Diversity of seed plants and their systematics

and scales, ultimately bearing another male cone and the process is repeated (Fig. 12C). The stem of the male
plant of cycas is therefore a sympodium.
When young, the surface of the cone is covered all over by brown scales. An old male cone is very large hard &
woody oval or conical in shape. (Fig. 12A, B). It emits a very characteristic odour which can be smelt from a
distance. It has a central cone axis that bears numerous, almost perpendicularly attached microsporophylls (Fig.
12D) arranged in a spiral manner. The microsporophylls at the apex and the base of the cone are sterile a mature
microsporophylls is a hard, horizontally flattened woody structure consisting of a wedge shaped portion and a
sterile extension tapering into an upcurved apex (Apophysis) (Fig. 12E,F). Numerous sparangia are borne on
the lower or abaxial surface (Fig.12 E-H; 13A) microsporangia occur in groups of 3,4 or 5 and such groups are
called sori (single sorus)
The average number at sporangia per sporophylls is 700 in C. circinalis and 1160 in C.
media. the sporangia dehisce by longitudinal slits radiating outwards from the center
(Fig.12H,I)
Microsporanglum:-
Microsporangium of Cycas (at a fairly well advanced stage of development) shows 5 or 6 wall layers. The
epidermal cells are thin walled of uniform size and covered by a thick cuticle. The cells of the tapetum are small
densely cyloplasmic and completely surround the developing sporogenous tissue. The tapetum is of secretary
type and starts degenerating during meiosis in the microspore mother cells. By the time the pollen grains are
mature, the tapetal cells are completely absorbed. The sporogenous cells are large with prominent nuclei and
their cytoplasm rounds upto form the microspone mother cells. (Fig. 14A) The microspore mother cells undergo
meiotic division to give rise to tetrads of microsposes enclosed in a sheath of callose.
Microsporogenesis. :-
The microspore mother cells undergo reduction divisions (Fig. 14A,B) after meiosis-I, a ring-like thickening
appears on the callose wall of the mother cell in the equatorial plate. Usually, the wall laid down after meiosis-I
seems to be thicker than the one which develops after meiosis-II. It is likely that the abundance of starch,
especially on the equatorial plate, obstructs a clear view of the cell plate. The cytoplasm of each microspore
contracts and secretes a membrane which forms its independent wall. Thus, each member of a tetrad is
contained within its own chamber. The original wall of the mother cell, which appears to be made up of peptic
substances, collapses and no trace is left after a week of division. The pollen grain of cycads shows an oval
germinal furrow or sulcus on its distal face (Millay and Taylor 1976). The furrow develops by a modification of
the exine in the region and can be differentiated into a peripheral and a central zone (Audran 1965, 1971).
According to Gulliver (1966) in C. Revoluta the exine is made up of two layers. The outer one comprises
orbicules and/or a sculptured granular layer, while the inner layer is lamellated. The newly formed micropsers
are uninveleate. Excluding the epidermis, the sporangial wall now consist of tree or four layers of thin walled
polygonal cells; most of the tapetum has already been consumed.
Male Gametophyte:-
The microspores are set free from the callose wall and each uninucleate microspore develops a thick exine and
thin intine Fig. 14C). The latter is the last layer to be laid down and is of variable thickness (Audran 1965),
thinnest at the proximal or prothallial cells end, and thichest at the distal end of the grain. (The exine is thicker
on the proximal side of the spore in the tetrad which thins out on the distal end.)
The Microspore nucleus divides to form a small prothallial cells and a large antheridial initial (Fig. 14D) The
former does not divide any further and remains as a small less-shaped cell. The latter divides into an antheridial
cell and a tube cell (Fig. 14E). The pollen grains are shed at the 3- celled stage. They are boat – shaped due to
the presence of a depression on the distal side (Fig. 14F-H) SEM studies (Sahashi & Ueno, 1986) on the pollen
grains of cycas revoluta have shown them to be oblong with sulcate shrunken aperture when dry. The fullly
expanded grain is almost spherical with a large rounded germinal aperture. The inner layer of exine shows
reliculum like sculpturing.
Structure of mature microsporangium:
The wall layers of the sporangium become disorganized and only the epidermis is conspicuous which forms the
exothecium (Fig.13C) The cells at the upper corners of the micro sporangium are very much alongated,
gradually becoming smaller at the region of dehiscence which comprises two rows of thin walled cells. All the
cells develop heavy thickenings on the radial and inner tangential walls and remain thin on the outer tangential
side. The Poller grains are at three- celled stage: Prothallial, antheridial cell and tube cell. The antheridial cell is
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