TRANSPIRATION 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. 
Fic, 70.—ENDING oF A Frpro-vaAscuLAR BUNDLE IN THE 
PARENCHYMA OF A Lgar. 
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 
