Factors Governing Seasonal Changes in Transpiration of Encelia farinosa

In a former paper it has been shown that Encelia farinosa, a desert perennial having a mesophytic form of leaf in the cool moist months and a xerophytic form in the arid season, reduces its response to the maximum evaporate power of the air in June to about one-fifth the January value. 2. The difference in the anatomical structure of the mesophytic and xerophytic leaves does not account for the greater resistance to water loss in the arid season. 3. When disks of equal areas are cut from the two types of leaves immediately before sunrise and placed under identical external conditions, the water loss from them is found to vary inversely with the total imbibitional capacity and directly with the original water content. Even though the external temperature varies as much as 10° F., the expression Ed/Ea x M/O gives a value which approaches a constant to an amount remarkable for work with water relations of plants. 4. The total imbibitional capacity of both types of leaves varies with the original water content and the dry weight. 5. The lower the original water content, the greater the amount of soluble material that diffuses into the water which surrounds imbibing disks. The dry weights after imbibition have one constant value for mesophytic leaves and another for xerophytic ones. 6. The ratio of evaporation to soil moisture affects the water content of leaf tissue; this in turn affects the imbibitional capacity and resistance to water loss of leaf tissue. The changes in imbibitional capacity are accompanied by changes in the amount of soluble material in the tissue. These changes in diffusible material are presumably the resultants of reversible chemical changes which take up or give up water according to the amount available