196 
MORPHOLOGY 
Ovules. The structure of the ovule (fig. 446) ? s 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 strobilus of Zamia. 
