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may do so, though the existing periodicity is so sensitive to the 

 action of external stimuli that it becomes very difficult to elim- 

 inate their influence. The transpiration of watery vapour in 

 plants is precisely analogous to the transpiration of vapour in 

 the respiration of animals. The physiological import or sig- 

 nificance of transpiration with regard to the general economy 

 of the whole plant may be briefly stated, as follows : — There is 

 very little doubt that transpiration exercises an important in- 

 fluence on the amount of absorption by the roots, although there 

 is no direct relation between these two functions. The loss of 

 water by transpiration from the plant must be replaced, or 

 else fading speedily ensues. Under normal circumstances it is 

 replaced by the water containing salts in solution, which is 

 absorbed by the roots from the soil being conveyed to the 

 leaves. The greater the loss by transpiration, the greater will 

 be, therefore, the amount of absorption, and, consequently, the 

 greater the amount of salts in solution taken into the plant, 

 and in this way the passage of an enormous quantity of water 

 through the plant assists and promotes the absorption of water 

 containing substances in solution from the soil by the roots. 



Transpiration, then, causes a varying flow of water bearing 

 certain nutritive substances; and as transpiration takes place in 

 the leaves which are situated at one extremity of the axis of 

 the plant, and the great organs of absorption, as we have 

 seen, are the roots, which are placed at the other end, the 

 general movement of water from the roots to the leaves through , 

 the stem is an upward one, and used to be spoken of as the rising 

 sap. It is determined by two principal causes: I, suction from 

 above, due to transpiration ; 2, pressure from below, due to the 

 hydrostatic pressure exercised by the parenchyma cells of the 

 root. The temperature of the soil varies less than that of the 

 air ; consequently the water taken up is at a more or less uni- 

 form temperature, and hence the rush of this water up the stem 

 produces the effect of equalizing the temperature of the plant. 

 Transpiration also lowers the temperature of the surface parts 

 of the plant in hot weather, just as, e. g., the evaporation of 

 spirit on the hand produces a sensation of cold, since the heat 



