102 VEGETABLE PHYSIOLOGY 



from the cells into the intercellular spaces is not at first 

 affected. 



The influence of the hygrometric condition of the air, 

 apart from changes of temperature, can be seen when a 

 plant which has been exposed to a dry atmosphere till its 

 leaves have become flaccid is transferred to one saturated 

 with moisture. After a short time the drooping leaves 

 again become turgid. This is not due to an absorption of 

 water in the form of vapour by the leaves, but to a diminished 

 loss by the checking of transpiration. The return of turgidity 

 is caused by the accumulation of the store drawn from the 

 earth by the roots. This can be shown by comparing the 

 behaviour of two plants treated in the way described, one 

 of which is allowed to remain rooted in soil, while the other 

 is taken up from the earth and exposed in that condition to 

 the saturated air. There is in the latter case no recovery of 

 turgescence. 



The temperature of the soil in which the roots of a plant 

 are embedded has also an influence upon the exhalation 

 of watery vapour, which increases as the soil is warmed 

 and diminishes as it becomes cooler. 



If the protoplasts of the cells of the turgid leaves of a 

 branch are stimulated by violently shaking it, the leaves 

 become flaccid. The protoplasm under the stimulus allows 

 more water to pass through it to the cell- walls, and hence 

 evaporation is promoted. The effect may be compared 

 with that which has already been mentioned as set up in 

 the cells of the cortex of the root by their over-distension 

 by the water which accumulates in them in consequence of 

 the continuous osmotic activity of the root-hairs. The 

 stimulus of this distension is responded to by the proto- 

 plasm by its becoming more permeable by the water of the 

 vacuoles of the cells. The response made by the protoplasts 

 of the leaves to the stimulus of shaking may help to explain 

 the flaccid condition observable in the foliage of certain 

 trees after the prevalence of a high wind. Besides this 

 effect upon the protoplasm, the continuous removal of the 



