2 PLANT RESPONSE 



The few instances enumerated by no means exhaust the 

 activities that go on in the Hving machine of the plant ; they 

 only suffice to give us a glimpse into the complexity of its 

 functions. We can arrive at a comprehensive idea of such 

 multifarious and obscure phenomena only by coming to 

 understand the machine itself, and trying to disentangle the 

 processes by which the various stimuli supplied by the 

 environment bring about the appropriate responsive move- 

 ments in the organism. This is difficult to do, inasmuch as 

 the intricate internal machinery is hidden from our view. 



Indicator-diagrams.— Though the interior be thus con- 

 cealed, however, it is still not impossible,. by careful observation 

 of external actions, to gain some conception of the hidden 

 mechanism. Let us take, for example, the analogous case 

 of a steam-engine. That we may be able to infer at any 

 moment the efficiency of various hidden parts of the machine, 

 we attach, to the moving piston, a recording apparatus, and 

 from the diagram thus obtained we are able to judge of the 

 working efficiency of the engine. The upstroke is followed 

 by a downstroke, and a recording pen traces for us, on 

 moving paper, the responsive movements. But an irregu- 

 larity may suddenly take place in the curve. This is due 

 to some internal obstruction. On removal of the cause, the 

 amplitude of the record is restored, and the pulsating strokes 

 resume their normal frequency. 



In dealing with living machines also, we may use 

 similar contrivances, in order to gain some indication of 

 their efficiency ; and by means of indicator-diagrams, or 

 ' response-curves,' thus obtained, we are able to gather much 

 information as to the physiological perfection or imperfection 

 of the living machine. 



Pulse-records as indicators of physiological efficiency. 

 We shall first take as an example that responsive pulsation 

 with which we are most familiar, our own heart-beat. As 

 in the steam-engine the energy of heat brings about the 

 responsive movement of the piston, so in the heart, some 

 internal stimulus brings about responsive pulsations. A 



