TEANSPIKATION 107 



continuity of the fluid in the channels of the transpiration 

 current. From this point onward, instead of evaporation 

 sucking up water from the root, it gradually leads to a drying 

 of the leaf. A similar result is brought about by raising 

 the temperature of the transpiring branch to such a point 

 as will kill the protoplasm of the cells. As these die the 

 evaporation is unchecked at first, but gradually the water 

 is taken from their interior and no more is supplied. The 

 cells rapidly become flaccid, the leaves droop, and the 

 total quantity of vapour exhaled is materially lessened, the 

 intercellular passages soon becoming partially obstructed 

 by the collapse of the cells abutting upon them. The 

 experiment does not interfere with the continuity of the 

 water-stream, but as soon as the cells are made unable to 

 retain their turgidity by the interference with osmosis 

 which follows the death of the protoplasm, the evaporation 

 empties the cells and no more water enters them to replace 

 what has been lost. As we have seen in other cases, the 

 death of the protoplasm is followed by the escape of the 

 osmotic substances, which do not leave the cells during 

 their life. The mechanical effects which follow the collapse 

 of the tissue are the consequence of the assumption of a 

 flaccid condition, and they intensify the check to the escape 

 of watery vapour from the affected organ. 



The course of events in a normal leaf during active tran- 

 spiration appears to be, then, the setting up of a tension 

 in the parenchymatous cells of the leaf by evaporation 

 from their surfaces, which tends to cause them to collapse 

 and become flaccid. This tendency is opposed and over- 

 come by a greater force excited by the turgescence of those 

 cells whose osmotic properties exert a traction upon the 

 water in the conduits or wood-vessels. Water is thus 

 supplied through the inner walls of the evaporating cells as 

 quickly as it is lost by evaporation from the surfaces which 

 abut upon the intercellular passages. 



Dixon ascertained that the osmotic pressure in the 

 leaves of transpiring branches of the Laburnum amounted 



