368 COMPARATIVE ELECTRO-PHYSIOLOGY 



This instrument consists of (i) an arrangement by which 

 the specimen may be subjected rapidly to the action of 

 different excitatory or depressing agents ; (2) a potometric 

 tube for the measurement of changes of suctional activity, 

 under different external conditions ; and (3) a contrivance by 

 means of which the movements of the water-index, with 

 their time-relations, are recorded. The principal parts of the 

 apparatus are seen in fig. 218. V is the plant-vessel, in 

 which the specimen is mounted, with or without roots, by 

 means of a watertight india-rubber cork. R is the 

 reservoir, which may be filled with hot or cold water, or with 

 the required chemical solution. By appropriate manipula- 

 tion of stopcocks, by means of key K, the water in the plant- 

 vessel may be replaced quickly by any of these, and the 

 effect of the changed condition on the rate of water-move- 

 ment observed. 



A direct record of the rate of movement may be made on 

 the revolving drum, actuated by clockwork, as shown in the 

 figure. For this purpose, a pen, P, is fitted over the 

 potometric tube, by means of a brass collar, which has a 

 rectangular opening, kept always coincident with the water- 

 index. The collar, carrying the pen, is attached to a thread, 

 which passes round small pulleys. One end of this thread 

 carries a counterpoise, M, and the other is wound round a 

 wheel, W, which can be so manipulated as to make the pen 

 follow the movements of the water-column. When the 

 water- index is followed in the way described, a direct record 

 of the water-movement in the plant is obtained. A curve 

 is thus traced, the ordinate of which represents the quantity 

 of water sucked up, and the abscissa the time. The slope of 

 the curve thus gives the rate of movement. As long as 

 suction is uniform, this slope remains constant. If, however, 

 any exciting agent increases the rate of suction, there is an 

 immediate flexure in the curve, which thus becomes steeper. 

 A depressing agent lessens the slope, and when suction is 

 abolished, the record becomes horizontal. For the detection 

 of the slightest variation in the rate of suction, the Method of 



