TKANSPIKATION 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 



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