Increases in the sap concentration of leaves of Eureka lemon and Valencia orange occur when the leaves are exposed to direct sunlight. The sap of the lemon leaves on exposure to direct sunlight increases its concentration more rapidly than does that of the orange leaves. Increases in sap concentration due to action of sunlight on the leaves are due entirely to photosynthetic products, the ash of the sap remaining the same under both conditions. A greater % of the cross-sectional diam. of the lemon leaves is occupied by palisade tissue than with the orange leaves. This, together with the fact that more Mg is found in the sap of the lemon leaves than in that of the orange leaves, and the more rapid response of leafy lemon cuttings to vegetative propagation, lends support to the view that greater increase in lemon leaf sap concentration in direct sunlight over that of orange leaf sap may be due to a more efficient photo-synthetic system. The great variation in sap concentration of citrus leaves brought about by direct sunlight indicates that large errors may result through indiscriminate sampling.