if 



292 A TEXTBOOK OP BOTANY [Ch. VI, 5 



are the most prominent, in which pollen and stigmas are of 

 two reciprocally corresponding kinds, though the two never 

 occur in the same flower, an arrangement called dimorphism 

 (Fig. 201). These and other arrangements bring it to pass 

 that cross-pollination is in fact the rule in showy flowers. It 

 is not, however, invariable, for with many kinds of flowers, 

 especially of the simpler sorts, close-pollination is perfectly 



Fig. 204. — Cypripedium Calceolus. 

 The insect can enter only by the front or upper opening, and leave only 

 by a side opening ; thus it must pass in succession the stigma and anthers. 

 (From Kerner.) 



possible, and is actually self -effected, if no foreign pollen 

 be present. Still further, there exist a few flowers, of which 

 some Violets are examples (Fig. 202), in which self-pollination 

 is not only the rule, but the invariable rule ; for the flowers 

 never open and the anthers shed the pollen directly upon 

 the stigmas, an arrangement called cleistogamy. All such 

 plants, however, possess also showy flowers, cross-pollinated 

 in the usual way. 



