TRANSPIRATION 97 
Consequently, while transpiration is active, there is a nega- 
tive gaseous pressure existing in the wood vessels. This 
continues 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 
pressure in the turgid cortex, upon which it exerts a con- 
siderable 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 considerable 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 
certainly 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 dries 
up rapidly, giving off its water more quickly 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 
7 
