Physical Conditions in Sun and Shade

Abstract

Students of the relation of environmental conditions to the distribution of organisms have long recognized the fact that relatively great differences in conditions nay occur within areas of very restricted size. Certain environmental conditions are more constant from spot to spot than are others; but the entire complex upon which the life and distribution of the organism depends, is to be thought of as uniform only throughout areas or volumes of extremely limited dimensions. It follows that the customary manner of measuring climatic conditions, so well suited to the purely physical requirements of meteorology, can give the student of environment nothing more than a normal base from which to determine the departures in the various habitats of an area. Physiological work has given us a large body of facts relative to the influence which difference intensities of a given condition are found to exert on various physiological processes. Very few of these relationships can be expressed by straight line curves. Not only may a small difference between 2 environments be of importance, but the same number of units of difference may vary in effect at different intensities of the condition under investigation. The importance of a given degree of difference can be ascertained only by determining its influence upon some specific life process. The study of environmental conditions can be fruitful for biological purposes only when it is carried on in close conjunction with the investigation of the physiology of the organisms which live under that environment. An important part of the program of the Desert Laboratory comprises such a study of the physiological behavior of a small group of plants, together with the intensive study of the conditions in their natural habitats. Some of the larger habitat differences ill the vicinity of the Desert Laboratory have been determined in the work of past years. Further work is being directed to determining the conditions for a number of localities in which differences in the dominant vegetation, or in the seasonal behavior of the vegetation, would seem to indicate that differences in the environment are operative on a small scale.