412 PLANT RESPONSE 



appears to be continuous. But when this rapidity is not 

 excessive, it should be possible, by employing sufficient 

 magnification and a suitably quick rate of movement of 

 the recording surface, to display its actual pulsatory cha- 

 racter. The sensitiveness of this mode of detection becomes 

 again very much increased if we employ the method of 

 balance, or compensation, which will be described presently. 

 We thus require a high magnification, and some means of 

 continuous record. 



A high magnification may be produced by microscopic 

 optical projection, but this labours under the great dis- 

 advantage that the specimen is subjected to the strong and 

 unilateral stimulus of light, by which its normal growth- 

 movements are greatly modified. The ordinary auxano- 

 metric method, again, cannot be employed, (i) because the 

 magnification produced is not sufficiently great ; and (2) 

 because the inertia of the wheel, and the unavoidable friction 

 of the apparatus, themselves combine to obliterate the quick 

 pulsations of the growth-response. 



The Crescograph. — All these difficulties were over- 

 come by the use of my Optical Lever, for making growth- 

 records. The lever is made extremely light, and the fulcrum- 

 rod rests on agate planes. When the tip of the growing 

 organ is attached by a thread to the short arm of the Lever, 

 the length of the latter being "5 cm., and when the record- 

 ing surface is at a distance of 2*5 metres, a magnification 

 of 1 ,000 times is obtained. This is in most cases more than 

 sufficient. But, when necessary, a magnification of 10,000 

 times can easily be secured. In order to avoid any dis- 

 turbance due to vibration in the room, the apparatus is 

 supported on a steady bracket, fixed on the wall. Using 

 these ordinary precautions, records are obtained with this 

 instrument which are absolutely free from external disturb- 

 ance. 



The Balanced Crescograph. — In records of growth 

 we obtain a sloping curve whose abscissa represents time ; 

 and ordinate, the elongation produced in the organ during 



