FERTILIZATION 175 



do not invite insects to do the work of pollination. 

 There are other pollinating agencies, as we shall see 

 later. 



On germination, a pollen grain protrudes a pollen 

 tube, which forces its way between the cells of the 

 stigma, penetrates into the tissue of the style, and ulti- 

 mately reaches the ovary and one of the ovules contained 

 therein. The pollen tube enters the ovule through an 

 opening known as the " micropyle," and serves as a 

 passage for the fertilizing male element, which, in the 

 end, fuses with the egg-cell, or ovum, in the embryo-sac, 

 such fusion being the real act of fertilization. After 

 fertilization, the egg-cell develops into an embryo, and 

 the ovule containing the latter becomes a fertile seed. 

 The pistil, which encloses the ovules, becomes the fruit. 

 We have said that the pollen grain corresponds to the 

 microspore of the heterosporous Cryptogams; it is 

 necessary to add that on germination it produces the 

 equivalent of an antheridium, in which the male element, 

 corresponding to a spermatozoid, is formed. The ger- 

 mination of pollen grains may be induced by placing 

 them in a watch-glass containing a weak solution of 

 cane-sugar; the glass should be covered by another and 

 kept in a warm place. This is a good method for the 

 student to adopt if he wishes to watch the development 

 of pollen tubes and their contents. Of course, the 

 process cannot be followed without the use of the 

 microscope. 



We now turn to the longitudinal section through an 

 ovule and part of the ovary wall of a Wallflower, in 

 Fig. 59, where f.t. is a pollen tube that has entered the 

 ovule through the passage of the micropyle, m; the 



