CHAPTER XII. — POLLINATION AND FERTILIZATION. 



I03 



walls, and the further growth of the endosperm now goes on by 

 cell-division, which proceeds from without inwards. This is 

 the more usual process. Sometimes, however, the first division 

 of the nucleus results in the formation of two perfect cells, and 

 the development of the endosperm takes 

 place from the very first by cell-division, 

 and not by nuclear division. In most 

 cases, the embryo-sac enlarges very ma- 

 terially after fertilization and during 

 the formation of the endosperm. Com- 

 monly, also, the cells of the latter and of 

 the embryo completely fill it when the 

 seed is mature, but there are instances, 

 of which the Cocoa-nut affords a conspic- 

 uous example, where only the peripheral 

 portion of the interior of the sac be- 

 comes cellular, while the interior remains 



Fig. 302. — Diagram of _ . , , ... 



the lower portion of the em- fluid. The so-called "milk in the 



bryo-sac of the Pansy Violet „, . . c x , e 



after fertilization, showing Cocoa-nut, is the portion 01 the cavity Of 



the development of the endo- ,, , 1 • i_ i_ /■•ij i i_ 



sperm. «, one of the nuclei the embryo-sac which has tailed to be- 

 pusm: >° ffu r id nU interi P o r r 0t of come cellular, while the " meat " that 

 embryo-sac. surrounds it is the endosperm. 



In most cases, as the embryo-sac and its contents develop, 

 the cellular tissue outside it in the nucellus is absorbed and dis- 

 appears ; but in some instances it remains or even increases in 

 quantity, and is termed the pcrispenn. In the seeds of Canna, 

 all of the nutritive tissue exterior to the embryo and within the 

 seed-coats is perisperm. The seeds of the Peppers contain 

 much perisperm and a comparatively small quantity of endo- 

 sperm. Its use to the plant is the same as that of the endosperm, 

 namely, to supply food to the germinating embryo. The endo- 

 sperm that is formed dees not always remain until the seed is 

 ripened, but not infrequently, as in the Melon and Bean, it is 

 completely absorbed by the growing embryo, and all within the 

 seed-coats comes to consist of embryo. In a few cases also an 

 endosperm is never developed at all. Beside the changes which 

 take place in the interior of the nucellus as the result of fertili- 

 zation, very important ones also occur outside of it. The ovary 

 alwavs increases in size ; sometimes to a remarkable extent, and 



