THE WATER -RELATION 



IOI 



well supplied with water, it is probable that transpiration has i ertain 

 advantages. In the first place, evaporation has a cooling effect which 

 may prevent harmful overheating of the leaf especially when exposed 

 to strong sunshine. Secondly, the flow of water up the plant that 

 is initiated by transpiration, probably accelerates the movement 

 of mineral salts from the roots to the upper part of the plant (see also 

 p. no). The fact that there is a subsidiary mechanism which tends 

 to provide for a current of water through the plant in the absence 

 of transpiration suggests that the maintenance of such a current 

 is of importance to the plant (see Root Pressure, p. 108). 



Fie. 69. 



Potometer of the Ganong pattern. When necessary the air-bubble (shown in 

 black) can be driven back to the right-hand end of the capillary tube by admitting 

 water from the funnel. 



The actual amount of water lost in transpiration may be consider- 

 able. For example it has been estimated that a Birch tree may lose 

 as much as 600 lbs. of water during a hot dry day. Measurements 

 of transpiration can conveniently be made on plants growing in pots. 

 The pot and soil must be carefully sealed up in metal shells roofed 

 over with sheet rubber so that evaporation can occur only from the 

 plant. The whole arrangement is weighed at intervals and the loss 

 in weight during a period represents the transpiration. An instrument 

 called the potometer is frequently used in experiments on transpiration, 

 though what is thus measured is actually the rate of absorption of 

 water by the cut shoot fixed into the apparatus (see Fig. 69). The 

 whole apparatus being initially rilled with water, absorption by the 



