CLASSIFICATION, STRUCTURE OF SPOROPHYTE, REPRODUCTION, STRUCTURE OF GAMETOPHYTE AND FERTILIZATION IN PSILOTUM
A. CLASSIFICATION:
Division – Psilophyta
Class – Psilotopsida
Order – Psilotales
Family – Psilotaceae
Genus – Psilotum
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.
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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