96 THE LIVING CYCADS 



until there are a large number of nuclei — in Dioon as 

 many as a thousand — not separated from each other by 

 cell walls but lying in a common mass of protoplasm 

 (Fig. 39). The growth of the megaspore is so rapid 

 during this stage that the mass of protoplasm does not 

 keep pace with the increasing size, and consequently a 

 large, central vacuole appears, so that the protoplasm, 

 with its numerous nuclei, forms a thin layer pressed 

 against the wall of the megaspore, as shown in Fig. 39. 



Cell walls then appear at the periphery, the increase 

 in the mass of protoplasm keeping pace with the increase 

 in size and then even exceeding it, so that the protoplasm 

 encroaches upon the central vacuole, which becomes 

 smaller and smaller, until it finally disappears. Mean- 

 while the formation of cell walls proceeds from the 

 periphery toward the center, until the entire female 

 gametophyte consists of cells, each cell with its own 

 nucleus (Fig. 40). 



In Dioon edule the female gametophyte is about an 

 eighth of an inch in diameter when it reaches the stage 

 at which it has become cellular throughout; but the 

 cells continue to divide, enlarge, and divide again, until 

 the length is an inch or even more. In Dioon spinulosum, 

 Cycas circinalis, and some species of Macrozamia the 

 length may be twice as great. During this growth vari- 

 ous foodstuffs, but principally starch, are stored in the 

 cells, the materials for growth being brought in largely 

 by the vascular system. Immediately surrounding the 

 gametophyte is a jacket, one or two cells in thickness, 

 and in this the nutritive substances undergo more or less 

 modification before they pass into the gametophyte. 

 The jacket is very prominent during the earlier stages 



/4 





