Marsilea, commonly known as ‘water fern’, grows in shallow water or in wet places, often creeping upon the banks and growing in all directions. A few species however, grow in dry situations throughout the year.
The genus comprises of 70 species. The common Indian species are M. minuta, M. quadrifolia, M. condensata, and M. poonensis.
The Sporophyte of Marsilea:
The stem is rhizomatous, dichotomously branched and creeps on or just below the surface of the substratum. The internodes may be short or long. On the upper side of the rhizome, there are two rows of alternating erect leaves. The branches are either axillary or arising below or lateral to each leaf.
Adventitious roots develop on the outer side of each node of the rhizome. The leaves are typically peltate and quadrifoliate, each with a considerably long petiole. Young leaves, towards the growing apex of stem, show characteristic circinate ptyxis. The veins in the awl-shaped leaflets are dichotomously branched and are ultimately connected by short veinlets to form a reticulum.
Internally, the rhizomes of all species of Marsilea have an amphiphloic siphonostele, being limited internally and externally by endodermal layers, and with a central pith. The pith may be either parenchymatous or sclerotic, depending upon environmental conditions in which the plant grows, and is separated from the inner pericycle by a single layer of parenchymatous endodermis.
In some species the protoxylem elements are not recognizable. Inside the outer endodermis, there is a single layer of parenchymatous pericycle. External to the outer endodermis lies a comparatively thick cortex extending up to the epidermis.
This is differentiated into an inner solid thin-walled or sclerotic parenchyma and an outer parenchyma with a single ring of conspicuous air space, being limited externally by one to several layers of parenchymatous tissue.
At the time of reproduction prominent reproductive structures, known as sporocarps, which are usually borne laterally on short or long peduncles inserted a little above the base of the petioles (sometimes apparently adaxially or axillary). The sporocarps are stalked and somewhat bean-shaped to ovoid nut-like bodies.
The wall is very hard and thick, strongly resistant to external influences and is capable of retaining its vitality for several years. The peduncle may be unbranched with a solitary terminal sporocarp, or in some species branches dichotomously and bears 2-20 or more sporocarps. It has generally been interpreted that the sporocarp of Marsilea is ‘a modified fertile segment from the lower part of a leaf, but it has also been considered to be homologous with the whole leaf’.
Morphology of Sporocarp:
The following views have been put forward to explain the morphology of the sporocarp of Marsilea:
1. The sporocarp has been interpreted as homologous with the whole leaf by Johnson (1933), because the apical growth of the sporocarp and an entire leaf is similar.
The argument against the view is that the vascular supply of the peduncle and a sporocarp is similar to that of a leaf segment.
2. According to Busgen (1890) the sporocarp is formed by two pinnae. But the single bundle in the peduncle and single main vein in the sporocarp show that it is not two pinnae but a single pinna.
Bower (1926) and Campbell (1903) believed that the sporocarp is formed by a pinnately divided pinna and the pinnules becoming fused.
3. Smith holds that in case of a single sporocarp at the base of a leaf (M. vestita) the infolding pinna is undivided. He suggested that the lateral margins of the abaxial side of the pinna is reflexed to oppose each other. Single pinna nature is evidenced by the presence of single main vein in the sporocarp and single bundle in the peduncle.
It is held that the sporocarp of Marsilea has been evolved due to an infolding of a fertile pinna of Cyathacious type of leaf. The Cyathacious nature is indicated by the presence of gradate sorus and enclosure of the receptacle by an indusium.
Within the inner wall of the sporocarp and attached to it, there are two rows of elongated, closely-packed sori, one on each side filling up the sporocarp cavity, and these extend transversely to the longitudinal axis of the sporocarp. Each sorus is enclosed by a delicate indusium. Marsilea is heterosporous and each sorus contains a row of mega-sporangia surrounded by many micro-sporangia.
These, sori are attached to a mucilagenous tissue, which swells remarkably on coming in contact with water at the time of germination. All the mother cells of the microsporangium undergo reduction division and form microspores.
Within the mega-sporangium only one mother cell matures and by reduction division forms a sporetetrad, of which only one megaspore matures, while others degenerate. With reduction division and formation of spores the gametophytic or haploid generation begins.
The Gametophytes and The New Sporophyte of Marsilea:
When the spores attain maturity, the sporocarp germinates and opens in water along its ventral side and apex, splitting in a spreading bi-valved structure. The ring of internal mucilaginous tissue within the sori absorbs water, swells remarkably, comes out of the ruptured sporocarp and thus drags out the sori which are attached to the ring by their ends. Gradually the indusia and the sporangial jackets undergo gelatinization and the spores remain embedded in the gelatinous matrix during early stages of germination.
1. Male Gametophyte:
The development of the male gametophyte from the microspore resembles that of Selaginella and Isoetes. It does not emerge out of the spore wall. A small prothallial cell is first cut off as usual, and a single antheridium is ultimately formed making up the whole gametophyte.
The antheridium consists of a jacket of sterile cells surrounding two inner spermatogenous cells, by the successive divisions of which 16, multiflagellate, spirally-coiled spermatozoids are produced. The spermatozoids are ultimately set free by the bursting of the spore wall.
2. Female Gametophyte:
The megaspore germinates soon after it comes in contact with water. Its nucleus divides near its apex and a small apical cell is cut off by a wall. The large basal cell, with its undivided nucleus, contains abundant food materials, including starch grains, and is nutritive in function.
The small apical cell protrudes through the megaspore wall and forms a small group of cells, the gametophyte proper, bearing a small and simple archegonium. The archegonium is broad but low and has only one neck canal cell.
The spermatozoids swim towards an archegonium and only one finding its way down through the neck fertilizes the ovum and forms an oospore. With fertilization and formation of oospore, the sporophytic or diploid generation begins.
The New Sporophyte of Marsilea:
The oospore develops into an embryo consisting of a leaf, a root, a foot and the stem, and from this embryo the Marsilea plant is ultimately developed.
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