1918] BAKKE—WILTING 113 
Summary 
1. The transpiring power of plants as determined by standard- 
ized hygrometric paper gives an accurate knowledge of the internal 
water relations of a plant. The exact wilting point as determined 
by this method occurs when there is a serious rupture in the water 
columns. 
2. During the daily march of foliar transpiring power obtained 
by making consecutive hourly readings for 24 hours, the maximum 
is attained at a time previous to the greatest evaporation. During 
the time of approximate maximum evaporation there is a marked 
fall in the foliar transpiring power index, followed shortly by a rise. 
The ratio between the maximum and the minimum is more or less 
definite, but not sufficiently so for the formation of any law. When 
the ratio is reduced to the point where it is in the neighborhood 
of unity, the plant is in a state of intense incipient drying. When 
the maximum value does not exceed the usual minimum, the plant 
is in a soil environment which is critical from the point of water 
supply, or almost at its wilting coefficient. It is then merely a 
question of time before the plant wilts. 
3. Evaporation plays an important part in the experiment 
upon transpiration. A high evaporation gives an increased tran- 
spiring power value, but during the process of wilting the index 
of foliar transpiring power comes to be independent of evaporation. 
4. During the process of the march of wilting an equilibrium 
point is reached where the indices of foliar transpiring power do 
not show much variation. It is suggested that the duration of the 
equilibrium gives a measure of the comparative drought resistance 
of different plants. Helianthus grown in 1915 during a rainy 
Season is different from Helianthus grown during 1916, when the 
‘S€ason was unusually dry. The equilibrium period of 1915 was 
much shorter than for 1916. 
5. There is a decided difference in the time at which permanent 
wilting occurs in old and young leaves. The older leaves will 
wilt long before the younger ones. The time interval varies 
according to age. 
6. Stomatal movements or changes are not important factors 
when the plant is in an intense state of wilting. 
