68 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE 
It is clear that by the action of nitrifying bacteria and tubercle- 
fungi the nitrogen of the air is steadily diminished, but here again 
we find a means of compensation. For there are certain denztrzfy- 
ing bacteria, which, in the absence of oxygen, act upon decaying 
organic matter in such a way that free nitrogen is liberated. 
RELATIONS BETWEEN THE NUTRITION OF PLANTS AND ANIMALS. 
—A little reflection will show that animals are entirely dependent 
upon plants in the matter of food. This is obviously so as 
regards purely vegetarian animals, while carnivorous forms are 
indirectly dependent upon the vegetable world. Many flesh- 
eaters feed entirely upon vegetarians, but if they prey upon other 
flesh-eaters, and these again upon still other carnivorous creatures, 
and so on, we get to plants in the end. 
Plants, considered as food for animals, have been concerned 
in the evolution of burrowing, climbing, parachuting, and flying 
forms, especially the last three. (For details, see vol. iii, pp. 
291, 281, 292.) 
On the other hand, animals contribute to the store of plant 
food. For, as we have already seen (pp. 65-67), they breathe 
out carbon dioxide, which green plants take up, while their nitro- 
genous excreta and dead bodies are partly converted into nitrates, 
which the same plants are able to use for the purposes of nutrition. 
There are also a number of Carnivorous Piants, which do 
not altogether rely upon simple substances as food, but are 
provided with “traps” for the capture and digestion of insects or 
other small creatures. One of Darwin’s most interesting books 
(Lusectivorous Plants) is devoted to these forms, some of which 
are native to our own country, while several others may be seen 
in botanic gardens. One of the simplest kinds of arrangement is 
seen in the Butterwort (Pzmguzcula), that is often to be found 
growing in damp places among our mountains. The pale-green 
slippery leaves are arranged in a rosette, from the centre of 
which violet flowers grow up. Small flies alighting on the leaves 
are held fast by a sticky fluid, secreted by a multitude of little 
knobbed hairs which project from the surface. The edges of the 
leaves then curl over the prey, and there is an increased exudation 
of the fluid in question, which acts very much like gastric juice, 
converting the flesh of the booty into a soluble form that is 
then absorbed as food. The widely distributed members of the 
Sundew family (Dvoseracee) exhibit greater specialization in 
