148 TEXTBOOK OF BOTANY 



from the anthers of one flower upon the stigma of another 

 flower. 



When pollen is thus carried by insects from one flower to 

 another, it does not necessarily follow that the second flower 

 will be on another plant. But this will certainly be true in a 

 large proportion of cases, especially since the different flowers 

 of a plant open in general at different times ; consequently 

 the stigma of an older flower is likely to have been pollinated 

 before a younger flower of the same plant opens. But it 

 appears that if insects do not visit a bean flower, seeds are 

 nevertheless sometimes formed. So it is evident that in 

 such cases the pollen in some way reaches the stigma of the 

 same flower that is, self-pollination occurs. Just how this 

 comes about is not yet known. The place in which the pol- 

 len grains of the bean finally land is very different from that 

 which is reached by the pollen grains of the pine. This must 

 be so, because the ovules of the bean are shut up inside the 

 ovary, so that the pollen cannot, as in the pine, be carried 

 to the ovules themselves. 



Since a pollen grain of the bean must germinate upon the 

 stigma far from any of the female plants (which are inside 

 the ovules), the pollen tube has a much greater distance 

 through which to grow than had that of the pine. The size 

 and colors of the sepals and petals of many flowers seem to 

 be of use in attracting the attention of insects to the places 

 where nectar is to be found. 



In general it is true that those angiosperms which, like the 

 bean, have more or less showy flowers are insect-polli- 

 nated ; and that others, such as the grasses and most of the 

 forest trees, which have small, inconspicuous flowers, are 

 wind-pollinated. There are exceptions to the latter rule, 

 because some insects seem to be attracted by the odors of 

 the flowers as well as by their size and color, and so they 

 may visit small flowers, such as the mignonette, that are 

 fragrant. 



