ECOLOGY OF FLOWERS 



363 



incapable of it. Frequently the pollen from another plant 

 of the same species prevails over that which the flower 

 may shed on its own pistil, so that when both kinds are 

 placed on the stigma at the same time it is the foreign 

 -pollen which causes fertilization. But apart from this 

 fact there are several means of insuring the presence of 

 foreign pollen, and only that, upon the stigma, just when 

 it is mature enough to 

 receive pollen tubes. 



435. Stamens and Pistils 

 maturing at Different 

 Times. — If the stamens 

 mature at a different time 

 from the pistils, self-polli- 

 nation is as effectually pre- 

 vented as though the plant 

 were dioecious. This un- 

 equal maturing or dichog- 

 amy occurs ia many kinds 

 of flowers. In some, the 



figWOrt and the common in ^ (earlier stage) the stamens are mature, 

 nlanfain for pxamnlp the while the pistil is stm undeveloped and bent 

 plan tarn lOr example, ine ^^^^^^^^^_ in ^ (later stage) the stamens 



pistil develops before the hare wittered and the stigmas have sepa^ 

 , Ti ji rated, ready for the reception of pollen. 



stamens, but usually the 



reverse is the case. The Clerodendron,^ a tropical African 

 flower (Fig. 255), illustrates in a most striking way the 

 development of stamens before the pistil. The insect visitor, 

 on its way to the nectary, can hardly fail to brush against 

 the protruding stamens of the flower in its earlier stage 

 (at A), but it cannot deposit any pollen on the stigmas, 

 1 C. Thompsoniss. 



Fig. 255. — Flower of Clerodendron in Two 



