166 ELEMENTARY STUDIES IN BOTANY 



ber of each set is definite, the prevailing number of flower 

 parts among Dicotyledons being five or four, and among 

 Monocotyledons three. There is also a strong tendency for 

 the members of any set to develop together so as to appear 

 united. This is most common in the case of carpels, so that 

 a pistil most frequently consists of more than one carpel, but 

 it is most regular in the case of petals, characterizing one of 

 the three great groups of Angiosperms (Sympetalse) . There 

 is also a tendency for two or more sets to develop together 

 so as to appear united, the extreme case being epigynous 

 flowers, which are characteristic of the highest members of 

 both Dicotyledons and Monocotyledons. 



Many of the variations of the flower are associated with 

 the visits of insects which feed upon the nectar or pollen of 

 flowers and thus become the agents of cross-pollination. 

 This relation between flowers and insects is often general, 

 but it may be so special that only a particular kind of insect 

 can act as the agent of pollination for a particular kind of 

 flower (as among the orchids). 



The use of artificial cross-pollination in plant-breeding is 

 to secure hybrids that combine certain desirable qualities 

 of two different kinds of parents, but the desired combination 

 usually appears in an extremely small number of the hybrids 

 produced. 



