Turning to the other side of the picture, no such general dependence of the plant world upon the animal world is found, but the inter-relations of the two are many and varied, and in the absence of animals of one kind or another whole groups of plants would become extinct. The cases where plants derive their food-supply wholly from animals are indeed rare, save near the bottom of the vegetable scale, and most of such parasites are minute; one of the most noticeable in our own country is the fungus _Cordyceps militaris_, which may be found growing on the dead bodies of larvæ or pupæ which it has killed--a little scarlet, club-shaped plant, about an inch in height. But some of the most highly organized plants obtain _portions_ of their food-supply from animal sources. Mention has already been made of the Sundews (_Drosera_), Butterworts (_Pinguicula_), and Bladderworts (_Utricularia_), which capture live insects, etc., by means of sensitive organs (as in the first two cases) or ingenious traps (as in the last), and subsequently digest them, and they will be dealt with later on (p. 186). Then there is the Venus’ Fly-trap (_Dionæa_) and the well-known Pitcher Plants (_Nepenthes_), which actively, as in the former case, or passively, as in the latter, catch insects and digest them, by means of leaves modified in very extraordinary ways. In all these instances the advantage lies entirely on the side of the plant, just as in the case of most of the plant-eating animals the advantage is wholly with the animal. But in a large number of instances--many of them of a most interesting nature--the inter-relations are such as to benefit both the actors, each obtaining from the other what is useful to it. One of the most conspicuous and widespread relationships of this kind is that prevailing between flowers and insects, the insect receiving food in the form of nectar, and at the same time carrying pollen from flower to flower, without which transfer no fertile seed would be formed. To this interchange of favours we shall return later (p. 81); meanwhile, it will be well to consider a few of the cases in which the relationship between plant and animal is continuous and more intimate, the two living in very close relations to each other: to such cases the term _symbiosis_ or “living together” is applied by naturalists. The relations existing between certain trees and some species of ant are of high interest, and illustrate well this phase of life. The Candelabra Tree (_Cecropia peltata_) of the South American forests is liable to attack by leaf-cutting ants (_Œcodoma_), which climb trees and bite off thousands of leaves; these they cut up on the ground and carry to their nests, where they form a basis for the growth of certain small fungi which are a favourite food of the ants (compare the cultivation of mushrooms as practised by gardeners). The Candelabra Tree protects itself from these ravages by forming an alliance with another kind of ant (_Azteca_).