152 



INTRODUCTION TO BOTANY 



it seems probable that the colors of flowers are an important 

 means of attraction for many flower-frequenting insects. 1 



The commonest method of color display is that in which 

 the color (other than green) is mainly found in the corolla, 

 as in the flowers of the poppy, rose, sweet pea, and morning- 

 glory. Sometimes the calyx also is bright-colored, or, as in 



the Hepatica, the Anemone, and 

 the Clematis, the corolla is want- 

 ing and the showy calyx looks 

 like a corolla. Not infrequently 

 the display is all made by an 

 enlarged and conspicuous set of 

 specialized leaves (bracts) sur- 

 rounding the flower, as in the 

 flowering dogwood and many 

 euphorbias (fig. 67), or even by 

 highly colored ordinary leaves, 

 like those of the poinsettia. 



144. Prevention of self-polli- 

 nation ; dichogamy. Of course, 

 dioecious flowers, like those 

 of the willow, cannot be self- 

 pollinated. In the culture of 

 date palms it is usual to polli- 

 nate the pistillate flowers by 

 hand. MoiHEcious flowers, like 



those of Indian corn (figs. 126 and 127) are likely to be pol- 

 linated with pollen from another plant. As regards bisexual 

 flowers, it is evident that there are many opportunities for self- 

 pollination ; but in all cases in which cross-pollination produces 

 more seed or stronger plants, or both, it is clear that anything 

 in the structure or mode of development of the flower which 

 tends to secure cross-pollination is highly advantageous. One 



1 See Kerner-Oliver, Natural History of Plants, Vol. II. Henry Holt and 

 Company, New York. Also Knuth-Davis, Handbook of Flower Pollination, 

 Vol. I; 



A B 



FIG. 136. Dichogamous flowers of 

 plantain (Plantago lanceolata) 



A, earlier stage, pistil mature, stamens 

 not yet appearing outside the corolla ; 



B, later stage, pistil withered, stamens 



mature. Six times natural size 



