precarious, and upon the compensation for water supply furnished by water loss where the supply is excessive, submerged plants excepted. =155. The measurement of absorption.= As responses to measured stimuli of water-content and humidity, it is imperative that the amount of absorption and of transpiration be determined quantitatively. It is also extremely desirable that this be done in the normal habitat of the plant. A careful examination of the problems to be met quickly discloses the great difficulty of obtaining a direct and accurate measure of absorption under normal conditions, especially in the field. For this purpose, the ordinary potometric experiments by means of cut stems are valueless. The use of the entire plant in a potometer yields much more trustworthy results, though the fact that the root is under abnormal conditions can not be overlooked, especially in the case of mesophytes and xerophytes. While potometric conditions are less abnormal for amphibious plants, the error is not wholly eliminated, since the roots normally grow in the soil. The potometer can be made of value for quantitative work only by checking the results it gives by means of an instrument or a method in which the plant functions normally. In consequence, the potometer can not at present be used to measure absorption directly, though, as is further indicated in the discussion of transpiration, it is a valuable supplementary instrument, after the check mentioned has been applied to its use with a particular species.