348 PLANT RESPONSE 



rhythmic action in cardiac muscle, by the consideration of 

 similar phenomena in plants, especially since it can be shown 

 that their similarity is manifested under numerous varying 

 conditions, and extends to fundamental characteristics. 



We have already seen that there is a similarity of funda- 

 mental characteristics between the response of cardiac muscle 

 and that of rhythmic vegetable tissue. 



Similarities of rhythmic tissues, animal and vege- 

 table. — We shall now, therefore, observe in detail those other 

 and more special similarities which are exhibited in a com- 

 mon modification of response under varying external condi- 

 tions, by Desmodium and cardiac muscle alike, and I shall first 

 describe the remarkable effect produced on both by internal 

 pressure. 



( i ) The effects of internal hydrostatic pressure. — It is found 

 that a heart which has come to a condition of standstill may 

 be set into rhythmic activity by filling the cavity of the heart 

 with liquid. Endo-cardiac pressure is thus found to act as a 

 stimulus. 



In Desmodium, when the season is favourable, the tissue 

 is in a turgid condition, and there is a considerable internal 

 hydrostatic pressure, which we have seen to be advantageous 

 to the maintenance of rhythm. This turgid condition 

 depends on the ascent of sap, which, as I shall show in 

 Chapter XXVIII., depends again on the rhythmic activity of 

 certain tissues. Thus, in the summer season, we have the 

 conditions most favourable for the maintenance of rhythmic 

 activity in Desmodium. But with the approach of winter 

 the vigour of the plant and its turgid condition undergo a 

 marked decline, and the autonomous movement of the leaflet 

 then comes to a stop. 



It appeared to me that this cessation of movement might 

 to a great extent be due to the diminution of internal 

 hydrostatic pressure, and I undertook experiments to see 

 whether the pulsatory movement could be renewed by an 

 increase of this pressure, just as increased endo-cardiac pres- 

 sure was found to renew the beating of the heart. The 



