134 
Journal of Agricultural Research 
Vol. XXIV. Na t 
The specimens were collected on December 3, 1917, before the ground 
was frozen, and when there had as yet been no drying winds. From 
three to eight trees of each species were taken so as to secure a con¬ 
siderable mass of material. The tops were cut off at the root collar and 
those parts only were used. 
PROCEDURE IN DETERMINING SAP DENSITY 
The necessary data for determining the momentary density of the 
sap in a plant appeared to be— 
(1) The weight of the green material, determined as quickly as possible 
sitkr the material was collected. 
(2) The weight of the soluble matter leached out with an abundance of 
water and evaporated. 
(3) The weight of the insoluble pulp, oven-dried. 
By adding together (2) and (3) and deducting from (i), the original 
amoimt of wat^ (and other solvents) in the plant is obtained, and this, 
when divided into (2), gives the sap density, usually expressed as a 
percentage. 
In these original tests the plant material for each species was ground to 
a pulp, and these pulps were allowed to stand in cold water of about 
10 times the pulp volume, 3 waters being used for each. Finally the 
pulp was all accumulated on a filter and dried with the filter paper. The 
^uepus solutions w^e evaporated at temperatures not exceeding that 
of lx)i|(ngjwater. In all cases the so-called “sugars'* thus secured, after 
not wholly resoluble, indicating that colloidal matter 
haH heen included and had passed through the filters. This matter was 
a small proportion of the total solids, however, and may be assumed to 
have equally affected all samples. 
RESULTS 
The sap density percentages obtained for the nursery seedlings, in< the 
first tests made, were as shown in Table XIII, in which the water re¬ 
quirements are again given. Figure 3 shows that the sap densities and 
wat» requirements plot a curve which is remarkably perfect, considering 
the changes that have been noted in relative transpiration rates during 
the season, and the somewhat questionable value of the water require¬ 
ment for limber pine, which must be based on the performance of only 
one specimen. 
Table XIII .—Sap densities and water requirements in igiy 
Species. 
Sap density 
in material 
of Dec, 3, 
X917 (tops 
only). 
Water re¬ 
quirements 
of trees in 
transpira¬ 
tion tests. 
I^imber pine..... 
Per cent, 
19. 6 
21. 8 
22. 4 
23.2 
27.9 
29-5 
Gm, 
144 
1,555 
' tj iio 
684 
525 
Yellow pine........ 
pine...... 
Lddireoole oinei................ 
Dougl^ fir. 
T^gglmatiti spfUCe... 
