358 PLANT LIFE. 



and loJt^'cd there. It has been tlearl)- shown not only that 

 adaptations for securing this result have been developed, but 

 also that tliere ha\'c arisen various ingenious adaptations to 

 secure cross-pollination and to jirevent close-pollination. 

 (See *l 358. ) Some of these ma\- be here enumerated. 



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

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

 late flowers being produced upon the same plant or e\'en upon 

 different plants of the same species ; (/') the earl}' ripening of 

 the stamens so that they discharge their spores before the 

 stigma of the same flower is exjiosed or recepti\'e, or vice 

 versa; (c) arrangements pre\'enting the jtoUen from reaching 

 the stigma of the same tlower, whi( h vary according to the 

 different modes by which the transfer of the jjollen is made; 

 (d) the failure of fertilization to occin- when close-jiollination 

 hap|iens. In such cases the pollen is said to be impotent. 

 This means that the male plants are either not completely 

 formed by it, or that their sperms do not stiuiulate the egg 

 to development. 



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

 ation, even tliough it results in weaker offspjring, is better 

 than entire failure to produce progenv. Therefore, some 

 plants permit close-pollination in the e\'ent of failure to 

 secure cross-pollination, while a feu- ha^x adaptations which 

 insure it. Our countion \iolets produce in the late spring 

 and early sunnner inconspi<:uous blossoms which do not open, 

 containing stamens with few ])ollen grains. These flowers, 

 however, produce seed abundantly, and alwa)s b\' tlose- 

 poUination. Various other species have similar arrange- 

 ments. 



484. Adaptations to insects. — The adai^tations to secure 

 cross pollination through the visits of insects are so numerou 

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

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



