1912] LIVINGSTON & BROWN—TRANSPIRATION 317 
applicable in all cases when leaves wilt and dry through the influ- 
ence of too great an intensity of transpiration or too low a maximum 
possible rate of water supply, these two causes being merely differ- 
ent aspects of the same condition, namely, that the ratio of supply 
to demand is less than unity. 
(2) OTHER POSSIBLE FACTORS IN THE PRODUCTION OF AN 
INTERNAL RESISTANCE TO WATER LOSS 
Logically, there are other possible causes for the observed fall 
in relative transpiration; it might be regarded as brought about 
by increased concentration either of the extracellular liquid (in and 
upon the cell walls), or of the cell sap within the cell vacuoles (as 
by photosynthesis), or by decreased permeability (perhaps some 
sort of hardening or coagulation) of the protoplasmic layer itself. 
The recent work of Frrrrmnc? shows that the osmotic pressures in 
the vacuoles of the foliage of desert plants in moist soil (these con-- 
ditions agree with those of LrvrncsTon’s plants) are usually isos- 
motic with a solution less concentrated than 2-molecular potassium 
nitrate. The fact that no wilting was apparent in the Arizona 
experiments upon which we are basing our work (which means that 
turgidity was maintained) indicates clearly that the concentration 
of the cell sap must have been higher than that of the extracellular 
solutions. Thus the maximum concentration of the evaporating 
solutions which can be postulated is surely no greater than that of 
a 2-molecular solution of potassium nitrate, and in order to be certain 
that our error is in the right direction, we may assume maximum 
concentrations isosmotic with a 3-molecular solution of this salt. 
The minimum concentration (as in the early morning) can never 
be zero, but we once more take this as an assumed limit far beyond 
the actual, and ask the question, if the foliar solutions vary 
from pure water to a concentration isosmotic with 3-molecular 
potassium nitrate, what may be the relative magnitude of the re- 
sulting retardation of evaporation? Other conditions being equal, 
evaporation is known to be proportional to the vapor tension of 
*Firrinc, Hans, Die Wasserversorgung rg ral osmotischen Druckverhiltnisse 
der Wiistenpflanzen. Zeitschr. Bot. 3:209-275. See also Livincston, B. E., 
The relation of the osmotic pressure of the cell = in iy to arid ioe Plant 
World 14:153-164. 1911. 
