88 Ewart and Rees.—Transpiration and the Ascent of 
through a similar branch taken from an equally actively transpiring plant 
and subjected to its own head of water was also measured, and it will 
be seen that assuming the flow to be proportional to the head, it would 
require heads several times the length of the stem to produce the transpira- 
tory rate of flow. As a matter of fact, the head required in a transpiring 
plant to maintain the transpiration current increases more rapidly than the 
rate of flow as the velocity of flow increases. If the stems are previously 
specially prepared and fully saturated, the resistance is of course greatly 
decreased, but we are concerned not with theoretical conditions, but with 
the actual ones existing in living trees. 
A. Cut Branches. 
I. Eucalyptus macidata var. citriodora. 
Branch, cut end placed in vessel of eosin. Evaporation from vessel 
prevented by enveloping in rubber sheeting. 
In direct sunlight, temp, (shade) 24° C. 
Diameter of stem, 3*6 cm. 
Duration of exp., 2 hours. 
Loss by transp ., 99-386 grams (49-693 grams per hr.). 
No. of leaves, 7,600. 
Average area, 10-4 sq. cm. 
Total area of leaves, 79,040 sq. cm. (stomata on both surfaces). 
Total area of stomatal surface, 158,080 sq. cm. 
Total number of stomata, 1,011,712,000 (64 per sq. mm.). 
Loss per hour per sq. metre, 3-1 grams. 
Loss per hour per stoma, -000048 milligram. 
Rate of ascent, 128-5 cm - P er hour- 
Amount passed through similar branch 'under its own head of water — 
i2-5 grams per hour. 
Hence head required to maintain transpiratory rate of flow should be 
not less than seven to eight times the length of the stem. 
II. Nerimn Oleander. 
Stem set up as in I.; shade temp. 30° C. 
Duration of exp., 1 hour. 
No. of leaves, 131. 
Average area, 15-6 sq. cm. 
Total area of leaves, 2,013-6 sq. cm. (stomata on both surfaces). 
Total area of stomatal surface, 4,087-2 sq. cm. 
Loss by transpiration, 16 grams. 
Average no. of stomata, 318,801,601 (78 per sq. mm.). 1 
1 The fact that the stomata are sunk in pits below the surface makes exact counting difficult. 
