444 Vines. — Suction-force of Transpiring Branches . 
plausible it may be. For my own part, I am at a loss to 
explain on this theory the results which I have obtained as to 
the suction-force of leafless branches, whether living or dead : 
for whilst the transpiration of a leafy branch may theoretically 
suffice to set up a considerable suction-force of the nature of 
a simple tensile stress on the water in its wood, it is, I think, 
questionable if the transpiration of a leafless branch would 
suffice to set up a tensile stress corresponding to the suction- 
force observed. I am, in fact, inclined to conclude that the 
imbibition-force of the wood plays .a more important part 
in the conduction of the transpiration-current than is gener- 
ally admitted at present. It may be that von Sachs’ 
imbibition-theory does not afford a complete explanation of 
the mechanism of the transpiration-current ; but it seems to 
me that his theory includes some at least of the essential 
elements of a complete explanation. 
But whatever may be the ultimate physical nature of the 
suction-force, my observations show how considerable is the 
force which a relatively small shoot can exert ; and though 
there are no data available for a conclusion as to the mode 
of summation of the suction-forces of the various branches of 
a tree, yet it is intelligible how that they together constitute 
a force which suffices to raise water from the roots to the 
topmost twig. At the same time, I do not consider it pro- 
bable that the suction-force of branches such as I used, is ever 
so great in nature as I have found it to be. For in these 
experiments the branches continued to transpire for hours 
without absorbing any water ; whereas, in nature, the demand 
of the branches for water would doubtless be supplied by the 
roots long before the suction-force had attained anything 
like such dimensions. These experiments illustrate an 
extreme case. 
I am conscious that these observations are by no means 
complete — in fact they are merely preliminary. I will there- 
fore not at present attempt any further discussion, still less 
any more definite conclusion. 
