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 How takes 



FIG. 70. ENDING OF A FIBRO-VASCULAR BUNDLE IN THE 

 PABENCHYMA OF A LEAF. 



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 as much as is that of the cortex of the axis. 

 Kesearches 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 



