314 PLANT RESPONSE 



The energy which expresses itself in pulsatory movements 

 may be derived by the plant, either directly from immediate 

 external sources, or from the excess of such energy, already 

 accumulated and held latent in the tissue, aided by the 

 incidence of external stimulation, or from an excessive 

 accumulation of such latent energy alone. 



By ' tonic ' condition is meant the latent excitatory con- 

 dition of the plant, as determined by the sum total of the 

 stimulating factors which are, or have been, derived from its 

 environment. In other words, the tonic condition depends 

 on the internal energy of the plant. 



In rhythmic tissues, a constant stimulus, external or 

 internal, finds pulsatory expression in consequence of the 

 oscillatory variation of conductivity and excitability. 



The duration of rhythmic movements, in the absence of 

 any external exciting cause, depends on the amount of 

 energy previously absorbed and held latent in the plant. 

 The persistence of this after-effect, therefore, depends also on 

 the greater or less capacity of the tissue for storage of energy. 

 These rhythmic movements thus appear to be automatic, but 

 when the reserve is exhausted, the so-called automatic move- 

 ments come to a stop. Renewal of pulsatory movements can 

 then take place only on the supply of fresh energy from 

 without. 



