196 



MORPHOLOGY 



Ovules. — The structure of the ovule (fig. 446) is of the same general 

 type as those of Cycadofilicales and Bennettitales. The thick integu- 

 ment is free from the nucellus in the region of the nucellar beak, and 

 develops a testa of three layers : a stony layer between an outer and an 

 inner fleshy layer. Among the Cycadofilicales, it will be remembered, 

 the vascular supply to the ovule is divided into two sets of vascular 

 strands, the outer set traversing the integument, the inner set the peri- 

 pheral region of the nucellus; but in that case the integument and 



nucellus are almost com- 

 pletely free. Among the 

 Cycadales, where the integu- 

 ment and nucellus are free 

 only above, the outer set of 

 vascular strands traverses 

 the outer fleshy layer of the 

 testa and the inner set trav- 

 erses the inner fleshy layer 

 (fig. 446). The nucellus de- 

 velops a sharp beak, within 

 which a conspicuous pollen 

 chamber is formed. The 

 first evidence of sporogenous 

 tissue is the appearance of a 

 spore mother cell deep within 

 the nucellus, which soon 

 differs conspicuously from 

 the neighboring cells in size 

 and contents. This mother 

 cell, by the reduction divi- 

 sions, forms a linear tetrad, 

 the innermost megaspore functioning, and in its growth encroaching 

 upon the other megaspores and the neighboring cells. 



Female gametophyte. — The female gametophyte develops in a 

 general way as in Selaginella and Isoetes, and as in nearly all gymno- 

 sperms. At least five stages in the development should be borne in 

 mind : (i) free nuclear division, by which a varying number of free 

 nuclei are distributed through the cytoplasm of the megaspore; (2) pa- 

 rietal placing of these nuclei by the development of a central vacuole; 

 (3) continued free nuclear division; (4) formation of parietal tissue by 



Fig. 442 



- Ovulate strohilu-i of Zamia. 



