TRANSPIRATION 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 ig 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 hag 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 
