There is something rather horrible in the sight of a large insect struggling with these slow, remorseless, well-aimed tentacles; most people free the insect unless, at least, it happens to be a midge. The point which is so difficult to understand is to know how those untouched tentacles know that the insect is there and exactly where it is. There is no doubt that they do know, for they behave exactly as if they were the arms of a spider. If you put two insects on either side of the middle of the leaf, half the tentacles will pin down one and the other half will deal with the other insect. At the same time acids and ferments are poured out which digest the insect. It takes about two days for a leaf to finish off an insect, and then the tentacles again unclose. Moreover it is difficult to deceive those tentacles. They will bend in for the tiniest piece of useful substance; for instance, a length of one-seventy-fifth of an inch of woman's hair will make them secrete digestive fluid. One millionth part of a pound of ammonium phosphate will also produce secretion. But a shower of heavy rain, grains of sand, or other useless material, will not cause any secretion, and even if they do bend in a little, they soon discover their mistake and stand out again. If you try the same experiment under a bell-glass from which the oxygen has been withdrawn by an air-pump, nothing happens; or if you chloroform the Sundew it will pay no attention to small pieces of meat until it recovers from the effects of the chloroform. When these Droseras are taken to a greenhouse and experiments are made on them, they run into very great danger. They are almost certain to die of overfeeding or indigestion. It is impossible to keep people from giving them too much to eat. This wonderful little plant shows quite distinctly that there must be some way of sending messages in its leaves. Somehow the message travels from the tentacle which the fly has touched, down the stalk into the leaf, and up into the other tentacles, and tells them that there is something worth stooping for. No one has explained this, and probably no one will ever do so. The last, and in some ways the most interesting, of all these carnivorous plants is Venus' Fly-trap (_Dionæa muscipula_), which grows in North America from Rhode Island to Florida. It is a quite small herb with a small circle of leaves which lie flat on the ground. Each leaf ends in a nearly circular piece which is divided by a very marked midrib. The two semicircular halves have a series of teeth along their edges; these margin teeth are stiff and a little bent upwards. In the centre of each half there are three small hairs. On looking closely at these hairs one finds that each has a joint near the base; all over the centre of these leaf halves there are scattered glands which secrete ferments intended to digest any animal matter.