244 



General Botany 



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 having it blown 

 about and reaching a stigma by mere chance. 

 If the amounts of pollen produced by the 

 pine, corn, ragweed, and other wind-polli- 

 nated plants are compared with the amounts 

 produced by plants that are pollinated by 

 insects, it will be seen that insect-pollinated 

 plants generally produce less pollen and are 

 no less effectively pollinated. 



Why insects visit flowers. Insects 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, 

 inside the flower. The insects visit the flow- 

 ers and secure food for themselves, but as 

 they make their visits they brush against the 

 anthers and become covered with pollen. 

 Later they come in contact with the stigmas 

 of other flowers and leave pollen adhering to 

 the stigmatic surface. In this way they per- 

 form a service for the plants. The perfumes 

 of flowers assist the insects in finding them, 

 and conspicuous white or brightly colored 

 parts of flov/ers may aid in the same way. The 

 massing of many small flowers in clusters and 

 heads certainly makes them more conspicuous. 



Fig. 151. Diagram of 

 a pistil with germinat- 

 ing pollen grains and 

 pollen tubes of various 

 lengths. The embryo 

 sac is in the seven- 

 celled stage, with a 

 central fusion nucleus 

 and an egg (below)- 

 Fertilization occurs 

 when one of these pol- 

 len tubes reaches the 

 egg. {After Buchholz.) 



