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 If you try the same experiment under a bell-glass from which the oxygen has been withdrawn by an air-pump, nothing happens; or if you chloroform the Sundew it will pay no attention to small pieces of meat until it recovers from the effects of the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.
The really interesting point is connected with these central jointed or trigger hairs; they are extremely sensitive. But when they are touched it is not they themselves that are affected, but the entire circular end of the leaf!
Suppose an insect wanders on to the leaf and reaches one of these semicircular halves, nothing happens until it touches one of these hairs, but then both halves suddenly close together, exactly like an ordinary rat-trap! The teeth on the edges of the halves interlock like the teeth of a trap, and the insect is caught and imprisoned.
Its body is slowly digested away and goes to nourish the plant. The use of the joint in the sensitive hairs can be easily perceived, for when the two halves shut up together, the hairs fold down exactly like the funnel of a river steamboat when it passes under a bridge.
The closing of the two halves, which has been well compared to shutting up a half-open book, is very quick, as it does not take more than ten to thirty seconds. There is an abundant flow of "gastric juice," but the leaf takes a long time to digest its food. It may require three weeks to finish one insect. Moreover, if overfed, it may turn a bilious or dyspeptic yellow colour, and wither or even die. It only shuts for a short time if a grain of sand touches the sensitive hair, and, like Drosera, is not deceived in its food.