216 



TRANSPIRATION. 



fill the reservoir and the shoot-chamber with boiled-out water, cut 

 the shoot under water and fix it into the rubber stopper, sealing 

 with soft wax ; then push the stopper, with the lower end of the 

 shoot projecting, into the chamber neck ; open the air-hole to let 

 the record tube fill, close the stop-cock, and the apparatus is ready. 

 Transpiration at once draws upon the water, so that air enters the 

 air-hole ; the bent end of the tube is then placed in a vial of boiled 

 water, after an air-bubble has been admitted to act as an index. 

 When the' index bubble reaches the other end of the record tube, 

 open the stop-cock so as to drive the bubble back again. When no 

 observation is being made, close the air-hole and open the stop-cock, 

 when the plant will be supplied from the reservoir tube. 



277. Fotometer Experiments. The potometer affords 

 an extremely neat and effective method for demonstrating 

 and measuring the rate of the transpiration current in the 

 same plant under different external 

 conditions. In making compara- 

 tive readings we may either ob- 

 serve the distance travelled by the 

 index bubble in a given time, or 

 the time required for the bubble 

 to travel a given distance. 



(a) The influence of varying 

 external conditions may be deter- 

 mined by simply carrying the 

 potometer into different positions 

 and taking readings, for instance, 

 (1) in a warm position and in a 

 cool position; (2) in bright light, 

 in diffuse light, in shade, and in 

 darkness ; (3) under a bell- jar or 

 glass-sided box with air dried by 

 calcium chloride, or kept moist by 

 means of a dish of water ; (4) in 



, .,, . -, . , , J V i 



still air and in a draught. In each 

 case allow a few minutes under 

 the new conditions for adjustment to temperature, etc. 



(6) For more exact experiments the shoot should be passed 

 through a plate into a supported tubulated bell-jar, the glass 

 stopper of which is replaced by a rubber one carrying inlet and 

 outlet tubes (Fig. 49) ; a thermometer should be placed in the jar. 



Fig. 49. Ganong's Potometer, 



fitted into a supported Bell- 



jar. 



