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The following points highlight the three main steps involved in embryogency in cycas. The steps are: 1. Development of Zygote 2. Structure of Seed 3. Germination of Seed.
Embryogency in Cycas: Step # 1. Development of Zygote:
The diploid osspore or zygote is the first cell of the sporophyte. It contains dense cytoplasm and a large nucleus. The nucleus moves towards the base and starts dividing by repeated free-nuclear divisions to form hundreds of nuclei (Fig. 8.51). They lie scattered throughout the cytoplasm.
A vacuole appears in the centre after some time. Subsequently the wall formation starts from the base and progresses towards the upper side to form a small mass of the cells. This embryonal cellular mass represents the pro-embryo.
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It is meristematic in nature. Some of the nuclei in its upper region remain free and are seen scattered in the cytoplasm.
Following three regions become differentiated soon in the pro-embryo (Fig. 8.52).
(a) Haustorial Region:
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It is the upper part of pro-embryo. It remains in contact with the upper free-nuclear region. It absorbs food material for the developing embryo. The cells of this region possess feebly-developed walls, which soon disappear.
(b) Suspensor Zone:
It is the middle zone. It lies next to the haustorial region. Its cells elongate enormously and form a long and coiled suspensor. It pushes the embryo into the food-containing cells of the endosperm.
(c) Embryonal Zone:
The cells of this zone are present at the tip of th suspensor. It is the basal part of the pro-embryo. Its cells develop into embryo proper by further divisions. By the cessation of growth in the centre and its continuance at the margins, the two cotyledons are differentiated.
The plumule gets differentiated in the central depressed portion. The radicle gets differentiated below the hypocotyle. A pad of tissue develops at the radicle end of the embryo. It represents coleorrhiza. It protects the delicate tip of the radicle against injury.
Embryogency in Cycas: Step # 2. Structure of Seed:
The mature fleshy Cycas seed (Fig. 8.53) consists of an orange red- coloured outer thick testa or seed coat which is formed from the 3-layered integument of the ovule. The inner fleshy layer and the nucellus are used up during the embryo development, and, therefore, these are represented only by thin layers.
A well- developed endosperm, which stores a large amount of food material, is present. The embryo remains embedded within the endosperm. It consists of plumule, radicle and two large cotyledons The embryo remains suspended in the endosperm with the help of a long and spirally coiled suspensor.
Thus, in a mature Cycas seed, the parent sporophytic generation is represented by the integument, the gametophytic generation is represented by the endosperm and the new sporophytic generation is represented by the embryo.
Embryogency in Cycas: Step # 3. Germination of Seed:
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There is no resting period for Cycas seed and its viability is very short. Under favourable conditions the seed starts immediate germination. The germination in Cycas is epigeal (Fig. 8.54). The seed coat breaks open and the radicle comes out through the micropyle to form the primary root. The roots takes a downward curve and grows rapidly into the soil. The cotyledons do not come out of the seed.
Instead they absorb the food material from the endosperm for the terminating seedlings. In the later stages, when the endosperm is exhausted, the cotyledons dry up and get slightly exposed. The plumule comes out and starts forming a few scaly leaves and then a foliage leaf. The circinate vernation is exhibited by the young foliage leaves. The first crown of leaves is formed after several years.
