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 acorolla. 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, 
Fic. 186. Dichogamous flowers of dicecious flowers, like those 
plantain (Plantago lanceolata) of the willow, cannot be self- 
A, earlier stage, pistil mature, stamens pollinated. In the culture of 
not yet appearing outside the corolla; date palms it is usual to polli- 
B, later stage, pistil withered, stamens we 
mature. Six times natural size nate the pistillate flowers by 
hand. Moncecious 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 ix highly advantageous. One 
1 See Kerner-Oliver, Natural History of Plants, Vol. TI. Henry Holt and 
Company, New York. Also Knuth-Davis, Handbook of Flower Pollination, 
Vol. I. 
