Pn^NOGAMIA. 



159 



there has arisen 



(Fig. 272), immediately within the second whorl, makes 

 its appearance. These blunt protu- 

 berances (Fig. 272, an) develop later 

 into stamens, with anthers, a portion 

 of whose interior tissue undergoes 

 repeated cell-division, and from it are 

 formed isolated double-walled cells — 

 the pollen grains. In the meantime, 

 •within the innermost whorl, a circular elevation, or wall 

 (composed of one or several parts, or carpellary leaves), 

 which, arising and arching over at the top, forms a cavity 

 (Fig. 273), destined to contain the ovules later developed 

 from a point, or line (called the placenta, and correspond- 

 ing to the edge of the carpellary leaf), on the inner side of 

 this cavity, or ovary. This struct- 

 ure, terminating the axis, is the 

 pistil (Fig. 273, p). The ovule, 

 at its first appearance, is a blunt 

 protuberance (Fig. 274); after grow- 

 ing to considerable size, an eleva- 

 tion, or ring, near its base appears, 

 and gradually grows as a covering, 

 or integument (Fig. 275), over the 

 nucleus or body of the ovule. In 

 many eases a second integument 

 arises, and in a similar manner 

 encloses the first. At the apex an 

 orifice, the micropyle (Fig. 275, 

 to), invariably remains, which, 



Figs. 274, 275, Successive stages in the early development of the ovule. Fig. 276. 

 Longitudinal section through an ovary and ovule ; ^, pollen ; ^m, pollen-tube ; ov.Wy 

 ovary wall ; pol-tu^ pollen-tube ; mic, micropyle ; ou. in, outer integument ; in. in, 

 inner integument ; em. s, embryo-sac ; n-uc, nucleus, or body of ovule. 



pol 'u. 



