in this article we will discuss about the modes of reproduction in psilotum with the help of diagrams.
Stout aerial branches bear large conspicuous sporangia (Fig. 7.1 A). Each sporangium is a 3-lobed structure (Fig. 7. 1B, C). It is borne on a forked appendage.
It has been described as:
(a) Trilocular sporangium
(b) Trisporangiate structure, and
(c) A synangium; a group of fused sporangia with distinct partition walls (Fig. 7.1D). Rarely, more than three sporangia are also seen.
In a collection from New Zealand, are recorded free sporangia, two fused sporangia and a single sporangium. Occasionally, sporangia terminate small lateral branches (Fig 7.1E). At times, the main axis also terminates into clusters of synangia (Fig. 7.1F) each with a variable number of sporangia. Presence of terminal synangia on an appendageless Psilotum as well as typical appendaged Psilotum is a regular feature.
These terminal synangia have been interpreted as crowded or fused lateral fertile appendages (sporophylls). In an extension of this concept, aerial axis of Psilotum is interpreted as phyletically reduced leafy shoot bearing sterile and fertile appendages. To what extent this reduction has taken place and in which direction it has proceeded remains an academic exercise.
Morphological Nature of Fertile Appendage:
The morphological nature of fertile appendage has been controversial. It has been regarded either as a bifid sporophyll or a short lateral branch. Some investigators describe the synangium to be axillary in origin whereas others describe the original primordium as fertile axis and the forked appendage as a lateral outgrowth on it.
However, the concept that the fertile axis (appendage as well as synangium) is a condensed branch system is favoured. This is substantiated by experimental evidence. Under long days (16hr at 200-300 ft.c.) the fertile axes of P. nudum elongated into definite branch-like structures. Rarely, such a situation is also encountered under natural conditions. On an appendageless Psilotum from Japan, the synangia are borne at the tips of branches.
Origin and Structure of Synangium:
A synangium (Fig. 7.1D) on the outside of its loculi is bound by a single layer of large cells, followed by it are a few layers of smaller cells. The septum has elongate cells. Unlike foliar structures, the synangium is vascularized.
A vasuclar bundle extends into a synangium and subdivides into three parts, corresponding to three sporangia. Occasionally are seen multiple sporangium lobes in P. nudum which are not represented by individual vascular bundles.
The numerous colourless kidney-shaped spores formed in tetrads, are all alike and disperse through slits in radial walls of the synangium.
Evolution of Synangium:
About the evolution of a synangium it has been proposed that it might have evolved either by fusion of a group of individual sporangia, each borne on a separate axis, or by an evolutionary segregation of the sporogenous tissue of a single sporangium.
Anatomical and morphological data are supportive evidences showing that a synangium is derived from terminal subdivisions or bifurcations of the apical meristem. This is in favour of phyletic concept that synangium of Psilotaceae is basically terminal to an axis or axis homologue and has probably evolved from terminal bifurcative branching.
Two phyletic models of synangium evolution are proposed that can be used to explain this phenomenon and which should be tested by further evidence. According to the first hypothesis, which is in essence based on telome concept, the synangium is derived from phyletic fusion of several ancestral terminal bifurcations each ending into an unlobed sporangium, supplied with a single vascular bundle. The second hypothesis derives the synangium from several massive and lobed sporangia.
Despite these explanations and supportive evidences from fossil record the evolution of Psilotum synangium remains to be answered convincingly. The main limitation is lack of substantive evidence.
Gametophyte:
The spores germinate after four months and form gametophytes. The gametophytes are irregularly branched cylindrical structures covered with rhizoids (Fig. 7.2A). They closely resemble pieces of rhizome and remain either underneath the humus in soil or in organic accumulation on tree trunks and are, therefore, difficult to trace.
Each gametophyte is a simple parenchymatous structure with a prominent apical cell. It lacks differentiation and quite early in its development comes to have mycorrhizic association. The gametophytes are non-green saprophytic structures, supported by fungal association.
The fungus enters through rhizoids and invades all the cells except the apical meristem. The hyphae are a good source of lipids, which are used as energy source by the gametophyte. The entire gametophytic surface, including rhizoids, has a cuticular covering.
The gametophytes of tetraploid P. nudum are known to have a central vascular cylinder. Occasionally, it is a complete stele with 1-3 tracheids surrounded by phloem and endodermis or is represented merely by a few elongate thick-walled cells.
Sex organs, antheridia and archegonia, occur in large number all over the gametophyte (Fig. 7.2A, B). The archegonium (Fig. 7.2A, C) is embedded in gametophyte at its venter, the neck is projecting. The antheridium is large superficial structure (Fig. 7.2A, D) with one-cell thick jacket of variable cells and encloses numerous spirally coiled multiflagellate antherozoids.
First division of zygote is transverse (Fig. 7.2E), forming outer epibasal cell and inner hypobasal cell. The latter divides repeatedly to form a lobed ‘foot’ (Fig. 7.3E); from the former is derived the rhizome.
This embryogenic pattern in which shoot-forming apical cell is directed outwards is described as ‘exoscopic’ and is characteristic of liverworts and mosses and is quite uncommon in pteridophytes. Also during embryogeny the formation of calyptra-like outgrowth is due to proliferation of gametophytic tissue from which the sporophyte has to emerge.
