38 



FLOWERS AND THEIR WORK 



a sweet-tasting substance manufactured by certain parts of the 

 flower known as the nectar glands. Sprengel further discov- 

 ered the fact that pollen could be and was carried by the 

 insect visitors from tli§ anthers of the flower to its stigma. 

 It was not until the middle of the nineteenth century, however, 

 that an Englishman, Charles Darwin, worked out the true relation 

 of insects to flowers by his investigations upon the cross-pollination 



of flowers. By pollination we 

 mean the transfer of yollen from 

 an anther to the stigma of a 

 flower. Self-pollination is the 

 transfer of pollen from the an- 

 ther to the stigma of the same 

 flower; cross-pollination is the 

 transfer of pollen from the an- 

 thers of one flower to the stigma 

 of another flower of the same 

 kind. Many species of flowers 

 are self-pollinated and do not 

 do so well in seed production 

 if cross-pollinated, but Charles 

 Darwin found that some flowers 

 which were self-pollinated did 

 not produce so many seeds, 

 and that the plants which grew 

 from their seeds were smaller 

 and weaker than plants from 

 seeds produced by cross-pol- 

 linated flowers of the same 

 kind. He also found that plants grown from cross-pollinated 

 seeds tended to vary more than those grown from self-pollinated 

 seed. This has an important bearing, as we shall see later, in the 

 production of new varieties of plants. Microscopic examination 

 of the stigma at the time of pollination also shows that the pollen 

 from another flower germinates before the pollen which has fallen 

 from the anthers of the same flower. This latter fact alone in most 

 cases renders it unlikely for a flower to produce seeds by its own 

 pollen. Darwin worked for many years on the pollination of many 



A wild orchid, a flower of the type from 

 which Charles Darwin worked out his 

 theory of cross-pollination by insects. 



