But though the Sensitive Plant is certainly protected from grazing animals by these movements, other advantages may be derived. Heavy rain, for instance, such as occurs in the tropics, will not injure its delicate leaves. Dust-storms will not damage it, and at night there will be no loss of heat by radiation. The "shrunk" or folded condition of the leaflets will decrease any chance of injury by raindrops, for the rain will not fall on the broad surface of the leaflets. A nearly vertical leaf also will not suffer the loss of heat which a horizontal one would endure. Besides the plants mentioned above, there are several others in which by a rather severe shaking the leaves can be made to fold up. This is the case with the common Woodsorrel (_Oxalis acetosella_), with the False Acacia (_Robinia_), and a few others. The former has a peculiarly delicate leaf. In cold, wet weather its leaflets hang limp and numb from the leaf-stalk all day. In fine weather they are spread out horizontally. On a fine sunny afternoon its leaflets may sometimes take a mid-day sleep, for they hang loosely down in the same way that they do in cold, wet weather or at night. But in the Woodsorrel these movements are not for protection against grazing animals. There are other examples amongst plants of a distinct sudden movement which begins whenever part of the plant is touched. The movements of tendrils have been already referred to. The Venus' Fly Trap and the Sundew will be mentioned when we are discussing Insectivorous Plants. There are also several flowers in which the stamens suddenly spring up when they are touched by an insect (Barberry, Centaurea, and Sparmannia), and in Mimulus the style-flaps close when touched (see p. 70). All these cases seem to involve some sort of mechanism which replaces the nervous system of animals. No very definite laws have yet been discovered as to the way in which plants are affected by electricity, but enough is known to show that there are many interesting discoveries in prospect. Professor Lemström has made some interesting experiments in the Polar regions which go to show that the rich development of plant life in that desolate region may be connected with the peculiar electrical conditions of the Polar atmosphere; the aurora borealis, which is a common phenomenon there, being also produced by those conditions. Several writers have claimed that slight electric shocks given at frequent intervals help the growth of plants and especially quicken the germination of seeds, but it can scarcely be said that this has been proved. When a branch or leaf-stalk is wounded or injured by being tightly clamped in a vice, then it will be found that a current of electricity passes from the injured spot to the part that is untouched, and then in the reverse direction. Changes of current are also produced when a leaf is suddenly exposed to light for a short time and then shaded.