698 PART IV. THE PHYSIOLOGY OF PLANTS. 



regulation of transpiration. As already mentioned (p. 156), the 

 stomata open and close, their opening and closing being dependent 

 upon variations in the turgidity of the guard-cells. When the 

 guard-cells are highly turgid, they curve so as to separate from 

 each other in the middle line, thus opening the stoma ; when they 

 are flaccid, their free surfaces are brought into contact, and the 

 stoma is closed. It was held for a long time that the opening and 

 closing of the stomata depended mainly on whether or not the plant 

 was exposed to light; that the stomata opened in bright light 

 and closed in darkness; and that the more active transpiration 

 in light than in darkness was attributable to the condition of the 

 stomata. But it is now known that the opening or closing of the 

 stomata is not thus directly dependent on light, but is a function 

 of transpiration as affected by the hygrometric condition of the 

 air, and by the supply of water in the plant : so that when the 

 transpiration is normal, as determined by a certain relation exist- 

 ing between the hygrometric condition of the air and the supply 

 of water to the transpiring leaf, the stomata are open ; but when 

 transpiration becomes excessive, by the air becoming drier, or by 

 a diminution in the supply of water to the leaf, the stomata close, 

 even before any trace of flagging is shown by the leaf. Thus the 

 stomata act as regulators of transpiration, and their opening or 

 closing depends partly on external and partly on internal condi- 

 :ions. 



The water lost by transpiration is supplied to the transpiring 

 organs from the roots. If the loss by transpiration is compensated 

 by the absorbent activity of the roots, the transpiring organs 

 remain fresh and turgid. But if, as is frequently the case on a 

 hot summer day, the loss of water by transpiration is greater than 

 the supply from the roots, the transpiring organs, more especially 

 the leaves, become flaccid and droop, and they are only restored 

 to the turgid condition in the evening when the temperature of 

 the air falls and the intensity of the light diminishes ; in a word, 

 when the external conditions become such as to lead to a diminu- 

 tion of the transpiration. 



There is, however, besides the flaccidity of the herbaceous 

 members of the plant, another means of observing the effect of 

 transpiration upon the amount of water contained in the tissues. 

 If the stem, or a branch, of an actively transpiring plant be cut 

 through under mercury or some other liquid, it will be observed 

 that the liquid will at once make its way for a considerable 



