The Water Relations of the Pine and the Silver Tree. 
15 
both per twig and per unit of leaf area. The Silver Tree, on the other 
hand, possesses wood of fairly high water-conducting power, its conductivity 
being rather more than double that of the wood of the Pine, but twigs with 
their cut ends in water show a much lower rate of transpiration than do 
similar twigs of the Pine. 
When transpiration is followed in twigs, as they dry, it is found that 
the Pine twigs at first transpire more rapidly than the Silver Tree twigs. 
After a short time, however, the transpiration of the Pine twigs suddenly 
drops and becomes less than that of the Silver Tree twigs. 
Determinations of water content show that Silver Tree leaves have a 
greater water content expressed as a percentage of fresh weight than have 
Pine needles. 
Comparing the relation between transpiration and water transmission 
in the two cases, we find that the ratio transpiration to transmission is far 
higher in the Pine than in the Silver Tree. 
Great caution is necessary in the interpretation of these results owing 
to the absence of information as to how far experiments with isolated twigs 
afford a true idea of processes taking place in the intact tree For instance, 
the fact that isolated Pine twigs transpire more rapidly than do Leuca- 
dendron twigs does not enable us to draw any conclusions as to the rate of 
transpiration of twigs attached to living trees. In all the experiments 
comparatively short twigs were used, and these were saturated with water 
overnight. Consequently the resistance to the flow of water to the leaves 
during transpiration would be minimal, and far lower than when the supply 
must be drawn from the roots of the tree through the trunk and branches. 
The measurements of conductivity show that the resistance to the flow of 
water to the leaves is very much greater in the Pine than in the Silver 
Tree, and this in itself would tend to lower the rate of transpiration of 
Pine twigs while attached to the tree. 
One conclusion which may safely be drawn is that under the experimental 
conditions Pine needles exert a greater suction on the water in the stem 
than do Silver Tree leaves. In experiment No. 2, for example, the " standard 
transmission 11 of the twigs was — 
Leucadendron 3 9 c.c. Pine'2'5 c.c. 
Therefore to force water through the Pine twig at the same rate as through 
the Leucadendron twig would require a pressure or suction 156 times as 
great as that applied to the Leucadendron twig. The rate of transpiration 
of the Pine, however, was 2*58 times as great as that of the Leucadendron. 
Therefore the Pine leaves must have exerted a suction 2 - 58 x P56, ?'. e. 
about four times as great as the Leucadendron leaves. In experiment No. 3 
the Pine leaves must have exerted a suction twice as great as the Silver 
Tree leaves, and in Nos. 13 and 14 four and a half times as great. 
One is tempted to infer from this that the greater suction, which our 
