478 PLANT RESPONSE 



in others fail to do so, the plant being ultimately able to 

 shake off the depressing influence after an interval of 

 struggle. In studying suctional response, again, we found 

 that copper sulphate applied at the roots induced, in some 

 cases, an immediate arrest of suction, while in other instances 

 this arrest did not take place till after a long time. The 

 depression of suction which was induced by the application 

 of strong sodium chloride, again, was in some cases imme- 

 diate, and in others preceded by a fairly long period of an 

 exalted rate of suction (pp. 384, 385). All these variations 

 of results we regarded as due to individual differences of 

 constitution, or of tonic condition. 



Thus we can only hope to arrive at a complete knowledge 

 regarding the action of any given drug if we first obtain a 

 precise understanding of what is meant by constitution, and 

 of how, for experimental purposes, specimens of a definite 

 characteristic constitution can be secured, while others can be 

 subjected to an ascertained variation in a pre-determined 

 manner. It will thus be made possible to study the effect of 

 a drug, by applying it (1) in solutions of different strengths 

 to a number of plants of identical constitution ; and (2) in a 

 single strength of solution to specimens of definitely varying 

 constitutions. 



' Constitution ' and the elements which determine it. — 

 The first factor in determining constitution will consist of those 

 properties which have been impressed upon the plant by its 

 heredity. The second will depend upon its environment. 

 The sum total of the energy absorbed by the plant from its 

 surroundings we have already designated as the tonic con- 

 dition. It is clear that we may secure the factor of a constant 

 heredity by taking either seedlings from the same batch of 

 seeds, or organs from the same plant. These, again, when 

 maintained under the same environmental conditions, in 

 respect of temperature and other circumstances, will give us 

 plants having practically the same constitution. In order, 

 next, to obtain specimens of different but well-ascertained 

 constitutions, it is only necessary to keep these under known 



