354 



ECOLOG y. 



i.e. the pollenation of the pistil of one flower by pollen from another, is 

 sure to take place, if it is pollenated at all. Even in monoecious plants cross 

 pollenation often takes place between flowers of different individuals, so that 



Fig. 451. 

 Viola cucullata ; blue flowers above, cleistogamous flowers smaller and curved below. 

 Section of pistil atright. 



more widely different stocks are united in the fertilized egg, and the strain 

 is kept more vigorous than if very close or identical strains were united. 



652. But there are many flowers in which both stamens and pistils are pres- 

 ent, and yet in which cross pollenation is accomplished through the agency of 

 insects. 



653. Pollenation of the hluet. — In the pretty bluet the -stamens and 

 styles of the flowers are of different length as shown in figures 452, 453. 

 The stamens of the long-styled flower are at about the same level as the 

 stigma of the short-styled flower, while the stamens of the latter are on 



