RHYTHMIC RESPONSE IN PLANT AND ANIMAL 355 



It is impossible to conceive that there could be movement 

 without an exciting cause. Automatism is said to be one of 

 the properties of protoplasm. It will be seen, however, from 

 the evidence which I have adduced in cases where experi- 

 mental investigation is possible, that, strictly speaking, there 

 is no such thing as automatism. Only under the action of a 

 stimulus can a living tissue give responsive indications. The 

 impact of an external stimulus may give rise to an imme- 

 diate expression, or it may partly or wholly be reserved in 

 latent form for subsequent manifestation. ' Inner stimuli ' are 

 simply external stimuli absorbed previously and held latent. 

 An animal or a plant is thus an accumulator which is constantly 

 storing up energy from external sources, and numerous mani- 

 festations of life — often periodic in their character — are but 

 responsive expressions of energy which has been derived 

 from external sources and held latent in the tissue. 



Summary 



The rhythmic tissue of Biophytum may be regarded as 

 equivalent to that of Desmodium in a state of standstill. 



Both alike, when at standstill, give a single response to a 

 single moderate stimulus, and multiple response to a strong 

 stimulus. 



Both, when the sum total of latent energy is above par, 

 give apparently ' automatic ' responses. 



The rhythmic tissues of both plants exhibit a long re- 

 fractory period. 



The automatically responding leaflet of Desmodium is 

 incapable of being tetanised. 



An artificial increase of internal hydrostatic pressure 

 renews pulsation in a Desmodium which was previously in a 

 state of standstill. 



The effect of rise of temperature on Desmodium is to 

 produce a shortening of period and decrease of amplitude of 

 oscillation. 



The automatic responses of Desmodium often exhibit 

 periodic groupings, of various degrees of complexity. 



