SPERMATOPHYTA 



327 



The proembryo soon becomes differentiated into three regions: an upper 

 haiistorial portion in contact with the nutritive material above, a middle 

 zone of elongating cells forming the suspensor, and a terminal group of 

 cells constituting the embryo itself (Fig. 279C). The suspensor becomes 

 enormously elongated and highly coiled, pushing the embryo deep within 



B 



Fig. 275. Development of the archegonium of Dioon edule, X85. A, archegonium 

 initial; B, formation of primary neck cell and central cell; C, later stage with two neck 

 cells; D, upper portion of archegonium with two neck cells and central cell nucleus. {After 

 Chamberlain.) 



the tissue of the female gametophyte. When mature, the embryo has a 

 short axis, the hypocotyl, terminating at the end next to the suspensor in a 

 root tip, or radicle. This is enclosed in a hard covering, the coleorhiza. 

 At the opposite end of the hypocotyl is a minute stem tip, the plumule, 

 lying between a pair of seed leaves, or cotyledons. The presence of two 

 cotyledons is a constant feature of the Cycadales. As the embryo devel- 

 ops, food is stored in the vegetative tissue of the female gametophyte, 

 forming ''endosperm." This is a feature of all gymnosperms. The 

 stored food is later absorbed by the embryo when the seed germinates. 

 The ripe seed is usually white, cream-colored, orange, or red. 



