CLASSIFICATION, STRUCTURE OF SPOROPHYTE, REPRODUCTION, STRUCTURE OF GAMETOPHYTE, FERTILIZATION, MORPHOLOGY OF RHIZOPHORE OF SELAGINELLA
Family – Selaginellaceae
The genus Selaginella is
commonly known as “Club-moss” or “Spike-moss”. Selaginella is
a large genus comprising about 700 species and is world-wide in distribution.
Some species of Selaginella are found to grow in temperate
regions but majority of them are found to occur in the rain forests of tropical
countries. The species of Selaginella is found to grow on the
ground, on damp, shaded and humid conditions. Some species are also occurs in
arid regions of the world. Temperate species are found to grow on damp shaded
sides of the hills.
About 55 species are found to occur in India.
Of these the common species are S. rupestris, S. chrysocaulos, S.
pallidissima, S. jacquemonth, S. megaphylla, S. pentagona, etc.
B. STRUCTURE OF THE SPOROPHYTE:
1. External structure:- The sporophyte, i.e., the plant body is
well differentiated into – stem, roots and leaves –
Stem – The stem is long, slender, usually
dorsiventral and prostrate with erect branches. In some species the stem is
erect. The stem may be un-branched or dichotomously branched. From each
ramification of the stem, colourless, leafless, elongated and cylindrical
appendages known as rhizophoresdevelop. The rhizophore develops
downwardly into the soil and gives rise to small tuft of adventitious roots at
their free ends.
Leaves – Stems and branches bear numerous
small, lanceolate, ovate to filiform leaves which are arranged in spirals,
decussate pairs or four longitudinal rows. Leaves are generally thin and
delicate in texture, and are provided with unbranched mid-vein.
Sporophyte of Selaginella reproduces
both by vegetative means and by production of spores –
1. Vegetative Reproduction:- Vegetative reproduction takes place by
following methods –
(a) Fragmentation – It is affected only in species that
grow under humid conditions (S. rupestris). In this case the trailing
branches of the stem develop adventitious branches and later get disconnected from
their parent plant and grow into separate individual plants.
(b) Tubers – Formation of tubers have been reported
in S. chrysorrhoizos and S. abyssinica. The tubers
bear rudimentary scales and appear towards the end of the growing season at the
tip of the underground branches that arise from the base of the stem. During
unfavourable condition the aerial parts of the plant die and the tubers enable
the plant to perennate. At the advent of favourable conditions the
tubers germinate to produce new plant.
(c) Resting Buds – Resting buds have been reported to develop at the ends of
some aerial branches in S. chrysocaulos. It is formed as a result
of compact arrangement of the leaves. The buds give off rhizophores that bear
roots at their tips and fix them to the soil. The resting buds survive the
unfavourable periods when the rest of the plant dies. They grow into new
individuals at the return of the favourable conditions.
2. Spore Formation:- In Selaginella, the spores
are formed in a specialized reproductive structure known as strobili (singular
: Strobilus) or cone.
The cone i.e., strobilus varies in size from
5mm to 7cm . They are cylindrical or quadriangular and are borne at the apices
of the main stem or on lateral branches. Each strobilus consists of an axis
upon which two types of sporophylls viz., megasporophyllsand microsporophylls are
arranged spirally. The megasporophylls appear at the base and microsporophylls
at the above portion of the strobilus.
Each megasporophyll bears a single
stalked megasporangium in its axil on its upper side.
Simillarly, microsporophyll bears a single microsporangium in its axil on its
upper side. Megasporangia are larger in size than the microsporangia.
Both the types of sporangia are provided with a jacket wall of sterile cells of
two-celled thickness. Within the jacket wall lies the sporogenous tissue, which
is surrounded externally by a prominent layer of nutritive tissue known
as tapetum.
Sporogenous tissue of each megasporangium
differentiates into megaspore mother cells, but all of them except
one degenerates. The surviving megaspore mother cell by reduction division
gives rise to four megaspores. In some cases, out of the four
megaspores only one or two survive, others degenerate.
Within the microsporangiun sporogenous tissue
later on differentiates into microspore mother cells, all of which except a
very few, by reduction division gives rise to spore-tetrads. Thus, each
microsporangium contains numerous microspores.
