146 NATURE TEACHING 



insects were absolutely necessary to plants in which the 

 stamens and pistils were not in the same flower, that 

 they were unnecessary to those plants (by far the 

 greater number) in which both these essential organs 

 are in the same flower. This, however, is not so. The 

 stamens and pistils of such " complete " flowers commonly 

 ripen at different times, so that when the stigmas are 

 ready to receive pollen, the stamens of that particular 

 flower have already shed their pollen. In other cases 

 various arrangements are found whereby the pollen of a 

 flower is prevented from reaching the stigma of the same 

 flower. The result is that cross-fertilisation , the fertilisa- 

 tion of a flower by the pollen of another flower, is the 

 general rule, and self-fertilisation that is, by the pollen 

 of the same flower is comparatively rare even in plants 

 which have both stamens and pistils in one flower. 



Many flowers can be pollinated by almost any insect. 

 Others have very complicated arrangements, and are 

 specially adapted to particular insects. This is well 

 illustrated by the flower of the vanilla plant (a climbing 

 orchid, grown in many parts of the tropics), which is so 

 elaborately made that it must be visited by certain 

 insects before it can be pollinated naturally; and, as these 

 particular insects are not found in most of the countries 

 where vanilla is now grown, the cultivator of vanilla, in 

 order to be certain of obtaining pods, has to place pollen 

 upon the stigma of every flower by hand. 



Wind- Pollinated Flowers. 



As a general rule the flowers visited by insects 

 are brightly coloured, sweet-scented, and secrete honey. 



