PSILOTUM - CLASSIFICATION, STRUCTURE OF SPOROPHYTE, REPRODUCTION, STRUCTURE OF GAMETOPHYTE AND FERTILIZATION

A. CLASSIFICATION:
Division – Psilophyta
    Class – Psilotopsida
        Order – Psilotales
             Family – Psilotaceae
                Genus – Psilotum

The genus Psilotum is widely distributed in the tropical and sub-tropical regions of both Northern and Southern hemisphere, (e.g. – P. nudum and P. flaccidium). The plant is xerophytic in nature and grows in various habitats from very dry to moist places. They are also found to grow on tree ferns and palms.

P. nudum is found to grow in India, and hence it is the only Indian species.

B. STRUCTURE OF THE SPOROPHYTE:

1. External structure:- The sporophyte, i.e., the plant body is well differentiated into – a slender rootless rhizome and an aerial shoot.

Rhizome – The rhizome contains a mycorrhizal fungus and is covered with hair like rhizoids. The branching of the rhizome is dichotomus. Some of the dichotomy may become erect and develop into green aerial leafy shoot.

Aerial shoot – Aerial shoots are either pendent (in case of epiphytic condition), or erect (in case of terrestrial condition). The branching of aerial shoot is strictly dichotomous.

Leaves – The aerial branches bear on the upper part, small, scale like appendages, i.e., leaves, arranged in 2-3 rows.

2. Internal structure:- (a) T.S of Aerial Shoot: Internal structure of aerial shoot shows the following tissue systems –

Epidermis – Epidermis of aerial shoot is one-celled in thickness with heavily cutinized outer wall. The epidermis is interrupted by the presence of stomata. 

Cortex – Cortex is broad and differentiated into 3-regions – (a) hypodermal outer cortex composed of chlorenchymatous cells, 2-5 layers thick, (b) middle cortex composed of thick walled sclerenchymatous cells, 4-5 layers thick, and (c) an inner cortex composed of thin walled parenchymatous cells, containing abundant starch grains. The innermost layer of the cortex is the single layered endodermis with casparian strips.

In rhizome, cortex is composed of only thin walled parenchymatous cells containing mycorrhizal fungus.

Stele – The stele of aerial shoot is actinostelic protostele, with 5-6 xylem rays. The xylem rays are exarch. Phloem lies in the outer portion, covering the xylem, beneath the endodermis. Phloem cells are made up of thin-walled cell elements and sieve tubes.

The stele of rhizome is haplostelic or actinostelic type of protostele. Central xylem core is completely surrounded by the phloem.

(b) T.S of Leaf: The T.S of leaf shows a single layered epidermis. Stomata are absent in the epidermal layer. Beneath the epidermis is an undifferentiated chlorenchymatous mesophyll cells. There is no vascular tissue in the leaf of Psilotum.

C. REPRODUCTION:

The sporophyte of Psilotum reproduces both by vegetative means and by production of spores –

1. Vegetative Reproduction:- Vegetative reproduction in Psilotum takes place by the formation of gemmae. The gemmae are developed in large numbers on the rhizomes. The gemma may germinate into new plant when it is still attached to the parent plant or when they fall on the suitable substratum after detachment.

2. Spore Formation:- In Psilotum, the spores are formed in a specialized spore bearing structure, known as sporangia. The sporangia are borne in triads on the vertical side of the appendage, i.e., leaf, at the point of dichotomy and are slightly raised by short stalk. The sporangia are fused with one another, and such group of sporangia is known as synangium.

Each mature synangium is 2-3 mm wide, 3-lobbed and each lobe contains a spore sac, in which numerous spores are present. Psilotum is homosporous, i.e., it produces only one type of spores.

D. STRUCTURE OF THE GAMETOPHYTE:

Spore is the first cell of the gametophyte. Each spore is bilaterally symmetrical with outer delicate and thin reticulate wall. The spore germinates after about 4 months of liberation, and develops into a gametophytic plant body.

The gametophytic plant body is very small (about 18 mm long), brown in colour, sub-terranean and sporophytic in nature. The mature gametophyte is dichotomously or irregularly branched and is covered with dense, brown, hair-like rhizoids and is filled with mycorrhizal fungus.

            Gametophytes are monoecious, i.e., they are homothallic. Both the sex organs, i.e., antheridia and archegonia are borne on the same gametophytic plant body.

1. Antheridia:- Antheridia begin to develop on the gametophyte earlier than the archegonia. Each antheridium is a projected spherical body, covered by a single layer of jacket cells. Within the jacket layer, lies numerous sperm mother cells, which metamorphose into numerous spiral, multiflagellate sperms, i.e., antherozoids.

2. Archegonia:- The archegonia are shunken with short projecting neck, which breaks away at maturity. The neck consists of 4-5 cells long, and consists of 2 neck canal cells. The venter consists of one ventral canal cell and an egg cell.

            3. Fertilization:- At maturity, the neck of the archegonium breaks, followed by the disintegration of the neck canal cells, which leaves a passage for the entry of sperms. The sperms after liberation from the antheridium, makes its way through the archegonial neck and finally reaches the egg. On reaching the egg, one sperm fuses with the egg to complete the fertilization.

            As a result of fertilization, a diploid zygote (2n) is formed. With the formation of zygote, diploid sporophytic generation begins.

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