There are one or two plants which are extremely like the nettle at first sight. Lord Avebury has an illustration in his excellent little book[99] in which it is most difficult to tell which are White Deadnettles and which are stinging nettles. No doubt the harmless deadnettle is helped to escape injury by this resemblance. The Hemp Deadnettle and some Campanulas are also very like it when growing. These also are sham nettles and may escape in the same way.
There are several common greenhouse Primulas which also produce irritation of the skin. When handled by gardeners a painful smart is set up which lasts for some time. Primula obconica is the worst of these, but P. sinensis, P. cortusoides, and P. Sieboldii sometimes have the same effect. In all these cases it is due to a peculiar secretion of certain glandular hairs.[100]
The methods of protection against grazing animals so far described, such as stinging hairs, thorns, spines, etc. (see page [190]), are obvious enough, but perhaps the most ingenious system of defence is that exhibited by the Sensitive Plant and a few others.
When man or any heavy animal is approaching certain Indian plants, their leaves suddenly drop, and the leaflets close together. The mere shaking of the ground or of the air produces these extraordinary movements in the sensitive Woodsorrel (Oxalis sensitiva), in two Leguminous plants (Smithia sensitiva and Aeschynomene indica), and in several Mimosas.
When one leaf-tip of Mimosa pudica, the Sensitive Plant (par excellence), is touched or injured, a series of changes begin. All the little leaflets shut up one after the other; then the secondary stalks drop; after this the main stalk of the leaf suddenly droops downwards. After a short interval, the next leaf above goes through identically the same movements. If the shaking or injury is severe, every leaf from below upwards moves in the same way.
One probable advantage of these movements can be understood from the behaviour of flies, which alight upon the leaves and make them drop. The flies are startled and go away. Grazing animals will consider such behaviour in a vegetable as very uncanny, and will probably go to some other less ingeniously protected plant.
Of course such extraordinary behaviour has been a challenge to the botanical world, and there is an overwhelming mass of speculation, and observations about the Sensitive Plant.
It has been proved that the movements are caused by the thickened part at the base of the main stalk of the leaf. This is swollen, and full of water, and much thicker than the stalk itself. It is by this thickened portion that the leaf is kept at its proper angle. When the tip of the leaf is shaken or injured, the cells on the under side of this swollen part allow their water to exude into the spaces between them, and in consequence down comes the leaf-stalk.
This is not, by any means, a full or even a sufficient explanation. There is certainly some peculiar sending of messages from the tip of the leaf to the swollen part itself. It is not safe to say that it is a nerve message, but the process resembles the way in which messages are sent by the nerves in animals. Not only so, but the contraction of the under side and a corresponding expansion on the upper side, resembles the muscular movements of contraction and expansion in animals.
It must always be remembered that plants are alive; their living matter is not in any way (so far as we know) essentially different from that of animals or of man. Their living matter (protoplasm) in leaf-stalks and leaves is cut up into boxes or cells, each enclosed in a case or wall of its own. Yet these are not entirely independent and unconnected, for thin living threads run from cell to cell, so that there is an uninterrupted chain of protoplasm all along the leaf, leaf-stalk, and stem.