RELATION TO ENVIRONMENT 523 



the insects being inserted into the clefts through which the pol- 

 linia are withdrawn. It sometimes happens that the insect becomes 

 entrapped, by not being able to withdraw the member, and may thus 

 perish. A plant belonging to the Milkweed family, Physianthus 

 albens, a climber occasionally cultivated for ornament, is especially 

 noted for its habit of capturing insects. 



Orchids. The Orchids offer innumerable examples of special 

 adaptation to insect pollination, but only one or two of the simpler 

 types can be given here. The numerous species of Orchis, and in 

 America the species of Habenaria, including the handsome Fringed 

 Orchids, are examples of a characteristic arrangement. As in As- 

 clepias, the pollen is aggregated into two pollinia, which are sepa- 

 rate, each occupying one lobe of the single stamen. Each lies in 

 a sort of pocket, its lower tapering end terminating in a little vis- 

 cid disk, which is covered by a delicate membrane. The disks lie just 

 above the opening of the spur into which the lip of the flower is pro- 

 longed, and which is the nectary. An insect searching for honey is 

 sure to strike against the membrane covering the viscid disks, and 

 this being broken, the insect's body is brought into direct contact with 

 the viscid substance of the disks, which become at once firmly ce- 

 mented to it. The insect, as it withdraws from the flower, drags 

 the pollinia out of their receptacles and carries them away. The 

 two pollinia change their position almost at once, and bend forward 

 in such a way, that if the insect visits another flower, they strike 

 against the two stigmatic surfaces which lie on each side of the 

 entrance to the spur, below the disks of the pollinia (Fig. 492). 



In the Lady's-slipper (Cypripedium), unlike most Orchids, there are 

 two perfect stamens, one on each side of the column. These are pol- 

 linated mostly by bees, which enter the sac-shaped lip in front, where 

 they can strike the stigma, but they can only escape at the sides, 

 where they scrape off the pollen from the stamens. There are no 

 pollinia, but the pollen-grains are separate, as in most other flowers, 

 but covered with a viscid secretion which makes them adhere to the 

 stigma of the next flower which is visited. 



Araceae. In many Aracese and Aristolochiaceae the large spathe, 

 or tubular perianth, is constricted near the base, and the cavity below 

 the constriction has above it downward-pointing, stiff hairs, which 

 enable the insects to enter it, but imprisons them, as they cannot 

 creep out against the hairs. This continues until the stamens have 

 shed their pollen, when the hairs wither, allowing the imprisoned 

 insects, with their load of pollen, to escape. 



Yucca. One of the most extraordinary cases of the mutual de- 

 pendence of an insect and a flower is found in the species of Yucca, 

 a peculiarly American genus, especially abundant in the deserts of 

 the Southwest, but better known by the common Y. Jilamentosa, of 



