1 66 PLANT RESPONSE 



The circulaf brass stand on which the helix is mounted 

 has at the centre a small tube, in which the lower part of 

 the specimen is clamped. The plant-organ thus occupies the 

 vertical axis of the spiral, its upper end being connected by 

 a thread with the short arm of the Optic Lever. It may be 

 pointed out here, as is better explained in the diagram, that 

 in order to give room to the specimen, the axis of the Lever 

 is made to rest upon T-pieces, which are erected at the two 

 ends of the diameter of the helix. The plant-organ being 

 thus placed at the centre, the inclosing spiral thermometer 

 gives an accurate indication of the temperature to which it is 

 exposed. 



The circular stand, supporting both the specimen and the 

 recording apparatus, is placed in what I shall describe as the 

 inner thermal cylinder, within the circumference of which 

 the base fits exactly, while the helix is free, to the extent of 

 •25 cm. all round. This internal cylinder is made of copper, 

 coated with silver. It is filled with water and placed inside 

 an outer, or heating, cylinder of brass, which is also filled 

 with water. Heat is applied, by means of a spirit-lamp, to 

 the bottom of the outer cylinder ; thus the water in the inner 

 vessel is subjected to equal heat, on all sides at the same 

 time. Had the heat been applied directly to the inner 

 cylinder, convection-currents would have caused great dis- 

 turbance of the recording spot. With these precautions, 

 however, there is no trace of such disturbance. 



The whole apparatus is supported on a steady stand. 

 Below it is the spirit-lamp, which may be raised or lowered 

 till a distance is found which gives us the standard rate of 

 rise of temperature, that is to say, i° C. per minute and a 

 half. Above the apparatus and on a sliding holder is the 

 electric lamp, with focussing lens ; the light from this falls 

 veitically on the mirror of the Optic Lever, which is inclined 

 at an angle of 45 ° to the horizon. The horizontally reflected 

 light is then thrown on a screen, which carries either semi- 

 transparent recording paper or a photographic plate. In 

 the former case the observer, standing behind the screen, 



