338 



PLANT MORPHOLOGY 



layered, consisting of an outer fleshy, a middle stony, and an inner fleshy 

 layer. A prominent nucellar beak and pollen chamber are present (Fig. 

 288-4). Vascular strands are present in the outer fleshy layer, the inner 

 set being suppressed. The megaspore mother cell is deep-seated but may 

 be easily recognized, as it becomes invested with a glandular digestive 



Fig. 289. Archegonium of Ginkgo biloba. A, micropjlar end of female gametophyte, 

 showing two archegonia, X40; fi, median longitudinal section of archegonium surrounded 

 by tissue of the female gametophyte, showing central cell and two neck cells, X 100. 



tissue. A linear tetrad is formed and only the innermost megaspore is 

 functional (Fig. 2885). 



Female Gametophyte. As in the Cycadales, the development of the 

 female gametophyte is initiated by free-nuclear division, but the nuclei 

 are parietally placed from the beginning. Wall formation is centripetal. 

 A remarkable condition is seen in that the vegetative tissue of the female 

 gametophyte develops chlorophyll and becomes bright green. As it 

 develops, it encroaches upon the nucellar tissue and destroys nearly all 

 of it. As a rule, only two archegonia appear, developing as in the cycads. 



