8 PLANT RESPONSE 



machinery may now be observed, as graphically indicated in 

 the waning and final arrest of the pulse-record (fig. 5). Or 

 the plant may be killed by passing through it excessively 

 strong electric shocks, after which the occurrence of death 

 will be indicated by the arrest of pulsation (fig. 6). 



Thus we see not only the similarity between the pulsations 

 of Desmodium and those of cardiac muscle, but also how 

 similarly both are affected by external agencies, such as poison. 

 Later, we shall study the effects of other physiological in- 



Fig. 5. Death of Plant, and Arrest of 

 Pulsation, by Poison 



Fig. 6. Death, and Arrest of 

 Pulsation, in Leaflet of Desmo- 

 dium by Strong Electric Shock 



fluences on both. In the present chapter, however, it has been 

 my aim to show that these pulse-records give us a reliable 

 indication of the very obscure modifications of the life-processes 

 initiated in the living tissues by various external factors. 

 Speaking generally, we may say that an exciting reagent 

 exalts the pulse, a depressing reagent reduces the amplitude 

 of pulsation, and a poison arrests it permanently, this arrest 

 being death. 



In the cases which we have chosen as examples, there is 

 the advantage of a store of latent energy, which maintains the 

 pulsation by providing an internal source of stimulus. This 

 internal stimulation, as will be shown later, is really derived 

 from external sources, the absorbed energy having been 

 held latent in the plant. We shall in the next chapter take 

 up a very much simpler case, in which the plant has no such 

 reserve, but responds immediately to external stimulus. 



