92 VEGETABLE PHYSIOLOGY 
gaseous pressure existing in the wood vessels. This con- 
tinues after transpiration ceases, and no doubt, like the 
evaporation itself, it is of assistance in maintaining the 
upward flow, acting as it does in the same direction as the 
turgid cortex, upon which it exerts a considerable suction. 
It continues until the entry of water from the root causes 
the pressure of the air in the vessels to be equal to the 
atmospheric pressure. This negative pressure is of con- 
siderable importance also in assisting the movements of 
gases in the plants. 
The exhalation of watery vapour from the surface of 
the cells is not a process of simple evaporation. As in the 
other phenomena which we have examined, the proto- 
plasm exercises a regulating influence upon the escape of 
watery vapour from the cell. If the amount given off from 
a measured area of leaf-surface is compared with the 
quantity evaporated from an equal area of free water, the 
latter is found to be much the greater. This area. is 
probably much less than the area of the cell-walls actually 
involved, which abut upon the intercellular spaces opening 
by the stomata included in the measured area. That this 
difference is due to the life of the leaf, and consequently to 
the protoplasm, is seen from the fact that a dead leaf gives off 
its water and dries up more rapidly than a surface of freely 
exposed water. The cuticle of the living leaf and its cell-walls 
are consequently not the causes of the differences observed. 
The ultimate exhalation of watery vapour, we have seen, 
is chiefly carried out through the stomata of the green 
parts, at any rate in those plants which possess them. 
Each stoma is situated above a somewhat conspicuous 
intercellular space, to which it forms an outlet. The 
stoma originates by the vertical division into two of one 
of the cells of the epidermis which is usually somewhat 
elaborately differentiated from the rest. The partition 
which is formed between the two daughter cells thickens 
slightly and splits so as to form an opening between them, 
which does not, however, extend the whole length of the 
