56 HOW ARE ANIMALS AND PLANTS DEPENDENT? 



One of the most interesting relationships for study are those 

 that exist between insects and flowers. Flowering plants, as we 

 know, produce seeds and fruits, and from these come new genera- 

 tions of plants. Not all of us realize, however, the very close 

 dependence of these plants on the insects that visit them. If it 

 were not for these insect visits, many plants would not produce 

 seeds. 



In the latter part of the eighteenth century a German named 

 Christian Konrad Sprengel worked out the facts that the structure 

 of certain flowers seemed to be adapted to the visits of insects. 

 Certain facilities were offered to an insect in the way of easy foot- 

 hold, sweet odor, and food in the shape of pollen and nectar, the 

 latter a sweet-tasting substance manufactured by certain parts of 

 the flower known as the nectar glands. Sprengel further dis- 

 covered the fact that pollen could be and was carried by insect 

 visitors from the anthers or pollen-bearing organ of the flower to 

 the top of the part that produced the seeds. It was not until the 

 middle of the nineteenth century, however, that an Englishman, 

 Charles Darwin, worked out further the relation of insects to 

 flowers by his investigations on the cross-pollination of flowers. 

 By this we mean the transfer of pollen from the pollen-producing 

 organ of one flower to the seed-producing organ of another flower 

 of the same kind. 



Many species of flowers are self-pollinated, but Charles Darwin 

 found that some flowers which were self-pollinated did not produce 

 as many seeds, and that the plants which grew from their seeds 

 were 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 tended to vary more 

 than those grown from self-pollinated seeds. This has an important 

 bearing, as we shall see later, in the production of new varieties 

 of plants. Darwin studied for many years the pollination of 

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

 scented, or otherwise attractive flowers were most likely to be 

 cross-pollinated by insects. He also found that, in the case of 

 flowers that were inconspicuous in appearance, often a compensa- 

 tion appeared in the odor which apparently rendered them attrac- 



