180 PART III. PHYSIOLOGY. [ 43 



turgid, that is, when they are tensely filled with cell-sap, 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. The opening or 

 closing of the stomata 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 existing 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 conditions. 



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 diminution 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 

 distance into the woody tissue of the cut stem or branch. This is 

 due to the fact that, in consequence of the withdrawal of water 

 from them, the gases in the vessels are at a lower pressure than 

 that of the atmosphere. This is termed the negative pressure in 

 the vascular tissue. 



These various points can be readily observed in low-growing 

 plants, such as the Cabbage. On a hot summer day the leaves 

 become flaccid, and the existence of a negative pressure in the 

 vessels of the stem can be ascertained. In the evening, when the 



