Apr. 14, 1923 
Physiological Requirements of Rocky Mountain Trees 119 
1. That for a given saturation deficit a wide variation is possible in the 
amount of transpiration induced. It is believed, however, that this is 
largely the result of insufficient data for computing both saturation 
pressure in the leaf and atmospheric vapor pressure. 
2. Air movement increases transpiration somewhat more than has been 
allowed by the computations, which, in fact, made no allowance for this 
factor except as it might influence the depression of the psychrometer 
wet bulb. 
3. The amount of sunlight is not the cause of much variation in the 
results. If anything, the use of black-bulb temperatures has made a 
little too much allowance for sunlight effects; or, in other words, the 
plants are not quite so strongly affected by light as is the black-bulb 
temperature. 
4. The transpiration rate (average) is quite proportionate to vapor 
deficits until the former approaches a daily amount equal to the total 
weight of the plants, when transpiration, apparently, does not quite 
keep up with saturation deficits. 
5. The transpiration at the end of the season is less than in the earlier 
months, days of like valuations being compared. The size of the plants, 
of course, increased during the season, but the increase in leaf area was 
almost wholly during the first month. It seems safe to say that old leaf 
tissues do not permit as much transpiration as young tissues. 
6. The physical control of transpiration and the lack of plant control 
is fairly evident, though by no means proved. 
TRANSPIRATION IN I920 
The striking differences between the species, indicated by the transpira¬ 
tion rates in 1917, both on the basis of growth accretion and relative 
leaf areas or leaf exposures, called, first, for a reasonable explanation of 
the temporary circumstances wffiich produced such results, and, secondly, 
for a repetition to determine whether similar relations of the species 
might hold with new and different material and whether either the 
absolute or relative water requirements might be different under more 
natural environmental conditions. 
The first need had been definitely pursued in the interim, and the 
second was fulfilled during the summer of 1920. In repeating the trans¬ 
piration tests, it was especially sought to have as large an assortment of 
plant material as could be adequately dealt with. 
MATBRIAI. STUDIED 
Twenty-three pots of plants were used, compared with the 12 in 1917. 
Of these one containing a large yellow pine failed early in the season. 
An additional pot without trees served to measure the possible direct 
water loss from the soil. In a number of cases, two or three specimens 
were handled as one in a single pot, in order to give a better average 
result. (See Plates 4 to 6.) 
The limber and bristlecone pines and a single large spruce specimen 
were from the same lots of stock which were drawn upon in 1917, having 
been in the Fremont Station nursery during the interval. The remaining 
trees were all younger stock grown in the same nursery, and mainly of 
fairly definite origin as to seed. Where possible a variety of seed sources 
was represented in the selections for ea^ species, in order to determine 
the possibilities of variation between geo^aphic forms of the same species. 
The data on the trees used are given in Table VI. 
