1. From various lines of evidence it is shown that the dry weight of Larrea leaves undergoes diurnal fluctuations when there is enough water for growth; but under the usual desert conditions the dry weight probably changes very slowly, water conditions being chiefly responsible for the fluctuations of the wc:dw ratio. However, the usual disregard of dry weight changes in literature pertaining to percentage water content, and the use of the fresh weight basis for expressing the data, have often led to erroneous interpretations. 2. The amount of moisture relative to dry weight in leaves of Larrea differs with age, position on the stem, season, habitat, and time of day. 3. Apart from seasonal influences, water content increases with the age of the leaves, except during rare periods of ample soil water, when the younger more plastic leaves have the higher water content. 4. During periods of growth the water content of the foliage is not lower than that of many mesophytic tree leaves: between one and two times...