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 



