Transpiring Branches. 443 
c. Same bare shoot, injected with CuS 0 4 (14 hours) : — 
July 20, 11 a.m.-July 21, 9 a.m. — Max. tension = 4^ in. 
Fagns (Shoot F). 
a. Bare branch, after injection with water (21 hours) 
July 24, 9.30 a.m.-6 p.m. — Max. tension = in. 
b. Same branch, after injection with CuS 0 4 (13 hours) : — 
July 25, 9.15 a.m. -7. 30 p.m. — Max. tension = 8 in. 
It may be explained that the reason for injecting with water in several of the 
above observations was to remove any possible after-effect from previous transpira- 
tion ; and, further, to render the results of these observations strictly comparable 
with those obtained with branches injected with solution of copper sulphate. 
The question as to the influence of external conditions 
(heat, light, &c.) on the suction-force of branches is one to 
which I have been unable as yet to give much attention. 
I have, however, observed in many experiments that the 
tension increases more rapidly when a branch is directly 
exposed to the sun, but I have not specially investigated the 
point ; I hope to return to it on a future occasion. 
In conclusion, I may perhaps venture upon some sugges- 
tions as to the bearing of my observations upon the theory of 
the transpiration-current in entire plants. Whilst it must 
be admitted that the ultimate motive power is the transpira- 
tion of the living or the evaporation from a dead branch, it is 
by no means clear what the intermediate or subsidiary forms 
of force may be. Both Askenasy and Dixon attach import- 
ance to osmotic processes taking place in the tissues intervening 
between the conducting wood and the evaporating surface : 
but it would appear that these processes cannot be of primary 
importance, since, as my observations prove, a high suction- 
force can be generated by a branch which is saturated with 
solution of sulphate of copper. Facts such as this point to 
the importance of the imbibition-force of the cell- walls — to 
which Askenasy draws attention — rather than to that of the 
osmotic force of their contents. Then, again, Dixon and Joly 
— and Askenasy essentially agrees with them — state that the 
suction-force conveys water through the wood ‘ by exerting 
a simple tensile stress on the liquid in the conduits,’ a state- 
ment which they cannot be said to have proved, however 
