592 PLANT RESPONSE 



desired, sunlight may be reflected in the required direction 

 by a mirror. From whatever source, the Hght can be made 

 to strike the plant horizontally through the slit, which is 

 usually covered with a sliding shutter, S. By manipulating 

 a key, K, the shutter is raised. Thus exposure may be 

 made at any moment, and continued for the length of time 

 desired. 



That part of the record which is made on the revolving 

 drum before exposure begins, gives an indication of the 

 quiescent condition of the plant. The moment and duration 

 of exposure are found from corresponding marks made on 

 the recording surface, at the instants of opening and closing 

 the shutter. 



It will be seen that we have means of controlling the 

 intensity of illumination within wide limits by (i) the use of 

 different sources of light as enumerated above, and (2) varia- 

 tion of the distance of the source of light, at least when this 

 is artificial. 



Thus we are able to illuminate the plant from the right 

 or left flanks, or from both simultaneously, and by lights of 

 equal or of different intensities, at will. Again, by covering 

 the slit with a second plate, provided with suitable apertures, 

 we are enabled to subject any part of the plant, whether tip 

 or growing region, or both, as desired, to the stimulus of 

 light, and thus to determine the characteristic response of 

 each. All these considerations will show the facilities 

 afforded by this apparatus for carrying out a great number 

 of diverse experiments. I shall, however, content myself 

 here with the description of a few necessary examples. 



Heliotropic response of hypocotyl of Sinapis.— As 

 examples of radial organs, exhibiting the positive helio- 

 tropic effect, I took seedlings of Sinapis nigra. They were 

 attached to the lever, as indicated above, and at least half an 

 hour was permitted to elapse, in order to remove the last 

 possible trace of excitation due to contact. The attainment 

 of the quiescent condition was ascertained from the stationary 

 position of the spot of light. 



