124 
Journal of Agricultural Research 
Vol. XXIV, No. 2 
Water Requirements 
Comparing now the species, as was done for the data of 1917, we have 
them in 1920 aligned as in Table VIII. 
TabIvK VIII.— Comparison of different species as to water requirement 
Species. 
Transpira¬ 
tion per 
unit of dry- 
weight 
accretion. 
Probable 
error in 
average. 
Limber pine. 
Gm. 
2, 779 
730 
600 
571 
441 
436 
376 
29s 
Gm. 
850 
18 
160 
5 
6 
23 
49 
32 
Lodgepole pine. 
Douglas fir. 
Yellow pine. 
Engelmann spruce. 
Scotch pine. 
Bristlecone pine. 
Siberian larch... 
Resistance to Transpiration 
On the basis of leaf exposures we have a very different arrangement 
(Table IX). 
Table IX.— Comparison of species as to resistance to transpiration 
\ 
Species. 
\ 
Transpira¬ 
tion per 
square 
centimeter 
leaf expo¬ 
sure. 
Probable 
error in 
average. 
Lodgepole pine. 
Gm. 
15-27 
II. 12 
Gm. 
0.49 
. 21 
Engellmann spruce. 
Yellow pine.. 
10. 08 
.66 
Scotch pine. 
8-45 
8 . 00 
2. 91 
I. 69 
1.44 
I. 10 
Siberian larch. 
Limber pine. 
5 - 30 
<. 00 
Bristlecone pine. 
Douglas fir... 
0 * 
4-63 
•39 
EXPLANATION OE RESULTS 
On thorough consideration of the meaning of the results which have 
been given above for both 1917 and 1920 tests, we come to the conclusion 
that neither method of comparing the species is very satisfactory when 
the number of individuals involved is insufficient to cover all possible 
variations. In these tests considerable variation in growth rate is to 
be expected as the result of more or less incomplete recovery from trans¬ 
planting. The small spruces, for example, in 1920 showed no delay in 
starting new growth; the single large spruce came on satisfactorily after 
considerable delay; one large limber pine grew vigorously while the 
smaller one did not extend its terminal or branch buds over one-fourth 
