THE FLOWER 



261 



B. The Function of the Flower: Pollination. 

 1. Various Provisions for Cross-pollination. 



Most flowers have stamens and pistils growing together on 

 the same flower. It does not follow from this, however, that the 

 pistil of such a flower is pollinated from its own stamens. This 

 would be called self-pollination. 

 Although cases of this kind do 

 occur in nature, they are not at 

 all common. It has been proved 

 by many careful observations and 

 experiments that plants on which 

 the flowers have been self-pollinat- 

 ed, bear poor and insigniflcant 

 fruit. As a rule, pistils are polli- 

 nated by pollen from other flowers, 

 and to ensure such cross-pollination 

 many wonderful and interesting 

 arrangements exist, some of which 

 we shall now consider. 



(a) Stamens and pistils are 

 distributed over different flowers 

 (monoecious and dioecious plants). 



ip) In some plants, where the 

 stamens and pistils do occur in 

 the same flower, these organs mature at different times: in the 

 Sunflower the stamens open when the stigma is still undeveloped; 

 but in the .Aroide?e and in Aristolochia the stigma is mature 

 before the stamens develop their pollen. 



(c) If both mature at the same time, the stamens and pistils 

 are sometimes so placed that the pollen cannot easily reach the 

 stigma of its own flower (Hibiscus). 



{d) The flowers of the shrub Clerodendrou i}?fortunatum 

 (Kan. Ittevu ; Mai. Peragu) exhibit a remarkable contrivance for 

 cross-pollination. The white corolla and the strong, sweet scent 

 make the flower conspicuous at dusk to moths which stretch 



Fig. 245. — Aristolochia. 



A. Stigma mature before ripening of 



stamens. B. Pollen developed after 



fading of stigma. 



