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. Eesearches 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 



