Orchid Embryogeny

Development of the zygote begins after a rest period, the duration of which increases with the evolutionary status of the species . It takes two days in many of the Orchideae and Neottieae , 30 days in Cymbidiinae and more than 30 days in Sarcanthinae .

The first division of the Zygote is always transverse differentiating a basal cell towards the micropyle and a terminal cell towards the chalazal end. This is constant for all species of orchids studied so far . After this , in most species of orchids , the basal cell divides transversely to form a suspensor initial cell and a middle cell , while the terminal cell divides transversely to form a suspensor initial cell and a middle cell, while the terminal cell divides vertically . From this proembryonic stage tow types of embryo divide and enter into the formation of the embryo, eg. Cypripedium reginae (Treub, 1869), Epipactis palustris , Spiranthes (Coulter and Chamberlain , 1912) Neottia nidus-avis etc. (Dunce , 1910)

The second type always possesses a suspensor which is formed by the suspensor initial cell, while the terminal cell and middle cell form the mass of undiffentiated cells which is the embryo proper . This type of embryo shows several types of variation with regard to the mode of formation of the suspensor. According to this suspensor characteristics, Swamy (1949) has divided them into five types which are as given below :

Type I The suspensor initial cell elongates into a tube-like or sac-like suspensor with very often a constriction at the point of attachment with the embryo. Eg. Coelogyne , Spathoglottis , Bulbophyllu, Peristeria and Dendrobium . In Dendrobium microbulbon and D.graminifolium, the suspensor initial cell does not enlarge but remains as a vacuolated cell.

Type II Here the suspensor initial cell divides forming a row of 5-10 cells which then enlarge and grow beyond the microphyle behaving as an aggressive haustorium and getting food materials from the placenta ,eg. Ophrys , Habenaria , Satyrium etc.

Type III Suspensor initial cell undergoes a series of transverse divisions, followed by vertical and oblique divisions. Each cell thereafter swells, the whole susupensor thus giving the appearance of a bunch of grapes , eg. Epidendrum, Sobralia etc.

Type IV Here the susupensor initial cell divides thrice by vertical walls forming a plate of eight cells, which then elongate towards the chalazal end enveloping the embryo on all sides , eg. Luisia , Cottonia , Vanda , Saccolabium , etc.

Type V After the transverse division the cells divide irrefularly in all planes. A few of the cells towards the micropylar end elongate in all directions to form suspensors ,while one unelongated cell towards the chalazal end divides to form the filamentous embryo, eg. Cymbidium , Eulophia , Geodorum etc.

By the time the embryo completes its development into a mass of undifferentiated cells, the integuments convert themselves into the loose airfilled seed coats, by loosing their protoplasm and becoming transparent . The seed then enters its rest period, which continues till such time as it chances upon a suitable fungus.