The wide range in moisture content of different soils at the time of wilting of the plant cover appears to have been first clearly recognized by SACHS3 in i859. Later investigators, in extending this work, concluded that not only do soils show a wide range in moisture retentiveness, but that different groups of plants differ widely in their ability to reduce the moisture content of a given soil. Thus, the experimental work of GAIN,4 HEINRICH,5 HEDGCOCK,6 and CLEMENTS7, all indicates considerable variation in the moisture content of the soil at the time of wilting of different plants, which has been interpreted to mean that some plants are capable of reducing the moisture content of a given soil to a lower point than others; in other words, that the "non-available moisture" varies according to the kind of plant used as an indicator. In fact, this is the view which is usually presented in many of the standard works on plant physiology and plant ecology. The difference exhibited by plants in this respect has also been