CH. VI, 5] METHODS OF CROSS-POLLINATION 



291 



lore potent in this 



Tespect^and that in probingaboutjn their q,nf.ivft 

 "nectar, Tfaeir hairy bodies brush pnVjpp upon the_8tjgg}as and 



* < . 



%ece*~aqiew supplyjrom^the anthers. All of these matters 

 the student can confirm for himself in any garden in summer. 

 At first sight it would seem that insects must effect close- 

 rather than cross-pollination, but such in fact is not the 

 ca>e. Any one can see that insects in gathering nectar 

 usually visit flower after flower and plant after plant of the 

 same kind as long as 

 these are plentiful; 

 and since their 

 bodies possess a large 

 pollen-carrying ca- 

 pacity, it must usu- 

 ally happen that any 

 given stigma, even if 

 pollinated from the 

 same flower, be- 

 comes also pol- 

 linated by other 

 plants from the 

 abundant mixture 

 on the body of the insect, 

 is found to be the 

 pollen which effects 

 pollen tubes being 



FIG. 203. Flower of Sahia pratensis, pol- 

 linated by a Bee ; X T- 



Right, the flower ready for pollination, show- 

 ing position of stigma and stamens. The stigma 

 is touched by the insect in entering, and later 

 the stamens are brought down on its body by 

 operation of a hinged-lever arrangement. (From 

 Wiesner.) 



In such cases, as a rule, it 

 foreign and not the flower's own 

 the fertilization, the growth of the 

 more rapid in the former than in 

 the latter case. Flowers, indeed^are knawn jezhicJL-axe-Ba- 

 v sJirely sterile to their own pollen. Furthermore, in some 

 plants close-pollination is prevented by mechanical arrange- 

 ments, as with the Iris (Fig. 199), where the stigma is upon 

 a shelf struck by the insect when entering bu 

 leaving_khe flowes^. Still more common 

 which the stamens and pistils are not ripe at the same time, 

 as with Scroplmraria, an arrangement called DICHOGAMY 

 (Fig. 200). And there are plants, of which the Primroses 



