When a clod of earth dries it is said to be through evaporation; when moisture passes away from the surface of the leaf in the regular operation of growth it is called transpiration. In relation to these terms Prof. Bessey says in a recent number of the American Naturalist:

" It will puzzle any one to make out a good reason for using two terms for the process of water-loss in plants. We have it said that ' evaporation ' is the 'purely physical process,' while the experiments show that what is called 'transpiration ' is, after all, a physical process also; and when we are told, as in the last sentence above, that life has a retarding effect on evaporation,' the confusion of ideas becomes somewhat embarrassing. Why not use but one term, and that the more general one - evaporation? The fact of modification or control of evaporation is so common a phenomenon in nature that we cannot regard it as of great significance. Common salt or sugar added to water retards evaporation.

"The mutual attraction of the molecules of cellulose and water retards evaporation; so does the mutual attraction of the molecules of protoplasm and water. Heat increases the rate of evaporation, while a reduction of temperature (other things being equal) retards it, etc., etc. Why not call the loss of water in the plant what it is - evaporation, and then discuss the several modifying influences? Certainly such a course would contribute to clearness and accuracy, and would relieve the beginner of one of the difficulties in vegetable physiology".

That living things do evaporate moisture is cer tain. When a tree freezes dry during the winter, or dries by the excessive warmth of summer, evaporation must be charged with this result. In this case it is simply a physical process, that which is dryer absorbing that which is moister; an effort at equalization. But life - the principle of life whatever it may be, and not merely " molecules" of matter - resists this effort at drying out. A live creature thrown on a spot where it can get no more water, with a dead creature alongside of it, will retain some moisture for weeks or until it thirsts to death. The dead creature soon dries up. On the other hand, a living creature, though in an atmosphere filled with moisture to the saturation point will still "transpire," still have moisture pass away from the surface of its body. Here the action actually opposes the physical laws of evaporation. It seems to us that the two terms serve a useful purpose at times, though their lines do often run together.