The Water Relations of the Pine and the Silver Tree. 
7 
of the aspirator. A long tube descended from the base of the aspirator, 
and the twig was attached to the lower end of this tube. In this way a 
head of water of 186 cm. was obtained. This is equivalent to a pressure 
of 13*68 cm. of mercury. 
The amount of water passing through the twig in 15 minutes under this 
pressure was measured. The values thus obtained are referred to as the 
" standard transmission " (i. e. the amount of water transmitted under the 
standard conditions) of the twigs. In the early experiments four readings 
were taken for each twig, and it was found that in nearly every case the 
amount of water transmitted became less with each successive quarter of 
an-hour, e. g. : 
Pine, first quarter hour, 5 - 6 c.c. . Silver Tree, 1 6 c.c. 
That this falling-off was due to clogging at the cut surface seems clear 
from the facts that it was not nearly so marked in the Leucadendron as in 
the Pine, and that it did not occur when the surfaces were shaved after each 
reading. The first reading was therefore taken as most nearly representing 
the true water-conducting power of the wood. In later experiments only one 
or two readings were taken, and the first of these was used. 
Having measured the standard transmission of the twig it was bisected, 
and a cross-section cut from the centre. By means of a projection apparatus, 
fitted up at the suggestion of Prof. Thoday from materials available in the 
laboratory, a drawing of this section was made on squared paper, and the 
area of wood in the section was measured. 
All the data were then available for calculating the specific conductivity 
of the wood. 
b. Measurement of transpiration. — The twigs for these experiments were 
cut from the trees in the evening and kept overnight with their cut ends in 
water. They were then placed in small test-tubes of water, and the loss of 
weight per half hour measured. In some cases the twigs were kept in the 
laboratory throughout the experiment, in others they were exposed on the 
roof of the laboratory on both dull and bright days. 
c. Measurement of leaf area.- — In the case of Leucadendron the method 
was comparatively simple. The leaves were picked from the twig and 
classified according to their size into about six groups. A representative 
leaf was then chosen from each group, and placed under glass on a piece of 
photographic paper. The leaf shape thus obtained was cut out and weighed, 
and from this weight and the area and weight of the whole piece the area of 
the leaf print was calculated. Knowing this, and the number of leaves in 
each group, the leaf area was calculated for each group and then for the 
whole twig. 
second 
third 
fourth 
1-45 „ 
1-35 „ 
1-30 „ 
