POLLINATION. 151 



Impatiens and Wood Sorrel, besides the ordinary large 

 flowers, there are special small ones (known as cleistoga- 

 mous flowers) whose floral envelopes do not open, thus 

 I ling self-fertilization. But it is well established 

 that in a vast number of cases the ovules in any given 

 flower have to depend for fertilization upon the pollen of 

 some other flower. Nature seems to have provided 

 against self-fertilization by various contrivances. Some- 

 times the relative positions of the anthers and the stigma 

 in the same flower are such as to render it impossible. 

 Sometimes the pollen comes to maturity and is shed from 

 the anthers before the stigma is in a suitable condition to 

 receive it ; whilst, on the other hand, the stigma is often 

 developed first and has withered before the opening of 

 the anthers. (Flowers showing these peculiarities are 

 said to bo dichof/amous.) When for any reason cross- 

 fertilization has become a necessity, the conveyance of the 

 pollen from one flower to another is ensured in various 

 ways. When the flowers are inconspicuous, as in Grasses, 

 the wind is the great agent, and flowers so fertilized are 

 said to be an&rnophUous. In other cases the flowers, 

 either by their brightness or their odour, attract insects in 

 quest of honey, and these then become the carriers of the 

 pollen. Flo \vers of this sort are said to be entomophilous, 

 and are usually so constructed as to the situation of their 

 honey receptacles, and the relative position of anthers and 

 stigma, as to ensure the transfer of the pollen from the 

 anther of one flower to its destination upon the stigma of 

 another. The case of the Orchids has already been 

 referred to in section 92. 



248. After fertilization, the embryo, or young plantlet, 

 #s exhibited in the seed, begins its growth in that end of 



