EFFECTS OF DRUGS ON TLA.NTS 479 



differences of tonic condition. Thus, all other factors being 

 maintained constant, we may have three clearly defined 

 states, in the case, say, of Crinuvi Lily, according as it is 

 kept at a temperature of 30 C, 34 C, or 37 C. The first 

 of these we may regard as the normal ; the second as 

 near the optimum ; and the third as intermediate between 

 optimum and maximum. The excitability of the plant kept 

 at the optimum will be the greatest ; but though the excita- 

 bilities of those at 30 C. and 37 C. will be approximately 

 the same, yet, in the latter case, the plant will possess an 

 excess of latent energy which will be wanting in the former. 

 Having thus secured these definite artificial constitutions of 

 different values, some of the investigations given at the end 

 of this chapter will show how free from uncertainty the 

 action of drugs may be made, and how rational an expla- 

 nation can be given of the observed variations of effect. 



Methods of investigation. — I shall now proceed to 

 describe the general methods of experiment, in studying the 

 effects of drugs, from the modifications which they induce in 

 growth-response. There are two ways of doing this. Accord- 

 ing to the first, we take a record of the growth before and 

 after the application of the reagent. From the variation 

 then seen in the rate of growth the excitatory or depressing 

 nature of the drug may be ascertained. The second, or 

 Method of the Balanced Crescograph, is much more delicate ; 

 it exhibits each transient variation of response, and its time- 

 relations, with perfect clearness. The balanced horizontal 

 record, which is first taken, indicates the normal rate of 

 growth. A deviation upwards from this horizontal line will 

 indicate accelerated growth ; a return to the horizontal will 

 mean a regaining of the normal rate ; and a deviation down- 

 wards will show responsive retardation. The specimens used 

 for this investigation were Crinuni Lilies, and, unless stated to 

 the contrary, the experiments were carried out at the normal 

 temperature of 30 C. 



Action of carbonic acid gas. — I first give a record of 

 the effect of carbonic acid gas on growth (fig. 192), taken by 



