Journal of Agricultural Research 
Vol. XXIV, No. * 
ii6 
of water must result in inefficiency in photosynthesis, and it might possibly 
be stated as an axiom that a weed is a plant which through protective 
adaptations is facultative for a variety of environmental conditions but 
which for the same reasons is incapable of making a standard rate of 
growth. 
The impression is, then, obtained that yellow pine, lodgepole, Douglas 
fir, and spruce are resistant to transpiration in almost the same degree as 
they are capable of making use of water for their development, and that 
none of them possess any special adaptations for preventing water losses 
which interfere with growth. The cause for the differences between the 
several species should, then, be sought in those internal conditions which 
may determine their photosynthetic capacities and the behaviors of 
their cell contents as solutions, as was done by Salmon and Fleming 
(19) in the study of the winter hardiness of grains. 
Before turning to that subject, however, which will be considered 
under the heading “Sap density,” the transpiration rates should be fur¬ 
ther analyzed. 
Periodic Transpiration 
In Table II the transpiration of the trees by months has been shown, 
with the amount for each tree expressed as a percentage of that for all 
the trees. From these data it may be observed that only a few of the 
trees maintained stable positions with respect to the whole. The greatest 
significance of this is to indicate that, if the growth could have been 
measured for shorter periods, the relative water requirements might not 
have been the same as those for the whole season. 
Comparing the transpiration at the beginning and end of the season 
(April to June against October to November), it is found that the species 
may be arranged in the following order, those which show the greatest 
relative increases being placed ffist: Yellow pine, limber pine, spruce, 
bristlecone, Douglas ffi, and lodgepole. Lodgepole and Douglas fir 
show actual decreases. 
If we should eliminate trees 3 and 10, which were apparently affected 
by some unknown factor, it would scarcely change the relations of the 
species. 
These data at least indicate that the internal conditions which control 
transpiration are variable and probably are affected by the building of 
new tissue, accumulation and distribution of carbohydrates, and other 
changes which may occur in a season’s growth. 
Response of Transpiration to Light and Air Movement 
The variations of each tree during the season, as shown above, almost 
preclude the possibility of determining the responses of the species to 
the ditferent environmental conditions which were produced from day to 
day, since such comparisons, to use the available data, must include 
days during all parts of the season. 
Taking as a standard for each species the days during the season when 
there was no ventilation in the greenhouse and when the total lecorded 
sunlight was in excess of 400 minutes per day, it is found, as shown by 
Table V. 
I. That for days having from almost none to 400 minutes of sunlight, 
with other conditions equal, all the species show about 60 per cent of 
the transpiration for a standard day. 
