MECHANICAL RESrONSE TO STIMULUS 25 



The next figure (fig. 18) gives successive responses of 

 Biophyiuni to condenser discharge, the pulvinus of the leaflet 

 being directly excited. It will be noticed that in this case 

 of direct stimulation, no abnormal erectile twitch is present. 

 From the magnification of the record the absolute value of 

 the movement is known, and in the present case it was r88 

 mm. The force exerted by the leaflet during its responsive 

 movement was found equivalent to that exerted by the 

 weight of 17 milligrammes. The total work performed by 

 the leaflet during each responsive movement is therefore 

 nearly equal to 1,600 millimetre-milligrams. 



Effect of load. — In order to observe the effect of load 

 on the response-curve, I added a slight additional counter- 

 poise to the other arm of the 



lever. The record (fig. 19, a) 

 shows the response-curve when 

 the acting load is the weight of 

 the lever ; (^) shows the effect 

 of the additional load. It will 

 be seen that while the height of 

 the responses was diminished, yet 

 the period of recovery was very 

 much reduced, from five minutes to ^^^- i9-. Efteci of Load 



(a) without, and {b) with, 



less than three, under the increased additional load 



load. 



Isometric record. — The method of observing response 

 employed in the foregoing results was that of recording the 

 movement of the leaf. Similar methods are known in Animal 

 Physiology as isotonic. There is, however, an interesting 

 method corresponding to that known in Animal Physiology 

 as the isometric — where, in obtaining records of responses, 

 actual movement is almost abolished. The contraction of 

 the more excitable half of the pulvinus exerts a certain 

 tension, or pull. The object, under the isometric method of 

 experiment, is to obtain records of varying responsive ten- 

 sions of excited tissues, the physical movement being at the 

 same time restrained. This I have been able to accomplish, 



