CHAPTER II 



MECHANICAL RESPONSE TO STIMULUS 



Molecular derangement caused by stimulus — Expression in change of form, 

 contraction — Mechanical model — Myograph— Response by differential con- 

 traction in pulvinated plant-organs— Longitudinal response in plants — 

 Response of plant to all forms of stimulus — Plant chamber — Practicable forms 

 of graduated stimulus — Electro-thermic stimulator — Stimulation by condenser 

 discharge — Response-recorder — Advantage of counterpoise — Response of 

 Biophytum to thermal stimulation — Response to condenser discharge — Absolute 

 measurements of motile effect and of work performed — Effect of load — Definite 

 determination of threshold of response — Determination of variation of excita- 

 bility by measurement of minimally effective stimulus. 



FEW of the phenomena" of plant-life are so striking as the 

 conspicuous mechanical movements of certain plants, like 

 Mimosa, commonly known as ' sensitive ' in contradistinction 

 to * ordinary ' plants. These movements take place in 

 response to various forms of stimulation, such as is caused by 

 mechanical touch or application of heat. It will be shown, 

 however, in the course of the present book that this division 

 of plants into sensitive and ordinary is arbitrary, since all 

 plants are sensitive — that is to say, react to stimulus. The 

 plant, throughout its life, is constantly responding to stimuli, 

 external and internal. Some of its responses are manifested 

 in mechanical movements which are too striking to be over- 

 looked. Others, not so obvious, have passed hitherto un- 

 noticed. But in both these cases changes of form occur in 

 the tissue, in consequence of stimulation. In some instances, 

 owing to conditions which will be explained later, these 

 changes produce little visible effect. In others, the responsive 

 change of form is displayed in a striking manner, owing to 

 certain advantageous circumstances of structure, and to the 

 possession of a magnifying arrangement. 



