PROTEROGYNY AND PROTERANDRY 



95 



Its style is two-cleft, the stigmas existing upon the inner faces of the 

 branches, their outer faces being clothed with stiff hairs pointing 

 upward. It is obvious that until these style-branches separate, polli- 

 nation cannot take place. Before such separation occurs, the tip of the 

 style is, by elongation, slowly forced up through the tube of the anthers. 

 The anthers, with their contained pollen, are mature, and the pollen is, 

 by the stiff hairs upon the backs of the style-branches, torn out from 

 its receptacles and exposed to such agencies of transportation as may 

 be prepared to act upon it. Cases are even known in which the tearing 

 out of the pollen in this way is effected by a spasmodic shortening 



Fig. 269. Dichogamous flower of Mitchella in first stage. 270. The same, in second stage. 271. 

 Dichogamous flower of Vernonia in first stage. 272. Style of same in second stage. 



of the stamens upon the instant of contact by a visiting insect, the 

 pollen being by the same process at once discharged upon the body of the 

 latter. After the removal of the pollen, or after the death of such 

 grains as fail to be removed, the style-branches separate (Fig. 272) in 

 readiness to receive the pollen brought from some other flower. This 

 method, or some modification of it, is very common among the Com- 

 positae, and illustrates how the study of pollination serves to explain 

 many modifications of flower-structure otherwise inexplicable, and why 

 the possession of the latter is regarded by the biologist as indicating a 

 higher stage of development. 



Dichogamy is very common among perfect anemophilous flowers. 



