646 
Report of a Discussion 071 
of about twenty feet. The problem is a purely physical — as 
distinguished from a physiological — one. What are the physical 
forces active in this stick ? When we thoroughly understand the 
physics of a piece of dead stick, it will be easy to explain the 
physiology of the transpiration-current in a living tree — but not till 
then. 
The suction-force developed by pieces of stick is very variable, and 
I must admit that I have not yet succeeded so far as to be able to 
control or account for the variations which I have observed. In 
illustration I may cite the following experiments made with different 
lengths of the same piece of stick, the duration of the experiments 
being nearly equal 
Stick=36 in. long 
» =18 „ 
5 ) 9 >> 
A 1 
>> 42 >> 
max. suction-force = 16 J in. 
55 
— I 9 i 
5 5 
55 
= i6i 
55 
J) 
= 6i 
55 
duration =28 hrs. 
» — 2 7 », 
» — 26^ , , 
33 
I am quite unable to explain these results ; but no doubt they are 
susceptible of some simple physical explanation — the factor of 
resistance being taken into account — for which the data are not at 
present forthcoming. 
It is a matter of congratulation to botanists that the question of the 
transpiration-current is again attracting the attention of physicists. 
The firstfruits of this renewed interest are already in our hands in 
the form of the papers published by Messrs. Dixon and Joly; and we 
shall have the privilege, as I understand, of hearing more about their 
work from one of the authors this morning. It is not my intention to 
attempt a criticism of the theory of the transpiration-current which 
they propound : but I would venture to say this, that the observations 
of which I have given you some account lead me to associate myself 
with Mr. Darwin in the opinion that these authors do not seem to 
attach sufficient importance to the part played by the imbibition-force 
of the cell-walls. Messrs. Dixon and Joly assert, indeed, that in the 
transpiring leaf the operative force is given by the meniscuses ‘ formed 
in the membranous reseau of the evaporating cell-walls ’ ; but surely if 
the physical properties of the cell-wall are of so much importance in 
these superficial cells, they must be equally important in the case of 
more deeply placed cells ; for these too are receiving and giving up 
water like the superficial cells, though not by evaporation. Whilst 
