658 Report of a Discussion on 
effectiveness, and the entire transpiration current is reduced in cross 
section throughout its length by the absence of one of the elementary 
filaments. Such a view greatly exaggerates the effect of the presence 
of free gas here and there in the lumina. It should be remembered 
that the vessels and tracheides are not merely laid end to end with 
intercommunication confined to the conduit above and below, but 
that where a lumen is occupied by a bubble the path of the current 
from the conduit beneath, for example, is simply deviated into an 
adjoining conduit. Thus by the presence of an idle lumen, the 
transpiration current experiences an opposition which is evaded 
somewhat in the manner in which a river evades the opposition of 
a stone in its bed. Or, if the idle lumina are numerous, the retardation 
effect might be compared to that of sand opposing the percolation of 
water. The hydrostatic tension is transmitted virtually in all 
directions, the lumina communicating as they do through the closing 
membranes of the pits. 
The rarity of the occurrence of a liquid in tension (we do not 
know if hydrostatic tension has been detected anywhere else in 
nature) almost justifies the misconception which ascribes different laws 
to hydrostatic tension and hydrostatic pressure. Essentially the laws 
are the same, substituting for the idea of a pull, the idea of a push. 
I would like to say a word as to some views which appear to us to 
commend themselves regarding the nature of the root-action in with- 
drawing water from the soil. The tensile stress transmitted to the 
root has ultimately the effect, we may say, of drying up the root 
surface. We suggested this in our first paper. The dry surface has 
the same power of condensing water as dry oatmeal, paper, or flannel 
would have if buried in damp earth ; not necessarily by mechanical 
contact with damp particles, but by absorption of aqueous vapour. 
As a matter of fact the aqueous film which overlies solid bodies is so 
immobile that it is hardly conceivable that a root-hair in contact with 
a particle of silica, for example, could get more water than what 
immediately adjoined its point of contact. The quantity of water 
taken up by trees appears to be often so large, even from dry soils, 
that this older view of the phenomenon would we think be inadequate. 
Of course a great deal is taken up in the liquid form, and the 
necessary salts thus brought into the plant. Professor Askenasy 
seems to misconceive our ideas on this matter 1 . Taking this view of 
1 Beitrage, p. 5, 1896. 
