THE GYMNOSPERMAE : CYCADALES, ETC. 779 



suspensor, the lower becomes the embryo. The elongation of the suspensor 

 pushes the embryo downwards, either along the prothallial tube or by direct 

 penetration into the tissues of the prothallus, where it matures. Several 

 separate acts of fertilization may occur, and hence numerous embryos are 

 formed, but normally only one survives to maturity. 



The embryonal cell divides into four, and from these the following cells 

 are produced in downward succession : — 



1. Twenty-four basal cells, which surround the lower part of the sus- 



pensor as secondary suspensors, or inner cortical cells, to which are 

 sometimes added a second or even a third set of outer cortical cells. 



2. An embryonal plate of eight cells, from which the mature embryo is 



formed. 



3. A " ring " of sixteen cells of uncertain function. 



4. An apical " cap " of eight cells, which is finally discarded. 



It is probable that 3 and 4 are only protections for the young embryo as 

 it bores its way down into the prothallus, the physiological analogues of a 

 root cap. 



The later stages of embryo development have not been seen, but the 

 mature embryo is straight, has two cotyledons, as we have seen above, and 

 is surrounded by endosperm. The winged outer coat of the ovule develops 

 into a papery structure nearly an inch across and seems to be a means of 

 wind dispersal for the seeds (Fig. 781). 



