118 THE FLOWER 



(Fig. 161). The tube penetrates the tissue of the stigma 

 and style, and at length reaches the cavity of the ovary, 

 through which it descends until one of the ovules is 

 reached. Penetrating the ovule at a certain spot, the 

 tube conies in contact with the large cell, termed embryo 

 sac, in which the embryo is to be formed (Fig. 164). 



Before this time the original pollen nucleus has given 

 rise, by division, to several nuclei. One of these nuclei, 

 Mliich has followed the tube in its descent, now passes 

 over into the embryo sac and fuses with one of tlie sev- 

 eral nuclei to be found there. F\-oin the united body so 

 formed the new plant takes its start. New cells begin 

 to appear in the embi'yo sac and the embryo gradually 

 assumes form. At the same time the whole ovule, and 

 in fact the entire ovary, begins courses of development 

 resulting in seed and fruit respectively. 



229. While every step of lliis process — whicli can be 

 followed only l)y aid of the microscope and nunierous dis- 

 sections — may not be entirely clear to the l)eginner, tlie 

 lirief account here given shimld serve to hx in mind the fact 

 til at thi^ pollen and the ovule play very definite and neces- 

 sary parts in the life of plants; and the conception gained 

 of the metljod and results of fertilization, even if some- 

 what iiii'iimplete, will give tlie flower and its varied forms 

 an added meaning. 



ECOLOGY OF THE FLOWER 



230. Self-fertilization and cross-fertilization. — Self-fer- 

 tilization is the action of a flower's pollen on its own ovules; 

 cross-fertilization, on the ovules of some other flower of the 

 same species. 



231. A limited number of [ilants bear in addition to the 

 ordinary flowers certain specialized flowers wliich are fer- 

 tilized by tlieir own pollen alone. The Violet is one of 

 these. The vernal flowers are cross-fertilized. Later on 

 another set, of a different appearance, are ]:iroduced. Tlie 

 calyx remains permanently closed, ^\']lile tbe corolla is un- 

 developed. * >nly two stauieJis reach maturity, and their 



