- In arrangement, the sporocarps are sympodial.
- The shape of the sporocarp varies from globose to ovoid, occasionally flattened also with ridged surfaces.
- The wall of the sporocarp is made up of two layers of cells. The outermost layer when young is clad with hairs.
- Externally all sporocarps are alike. The sporocarps are always bisporangiate.
- Each sporocarp contain many sori.
- The sporocarp at its base has a stout columnar receptacle which carries the vascular supply into the sporocarp.
- The receptacle is un-branched in the sporocarps of S. natans.
> Internal structure of sporocarp:-
- A sectional view of mature sporocarp shows a well-developed wall enclosing the, sporangia.
- Each sprocarp has 4 valves.
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> Sporangia:-
i. Microspoangia:-
- The micro-sporangium consists of a short stalk and a generally ovoid capsule.
- Enclosed in the sporangium are 16 microspore mother cells and consequently 64 microspores are formed.
- All the spores attain maturity and survive.
- The microspores are embedded in the cytoplasmic fluid derived from the tapetum.
- As the spores mature, the tapetal cytoplasm encompassing the spores gets hardened and forms a mass called ‘massula’ or glochidium.
- The microspores are extremely small, triradiate in appearance and have two wall layers.
- There is a single nucleus in each microspore.
- Occasionally the number of microspores per microsporangia might vary. In S. auriculata, according to Loyal and Grewal (1966), there are only 8 microspore mother cells and consequently only 32 microspores are formed.
ii. Megasporangia:-
- The mega-sporangium consists of a short stalk and a generally ovoid capsule.
- The capsule is made up-of a single layer of cells. Internal to the wall of the capsule is a layer of tapetum.
- Enclosed by the tapetum are found 8 mega-sporocytes which divide meiotically to produce 32 megaspores. Of these spores, all except one degenerate.
> Gametophyte:-
- Two types of gametophytes may be expected since Salvinia is heterosporous.
- In both micro and mega gametophytes the development is endosporic.
- The gametophytes begin their development much earlier to the dehiscence of the sporangium.
i. Male gametophyte:-
- The microsporangium does not dehisce, hence the microspores develop within the microsporangium.
- At the first sign of development, the microspore gets divided into three cells. Of these, the lowest divides slightly obliquely to form a prothallial cell.
- Further divisions are restricted only to the upper two cells.
- From these two cells, by successive divisions, two spermatogenous cells and four sterile cells are formed.
- The four sterile cells constitute the jacket cells.
- The two spermatogenous cells by further divisions, form eight spermatocytes which appear in two clusters separated by means of a sterile cell.
- When the gametophytes are mature the sterile cells (Jacket cells) disintegrate, releasing the multi-flagellate spermatozoids which develop from the spermatocytes.
ii. Female gametophyte:-
- The megaspore generally floats on the surface of water in a prone position.
- Prior to the first division, the nucleus migrates to the apical portion of megaspore.
- First division of the nucleus results in forming two unequal cells.
- The upper cell is small and lenticular while the lower cell is very large and is filled with plenty of nutrition.
- Most of the gametophyte is derived from the upper lenticular cell, while in the lower large cell there are only free nuclear divisions.
- This multinucleate cell functions as a store house of reserve food material.
- The upper lenticular cell by means of several divisions forms a lobed apical cushion which comes out penetrating the spore wall and perispore.
- Archegonia appear on this apical cushion.
- Generally, two to four archegonia develop in the apical cushion.
- The archegonia are deeply sunk in the apical cushion. They have a very short neck, an egg cell, a venter canal cell and a two nucleate neck canal cell.
> Fertilization:-
- The female gametophyte at maturity exposes the archegonia.
- In the archegonium, the neck canal cell and venter canal cell disorganize to facilitate the entry of the sperms.
- A large number of sperms enter into the neck of the archegonium while only one succeeds in fusing with the egg.
- The resultant zygote develops into embryo.
> Embryogeny:-
- The first division of the zygote is longitudinal i.e., parallel to the long axis of the neck of the archegonium the next division is transverse resulting in the formation of a quadrant.
- Vertical division of the quadrant cell results in the octant stage.
- The basal cells of the octant form a foot which is haustorical in nature.
- Of the remaining upper four cells of the octant, the two anterior ones-develop into the first leaves.
- Of the remaining two cells, one gives rise to the stem apex while the other is functionless.
- There are no roots in Salvinia.
- Further divisions from the octant stage onwards are bit difficult to follow.
- From the octant cells, a cell plate is formed, which functioning like a column connects the foot with the leaf and stem apex.
- According to some workers the foot plus the column represents a vestigial root.
- The leaf segment enlarges and grows into a cordate leaf.
- The stem apex divides actively and forms the rhizome.
5. Phylogeny:-
> Phylogeny:- The history of the evolution of a species or group, especially in reference to lines of descent and relationships among broad groups of organisms.
> Of the three members of aquatic ferns Salvinia is closer to Azolla than Marselia. While Marselia may be called terrestrial or semiaquatic, Salvinia is purely aquatic.
> This aquatic environment has certainly stamped its impressions on the bodily features of Salvinia.
> The absence of roots, highly dissected filiform leaves and poor development of vasculature are all signs of a hydrophytic environment that houses the plant body.
> In reproductive structures too Salvania is closer to Azolla.
> Ontogenetic studies clearly reveal that in the group of aquatic ferns a bisporangiate sporocarp is probably more primative than a mono-sporangiate one.