Short essay on Embryo and seed of cycas



After fertilization the oospores enlarges. Its nucleus divides by free nuclear divisions into numerous free nuclei (64-256) which are distributed in the cytoplasm. Soon a vacuole appears at the centre. Thus, all the nuclei get pushed to the periphery.

There are more nuclei at the lower i.e. chalazal end than those at the micropylar end. The embryo is formed from a cellular part of the chalazal end. Hence, this structure is termed a pre-embryo but not an embryo. Now, wall formation begins at the base of the pre-embryo and extends up to the periphery until the entire structure becomes cellular.

The basal cells of the pre-embryo are smaller, more numerous and with dense cytoplasm. The embryo proper is formed from the basal region.

The upper cells of this embryonic region begin to elongate vigorously forming a long, spirally coiled structure called suspensor. The embryo with two prominent cotyledons develops from the tip cells through a series of divisions.

The suspensor pushes the embryo out of the oospores membrane and dip into the endosperm from which the embryo derives its nutrition. The embryo within the ovule enlarges. The nucleolus is completely used up. The thick integument becomes differentiated into a three layered seed coat (testa).

The ovule, as a whole changes into the seed. The seeds remain naked which is a characteristic feature of the group. Mature seed of Cycas is fleshy; red or orange brown in color. The seed is enclosed by a thick seed coat formed from the integument. The seeds are dispersed by birds. Seeds germinate hypogeal under favorable conditions.