After reading this article we will learn about:- 1. Occurrence and Distribution of Salvinia 2. Sporophyte of Salvinia 3. Gametophyte 4. Phylogeny.
Occurrence and Distribution of Salvinia:
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.
Sporophyte of Salvinia:
Morphology of the Plant:
The sporophytic plant body consists of a long slender floating rhizome which grows horizontally. The rhizome is completely covered by whorls of leaves.
1. 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 and
(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.
2. 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.
3. Roots:
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. 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.
Internal Structure:
A transverse section of the rhizome shows the following features. Outermost layer is epidermis. It is distinct, single layered and is made up of thin walled cells. Epidermis on its outer surface is covered by a thin cuticle. Stomata are absent. Next to the epidermis is the cortex. It consists of a number of lacunae or air cavities. In the central region is found the stele. The stele is surrounded by an endodermis and a pericycle.
True to the hydrophytic nature, the rhizome shows poorly developed vascular elements. Central region of the stele consists of parenchyma within which are found many tracheids. Xylem is surrounded by phloem. The vascular arrangement suggests an ectophloic siphonostele. In S. auriculate, the stele may be an amphiphloic siphonostele.
Anatomically, the floating leaves show a bifacial arrangement. There are two epidermal layers surrounding a mesophyll with plenty of air cavities. The mesophyll is undifferentiated. The epidermal layers are studded with multicellular hairs.
The submerged leaves, anatomically exhibit a siphonostele near their base. A little higher up, the single vascular strand first breaks up into two and ultimately into a number of small ectophloic steles. Surrounding the vasculature, pericycle and endodermis are distinguishable.
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. The foregoing account is mainly based on the investigation in S. natans. 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. But unlike in Marselia, the sporocarps are mono-sporangiate. The first one or two sporocarps in each cluster are megasporangiate whereas all the later formed ones are microsporangiate. 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 megasporangiate sporocarps of S. natans.
Development of Sporocarp:
The entire sporocarp is traceable to a single apical cell. This apical cell with its two cutting faces cuts off cells towards its left and right. Very soon a sorus primordium is resulted. A little later, an outgrowth surrounds the sorus primordium. This later develops into the inducium completely covering the sorus. The derivatives of soral primoridum give rise to sporangial initials, which later develop into the sporangia.
Structure of a Mature Sporocarp:
A sectional view of mature sporocarp shows a well-developed wall enclosing the, sporangia. In a megasporangiate sporocarp, only about 25 mega-sporangia are seen whereas in a microsporangiate sporocarp, the number is much larger because of the branching of the receptacle.
Structure and Development of Sporangia:
The sporangium initials whether of micro or mega-sporangium arise on the distal end of the soral primordium. The development of sporangium is of the leptosporangiate type. The first division of the sporangial initial is transverse resulting in two superposed cells. Of these, the upper one develops into the wall whereas the lower one gives rise to the sporogenous tissue.
The sporangial development upto the sporocyte stage is similar in both micro and mega-sporangia. While only a few spore mother cells survive in mega-sporangium, quite a large number do so in microsporangium.
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.
The degenerasting spores and the tapetum harden and surround the functional megaspores in the form of a thick layer. This is often called the ‘perispore’ or ‘epispore’.
The perispore towards the apex of the sporocarp forms a triangular chamber, which very much simulates the pollen chambers of gymnosperms. Arising from the floor of the pollen chamber is a central mound of tissue with three flaps. These three flaps get separated during the germination of megaspore.
The microsporangia are basically similar to mega-sporangia in their construction. 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 amass called ‘massula’. 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 eight microspore mother cells and consequently only 32 microspores are formed.
Dispersal of Sporocarp:
After the sporangia are mature the sporocarps get detached from the leaves and sink to the bottom of the pond. In annual species, this generally takes place during September, when the plant breaks up into bits. The sporocarps open up due to the mechanical decay of their walls. This results in the release of spores. In most of the cases the spores that float on the surface of water are still surrounded by the sporangial walls.
Gametophyte of Salvinia:
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.
Structure and Development of Mega-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.
Structure and Development of Micro-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.
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.
Chromosome Number:
Cytological studies on Salvinia molesta have shown that the 2n number is 45. Chromosomes range in size between 1.6-4.1μ in length. Schneller (1980) has reported 63 chromosomes in S.herzogii and regards it as octaploid. In another cytological study on S. auriculata Scheneller (1981) has reported 54 chromosomes and regards it as hexploid with n = 9 being the basic number.
Phylogeny of Salvinia:
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.
While the sporocarp of Marsilea is bisporangiate, Salvinia, is mono-sporangiate, and Azolla seems to be potentially bisporangiate. Ontogenetic studies clearly reveal that in the group of aquatic ferns a bisporangiate sporocarp is probably more primative than a mono-sporangiate one.
In this respect Azolla seems to be intermediate between Marsilea and Salvinia, because while the mono-sporangiate condition is established at maturity, ontogenetically it passes through a bisporangiate phase. In many of the morphological features (except for sporocarp) Salvinia recalls Hymenophyllaceae.