Apr X 4 , X9«3 
Physiological Requirements of Rocky Mountain Tfees 133 
The available facts, then, which have a bearing on the possible influence 
of sap concentration on the rates of evaporation from similar bodies are: 
I. At any given temperature the vapor pressure over a solution de¬ 
creases as the cdncentration increases, indicating that the solution does 
possess a stronger hold on the molecules than does the pure solvent and 
that therefore the sdute may at least decrease the rate of evaporation. 
There is nothing in the quantities involved, however, to indicate that 
this might be an important factor within the limits of cell-sap concen¬ 
trations. 
. 2. Calorimetric tests on the heat required for drying cones indicate 
an increase in the latent heat of vaporization as the concentration of solu¬ 
tions in the cone cells increases. Even admitting that the large number 
of tests puts the facts practically beyond question, there may be here a 
case of adsorbed water rather than a case of solutions, and with the 
molecular affinities and possible latent heats of the former we are not, 
just at present, concerned. 
3. Carefully conducted tests on free saline solutions indicate that the 
latent heat of vaporization is not appreciably affected by concentration 
or at least not more so than might be deduced from paragraph i. 
4. Observations on the heating and evaporation of solutions by low- 
temperature, exterior sources indicate an inability, increasing with con¬ 
centration, to absorb and transmit such heat in a manner conducive to 
evaporation. We shall not attempt to go into the theory of this. The 
important fact is that the heat of the air, and sunlight so far as it is 
absorbed by the exterior walls of the leaf or the interior cell walls, may 
be relatively ineffective in producing evaporation from a concentrated 
as compared with a dilute solution, while, apparently, such rays as were 
directly absorbed within the solution would be about equally effective 
in all cases. It goes almost without saying that, if such absorbed heat 
does not produce evaporation, it must increase the temperature of the 
leaf until a point is reached where absorption and radiation balance. 
In) view of these facts, when, at the close of the transpiration tests, it 
was i^covered that the several species showed such unaccountable and 
snrprisilng differences in transpiration rate, with respect to growth or 
tE^ss or leaf area, the first thought was that they must exhibit differ¬ 
ences which could be expressed in the qualities of the solutions from 
wMeh the evaporation of water takes place.® This thought was too 
hastily transformed into action by igniting the specimens which had 
served for the transpiration tests, in the expectation that the ash weights 
would be an index to the solutes in the plants and the densities of their 
cell solutions. This supposition was, of course, erroneous in taking no 
account of the soluble carbohydrates as well as some of the mineral 
oxids which would be lost in ignition and which comprise the greater 
mass of the solutes. As indicated in Table I, the ash percentages are 
irregularly variable and are found to bear no relation to transpiration 
rates. 
Having destroyed the best source of information on the physical quali¬ 
ties of the ori^nal specimens, the next step was to obtain specimens as 
hearly as possible like those used in the transpiration tests. This was 
by securing trees of the same classes as those taken from the nqrsery 
in the spring of 1917 which had spent the growing season in the nursery. 
4--___;________ 
■ * The miter wish^ to adcnowledee the very helpful suggestions of the article by Barrington Moore 
(jj), whidi was received in galley proof at such a time as to aid very materially in solving the current 
problem, and which reviews a number of the more recent researches on this problem. 
