222 



PRACTICAL COURSE IN BOTANY 



251. Fertilization. When a pollen tube has penetrated 

 to the ovary, it next enters one of the ovules, usually through 



the micropyle (Fig. 330, m). 

 There it penetrates the wall of 

 a baglike inclosure called the 

 embryo sac (Fig. 330, u, t, z), 

 where one of the generative 

 cells emitted by the pollen tube 

 fuses with a large cell contained 

 in the embryo sac, known as 

 the germ cell, or egg cell (Fig. 

 330, z). The fusion of these 

 two bodies is what constitutes 

 fertilization. The cell formed 

 by their union finally develops 

 into the embryo, and the other 

 contents of the sac into the 

 endosperm, and the ripened 

 ovules become seeds. 



252. Stability of the process 

 of fertilization. - - The phe- 

 nomena that characterize the 

 functions of fertilization and 

 reproduction are the most uni- 

 form and stable of all the life 

 processes, varying little not 

 only in different species and 

 orders, but throughout the whole vegetable kingdom. And 

 since these functions furnish a more reliable standard for 

 judging of the real affinities of the different groups than do 

 mere external resemblances, which are more liable to varia- 

 tion and may often be accidental, they have been chosen 

 by botanists as the ultimate basis for the classification of 

 plants. 



253. Embryology. - - The study of the developing plantlet, 

 known as embryology, is a comparatively recent branch of 



m 



FIG. 330. Diagram of a simple 

 flower, showing course of the pollen 

 tube : a, trans verse section of an 

 anther before its dehiscence; 6, an 

 anther dehiscing longitudinally, with 

 pollen ; c, filament ; d, base of floral 

 leaves ; e, nectaries ; /, wall of carpels ; 

 g, style; h, stigma; i, germinating 

 pollen grains ; m, a pollen tube which 

 has reached and entered the micropyle 

 of the ovule ; n, stalk of ovule ; o, base 

 of the inverted ovule ; p, outer integu- 

 ment or testa; q, inner integument; 

 s, rudimentary ovule ; t, cavity of the 

 embryo sac ; u, its basal portion ; z, 

 oosphere. 



