350 LIFE MOVEMENTS IN PLANTS 



inferred from the curve obtained by means of appropriate 

 magnetometric or galvanometrie methods. I reproduce the 

 characteristic curve of iron (Fig. 131) in which the abscissa 

 represents increasing magnetising force, and the ordinate, 

 the induced magnetisation. This characteristic curve, giving 

 the relation of cause and effect, will be found to be 

 highly suggestive as regards the similar characteristic 

 curve which gives the relation between increasing stimulus 

 and the resulting enhanced tropic effect in vegetable 

 tissues. The parallelism will be found to be very striking. 



Inspection of figure 131 shows that, broadly speaking, 

 the curve of magnetisation may be divided into four parts. 

 In the first part, under feeble magnetic force, the slope of 

 the curve is very small ; later, in the second part, as the 

 force increases, the curve becomes very steep ; in the third 

 part the slope of the curve remains fairly constant ; 

 and finally in the fourth part, the curve rounds off and 

 the rate of ascent again becomes very small. The suscep- 

 tihility for induced magnetisation is thus very feeble at 

 the beginning ; under increasing force, in the second stage, 

 the susceptibility becomes greatly enhanced; in the third 

 stage, the susceptibility remains approximately constant ; 

 and in the fourth stage it becomes diminished. We shall 

 presently find that the susceptibility for excitation also 

 undergoes a similar variation at the four different stages 

 of stimulation. 



CHARACTERISTICS OF SIMPLE PHOTOTROPIC CURVE. 



I have shown (Fig. 130) the relation between the 

 stimulus and the resulting excitation, the latter being deter- 

 mined from the diminution of the rate of growth. Under 

 unilateral action of light, the excitatory contraction gives 

 rise to tropic curvature. We may thus obtain the charac- 

 teristic excitation curve, by making the plant organ record 