D. STRUCTURE OF THE GAMETOPHYTE:
Selaginella is heterosporous,
hence it produces two types of gametophytes, viz., microgametophyte i.e., male
gametophyte from microspore and megagametophyte i.e., female
gametophyte from megaspore. Thus gametophytes are dioecious(heterothallic).
1. Male gametophyte:- Microspore is the first cell of the male
gametophyte. Each microspore is small, spherico-tetrahedral and provided with
two coats, viz., outer thick ornamental exine and an inner
delicate intine.
Germination of microspore takes place within
the microsporangium. Microspore nucleus first divides to form small
lense-shaped prothallial cell at one side and the larger antheridial
initial. The prothallial cell divides no further but the antheridial
initial divides and re-divides forming 12-celled structure, the so called antheridium.
Now the male gametophyte consists of 13 cells (12 cells from the divisions of
antheridial initial and 1 prothallial cell). Of these 13 cells, the central
four cells constitute the primary spermatogenous cells, the eight
cells surrounding the primary spermatogenous cells constitute the sterile
jacket cells. The primary spermatogenous cells divide and re-divide
forming 128 or 256 sperm mother cells i.e., androcytes. Each sperm
mother cell is then metamorphosed into biflagellate sperm.
2. Female gametophyte:- Megaspore is the first cell of the
female gametophyte. Megaspore are larger and tetrahedral in shape and consists
of outer sculptured thick exine and inner thin intine.
The female gametophyte also begins to
germinate while the megaspore is still within the megasporangium. The
germinating megaspore first enlarges in size and now consists of three wall
layers and a thin layer of peripheral cytoplasm enclosing the nucleus. Its
nucleus divides into two. Then the two nuclei, by free nuclear divisions,
divide continuously until the cytoplasmic layer contains many free nuclei,
surrounding the large central vacuole. As the nuclei increases in number, the
cytoplasmic layer becomes thicker and the vacuole becomes smaller, and
ultimately the vacuolar region is filled up with cytoplasm. Now wall formation
begins about the nuclei in the apical region. As a result a cushion of tissue
is formed which extend inwards filling the megaspore completely before
fertilization.
Shortly, after the formation of apical tissue,
the spore wall cracks along the tri-radiate ridge and the apical cushion of
tissue becomes exposed. This tissue of the gametophyte may become green and
rhizoids may develop from the gametophyte after they have fallen into the soil.
Most of the superficial cells of the apical
tissue are potential archegonial initial and several of these develop
into archegonia. Archegonia are developed from the centre of the
cushion. They are small and shunken in the gametophytic tissue.
Each archegonium consists of neck,
composed of two tiers of four cells each, one neck
canal cell, ventral canal cell and an egg.
3.
Fertilization:- It
may takes place while the female gametophyte is still within the magasporangium
or after the megasporangium has fallen to the ground. The sperm after
liberation swim to the archegonia in dew or in rain water and of them
ultimately fertilize the egg. As a result, a zygote (2n)is formed.
With the formation of zygote, diploid sporophytic generation begins.
MORPHOLOGY OF RHIZOPHORE OF SELAGINELLA
Rhizophores are the colourless, leafless,
elongated and cylindrical appendages which arise from each ramification of the
stem of Selaginella.
There are three different views regarding the
morphological nature of the rhizophore. These are –
1. Rhizophores are regarded as capless roots,
as they look like root, positively geotrophic, leafless and have the same
anatomical characteristic as that of a root (Van Tieghem and Harvey
Gibson, 1902; Uphof, 1920).
2. Rhizophores are regarded as leafless shoots
because rhizophores like the stems are exogenous in origin and develop from
angle meristem one above the other below the junction of two branches (Bruchmann,
1871 and Worsdell, 1910).
3. Rhizophore is neither shoot nor root, but
exhibit some of the characters of both (Sporne, 1966; Goebel, 1905;
Bower, 1908, 1935).
Some of the stem-like characteristics of
rhizophores are –
(b) They develop from special meristem called
angle meristem that are present in between the
two branches of the stem.
(c) They lack root caps.
(d) They have no root hairs.
(e) Experimental evidence proved that under
certain environmental conditions the rhizophores
develop into leaf bearing shoots.
Some of the root-like characteristics of
rhizophores are –
(a) They are positively geotrophic.
(b) They bear no leaves.
(c) Their internal structure resembles that of
a root.
(d)
Their stellar organization is always monostelic even if the
stems are polystelic.
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