MORE ABOUT POLLINATION 



307 



from other flowers are both upon the same stigma, that 

 from other flowers will produce pollen tubes of more vigor- 

 ous growth than those produced by the pollen from the 

 same flower. The more vigorous pollen 

 tubes will be the ones to reach the ovules. 

 Thus cross-polHnation is favored even 

 though close-polKnation is not absolutely 

 prevented. 



Fig. 128. — End of 

 the pistil of Hibis- 

 cus, much enlarged. 

 Note the hairy sur- 

 face of the stigma to 

 which pollen grains 

 are adhering. 



D. Wind-pollination. — • This is the 

 simplest form of polKnation. It is 

 also the most ancierlt. Insect-pollinated 

 plants came from ancestors that were 

 wind-pollinated. Those structures in 

 lower plants which correspond to the 

 pollen of seed plants are scattered by the 

 wind. 



The pollen of wind-polKnated plants 

 differs from that of insect-pollinated plants in two general 

 features. It is more buoyant and it is more abundant. 

 It is generally light and smooth and dry, whereas insect- 

 carried pollen is generally heavy and rough and moist. In 

 some cases the buoyancy is increased by wing-like out- 

 growths. (See Figure i2p.) 



The flowers of wind-pollinated plants have other features 

 which are distinctly favorable to this process. It is common 

 to find both stigmas and anthers prominently exposed. (See 

 Figures 106, iig, I2y, and 130.) It is also common, among 

 those which are diclinous, to find the staminate flowers 

 arranged in slender, pendent catkins from which, when ripe, 

 even the gentlest breeze may start a cloud of pollen. (See 

 Figures 104 and 105.) Such catkins are characteristic 



