118 THE FLOWER 



(Fig. 164). 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 comes 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, 

 which has followed the tube in its descent, now passes 

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

 eral nuclei to be found there. From the united body so 

 formed the new plant takes its start. New cells begin 

 to appear in the embryo 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 this process which can be 

 followed only by aid of the microscope and numerous dis- 

 sections may not be entirely clear to the beginner, the 

 brief account here given should serve to fix in mind the fact 

 that the pollen and the ovule play very definite and neces- 

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

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

 what incomplete, will give the 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 plants bear in addition to the 

 ordinary flowers certain specialized flowers which are fer- 

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

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

 another set, of a different appearance, are produced. The 

 calyx remains permanently closed, while the corolla is un- 

 developed. Only two stamens reach maturity, and their 



