136 Journal of Agricultural Research Voi.xxiv,no. 
effective use of water throughout the season. On the other hand, when we 
consider the low sap density of limber pine and its low water requirement 
in relation to either mass or leaf area, we obtain a suggestion that tran¬ 
spiration may be controlled by mechanical means rather than through the 
physical properties of the sap and that such control indicates a low state 
of development because it inevitably means the sacrifice of the absolute 
growth rate. Yellow and lodgepole pines, with relatively low sap densi¬ 
ties, appear not to exert the mechanical control over transpiration and 
are, as a result, perhaps more fastidious as to growing conditions than 
limber and bristlecone pines. 
While it seems important to have demonstrated that among the species 
of a|)proximately equal development from the forester’s standpoint, 
growth, photosynthetic activity, sap density, and the relative extrav¬ 
agance in wato use are thus interrelated through simple physical 
control, yet the really important question is whether high or low sap 
density exerts a control over the more absolute water loss. In considering 
this it seems unquestionably best to use the leaf-exposure basis, since 
the maximum area exposed to the sun must determine very largely the 
total amount of energy which might be available for the evaporating 
process. Without repeating the data which are given in Tables II and 
XIII (omitting the slow-growing specimen of limber pine), the relation¬ 
ship is shown in figure 4. It is to be noted that the transpiration rate of 
3[enow pine on this basis is relatively high, while bristlecone and limber 
pines are relatively low. In 1920, these relations are completely reversed, 
^e facts leave little doubt that high sap density does materially sup¬ 
press transpiration. 
RESULTS IN 1920 
I la view of the apparent relation between transpiration rate and internal 
Condition bf the tree, it is important to see whether the physical characters 
I hich we might ascribe to the several species are in any degree constant, 
et us first consider the transpiration material of 1920. 
I To obtain better data on the sap density of the trees whose transpira- 
non was observed during 1920, sample trees corresponding to those potted 
were treated at the beginning of the season, and the transpiration trees 
were themselves treated at the close of the primary test. 
the feesbly dug trees at the beginning of 1920 it was possible to 
grind and treat the whole plants in very much the same way as the tops 
w^e tr^t^ in December, 1917. Owing to the large number of lots 
involved, however, four sets of sample trees were merely dried, and it is 
necessary to deduce their approximate sap densities from other results 
for the same species. 
In the fall examination it was considered of first importance to deter- 
iniiie the dry weight of the trees, as a measure of growth, without the 
risk losing any material or whole results through aecidents. The trees 
Were^ therefore, first oven-dried. It was several months before oppor¬ 
tunity itself to bring out this dried material, grind it in a 
'Uiortarl make the extractions of sugars, and again dry the leached pulp, 
liiil this ^e no attempt was made to evaporate and weigh the sugars. 
^This liti^od is probably qp^ to the criticism that a longer period is 
'required to seeure the same degree of leaching of solutes that may be 
^pedted With green material, and also that the drying of the material 
probably coagulates and holds some collodial matter that would, from 
green mati^al, pa^ with the solutes. This tmy account in part for 
t^ telariveiyUow Oap denrities found at the fend of 1920. 
