154 TRANSPIRATION. 



is an air space, whence the air can pass into other 

 cells of the plant. The stomata, therefore, are the 

 breathing pores by means of which water is exhaled 

 from the plant, and carbonic acid gas is inhaled. 

 When the guardian cells absorb water they swell, 

 and this makes them curve farther apart and 

 widens the slit, so that the water can evaporate 

 faster. When the plant begins to wilt, which is 

 caused by a collapsing of the cell-walls for want 

 of sufficient water, the guardian cells contract, and 

 this closes the slit and checks the evaporation of 

 water, so that most of the liquid brought from the 

 roots is retained within the plant. 1 If plenty of 

 water be again supplied to the plant which has 

 begun to wilt, it will recover itself, the cells again 

 become turgid, the stomata open, and transpira- 

 tion is resumed. 



Stomata open more widely in light than in 

 darkness. According to some observers, they are 

 always open in sunlight. This will explain the 

 reason why stomata are generally more numerous 

 on the under side of the leaf. Transpiration would 

 go on too rapidly if they were exposed to the 

 direct rays of the sun. Where the leaves are 



1 The mechanism is too complicated for further explanation here. 

 See Sachs, " Physiology of Plants," p. 249. 



