Salvinia

Salvinia is a genus of floating ferns that are commonly found in still waters, such as ponds, lakes, and slow-moving rivers. This genus is included in the order Salviniales of the class Leptosporangiospsida. The name is given by Anton Maria Salvini. 

1.0Classification of Salvinia

2.0Distribution

Salvinia commonly known as water moss. Salvinia is heterosporous means it produces two kinds of spores.The genus Salvinia has 12 species, most of which are found in the African continent. Two species are found in India, namely S. natans and S. auriculata.  

Common Species of Salvinia 

• Salvinia auriculata 
• Salvinia biloba
• Salvinia cucullata
• Salvinia hastata
• Salvinia martynii
• Salvinia minima
• Salvinia natans
• Salvinia oblongifolia

3.0The Sporophyte Plant

The sporophyte features a long, slender, horizontally growing floating rhizome covered entirely by whorls of leaves.

Leaves

Leaves are oblong or hemispherical, typically appearing in groups of three. Two lateral leaves float on the water's surface, while the third is submerged, showing distinct morphological differences. A bud emerges at the stem node between the floating and submerged leaves. Floating leaves are green, ovate, or oblong, covered in stiff hairs and papillate projections to prevent wetting. Submerged leaves are long and filiform, segmented with numerous hair-like structures that resemble roots, possibly acting as balancers. These multicellular hairs may absorb water and nutrients, with spongy-tipped hairs suggesting a water absorption function.

While resembling roots, the function of the filiform leaves is uncertain in terms of root-like functions. Common suggestions propose that these structures serve two potential roles:

(i) Protection of the sporocarp.

(ii) Acting as ballast or stabilizers to prevent excessive plant drifting.

The multicellular hairs on these leaves may contribute to water and nutrient absorption, yet their primary purpose remains ambiguous.

Stem

The stem is divided into nodes and internodes, with a branch initially produced at every node. However, the stem is sparsely branched due to non-functional branch initials.

Roots

The sporophyte lacks roots and relies on general surface absorption. Hair-like structures on submerged filiform leaves may aid in water and nutrient absorption. Although multicellular, these hairs differ from root hairs. Their function is often suggested to protect the sporocarp and act as ballast, preventing excessive plant drift.

Diagram of Salvinia

4.0Reproduction in Salvinia

The plant's reproductive process involves structures called Sporangia, which gather together to form clusters known as sporocarps. These clusters, ranging from 4 to 20, emerge at the tips of plant segments. The first ones developed, called sporocarps, are specialized for producing larger spores (megasporangiate), containing up to 15 or more of them. As the process continues, later sporocarps are typically designed for smaller spores (microsporangia) and are both larger in size and more numerous. The shapes of these sporocarps can vary, being either spherical, globose, or flattened.

Megasporangium

The megasporangium has a short stalk and an oval capsule made of a single layer of cells. Inside the capsule is a layer called tapetum, which surrounds 8 megasporocytes. These megasporocytes undergo meiotic division, resulting in 32 megaspores. All spores except one degenerate, and the degenerating spores and the tapetum form a thick layer known as the 'perispore'. This layer, especially towards the top of the sporocarp, creates a triangular chamber resembling the pollen chambers of gymnosperms. In this chamber, a central mound of tissue with three flaps emerges, and these flaps separate during the germination of the megaspore. 

Microsporangium

Microsporangia share a similar structure with megasporangia. Inside each microsporangium, there are 16 microspore mother cells, resulting in the formation of 64 microspores. Unlike megaspores, all microspores mature and survive. These microspores are surrounded by cytoplasmic fluid derived from the tapetum, which later hardens to create a mass known as a 'massula.' Microspores are tiny, triangular, and have two wall layers, each containing a single nucleus.

Dispersal of Sporocarp

Mature sporocarps, housing developed sporangia, detach from the leaf segments and sink to the bottom. As the sporocarp wall decays, the sporangia are released intact, rising to the water surface and floating freely.

5.0The Gametophyte Generation

Salvinia is heterosporous, forming two types of gametophytes: (1) megagametophytes originating from germinated megaspores and (2) microgametophytes developing from microspores. Both types are endosporic,  their development starts before the sporangia dehisce.

Megagametophyte

The megaspore, usually found floating on water, repositions its nucleus to the top before its first division. After division, two cells form— a small upper cell contributing to most of the gametophyte and a large lower cell functioning as a nutrient storehouse. The upper cell undergoes several divisions to create a lobed apical cushion that protrudes through the spore wall. Archegonia, typically two to four, emerge on this cushion, each with a short neck, an egg cell, a ventral canal cell, and a bi-nucleate neck canal cell.

Microgametophyte

The microsporangium doesn't open; instead, microspores develop inside. The first developmental step divides the microspore into three cells, with the lower one forming a prothallial cell. Only the upper two cells undergo further divisions, resulting in two spermatogenous cells and four sterile cells (jacket cells). The jacket cells surround the spermatogenous cells. Through additional divisions, these spermatogenous cells produce eight spermatocytes arranged in two clusters separated by sterile cells. The jacket cells disintegrate upon gametophyte maturity, releasing multi-flagellate spermatozoids derived from the spermatocytes.

Life cycle of heterosporous Pteridophyte

Fertilization

When the female gametophyte matures, it reveals the archegonia. Within the archegonium, the neck canal cell and ventral canal cell undergo disorganization, allowing sperms to enter more easily. Numerous sperms enter the archegonium neck, but only one successfully fuses with the egg. The resulting zygote then develops into an embryo, which further develops into the new sporophytic plant.