104 Ewart and Rees .— Transpiration and the Ascent of 
frictional resistance of the ascending stream. In the case of the Oleander 
the maximal height reached in a saturated atmosphere (io cm.) was 
attained before the end of the day, and when once equilibrium has been 
reached, any further prolongation of the experiment simply lowers the 
apparent rate of flow. With experiments of equal and not too prolonged 
duration the liquid rises most rapidly in the saturated stem in dry air, less 
rapidly in the living saturated stem in saturated air, and still more slowly 
in the stem killed by mercuric chloride. 
Suggestive as these results are, they must only for the present be 
taken as indicating a possible mode of demonstrating the existence of 
a pumping action in the wood of transpiring trees. 
If such a pumping action exists we should expect to find the sap 
ascending in the stem of a tree after the leaves had been removed, especially 
if the tree was previously short of water. 
Accordingly the whole of the branches and leaves were removed from • 
a tree of Araucaria excelsa metres high. After attaching the trunk to 
scaffolding it was sawn across at the base under a jet of water, trimmed 
under water and lowered into a drum of eosin solution. After two months, 
no eosin having appeared at the top, it was sawn up and examined. 
The eosin had risen on one side in the second and fourth rings to 
a height of 134 cm. The wood was uniformly tinged only below 50 cm., 
but was moist and living at all points. The experiment was begun near 
the close of summer after a two months’ spell of dry weather, and yet the 
rate of ascent barely exceeded 2 cm. per day on the average for the whole 
time. This fact negatives the conclusions that might have been drawn 
from short lengths of stems, but leaves it still an open question as to 
whether or no a pumping action may exist but only act under the stimulus 
of demand, and be of such character as to allow a moderate suction of 
nearly equal intensity at all points to extend to the ground, each region 
in the path of the transpiring current maintaining its water in a labile 
condition so as to neither appreciably add to nor diminish the suction 
exerted by the leaves, which is transmitted by cohering columns of water 
as far as their tensile strength will allow and as far as they exist. 
Summary. 
The rate of evaporation per sq. metre of leaf-surface from cut branches, 
whether placed in water or not, is always less than from a plant rooted in 
the soil, under otherwise similar conditions. 
When the air is hot and dry the evaporation from a free surface of 
water undergoes an enormous increase, but that from a living plant under¬ 
goes a regulatory decrease, and may be only one-sixth as active as the 
former. Under optimal conditions a rooted plant of Eucalyptus corynocalyx 
