TEANSPIRATION 97 



tion 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 

 air around the transpiring organs has, no doubt, a consider- 

 able influence upon the removal of the watery vapour from 

 their intercellular passages. 



The effect of alteration of the external conditions upon 

 transpiration may be investigated by means of Darwin's 

 potometer, which enables approximately accurate determina- 

 tions of its amount to be made from time to time. This 

 instrument is shown in fig. 68. It consists of a glass tube 

 with a side arm which is bent upwards so as to be parallel 

 with the tube itself. A capillary tube of about -2 mm. bore 

 is fastened by an indiarubber cork into the lower opening 

 of the tube so as just to project beyond the cork. A con- 

 venient length of the capillary tube is about 20 cm. Its 

 lower end dips into a small vessel of water, arranged so as 

 to be easily withdrawn from the tube. The upper orifice 

 of the potometer is closed by a tightly fitting cork, and the 

 plant whose transpiration is to be observed is fitted into 

 the side arm by means of an indiarubber band or tube 

 which embraces the glass arm and the end of the cut 



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