102 VEGETABLE PHYSIOLOGY 
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 
transpiration 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 
to between six and eight atmospheres, a force which is 
capable of raising a column of water to a height of more 
than 200 feet. 
Careful consideration of the facts recorded in this 
chapter shows us that although we cannot fully explain the 
ascent of the transpiration current, we can see that it 
ultimately depends upon the behaviour of the protoplasm. 
All the factors which aid its progress, root-pressure, tran- 
spiration, osmosis in the cells of the leaves, are largely 
under the control of the living substance, and are particu- 
larly influenced by the power it possesses of allowing more 
or less water to pass through it, according to its condition. 
Moreover all the external influences which we have ex- 
amined, which are brought to bear upon these factors, are 
mainly efficient in as far as they affect the protoplasm in 
the exercise of this power. 
