10 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 1 A-a group 
of flowers is shown, and in Figure 1 B and C a single 
flower is split open to show the interior. In Ba 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. 
