324 PLANT MORPHOLOGY 



walls come in, forming a tissue. This tissue develops centripetally until 

 it reaches the center of the gametophyte. Two regions are now differen- 

 tiated — a region of smaller cells that develops archegonia, situated near 

 the micropylar end, and a deeper region of larger, nutritive cells. 



As a rule, 3 to 5 archegonia are formed, but there may be as many as 10. 

 In Microcycas, which is unique in this respect, as many as 200 archegonia 



Fig. 272. Longitudinal section of microsporangium of Zamia floridana, showing sporog- 

 enous tissue surrounded by the tapetum (both shaded) and a wall five or six layers thick, 

 X150. 



may appear. The archegonium initial is superficial and, by a periclinal 

 division, an outer primary neck cell is differentiated from a central cell 

 (Fig. 275 A, B). The former undergoes a vertical division, thus forming 

 two neck cells, a constant feature throughout the Cycadales (Fig. 275C, 

 D) . There are no neck canal cells. The central cell undergoes a marked 

 enlargement. Its nucleus divides to form a ventral canal nucleus and an 

 egg nucleus, but no wall is laid down between them. The ventral canal 

 nucleus soon disorganizes and the egg is now ready for fertilization. An 

 archegonial chamber is not present when the archegonia are young, but 

 arises later. It is a depression formed by upgrowth of the adjacent tissue 

 of the female gametophyte. 



