176 



Introduction to Botany. 



but (a) the pistils and anthers are not mature at the same 

 time. Thus the stigmas may be ready to receive the pollen, 

 but the anthers are not ready to break open and discharge 



the pollen, or vice versa ; such 

 flowers are called dichogamous. 

 If the stamens mature first, the 

 flowers are proterandrous ; if 

 the pistils mature first, they 

 are proterogynous. (b) The 

 stamens and pistils may differ 

 greatly in length, so that the 

 pollen would not be apt to at- 

 tain to the stigmas of the same 

 flower. Flowers of this kind 

 are called dimorphic if the 

 stamens and pistils are each 

 of two different lengths, or 

 trimorphic if of three different 

 lengths, (c) The relative posi- 

 tion of stamens and pistils 

 may in other ways be such as 

 to keep the pollen from the 

 stigma, or other parts of the 

 flower may intervene between 

 them. 



The devices to prevent self 

 fertilization are also in the 

 interest of cross fertilization. If the flowers are dioecious, 

 insects in going from one plant to another are quite certain 

 to carry pollen from the staminate flowers to the stigmas 

 of the pistillate, or the wind may accomplish the same 

 thing. An insect in visiting dichogamous flowers would, 

 in the case of proterandry, carry pollen from the stamens of 



FIG. 91. 



Cross pollination of Primula, a di- 

 morphic flower. In A a bee is 

 gathering nectar at the base of the 

 pistil while its head is in contact 

 with the anthers inserted in the 

 throat of the corolla. In gathering 

 nectar from the long-styled flower, 

 B, the pollen on its head will be 

 deposited on the stigma, and at the 

 same time pollen from the low 

 stamens will be brushed off on its 

 proboscis at the right height to be 

 transferred to the stigma of a short- 

 styled flower as in A. WISE. 



