100 VEGETABLE PHYSIOLOGY 
the force of transpiration was of considerable assistance in 
maintaining the upward flow of water from the roots. The 
apparatus shown in fig. 69 enables this to be demonstrated. 
The cut end of a branch is connected by an air-tight joint 
with a glass tube filled with water, the lower end of which 
dips into a vessel of mercury. As the water is transpired, 
a certain quantity of mercury enters the tube, and is drawn 
up for some considerable distance by the suction. 
The evaporation from the cells takes place, as we have 
seen, not immediately into the external air, but into the 
intercellular passages of the plant. The force causing this 
suction, so far as it is due to evaporation, is therefore localised 
in the surface film formed in the evaporating cell-walls. 
Such an evaporation has been shown by Strasburger to be 
capable of raising a current of water through pieces of dead- 
wood which have been soaked and injected with water. 
There is reason to believe, however, that a third factor 
in the ascent of the stream is interposed between the forces 
of root-pressure and the evaporation described. The water 
is passed from the wood-vessels or conduits to the evaporat- 
ing cells through a varying thickness of parenchyma (fig. 
70), which is kept turgid during active transpiration. The 
turgid condition of the cells is maintained by osmosis, just as 
is the similar condition in the roots. The vessels abutting 
on the parenchymatous cells are well supplied with water, 
which is in their cavities and which saturates their walls. 
The cells contain substances of an acid reaction, which 
possess a high osmotic equivalent. We cannot doubt that 
osmosis takes place through the walls of the cells, and that 
the turgidity of the tissue of the leaf is due to it as much 
as is that of the cortex of the axis. Researches carried 
out by Dixon show that this osmotic force plays a very 
important part in supplying the water to the evaporating 
surfaces. If the end of a cut branch is immersed, in 
any of the forms of apparatus described, in a solution of a 
salt which will plasmolyse these cells by detroying their 
turgescence, such as the sodium chloride which we have 
