CROSS POLLINATION 355 



the flower is a very wonderful and perfect mechanism 

 for securing this process. 



Cross-Pollination by Insects. — A very large number 

 of flowers, probably the great majority, are adapted to 

 cross-pollination by insects. Insects of different kinds 

 (mainly bees, flies, butterflies and moths) visit flowers 

 to feed on the nectar or on the pollen, or on both, or 

 to collect them (bees) for their young. While in the 



free ceretW. 



Fig. 6i. — A, ground plan of a flower (floral diagram) showing five 

 free sepals (black), five alternating free petals, ten stamens in 

 two alternating whorls of five each, and three free (apocarpous) 

 carpels. B, cross-sections of syncarpous ovaries of three carpels 

 each (a, b, c), showing different types of placentation : axiU 

 (note correspondence of placentae with those of the carpels in 

 A), free central (the infolded carpel walls have disappeared, leaving 

 one central placental column), and parietal (carpels not infolded, 

 margins joined to form placentae on inside of outer wall). 



flower they brush against the ripe anthers, and the 

 pollen grains stick to their hairy bodies. On visiting 

 another flower of the same species — and bees especially 

 often keep to one kind of flower on one journey — ^the 

 insect may brush against the stigma and rub off the 

 grains. The positions of the ripe anthers and ripe 

 stigmas in the flower are generally such that this probably 

 or even inevitably happens. 



The petals of insect-pollinated flowers are often 

 large, brightly coloured and conspicuous, so that the 



