106 VEGETABLE PHYSIOLOGY 
interposed between the forces of root-pressure and the 
evaporation described. The water is passed from the wood- 
vessels or conduits to the evaporating 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 sub- 
‘stances of an acid reaction, and exert a high osmotic 
pressure. We cannot doubt that an osmotic flow takes 
Fic. 70.—ENpDING oF A FrBro-vaScuLAR BUNDLE IN THE 
ParencuyMa oF A Lear¥, 
place from the vessels through their walls into the paren- 
chyma of the leaf, and that the turgidity of the tissue of the 
leaf is due to it ag 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 destroy- 
ing their turgescence, such as the sodium chloride which we 
have already seen capable of doing so, the rate of transpira- 
tion continues without much, if any, diminution till the 
salt can be detected in the leaves, when it suddenly falls off. 
This takes place though there is no interruption of the 
