Still other plants preserve more of their structures than either the annuals or the ground-loving plants. For example, most of our trees have cultivated what is known as the deciduous habit, that is, they merely drop their leaves, which are the endangered structures, at the approach of the unfavorable season, and renew them again when the favorable conditions return. It should be remarked that these leaves do not fall because they are broken off, but that in a certain sense it is a process of growing off, which is carefully prepared for. One of the most prominent features associated with the deciduous habit is the autumnal coloration. The vivid colors which appear in the leaves of many trees just before the time of falling have attracted a great deal of attention, but although it is so prominent, the causes for it are very obscure. It will be noticed that this autumnal coloration consists in the development of various shades of two typical colors, yellow and red. It is known that the yellow is due to the breaking down of the green substances, so that it simply indicates a post mortem change, as may be noticed in connection with the blanching of celery in which the leaves and upper part of the stem may be green, the green may shade gradually into yellow, and finally into the pure white of complete blanching. The red coloring matter, however, is very different. Certain experiments upon plant colors have indicated that the presence of the red slightly increases the temperature by absorbing more heat. It is suggested that the red color may be a slight protection to the living substance which is ceasing to work, and which is in danger of exposure to cold. If this be true, it may be that the same explanation will cover the case of the red flush so conspicuous in buds and young leaves in early spring. It must not be supposed that the need of protection has developed the coloring, but since it is developed it may be of some such service to the plant. Even the conditions which determine autumnal coloration have not been made out certainly. It is instructive to notice how differently the so-called evergreens, as pines, spruces, etc., have answered the problem of protection against the cold of winter. The evergreens, instead of dropping their leaves, have undertaken to protect them, giving them a small surface and very heavy walls. In this way protection has been secured at the expense of working power during the season of work. Reduced surface and thick walls are both obstacles to leaf work. On the other hand, the deciduous trees have developed the working power of their leaves to the greatest extent, giving them large surface exposure and comparatively delicate walls. It is out of the question to protect such an amount of surface during the winter, and hence the deciduous habit. The evergreens are saved the annual renewal of leaves, but lose in working power; the deciduous trees must renew their leaves annually, but gain greatly in working power.