INSECTS IN RELATION TO PLANTS 



221 



an 



encounters the guardian lip again, but .as this side of the lip cannot 

 iceive pollen, immediate close pollination is prevented. Of course, 

 it is possible for bees to enter another part of the same flower or another 

 lower of the same plant, but as a matter of fact, they habitually fly 

 tway to another plant; moreover, as Darwin found, foreign pollen is 

 >repotent over pollen from the same flower. It may be added .that 

 bees and other pollenizing insects ordi- 

 narily visit in succession several flowers 

 of the same kind. 



Orchids. The orchids, with their 

 fantastic forms, are really elaborate 

 traps to insure cross pollination. In 

 some orchids (Habenaria and others) 

 the nectar, lying at the bottom of a 

 long tube, is accessible only to the long- 

 tongued Sphingidae. While probing for 

 the nectar, a sphinx moth brings each 

 eye against a sticky disk to which a 

 pollen mass is attached, and flies away 

 carrying the mass on its eye. Then 

 these pollinia bend down on their stalks 

 in such a way that when the moth 

 thrusts its head into the next flower 

 they are in the proper position to 

 encounter and adhere to the stigma. 

 The orchid Angr&cum sesquipedalej of 



Madagascar, has a nectary tube more 

 than eleven inches long, from which Darwin inferred the existence of a 

 sphinx moth with a tongue equally long. 



Milkweed. The various milkweeds are fascinating subjects to the 

 student of the interrelations of flowers and insects. The flowers, like 

 those of orchids, are remarkably formed with reference to cross pollina- 

 tion by insects. As a honey bee or other insect crawls over the flowers 

 (Fig. 258, A) to get the nectar, its legs slip in between the peculiar nec- 

 tariferous hoods situated in front of each anther.^ As a leg is drawn up- 

 ward one of its claws, hairs, or spines frequently catches in a V-shaped 

 fissure (/, Fig. 258, B) and is guided along a slit to a notched disk, or cor- 

 puscle (Fig. 258, C, d). This disk clings to the leg of the insect, which 

 carries off by means of the disk a pair of pollen masses, or pollinia (Fig. 

 258, C). When first removed from their enclosing pockets, or anthers, 



FIG. 257- Section to illustrate 

 cross pollination of Iris, an, anther; 

 I, stigmatic lip; n, nectary; s, sepal. 



