MECHANICAL RESPONSE TO STIMULUS 15 



discover after what period of rest this disappears. We have 

 also to record the exact time-relations of these phenomena. 

 And further we have to study the effects of various external 

 agencies in modifying the response. 



In order to carry out these investigations, it will be 

 necessary first to arrange for placing the plant under suitable 

 conditions for experiment. The next point is the devising of 

 facilities for applying a stimulus of known intensity, which 

 can be repeated, or increased by definite amounts, at will. 

 And, lastly, there must be some means of obtaining an exact 

 record of the response, from which the absolute movement 

 of the responding organ and its time-relations may be 

 deduced. 



Experimental plant chamber. — As regards the first of 

 these, it is advisable to have a special plant chamber within 

 which the specimen can be subjected to the necessary con- 

 ditions. This chamber may consist of a base-board and a 

 movable cover. The framework of the latter is of wood, 

 with glass panes. In order to give easy access to the plant 

 during experiment, one side of the cover has a hinged 

 window. The recording Optical Lever is placed inside the 

 chamber, and the glass cover protects the recorder from any 

 accidental disturbance caused by air-currents. 



In connection with this, it is also important to provide 

 arrangements for producing changes of temperature, and 

 maintaining the changed condition uniform, for the required 

 length of time. This is most satisfactorily accomplished by 

 means of a heating coil placed inside the chamber, the 

 temperature being regulated by suitable adjustment of the 

 electrical current, sent into the coil through proper electrodes. 

 Other necessary accessories consist of appliances for the 

 purpose of stimulation, and facilities by which a constant 

 current can be made to flow through the tissue, in experiments 

 on the effect of electric currents on the excitability of plants. 

 Details regarding these will be given later. The plant may 

 be maintained in favourable humid conditions by placing 

 wet blotting-paper inside the chamber (fig. 12). 



