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. [99] _Flowers, Fruit, and Leaves._ 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] [100] Nestler, _Sitz. d. K. Akad. d. Wiss. Wien_, vol. 3, p. 27. 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.