In this article we will discuss about the morphology of psilotum with the help of a suitable diagram. Also learn about its phylogenetic position.
Two species of Psilotum recognized are P. nudum and P. flaccidum (P. complanatum). Of these P. flaccidum is a pendulous epiphyte, rare in distribution; restricted to Jamaica, Mexico, Hawaii and a few Pacific islands. On the other hand, P. nudum, which is frequent in tropics and subtropics, is an erect slender shrubby plant of 20-25 cm height. It is generally found in the crevices of rocks, but rarely also occurs as an epiphyte.
This species has been brought in cultivation in green houses, and is commonly known as ‘whisk fern’. Horticultural value of Psilotum has waxed and waned. In Japan there are many variants and cultivars, so are also Psilotum Societies of enthusiasts. Academic significance of Psilotum lies in its simple structure and plasticity. It is a phyletically reduced genus.
The plant consists of a subterranean rhizome and dichotomously branched aerial axes (Fig. 7.1 A). The rhizome that anchors the plant is a cylindrical, prostrate and dichotomously branched mycorrhizic structure with numerous rhizoids. Apparently, there is an apical dominance; rhizome with intact apices form very few branches but in plants growing in sandy soil, due to apical injury, regeneration is quite common.
The plants characteristically lack roots and instead the rhizoids, that are 1-3 cells long, serve for absorption. Mycorrhiza also assists in absorption. The fungus is considered to be intimately related to the physiology of plant. The tips of rhizoids in P. nudum proliferate to form gemmae which on regeneration produce entire plants, and serve for vegetative propagation.
The subterranean axis is neither leafy nor capped. In these respects as well as in its ill-organized apical meristem this axis is different from basic organs (stem and root) and seems to be an independent organ, representing a stage in evolution of plant organs.
Some branches of rhizome turn upwards and develop into aerial green branches of regular dichotomy. The rhizome and aerial branches are continuous. What induces some branches to turn upwards remains to be known; probably those nearer the surface and exposed to light behave that way.
The aerial branches are flattened in P. flaccidum and ridged in P. nudum; in that the lower portion is often pentagonal, and triangular towards distal dichotomy. On aerial branches, except the base, are present small awl-shaped leaf-like appendages (Fig. 7.1 A) which are without any vascular trace and stomata. Minute traces for these appendages do originate from stele in P. flaccidum but die out in cortex. An appendageless variant of P. nudum is on record.
Phylogenetic Position of Psilotales:
Psilotales resemble the early vascular plants Psilophytales in the following features:
(1) Dichotomously branched sporophytes with subterranean and erect axes.
(2) Absence of root system.
(3) Absence of well-defined leaves in Psilotum.
(4) Sporangia in rare instances are terminal and occasionally terminate short lateral branches in Psilotum.
These similarities with Psilophytales are suggestive of Psilotales as the most primitive among vascular plants. However, an absence of fossil record of these genera and a gap of 400 million years that separates these plants from the earliest known vascular plants are the causes of concern about their closeness and relationship.
Of interest, in this context is the presence of a specific phenolic compound (psilotin) in Psilotum and Tmesipteris which is not found in Lycopsida. This indicates that the two genera constitute a natural group in pteridophyta.
Also the electrophoretic patterns of histones of Psilotum are similar to those of mosses and unlike those of vascular plants. This places Psilotum at the beginning of vascular plants and should be placed before lycopsida. Phyletically, Psilotum is a reduced genus with terminal synangia.