354 PLANT RESPONSE 



hydrostatic pressure renews in them rhythmic activity. All 

 of them exhibit under certain circumstances similar cyclic 

 groupings. The effects of chemical reagents are similar on 

 both classes. Rise of temperature quickens the rhythm and 

 reduces the amplitude of pulsation in a manner exactly 

 similar in both ; and, finally, the effects of chemical reagents, 

 even in the matter of antagonistic actions, are alike in the two 

 classes. From a consideration of all these, it would appear 

 that in studying response in rhythmic animal and vegetable 

 tissues, we are dealing, not with two distinct but with a 

 single class of phenomena. We are thus justified in 

 ascribing the rhythmic action of the heart to those same 

 causes which we have found to originate and maintain the 

 rhythm of Desmodium or BiopJiytnvi. We have seen that 

 these plants absorb energy continuously from the various 

 forms of stimulus — mechanical, thermal, chemical, and other 

 — to which they are subjected. This absorbed energy 

 remains latent in the tissue, and determines its tonic con- 

 dition, which is simply the sum total of these latent stimu- 

 lating factors. When the sum of these factors exceeds a 

 certain value, it will find expression outwardly in the form of 

 excitatory discharges. This discharge, however, is not single 

 and continuous, but intermittent. After each partial excita- 

 tory discharge, there is a diminution of conductivity and 

 excitability which are only restored gradually. The long 

 refractory period is merely an expression of this peculiar 

 property. Owing to this periodic oscillation of conductivity 

 and excitability, the constant latent stimulus finds expression 

 in a rhythmic manner. Under favourable circumstances, 

 there is a large surplus of accumulated energy, and long- 

 continued responses, apparently automatic, are thus produced. 

 Under less favourable conditions, when the stored-up energy 

 is not great, a single stimulus gives rise to a single response, 

 or, when the stimulus is stronger, to a multiple series of 

 rhythmic responses ; and this statement is true of ti-ssues 

 exhibiting ' spontaneous movements,' not only in the case of 

 the plant, but also in that of cardiac muscle. 



