Among the common marsh plants in certain regions are the bladderworts, so called because their bodies are kept afloat in water by means of numerous little bladders. While these bladders are used in this fashion, they also serve as most effective traps for certain very small water animals related to the insects. Each bladder has a sort of opening which is guarded by a door like that of an ordinary rat trap. From the side of this entrance hairs are floating and waving in the water, and within the transparent bladder are other waving tufts of hairs. For some reason these things are attractive to the minute water animals, and they push aside the easily moved trap door, and entering the bladder find escape impossible, for the door, which was easy to push aside on entering, cannot possibly be moved outwards.

It must not be supposed that carnivorous plants are peculiar in the kind of food they use, but merely in the source from which they obtain it. There are other green plants which supplement their food supply by preying upon other plants. For example, the mistletoe is able to manufacture a certain amount of food for itself, but it adds to this supply by absorbing prepared food from the trees upon which it grows. The dodder is another illustration of a high grade plant which begins life independently, but presently breaks its connection with the soil and becomes entirely dependent upon the plants around which it twines and from which it absorbs.

A great many plants are known as root parasites, that is, they absorb from the underground parts of other plants. This is notably the case with the orchids and heaths, which have the appearance above ground of being entirely independent, but which really are quite dependent upon the underground parts of other plants.

One of the lowest groups of plants, known as the fungi, have cultivated most completely the habit of dependence on other organisms. They attack both plants and animals, and are often exceedingly destructive. Among the better known of these parasites are the rusts, which attack and destroy many of our most useful crops. To the fungi there also belong the well known bacteria, which are the cause of numerous contagious diseases both among plants and animals. It will be observed that these parasites are using exactly the same sort of food as do the carnivorous plants. This does not appear so striking in this case, simply because the attacking plants are so much smaller than the organisms attacked that they do not seem to capture them, although they are often none the less effective in destroying them.