PHJENOGAMIA. 



159 



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

 its appearance. These blunt protu- 

 berances (Fig. 272, ayi) 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, there has arisen 

 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 cases a second integument 

 arises, and in a similar manner 

 encloses the first. At the apex an 

 orifice, the micropyle (Fig. 275, 

 m), invariably remains, which. 



QV.v^. 



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

 Longitudinal section through an ovary and ovule ; ^, pollen ; tu, pollen-tube ; ov-iv^ 

 ovary wall ; pol-iu, pollen-tube ; Tnic, micropyle ; ou. in, outer integument ; in. in, 

 inner integument ; em, o', embryo-sac ; nuc, nucleus, or body of ovule. 



