Variation of The Transpiring Power of Leaves as Related to The Wilting of Plants

1. The variation of the foliar transpiring power of plants was studied by means of standardized hygrometric paper in Coleus and soy bean plants. 2. The ratio of the index of the foliar transpiring power at a difinite time of the day, at which the highest value of day-time indices most often found, to the index at night, when the transpiring power is generally low, was named \u22day-night ratio.\u22 This ratio was used for judging the degree of the daily fluctuation of the transpiring power of plants. 3. This \u22day night ratio\u22 of the plants approaches in the wilting process nearer and to unity, as wilting progresses, that is, there an equilibrium phase, in which the foliar transpiring power shows little variation is attained. But after that time it tends in many cases to become less than unity, because the night index becomes higher than the day index. 4. During the process of wilting, the index of the transpiring power itself decreases progessively, until it reaches its minimal value at the critical point of wilting, corresponding to the time of permanent wilting. After that time, it begins again to wise more or less, and then the plant falls into the drying phase of low transpiration or evaporation. The same phenomenon can be seen in the uprooted plants. 5. The value of the critical index in question seems to be practically constant for a given plant species, regardless of plants rooted or uprooted. However, it may be probably more or less affected by the external factors, for instance by the nature of the soil. 6. The critical indices found in the three mesophytic plant species used appeared to be approximately alike. But, although the difference between them was rather slight, it was suggested that the critical index in question is characteristic for each kind of plants. 7. The soil moisture residue at the time of critical state of wilting seems not to be constant for a given soil, but affected more or less by the nature of plants. 8. The index of the foliar transpiring power of a plant and the soil moisture residue at the time of the critical state of wilting may be applied as the measure of the comparative xerophytism of plants, in both the lower value showing the greater drought resistance