572 



Insects and Flowers 



which stand close together at about the same height near the mouth of the 

 corolla. Both cross- and self-pollination might be brought about in this 

 way, but, as Darwin has shown, the foreign pollen would probably possess 



FIG. 763. Hawk-moth posed before a jimson-weed, Datura stramonium. (After 

 Stevens; one-half natural size.) 



the greater potency, and cross-fertilization would be apt to result. Fig. 763 

 is a photograph of a sphinx moth and Datura-flower, posed to show the rela- 

 tive lengths of the moth's proboscis and the corolla tube." 



Another kind of specialization in flower structure which tends to pre- 

 serve the nectar for certain spe- 

 cific insect visitors is well illus- 

 trated by the salvias, the snap- 

 dragon, and other similarly 

 irregularly tubular flowers (La- 

 biatse, Leguminosae, Scrophu- 

 lariaceae, etc.). Probably all 

 such flowers are pollinated by 

 insects (a few species by hum- 

 ming-birds). The irregularity 

 in corolla is accompanied by a 

 specific disposition of the stamens 

 and pistil, so that the insect 

 visitors are compelled to visit 

 the nectary in one particular 

 manner, a manner devised to 

 insure their touching, or being 

 touched by, the anthers or stigma 

 or both. In the snapdragon (Fig. 760) the opening of the flower-cup is 

 normally closed, but when a bee alights on the broad keel or platform (com- 

 posed of two petals grown together) its weight so depresses this platform as 

 to open the way into the flower-cup, which closes at once when the bee goes 

 in and drinks the nectar. Scrambling and twisting about in the narrow 

 chamber it thus thoroughly dusts itself with pollen, or thoroughly dusts the 



FIG. 764. Salvia-flower. A, showing position 

 of pistil and stamens; B, anthers of stamens 

 in normal position; C, anthers of stamens 

 tipped down; D, bee entering flower; E, flower, 

 natural condition. (After Lubbock; natural 

 size.) 



