268 



TEXTBOOK OF BOTANY 



has full sweep. Under such circumstances each stigma has 

 a good chance of receiving pollen. Many shrubs and trees, 

 .such as the hazel, alder, elm, oak, and poplar, are wind- 

 pollinated. Their flowers are high up above the ground, 

 where they are reached by winds from all directions. Wind- 

 pollinated plants necessarily produce much more pollen than 

 is actually needed for pollination. Their pollen grains are 

 usually small, smooth, and dry, so that they may be blown 

 about easily and carried long distances. Their stigmas are 

 often large, and branched or covered with projections in a 



Fig. 157. — Types of pollen grains : 0, pollen of one of the pinks {Dian- 

 thus) ; b, of the pumpkin ; t, of ilorina pcrsica ; d, of the enchanter's 

 nightshade (Circcea alpina) ; e, of a passion-flower (Passiflora kcrmcnsina) ; 

 /, of a pine {Piniis piimilio) ; g, of Cobcsa scandens. After Kerner. 



brush-like or feather-like fashion ; this increases the chance 

 that some of the pollen grains will be caught and held. As 

 a rule, the flowers of wind-pollinated plants are small and 

 have no bright-colored parts, no odor, and no nectar. 



279. Pollination by Insects. — There are many more 

 species of insect-pollinated than of wind-pollinated plants. 

 As a rule, insect-pollinated plants produce less pollen than 

 do wind-pollinated plants ; on the other hand, their iiollen 

 grains are sometimes (though by no means always) com- 

 paratively large. Pollen grains which are to be carried by 

 insects are often covered with a sticky substance, and as a 

 rule their outer surfaces are rough or provided with small 



