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 one's 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. Perhaps the most general preparation for protection in our region is that which is made for the coming of the winter's cold. In many cases plants do not attempt to protect their delicate structures from the severity of winter, but disappear entirely, leaving only well-protected seeds to carry them over into the next growing season. This results in the so-called "annual habit," which has been learned by many plants in order to escape a season of danger. Other plants do not disappear so completely, but everything above the surface of the ground dies, while the plant continues in the form of underground bulbs, tubers, or various thickened structures. This habit of seeking a subterranean retreat at the approach of some dangerous season is a very good one, and is found in many of our early spring plants. This subterranean habit has a great advantage over the annual habit, since a seed is very slow in bringing the plant back again, while a bulb can produce its plant very rapidly.