36 INTERRELATIONS OF PLANTS AND ANIMALS 



anthers of a flower to its stigma. It was not until the middle of 

 the nineteenth century, however, that an Englishman, Charles 

 Darwin, applied Sprengel's discoveries on the relation of insects 

 to flowers by his investigations concerning cross-pollination. The 

 growth of the pollen on the stigma of the flower is a necessary step in 

 the production of seeds, and thus of new plants. Many kinds of 

 flowers are self -pollinated and do not do so well in seed production 

 if cross-pollinated, but Darwin found that most flowers which 

 are self-pollinated do not produce so many seeds, and that the 

 plants which grow from their seeds are smaller and weaker than 



plants from seeds produced 

 by cross-pollinated flowers 

 of the same kind. He also 

 found that plants grown 

 from cross-pollinated seeds 

 tend to vary more than 

 those grown from self-pol- 

 linated seed. This has an 

 important bearing, as we 

 shall see later, in the pro- 

 duction of new varieties 

 of plants. Microscopic ex- 

 amination of the stigma 

 at the time of pollination 

 also shows that the pollen 

 from another flower usu- 

 ally germinates more rap- 

 idly than the pollen which 

 falls from the anthers of 

 the same flower. This latter fact alone in most cases renders it un- 

 likeh^ for a flower to produce seeds by its own pollen. Darwin 

 worked for years on the pollination of many insect-visited flowers, 

 and discovered in almost every case that showy, sweet-scented, 

 or otherwise attractive flowers are adapted to be cross-pollinated 

 by insects. He also found that, for flowers that are inconspicuous 

 in appearance, often a compensation appears in an odor which 

 renders them attractive to certain insects. The so-called carrion 

 flowers, pollinated by flies, are examples, the odor in this case 



Two different -wild orchids. Flowers of this 

 t>T3e were used hx Charles Darwin to work out 

 iis theory of cross-pollination by insects. 



