ANTHOPHTTA. 



253 



cular ridge arises upon it, which often is soon followed by 

 another (Fig. 1 50, A and E). These ridges grow out and 

 upwards so rapidly that they overtake and enclose the 

 OTule-body, leaving but a small opening or pore. The body 

 of the ovule, called the nucellus, is relatively large in the 

 lower Angiosperms, while it is small in the higher orders. 



478. In the nucellus an axial cell develops into the ar- 

 chespore, which soon undergoes transverse division into four 



em 



Fig. 151.— Diagrammatic longitudinal sections of ovules. 7c, the nucellus 

 or body of the ovule with its embryo-sac, em ; of, the outer, a,nd ii. tlie 

 inner, coat ; m, the opening in ovule-coat (micropyle) ; c, the base of the 

 ovule ; /, the ovule-stallc ; A, a straight ovule ; B, an inverted ovule ; 

 the long stalk, /, has fused with the outer coat of one side of the ovule. 

 C, an inverted ovule with but one coat, and a slender nucellus. 



cells (rudimentary macrospores) ; one of these (usually the 

 lowermost) grows at the expense of the remainder, crowd- 

 ing and eventually destroying them. There is thus but 

 one mature macrospore in each nucellus (macrosporan-. 

 gium). In the further development (germination) of the 

 macrospore its nucleus divides, and the two daughter-nuclei 

 move to opposite ends of the macrospore-cavity ; there they 

 divide again and again, producing two terminal tetrads; 

 now one nucleus from each tetrad moves to the centre of 

 the macrospore-cavity, where they fuse into one, thus con- 

 stituting the nucleus of the embryo-sac. One of the 

 nuclei at the apex becomes the germ-cell (oosphere or egg- 



