In the last number of this journal it was shown how plants seek to avoid the visits of unsuitable insects to their flowers. This is one means of protection, but there are many others which are even more striking and vital. It is supposed by many that plants are helpless beings, which must submit to all sorts of unfavorable conditions which come upon them. This is far from true, for while plants as a rule are fixed and unable to escape from danger by flight, still they have very many ways of helping themselves.

Prominent among the dangers which come to active green plants are those which arise from too intense light, which may destroy the delicate working substances. Since the leaves are the great working organs in the manufacture of food, they are especially equipped for protection. Those leaves which must work in exposed places have many details of structure which are evidently for guarding them against the ill effects of too intense light. The most striking adaptations, however, are those which have to do with protective positions. Under ordinary circumstances leaves are placed so that their flat faces are exposed to the most intense light. In some cases this is so great a danger that the leaves are set edgewise, the edges being directed upwards and downwards. When a plant assumes this habit, the leaves are said to be in a profile position, and the plants are sometimes called "compass plants." The latter name has come from the fact that such leaves usually point north or south, and once it was assumed that this position was in response to some mysterious magnetic influence. It is found, however, that it is merely an effort on the part of the plant to protect its leaves from the intense light of midday, and at the same time to expose them to the morning and evening rays of much less intensity. If a leaf is to be placed with its edge upwards and its flat faces east and west, it follows of necessity that it will point either north or south.

Some leaves, however, have the power of shifting their position according to their needs, directing their flat surfaces toward the light, or more or less inclining them according to the danger. Perhaps the most completely adapted leaves of this kind are those of the "sensitive plants," whose leaves respond to various external influences by changing their positions. The sensitive plants abound in dry and hot regions, and one of the best known is represented in our illustration.

It will be noticed that the leaves of this Mimosa are divided into very numerous small leaflets, which stretch in pairs along the leaf branches. When the time of intense light and dryness approaches some of the pairs of leaflets fold together, slightly reducing the surface exposure. As the unfavorable condition continues, more leaflets fold together, then still others, until finally all the leaflets may be folded together, and the leaves themselves may bend against the stem. It is like a sailing vessel gradually taking in sail as a storm approaches, until finally nothing is exposed, and the vessel weathers the storm by presenting only bare poles. These are but a few illustrations of the very numerous devices for escaping too intense light and the dangers which accompany it.

One common danger in temperate regions comes from the lowering of the temperature each night, which sometimes may chill the living substances to the danger point. This is particularly dangerous to seedlings, whose tender structures have not yet developed the ordinary protective coats. In the spring the seed leaves of numerous seedlings may be seen at the approach of night to rise upward and come together, just as the palms of the hand may be placed together over ones head. This reduces the surface of exposure and the danger of chill at least one-half. Darwin experimented upon these seedlings, and discovered that by preventing some of the seed leaves from moving, the seedlings were seriously injured. The leaves of very many plants assume a peculiar night position which tends to meet the danger of loss of heat. Often the three leaflets of the common clover, if growing in an exposed place, may be observed to fold together into a sort of tent-like arrangement.

Many plants are also observed to protect themselves against rain, as it is necessary for leaves to avoid becoming wet. If the water is allowed to soak in, the work of the leaves is at once interfered with. Hence it will be noticed that most leaves are able to shed water, partly by their position, partly by their structure. In many plants the leaves are so arranged that the water runs off toward the stem; in other plants the rain is shed outwards as from the eaves of a house. Some of the structures which prevent the rain from soaking in are a smooth epidermis, layers of cuticle, hairy coverings, etc. Interesting experiments may be performed with different leaves to test their power of shedding water. If a gentle spray be allowed to play upon different plants it will be observed that the water glances off at once from the surfaces of some leaves, runs off more slightly from others, and may be more or less retained by others.