MECHANICAL RESPONSE TO STIMULUS 27 



excitability of the tissue be increased, the seven-volt con- 

 denser charge, before inadequate, will become adequate. 

 Conversely, if by the action of an external agent the excita- 

 bility of the tissue be depressed, the nine-volt charge, which 

 was formerly effective, will become now ineffective, I find, 

 for instance, that lowering of the temperature will, by in- 

 creasing molecular sluggishness, reduce excitability. Hence 

 a minimally effective stimulus becomes ineffective when 

 the tissue is cooled. Conversely, a rise of temperature 

 produces the reverse effect, namely, increase of excitability. 

 This was seen in a particular experiment with Biophytujii, 

 where the minimally effective stimulus necessary at 30° C. 

 was found to be reduced to two-thirds when the temperature 

 was raised to 35° C. 



Having thus obtained a reliable stimulus, whose value may 

 be measured with precision, and which is capable of being 

 repeated, and having also discovered an arrangement by which 

 the effect of a given stimulus is invariably exhibited by a 

 uniform response-record, we are now in a position to attack 

 various physiological problems, as regards the influence of 

 given external agencies on the conductivity and excitability 

 of the plant-tissue. 



Summary 



Longitudinal responses are given by radial organs. 



A differential response, causing lateral movement, is 

 given by an organ in which the excitability of one half is 

 different from that of the other ; and the movement takes 

 place in a direction perpendicular to the plane of separation 

 of the two halves. Such responses are characteristic of dorsi- 

 vcntral plant-organs. 



In the responses of the sensitive organs of plants we 

 notice : a short latent period ; a period during which the 

 excitatory movement attains its maximum ; and a period of 

 slow recovery. 



When the stimulus is applied at a distance, a preliminary 

 abnormal erectile twitch is occasionally observed, which is 



