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 



