io Evolution and Adaptation 



There can be no question that this contrivance is of some 

 use to the plant. In other insectivorous plants, the pitcher 

 plants, the leaves are transformed into pitchers. In Nepenthes 

 a digestive fluid is secreted from the walls. A line of glands 

 secreting a sweet fluid serves to attract insects to the top 

 of the pitcher, whence they may wander or fall into the fluid 

 inside, and there being drowned, they are digested. A lidlike 

 cover projecting over the opening of the pitcher is supposed 

 to be of use to keep out the rain. 



In Utricularia, a submerged water-plant, the tips of the 

 leaves are changed into small bladders, each having a small 

 entrance closed by an elastic valve opening inwards. Small 

 snails and crustaceans can pass into this opening, to which 

 they are guided by small outgrowths ; but once in the cup 

 they cannot get out again, and, in fact, small animals are 

 generally found in the bladders where they die and their 

 substance is absorbed by forked hairs projecting into the in- 

 terior of the bladder. 



The cactus is a plant that is well suited to a dry climate. 

 Its leaves have completely disappeared, and the stem has 

 become swollen into a water-reservoir. " It has been esti- 

 mated that the amount of water evaporated by a melon 

 cactus is reduced to one six-hundredth of that given off 

 by any equally heavy climbing-plant." 



Sachs gives the following account of the fertilization 

 process in Aristolochia Clematitis, which he refers to as a 

 conspicuous and peculiar adaptation. In Figure i A a group 

 of flowers is shown, and in Figure i B and C a single 

 flower is split open to show the interior. In B a small fly 

 has entered, and has brought in upon its back some pol- 

 len that has stuck to it in another flower. The fly has 

 entered through the long neck which is beset with hairs 

 which are turned inwards so that the fly can enter but 

 cannot get out. In roaming about, the pollen that is stick- 

 ing to its back will be rubbed against the stigmatic surface. 



