364 PLANT RESPONSE 



the rate of movement. But these readings are necessarily 

 discontinuous, and important phases of change are thus apt 

 to be overlooked. They are, again, subject to error ; nor do 

 they give us, at sight, the rate of flow, nor the changes in that 

 rate, at any given moment. In order to determine such varia- 

 tions, a laborious process of construction of curves, from the 

 experimental data, must, generally speaking, be undertaken. 



It was therefore necessary to devise an apparatus by 

 means of which curves might be obtained direct, so that a 

 mere inspection would be sufficient to inform us as to the 

 normal rate of suction, and the influence of external factors 

 on that rate. This suction will be shown to be a physio- 

 logical phenomenon ; its variation, therefore, will enable us 

 to measure the physiological influence of various external 

 agencies. It must be borne in mind that under ordinary 

 conditions there is a normal rate of suction. The incidence 

 of an external stimulus will change this rate, and the variation 

 of rate which results is thus a measure of the effect produced 

 by the stimulus. Similarly, we measure a force by noting 

 the variation which it produces in the rate of movement of a 

 uniformly moving body. The variation of the rate of suction 

 may thus be taken as constituting a form of response to 

 stimulus, which for the sake of simplicity we shall designate 

 as Suctional Response. 



The Shoshungraph. — For the purpose of subjecting the 

 plant to various conditions, and also in order to obtain the 

 record of the resultant suctional response, I have constructed 

 an apparatus to which I have given the name of the 

 Shoshungraph} It consists of (1) an arrangement by which 

 the specimen may be rapidly subjected to the action of 

 different excitatory or depressing agents ; (2) a potometric 

 tube, by which the constant changes of suctional activity are 

 measured ; (3) a contrivance by means of which the move- 

 ments of the water-index, with their time-relations, are 

 recorded. The principal parts of this instrument are shown 

 diagrammatically in fig. 156. V is the plant-vessel, in which 

 1 From Sanskrit, Shoshun — suction. 



