TEANSPIEATION 101 



already seen capable of doing so, the rate of transpiration 

 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 con- 

 tinuity 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. 



FIG. 70. ENDING OF A FIBBO-VASCULAB BUNDLE IN THE 

 PABENCHYMA OF A LEAF. 



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 



