128 
Journal of Agricultural Research vo1.xxiv.no. 
ABILITY OF THE SPECIES TO OBTAIN WATER 
A very considerable amount of light is thrown on the meaning of 
Table XI and our other discussions by considering the water require¬ 
ments of each species in 1920, with the considerably less abundant water 
supply, as a measure of the ability of each to extract water from the soil. 
Thus, in comparing Tables III and VIII it was particularly noted that 
bristlecone and yellow pines, which in 1917 were more generous users of 
water, in 1920 took very low positions in the scale. The relative changes 
are indicated in the following table, where the use in 1920 is expressed 
as a percentage of the use in 1917. Only the better growing limber pines 
are considered. Comparison is made on the basis of Tables III and VIII 
and Tables IV and IX. 
Species. 
Relative 
transpira¬ 
tion in 1920. 
Species. 
Relative 
transpira¬ 
tion in 1920. 
Engelmann spruce. 
Per cent. 
77. 2 
66. 2 
Limber pine. 
Per cent. 
48. 5 
33*0 
28.8 
Lodgepole pine. 
Yellow pine. 
Douglas fir. 
57-6 
Bristlecone pine. 
In some degree these observations are corroborated by the data in 
Table XII in which is shown the change in relative transpiration rates 
through a period in which the last pot listed for each species was given 
additional water. For the periods represented by September 28, 29, 
and 30, the water in each pot was at 70 gm. below standard, and except 
for the one pot of each species, the same amount was maintained on 
succeeding days. On the morning of September 30 the amounts in the 
special pots were increased 50 gm., and on October i they were brought 
up to standard. After this, they were not again watered until dried out 
to the original basis. If, then, the availability of the water, which was 
thus increased from about 0.461 to 0.594, as described under the heading 
'‘soil,’' has a bearing on the amount transpired, its effect should in all 
cases be apparent in the transpiration recorded October i and 2. In the 
table is shown what was the prevailing relative amount for the special 
tree, compared with others of the same species for the three days when 
water contents were the same. The relative amount on October 2 is 
then shown as a percentage increase. 
It is to be noted that after October 2 the relative rates of the specially 
watered trees steadily declined, and in some cases went below the pre¬ 
vious rates, until the day indicated by “ W,” when the first fresh water 
would take effect. This is plainly due to exhaustion of water close to 
the roots and is a commentary on the importance of transport within 
the soil. 
