INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 313 



minimally effective stimulus must remain latent in the tissue. 

 This is evidenced by the fact that on applying a very strong 

 stimulus we obtain, not a single but multiple responses. 

 Thus all the various forms of constant stimulus to which it is 

 exposed — warmth, light, moisture, and the different chemical 

 reagents, organic and inorganic, present in or absorbed by 

 it — become latent, and the sum total of all these stimulating 

 factors determines its ' tonic ' condition. 



When this accumulated latent energy exceeds a certain 

 value, it is visibly manifested in the form of the so-called 

 1 automatic ' movements. 



The periodicity of the excitatory discharges which give 

 rise to rhythmic movements in a plant that is under constant 

 latent stimulus, is brought about by the peculiarity which 

 has been demonstrated, that after each discharge the con- 

 ductivity and excitability of the tissue are diminished, and 

 are only gradually regained. This oscillation in the conduc- 

 tivity regulates the outflow of energy, and causes the 

 rhythmicity of the responsive movements. 



Summary 



In Desmodium, the seat of excitation of the lateral leaflets 

 lies in the motile organ itself. 



Multiple response is produced in the plant, as in the ani- 

 mal, under constant thermal, chemical, or electrical stimulus. 



The retina, under constant stimulus of light, exhibits 

 periodic visual pulsations. 



Similarly, the leaflet of Biophytum, and that of Desmodium 

 at standstill, under continuous stimulation of light, exhibit 

 rhythmic mechanical pulsations. 



Biophytum, when raised to a temperature of about 29 C, 

 becomes automatically responding. Under these circum- 

 stances, the only difference between the so-called automatism 

 of Biophytum and Desmodium is that, in the latter case, the 

 critical thermo-tonic condition is arrived at about 12 C. 

 earlier. 



