DISTRIBUTION OF SPORES AND SEEDS. 275 



388. Adaptations for cross-pollination. — (a) The sepa- 

 ration of the stamens and pistils, staminate flowers and pistil- 

 late ilowers being produced upon different parts of the same 

 plant or even upon different plants of the same species ; (i5) the 

 early ripening of the stamens so that they discharge their 

 spores before the stigma of the same flower is exposed or 

 receptive, or vice versa; (c) arrangements preventing the 

 pollen from reaching the stigma of the same flower, which 

 vary according to the different modes by which the transfer 

 of the pollen is made ; ((/) the failure to form good seed 

 when close-pollination happens. 



389. Adaptations for close-pollination. — But close-pollin- 

 ation, even though it results in weaker offspring, is better 

 than entire failure to produce progeny. Therefore, some 

 plants permit close-pollination in the event . of failure to 

 secure cross-pollination, while a few have adaptations which 

 insure it. Our common violets produce in the late spring 

 and early summer inconspicuous blossoms which do not open, 

 containing stamens with few pollen grains. These flowers, 

 however, produce seed abundantly, and always by close- 

 pollination. Various other species have similar arrange- 

 ments. 



390. Adaptations to insects. — The adaptations to secure 

 cross-pollination through the visits of insects are so numerous 

 and so varied, and the advantage in the number and weight 

 of seeds produced is so marked, that for most seed-plants 

 cross-pollination must be considered the far more desirable 

 process. Flowers are adapted to insect visitors in the fol- 

 lowing ways : 



391. (a) Food, etc. — ^They provide for their visitors edi- 

 ble substances, such as nectar and pollen, * material for nest 

 building, shelters, or breeding places. 



* The pollen is often produced* in great excess of the plant's own 

 needs. 



