CHAPTER XII. -POLLINATION AND FERTILIZATION. 99 



ground, so that insects cannot penetrate them, and in which the 

 pollen falls directly upon the stigma and fertilizes it. Such 

 flowers produce seed abundantly, and yet there is the closest of 

 in-and-in breeding. They appear to be a contrivance by means 

 of which the plants are able to produce a greater multitude of 

 seeds with a less expenditure of force than by the production of 

 an equal number of the ordinary flowers. They seem to multiply 

 in this way as other plants do by means of bulblets or tubers, 

 while the occasional crossing which occurs by means of the 

 showy flowers serves to keep the stock vigorous. 



It does not come within the scope of this work to treat this 

 subject more extensively, but those who are interested in it, and 

 wish to pursue it further, should read some or all of the following 

 works : Darwin's a Cross and Self Fertilization in the Vegetable 

 Kingdom;" Sir John Lubbock's "British Wild Flowers in 

 Relation to Insects;" Mueller's " The Fertilization of Flowers," 

 and the chapter in Prof. Gray's " Structural Botany " which 

 relates to this subject. 



II. — Fertilization. By Fertilization is meant the process 

 which takes place subsequent to the deposit of the pollen on the 

 stigma, resulting in the union of the contents of the two reproductive 

 cells, the pollen-cell and the germ-cell. 



The pollen-grain, conveyed by the wind, by insects, or by 

 some other means, is securely held and stimulated to germina- 

 tion by the adhesive secretion of the stigma on which it is de- 

 posited, and it soon forms a pollen-tube which readily finds its 

 way between the easily separable cells of the loose interior tis- 

 sues of the style, or through the canal in the latter, if there be 

 one, to the interior of the ovary, and enters the micropyle of 

 the ovule, as shown in Figs. 295 and 296. As many pollen-tubes 

 will be required as there are ovules in the ovary, every separate 

 ovule requiring one for its fertilization. 



The time that it takes for tl\e pollen-tube to reach the ovule 

 and effect its fertilization varies greatly in different plants. In 

 some it occupies only a few hours, while in others it may require 

 weeks, or even months, as in the Orchids. 



In the meanwhile, the ovule is preparing to receive the fer- 

 tilizing influence of the pollen. This preparation consists in the 

 formation of a large cell in the nucellus of the ovule, called the 



