1 98 



MORPHOLOGY 



The primary neck cell divides vertically, and these two neck cells, 

 lying side by side, are constant among the Cycadales. The central 

 cell then begins a remarkable enlargement, and becomes invested by 

 a special jacket of cells, known as the arckegonial jacket, which func- 

 tions as a nutritive layer. Finally the 

 nucleus of the central cell divides into the 

 ventral nucleus, which soon disorganizes, 

 and the egg nucleus, about which the cyto- 



Fig. 446. — Diagrammatic 

 section of ovule of Dioon, show- 

 ing the thick integument free 

 from nucellus at beak (in which 

 the pollen chamber is evident, 

 but not open); the three layers 

 of the testa, outer fleshy (trav- 

 ersed by vascular strands), stony 

 (thick and shaded), and inner 

 fleshy (distinct in region of beak, 

 but merged with nucellus be- 

 low, and traversed by vascular 

 strands); the embryo sac con- 

 taining the female gametophyte 

 (endosperm), in which two 

 archegonia are present. — After 

 Chamberlain. 



Fig. 447. — Micropylar end of a mature 

 archegonium of Dioon, showing the two neck 

 cells, (he ventral nucleus (in the apex), and the 

 egg nucleus (below). — After Chamberlain. 



plasm is organized to form the egg (fig. 447). The notable feature of 

 this archegonium, in contrast with the archegonia of bryophytes and 

 pteridophytes, is the complete elimination of neck canal cells. The 

 cycad egg and its nucleus are the largest known among plants. As the 



