1 86 



PLANT PHYSIOLOGY 



a draft on the water so that air enters the air-opening. One may then either 

 take the advancing end of this column as an index, or, better (since the fric- 

 tion of the water column in the record tube will then remain constant), may 

 use only a bubble some 2 mm. long, which is admitted before the bent end 

 is immersed in a vial of boiled water. When the index bubble has reached 

 the proximal end of the tube, a cautious opening of the stop-cock will cause 

 it to be driven back to the starting-point, and thus it is used over and over. 

 Whilst observation is suspended, it is only necessary to close the air-open- 

 ing and open the stop-cock, when the plant will be supplied from the reser- 

 voir. 



A form similar in general principle, but less compact, is among the 

 apparatus of the STOELTING Co. It is not difficult to adapt a closely similar 

 form from a large T tube, capillary tubing, and other simple materials as 

 illustrated in the accompanying figure (Fig. 49). 'The air-bubble is readily 



V, 



\ 



FlG. 49. POTOMETER ADAPTED FROM LABORATORY MATERIALS; Xi- 



Heavy black is sealing-wax; the vial is suspended by wire from the hook just above it. 



and accurately driven back by slightly twisting and pushing the solid glass 

 rod in the reservoir tube, to which is attached a supporting clamp. 



The potometer is quite untrustworthy as a measurer of the natural tran- 

 spiration of a plant, that is, of its transpiration upon its own roots; but it 

 is of much value for measuring, and especially for demonstrating, the rela- 

 tive rates of transpiration of the same plant under different external con- 

 ditions. For this purpose, however, it must be used with certain precau- 



