Atmometers of Porous Porcelain and Paper, Their Use in Physiological Ecology

This paper is de(licate(l to mn life-long friend and colleague, Doctor Henry Chandler Cowles, whose thought and enthusiasm have exerted such notable influence in the rapid (levelol)ment of American ecology throughout a third of a century. His remarkable paper on sandl-(lune vegetation and his early al)l)reciation of the dynamic relations of plant distril)ution to physiographic development came as enlightening encouragement to those who had dreamedd that it might sometime be possible to approach problems of plant distribution from the physiological viewpoint. Studies like those of Cowles furnished a logical method by which the environmental complexes of plant habitats might be usefully characterized and classified according to soils and exposure, inl terms of physiographic features that naturally pass through progressive changes with the lapse of years and centuries. Such simply descriptive terms as moraine, bluff, talus, bajada, beach, strand, lagoon, swamp, thus came to imply slowly changing systems of environmental conditions. Of course the need for a still more penetrating examination of environmental complexes was realized. It was desirable that habitat features should be described in such terms as might replresent conditions to which the indiviclual plant is sensitive, which is not immediately true of geographic or physiographic concepts; at the same time, the terms of habitat description should be susceptible of physical and chleical measurement and integration whenever suitable techniques therefor might be developed. Accordingly, there has arisen a special branch of ecology, which has been called physiological ecology. Considerable progress in physiological ecology had been made by agricultural chemists before ecology took its place among the biological sciences, and their studies on the soil relations of cultivated plants constituted an important part of the fundamental basis for the new science, although it is still sometimes forgotten that ecology must logically include cultivated forms and weeds in its purview, as well as the components of natural plant associations and other natural vegetational units. Agronomy and horticulture furnish the most readily approachable fields for fundamental ecological research, and workers in these fields early turned their attention to soil characteristics as effective environmental features. It was soon realized that the soil condi