TRANSPIRATION. 



215 



(a) A fairly good simple potometer may be made as follows. Fit 

 a wide-mouthed bottle with a rubber stopper having two holes ; 

 through one hole fit tightly the stem of a cut shoot or the stalk of 

 a large leaf, and through the other hole one end of a twice bent 

 narrow glass tube the straight horizontal part of which should be 

 at least 20 cm. long. The other end of this tube dips into a dish 

 of water supported on two or more blocks of wood or other objects 

 that can be slipped from under it when desired. Fill the bottle to 

 the brim with boiled water, so that when the stopper (carrying the 

 plant and the glass tube) is forced into the neck enough water will 

 pass into the narrow tube to fill it if not, fill the tube with water 

 before forcing in the stopper, keeping the water from running out 

 by placing a finger on 

 the end. When about 

 to take a reading, re- 

 move the dish of water, 

 let a short air-bubble 

 enter the open end of 

 the tube, and time the 

 passage of the bubble 

 along the tube. 



The obvious disadvan- 

 tage of this makeshift 

 potometer is the neces- 

 sity for taking out the 

 plant -carrying stopper 

 at the beginning of each 

 experiment, in order to 

 pour more water into 

 the jar. 



(&) One of the best 

 orms of potometer is 

 that shown in Fig. 48. 

 In this instrument, be- 

 longing to Prof. Ganong's "Normal" set, and supplied by the 

 Bausch and Lomb Optical Company, the shoot chamber is made 

 small, so that the water may quickly take the temperature of its 

 surroundings and not vitiate the readings by volume changes ; the 

 record tube is horizontal, so as to prevent buoyant rise of the air- 

 bubble used as the index of movement of the water. The record 

 tube is calibrated as well as graduated in c.c., so that transpira- 

 tion may be determined absolutely as well as relatively ; and it has 

 a small lateral air-opening, readily closed by a sliding piece of 

 rubber tubing. The reservoir of water, which can be used either 

 to supply a reserve to the plant or to drive the air-bubble back to 

 the starting-point, is connected with the^ rest of the apparatus by 

 a stop-cock, and is made removable to permit the use of the poto- 

 meter with a supported bell-jar as shown in Fig. 49. 



To start an experiment, close the air-opening on the record tube, 



Fig. 48. Ganong's Potometer. 



