POLL IN A riON. 



285 



Fig. 92. Staminate Squash flower. 



a circular expansion of tissue (Fig. 97), but sometimes it is 

 divided into lobes and the lobes remain closed until it is ready 



for the pollen. The fuchsia 

 stigma is composed of four 

 lobes, which are closed in Fig. 

 go. Fig. 93 shows the two-lobed 

 stigma of the trumpet-creeper 

 or tecoma before the flower is 

 ready for pollination. Fig. 94 

 shows the stigma open, in condi- 

 tion to receive the pollen. In 

 these flowers the stamens are 

 hidden in the tube of the co- 

 rolla. 



The pollen is nearly always in 

 the form of very small grains, 

 which become dry when ripe. 

 In some plants, notably in 

 orchids, the pollen is borne in 

 large masses known as pollinia. When the anther is "ripe " it 

 assumes a yellow, orange or brownish cast and the pollen is dis- 

 charged through a split in the side, a chink at the apex or other 

 aperture. The pollen may fall upon and fertilize the stigma of 

 the same flower, in which case the flower is said to be self- 

 fertilized, or oftener it is carried to another flower by insects, 

 winds or other agencies. Most plants possess some contrivance 

 which renders salf-fertilization difficult and cross-fertilization 

 easy. 



There are many degrees of cross-fertilization. The cross 

 may take place between two flowers in the same cluster or 

 between two clusters upon the same plant ; or it may take place 

 between distinct plants, either of the same or of another 

 species. Fertilization between flowers on the same plant is 

 known as individual-fertilization. The limits within which 

 crossing is possible are not known, but the closer the species are 

 related the more readily, as a rule, will they cross. One of the 

 barriers which nature erects to prevent self or close-fertilization 



