212 COMPARATIVE ELECTRO-PHYSIOLOGY 



of the leaflet of Desmodium. The leaflet was in a quiescent 

 condition, but under the action of stimulus of light, it ex- 

 hibited multiple responses ; and these, owing to the increasing 

 absorption of energy, showed a staircase enhancement of 

 amplitude. On the cessation of light, the energy absorbed 

 maintained the pulsation for some time. 



It is thus the absorption of energy which is the cause of the 

 so-called autonomous movements. The energy, as already 

 stated, may be derived by the plant either directly from ex- 

 ternal sources ; or from the excess already accumulated and 

 held latent in the tissue, aided by incident external stimulus ; 

 or from an excess of latent energy previously accumulated. 



FIG. 141. Initiation of Multiple Response in Lateral Leaflet of 

 Desmodium originally at Standstill 



Light applied at x and continued till the end of the sixth response, as 

 shown by the thick line. The responses show a staircase increase 

 with increase of absorbed energy. Pulsations persist for a short time 

 even on the cessation of stimulus. 



It would be impossible to conceive of movement without 

 an exciting cause. Only under the action of stimulus 

 can a living tissue give responsive indications. An ex- 

 ternal stimulus may either give rise to an immediate 

 responsive expression, or be partly or wholly reserved in 

 latent form for subsequent manifestation. * Inner stimuli ' 

 are simply external stimuli previously absorbed and held 

 latent. A plant or animal is thus an accumulator which 

 is constantly storing up energy from external sources, and 

 numerous manifestations of life often periodic in their 

 character are but responsive expressions of energy which 

 has been derived from external sources and is held latent in 

 the tissue. 



