Transpiring Branches . 431 
the other end of the tube dipping into an open vessel con- 
taining mercury : as the water at the upper part of the tube 
was absorbed and transpired by the branch, the mercury rose 
in the tube from below. 
But it is clear that an experiment thus arranged is not 
well adapted to demonstrate and estimate the suction-force 
generated in connexion with transpiration : for it is obvious 
that the rise of the mercury in the tube, up to a height of 
thirty inches (760 mm.) or so, is not the result of a tensile 
stress exerted by the transpiring branch on the liquid in the 
tube, but is due to the atmospheric pressure acting upon the 
mercury in the open vessel. It is surprising how small were 
the results obtained by this method. Thus Hales observed, 
with an apple-branch two feet high, a rise of twelve inches of 
mercury in the tube : Meyen, with a shoot of Vicia Faba , 
seven inches in four hours : Unger, with a shoot of Eupatorium 
cannabinum , a rise of 1 10 mm. : von Sachs, with a small branch 
of Aesculus Hippocastamim , a rise of 20 mm. in nine hours ; 
and with a Cabbage-leaf, a rise of 30 mm. in twenty-four 
hours : von Hohnel, with a small branch of Syringa vulgaris, 
a rise of 300 mm. Boehm alone appears to have succeeded 
in observing a rise of the mercury in the tube above the 
height attributable to the atmospheric pressure : in his most 
recent (1893) account of his work, he mentions two experi- 
ments in which this occurred, both being made with branches 
of Thuja : in the one the mercury rose to a height of 864 mm., 
with the barometer at 751 mm. ; in the other it rose to 
906 mm., with the barometer at 745 mm. Hence, in the one 
case, the suction-force of the branch alone was (864-751) 
113 mm. ; in the other it was (906-745) 161 mm. 
This mode of experimentation has the disadvantage that 
the conditions under which the water is absorbed by the 
branch differ materially from those obtaining in nature, in 
that the atmospheric pressure intervenes ; whereas in the 
uninjured plant the atmospheric pressure does not directly 
affect the absorption and transmission of water by the wood 
which constitutes an air-tight system. It occurred to me that 
