202 Science of Plant Life 



merely falls by gravity on the stigma. Wheat and oats are 

 examples of plants that are pollinated in this way. In other 

 plants, like the pines, elms, birches, oaks, rye, and corn, the 

 pollen is carried by the wind. It is an interesting fact that 

 the stigmas of wind-pollinated flowers are usually roughened 

 by hairs, which probably make them more effective in hold- 

 ing the pollen. 



In the case of most plants with conspicuous flowers, the 

 pollen is carried by bees, flies, butterflies, and moths. As 

 the body of one of these insects is rough or hairy, pollen grains 

 become attached to it when the insect enters a flower. Then 

 when the insect passes to another flower, some of the pollen 

 from the first flower is brushed off on the stigma of the second. 

 Thus pollination is brought about by the insects in the course 

 of their visits to successive flowers. It is an advantage to 

 the plant to have its pollen carried by insects directly from 

 flower to flower instead of being blown about and reaching 

 a stigma by mere chance. If the amounts of pollen pro- 

 duced by the pine, corn, ragweed, and other wind-pollinated 

 plants are compared with the amounts produced by plants 

 that are poflinated by insects, it wiH be seen that insect- 

 pollinated plants generally produce less pollen. 



Why insects visit flowers. I.^.sects do not visit flowers to 

 carry pollen for the plants. They eat the pollen or feed their 

 young on it, and they also secure nectar from the flowers. 

 The nectar is a watery solution containing sugar, which is 

 secreted by glands called nectaries. One or more of these 

 nectaries is usually located near the base of the corolla, in- 

 side the flower. The insects visit the flowers, therefore, to 

 secure food for themselves, but as they make their visits they 

 brush against the stigmas and leave grains of pollen adhering 



