90 
Journal of Agricultural Research 
Vol. XXI, No. i 
accumulation of soluble materials in their tissues. The smaller water 
content of the slow-growing wood also contributes to a higher concen¬ 
tration in the sap. 
The increased concentration of soluble materials in the sap of both 
classes of trees as the season advanced is to be referred in part to the 
foregoing factors and in part to the increase in soluble organic products 
of photosynthetic activity. With the increase in leaf area and the 
advance of the season there should be an increase in the content of soluble 
carbohydrates in the cell sap. 
The water content of the soil has an important influence upon sap 
concentration of the plant (compare Hibbard and Harrington, 7). There 
are numerous instances to be seen in the table where the sap concentra¬ 
tion fell after the application of irrigation water; but it will be noticed 
that the drop in sap concentration was not evident until, in some cases, 
Fig. 4.—Graph showing regression of osmotic pressure oil growth increments of pruned apricot trees (1919). 
one or two weeks after the application of water. As the season advanced 
the amount of water in the soil gradually diminished, in spite of the 
irrigation, and there is little doubt that the gradually diminishing water 
supply is to some extent causally related to the gradually increasing 
concentration of the plant sap through the season. It is difficult to 
express the relationship by a correlation coefficient because the regres¬ 
sion between the two pairs of values is not linear. After a certain 
moisture content (say that of saturation) is reached, a further addition 
of water would have no effect on the sap concentration of the tree. It 
may not, however, be entirely’improper to attempt to express the rela¬ 
tionship as a coefficient of partial correlation. This figure will express 
the relationship between growth increments and sap concentration, 
assuming the soil moisture to have been constant. In the case of the 
heavily pruned trees, 
n P .w= - 0.525. 
