128 



BOTANY. 



[CHAP. iv. 



connection with preventing self-fertilization, and con- 

 sequently securing cross-fertilization, was the evolution 

 of unisexual flowers, and this was not brought about 



by the special formation of such 

 in addition to those already 

 existing, but, as already stated, 

 by the modification of the old 

 hermaphrodite form of flower. 

 So long as the two kinds of 

 flower stamina te and pistillate 

 were present on the same 

 plant, as in the hazel, the 

 prevention of self-fertilization 

 was not absolute, this point 

 being attained only when one 

 of the two sexes was completely 

 arrested on a given tree, as in 

 the willows, where the bright 

 yellow catkins of the goat wil- 

 low (Salix caprcea) , popularly 

 known as "palms'", are the 

 clusters of staminate flowers, 

 the trees of the same species 

 bearing the female catkins, that 

 are green and inconspicuous, 

 make but little show, and are 

 ignored for decorative purposes. 

 This early attempt to secure 

 cross-fertilization was certainly effective, but too ex- 

 pensive in many ways, and it will be seen that later 

 ideas in this direction were equally effective with a much 

 less expenditure of energy and bulk of material. All 



Fig. 35. The hazel (Cory- 

 lus avellana), a monoecious 

 plant ; $ , pistillate flowers 

 having bright crimson pro- 

 truding stigmas ; <r, cat- 

 kins of staminate flowers. 

 (Natural size.) 



