290 



A TEXTBOOK OF BOTANY 



[Ch. VI, 5 



nation merges over gradually to insect-pollination, the floral 

 structure being also intermediate, as some Maples and Wil- 

 lows illustrate. 



The most prominent of the agencies of cross-pollination, 

 however, are insects, to the utiUzation of which the most 

 typical flowers are fitted. The characteristics of insect- 

 pollinated flowers are 

 these. First, the flower 

 is conspicuous to the 

 eye, through a showy 

 corolla usually colored 

 in contrast to the back- 

 ground and set prom- 

 inently forth in the 

 light ; and often its 

 presence is indicated 

 also by strong odor. 

 Second, nectar is com- 

 monly present in the 

 base of the flower, and 

 when absent, is replaced 

 bjr more abundant 

 pollen which some in- 

 sects particularly de- 

 sire. Third, stamens 

 and pistils are usually 

 present in the same 

 flowers. Fourth, the 

 shape of the flower is 

 such that an insect in probing for nectar leaves some 

 poUen on the stigma, and receives some by contact with 

 the anthers. Fifth, the pollen grains are commonly angu- 

 lar, roughened, or adhesive (Fig. 198). Observation, 

 supplemented l)y experiment, proves that insects visit 

 flowers for the nectar, their usual food : that tliej' are 

 guided in general to the flowers by the bright spots of 



Fig. 202. — The cleistogamous flowers of 

 the common Blue Violet; reduced. 



The cleistogamous flowers arc on run- 

 ning stalks on the ground, a is a small 

 flower, and b a pod containing seeds. r)r(ii- 

 nary flowers, of natural size, on the right. 

 (From Bailey.) 



