Diversity of seed plants and their systematics

Differentiation of Embryo:- After the proembryo becomes cellular, the cells of the upper region elongate
to form a very long suspensor. It grows rapidly, and thrusts the basal embryonic region through the egg
membrane and archegonial jacket deep into the gametophytic cells, which have abundant food reserve. This is
due to enzymatic digestion of the gametophytic cells when they come in contact with the embryonic cells. Due
to rapid elongation, the suspensor becomes much coiled and twisted (Fig. 24E). Usually, more than one
archegonium in an ovule is fertilized, and the development of multiple zygotes leads to simple or archegonial
polyembryony. At first, all the zygotes in an ovule appear to develop with equal vigour, the embryos project into
the gametophytic tissue, and their suspensors become closely intertwined with each other. Due to competitive
growth, ultimately only one embryo develops while the rest abort at various stages of development (Fig. 24E).
The suspensor of the aborted embryos remain attached to the tough egg membrane, and persist for a long time.
The suspensor of the successful embryos is, therefore, a composite structure of the coiled suspensors of the
entire group of embryos in an ovule. When stretched, the suspensor may be up to 10 cm long.
Seed
The maturation of the embryo in seed takes over a year after fertilization. The seed is shed at any stage during
this period, and the development of the embryo is completed on the ground. To begin with, the embryo only
increases in size without differentiation into its organs. Then the coleorhiza develops at the micropylar end of
the embryo. The latter shows internal differentiation into two polar meristems, epidermis, cortex, procambium
and pith; the hypocotyl is rather short (fig. 24F). The number of cotyledons varies from one to three, closely
adpressed to one another and appear as a single structure. The coleorhiza is partly derived from the suspensor
and becomes quite "hard" in a mature embryo. It is morphologically equivalent to the root cap. At maturity
several outer layers of the root cap become especially thick-walled to form a distinctive caplike structure. There
are abundant mucilage cells in the tissues of the embryo. The integument hardens to form the seed coat. Its outer
fleshy layer adheres firmly to the stony middle layer forming a hard type of seed. The inner fleshy layer had
already shriveled. The nucellus is completely crushed. Within the seed coat lies the female gametophyte
(endosperm). It is fleshy and function as food storage region of seed lying embedded in the middle of female
gametophyte is the straight embryo (Fig. 22F).
The attractive, red or orange fleshy seeds contain abundant food reserve. Quite likely, they
are dispersed by birds and rodents. The cycad seeds do not have a resting period and the
viability is short. Germination of the seed is epigeal. (fig.24)
Temporal Consideration
As compared to the temperate gymnosperms, the reproductive cycle of cycads occurs at different times of the
year. The cones in the South Indian. C. circinalis are initiated probably in April, and show micro- and
megaspore mother cells during June-July (Rao 1963). The ovules have a free-nuclear gametophyte at the time of
pollination (8 months after initiation) in December. Fertilization occurs in May-June (5-6 months after
pollination), followed by a very slow development of the embryo, in which the cotyledons appear during
November-December. The seeds with an immature embryo are shed during May and June (1 year after
fertilization). The embryo matures and attains full size, and the seeds germinate during September-October (4
months after shedding). Thus, this taxon takes 2 years and 5 months to complete its life cycle, although there is
no winter rest. In C. rumphii, the ovules are pollinated in May (De Silva and Tambiah 1952), fertilization occurs
13 months later (i.e. the following June), and the seeds are shed in January but the embryo matures only by
March. Consequently, the time lapse between pollination and fertilization is much longer.
The general reproductive cycle of cycads is:
1. There is a long time gap from the time of ovule initiation to pollination (the ovules are in an advanced
stage of development at the time of pollination, as compared to conifers).
2. There is a short time lapse between pollination and fertilization. Most of this time seems to be taken up
in the maturation of archegonia and filling up of their cytoplasm.
3. The embryo takes a long time to mature, partly on the plant and partly after shedding.
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