1. The water economy of plants was studied in water-cultured tobacco plants reported in a previous paper3) by means of abrupt changing of culture solution level. 2. Equations 2 and 3 were formulated quantitatively to elucidate the water economy of plants, and they could clarify the main effective factors as follows: a) The active root/leaf area ratio (the ratio of mg. fresh weight of root active in water absorption Cw to leaf area of plant in sq. cm. F), which well indicated the grading of a water level lowering, had the direct effect on the water economy of an entire plant, and remained almost constant under normal conditions of the plant growth. b) The transpiration rate kept a constant value of 5-6mg./sq.cm./hr./cm. Hg in the range of 4.5-.7 of the Cw/F ratio, but below 4.5 of Cw/F, the transpiration rate decreased linearly with the depression of the active root/leaf area ratio. c) The water amount absorbed through 1mg. of submerged fresh roots was assessed to be 1mg./hr. under the given conditions (25°, ca. 1cm. Hg of saturation deficit). d) The water amount in leaves was expressed on leaf area basis, and designated in terms of leaf water index (LWI, mg. H2O/sq. cm.). The index represents the water condition of leaves more clearly than the water content expressed on dry weight basis does. 2) By substituting the above factors in Equation 3, the leaf water deficit of the tobacco leaves accompanied with lowering the culture solution level was calculated. The trend of the calculated curve was fairly similar to the observed. This may prove that the obtained equations will be applicable to the general discussion of water enocomy of plants with variously developed root systems.