Many interesting experiments have been performed with Dionaea to show its quickness and its recognition of suitable food material. For example, although it will snap shut at the touch of a pencil point, or any other indigestible substance, it soon opens again; while in the case of a digestible substance the trap remains closed until digestion has taken place. It has been claimed further that when the trap has closed its bristles do not interlock closely at first, so that between the crevices very small insects may crawl out and escape. In such an event the trap opens again and waits for other prey. If this be true, it follows that the leaf does not undertake the rather long process of digestion until an insect of suitable size has been captured, one which cannot escape through the meshes of the bristles. Digestion is slow work with Dionaea as with an anaconda, being said to occupy not less than two weeks. 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.