know that they allow greater openings during the night and reduce them during the day. When we reflect that the constant removal of water in the leaf, both as such, and as the only carrier of food supplies from the roots, depends in such large measure upon the functioning of these guard cells, then we come to some realization of their importance to the plant. They do not always work unaided, for in many places the transpiration, even with their best efforts, would exceed the rise of water in the plant and death must follow if such a condition exists for long. This may be the case in certain bog plants, where, even with their roots in the water, they actually are in danger of drying out because the composition of bog water makes it partially unfit for most plants. And, again, in very open dry or windy places, such as deserts or the mountain tops above timber line, the actual supply of water may be insufficient. Many plants growing in such places have their leaves, particularly the under surfaces of them, clothed with various kinds of hairs. These may be quite velvety or cottony, but in any event, either by their texture or their color, they tend to reduce transpiration. An extreme case is a desert plant from Arizona where the whole leaf surface is covered with an ashy gray velvety coating, which, of course, absorbs less heat than a normal green leaf, and in addition there are much fewer pores through which transpiration could be carried on. In ever so many leaves nature has provided them with a thick coating of hairs in early spring, which they lose later in the summer. Shrubs and herbs, especially those that start earlier than the trees under which they grow, very often may be found with a dense woolly or silky covering in early spring. As the shade becomes denser and the need of the protection less, the wool or silk is shed, sometimes completely. Some of the most conspicuous cases of this are certain kinds of our common shadbush, which in April are covered with a beautiful grayish-white silky coat, but by August are practically the ordinary green color of other leaves. The hairy covering of leaves is well worth observation, as it may hide not a few facts about transpiration and, in some leaves, has had much to do with their preservation from grazing animals. Some, like the common mullein, are never touched, and may be found standing like sentinels in fields otherwise cropped short.