657 
the Ascent of Water in Trees. 
In this table the letters prefixed indicate the identity of a branch 
used a second or third time. The branches B and b were, it will be 
seen, used in three succeeding experiments. 
As regards the condition of the leaves at the conclusion of the 
experiments, it is to be remarked that but little difference was to be 
detected in the degree of flagging exhibited by specimens which had 
transpired in air and in carbon dioxide ; on the whole the latter were 
the fresher. 
I do not think that any one considering these results can fail to be 
struck by the inference that the great differences in the rate of 
transpiration may in a great measure be due to a lowering of vital 
activity in the one case. But of course, pending further and more 
varied experiments on this complex matter, the question hardly admits 
of profitable discussion. 
The question has been raised as to what may happen in a tracheid 
containing air bubbles. I may say that our views on this subject 
are as follows : — 
There will be a critical size for the bubble corresponding to the 
prevailing tensile stress. If below this dimension, the surface-tension 
force of the bubble will be adequate to cope with the tensile stress 
of the sap and the vapour and air pressure within it. It will then 
retain its minute dimensions and no more interfere with the circulation 
of water than would an equally minute solid body. If, on the other 
hand, it is of a dimension incapable of withstanding the expanding 
forces enumerated, it will yield and expand so as to occupy all but 
the sharp corners of the conduit or deep depressions upon the walls. 
Except within the cell-wall, we think that, under these circumstances, 
there will be no motion of water past the bubble in the conduit under 
consideration. Increase of tension will not tend to further a free 
current past the bubble, rather it will cause this to expand still further 
and carry the gaseous space deeper into the surface of the cell-wall. 
In this connexion I may observe that the manner in which 
a hydrostatic tension is transmitted through the column of sap in the 
tree is sometimes a subject of some misconception, and many appear 
to regard our theory as postulating the existence of hanging thread- 
like filaments of water in tension, which must be supposed extending 
in continuous lengths from leaves to root in order to be effective. 
Hence, it is argued, the presence of a bubble anywhere in the path of 
one of these threads, by interrupting its continuity, destroys its 
