ANIMALS AND PLANTS 259 
and acts, to some extent at least, as a digestive 
fluid. 
The bladder-weeds (Utricularia) (Fig. 58, E, F) and 
the butterworts (Pinguicula) are also well-known ex- 
amples of carnivorous plants. The former are aquatics, 
whose finely dissected leaves are provided with little 
bladder-like vesicles, which form perfect traps for small 
crustacea, and, it is said, in some cases for young fish. 
In all of these carnivorous plants, this peculiar habit is 
evidently a provision for providing them with nitrogen. 
They are always either bog-plants, or actually aquatic, 
and the roots are poorly developed or quite wanting, so 
that they are inadequate to provide the plants with the 
amount of nitrogen necessary for their growth, especially 
as the medium in which they grow is apt to be deficient 
in nitrogenous matter. 
We find, among the higher plants especially, many 
devices for protecting them against the attacks of ani- 
mals which seek them for food. These protective de- 
vices are of very different character in different forms. 
Thus many plants, such as the majority of perennial 
grasses, have creeping underground stems which send 
up leaves at a great many points, and these leaves are 
capable of continued basal growth, and may be eaten 
down close to the ground, growing up again promptly, 
so that the destruction of the plant is almost impossi- 
ble. It is this tenacity of life which makes many of 
the grasses such troublesome weeds. It is extremely 
probable that the development of acrid or poisonous 
substances, or ill-scented essential oils, in the leaves of 
many plants, is primarily protective, and makes the 
plants offensive to animals. That these secretions do 
