112 LECTURE VII. 



deprived for some time of a supply of salts and are then placed 

 in a saline solution, they absorb it more rapidly than they do 

 distilled water ; and conversely, when they have been supplied 

 with salts, they absorb distilled water more rapidly than a 

 saline solution. The increased or diminished absorption of 

 water brought about in this way naturally affects the amount 

 of water transpired. 



The activity of transpiration is further affected by mechani- 

 cal disturbances. It is a matter of common observation that 

 leaves soon become flaccid when they are agitated by a strong 

 wind. The increased loss of water which the flaccidity indi- 

 cates may be attributed to some extent to the constant renewal 

 of the air in contact with the surface of the leaves, but this is 

 not the only effect of the wind. Baranetzky has found that 

 disturbances of short duration suffice to induce flaccidity, and 

 the leaves rapidly return to their normal condition when left 

 at rest. If the shaking be repeated at short intervals, the loss 

 of water gradually diminishes with each repetition, until finally 

 no further effect is produced. 



Baranetzky considers that the increase of transpiration is 

 a consequence of the rapid expulsion of saturated air through 

 the stomata at the time of shaking, and that the subsequent 

 diminution is due to the flaccid condition of the guard-cells of 

 the stomata which are partially closed. These conditions 

 doubtless contribute to produce the effects, but they do not 

 appear to afford a complete explanation. It is difficult to 

 imagine that the loss of water during the time of shaking is 

 so great as to induce flaccidity of all the mesophyll-cells of 

 the leaf. It seems more reasonable to suppose that the 

 shaking acts as a mechanical stimulus on the protoplasm of 

 these cells in such a way as to facilitate the escape of water 

 from them, and that the water thus set free is in part rapidly 

 transpired and in part reabsorbed by the cells when the leaf 

 is again at rest. From this point of view the flaccidity is the 

 result, not of increased transpiration, but of the shaking, and 

 the increased transpiration is due to the momentary presence 

 of free water outside the primordial utricles of the mesophyll- 

 cells, a condition which is very favourable to the process. 



