130 Dr. A. D. Waller. 



of black paper. Leaf and electrodes are enclosed in a box, provided 

 vith a shuttered aperture, through which light can be directed. A 

 water trough in the path of light serves to cut out more or less heat. 

 A glass jar inverted over the leaf and electrodes forms a moist chamber 

 to delay drying. During illumination the galvanometer spot is 

 deflected so as to indicate current in the leaf itself from excited 

 part to protected part, i.e., if B is shaded, light falling upon A 

 arouses current in the leaf from A to B ; if A is shaded, light 

 falling upon B arouses current from B to A. 



1. The deflection begins and ends sharply with the beginning and 

 end of illumination. 



2. It is provoked slightly by diffuse daylight, more considerably by 

 an electrical arc light, and in greatest degree by bright sunlight. 



3. It is abolished by boiling the leaf and by the action of anaesthetics. 

 These are the main facts proving that the living leaf responds 



electrically to the stimulus of light. 



At this preliminary stage two points of doubt occur to mind and 

 require to be tested, viz., possible effects of heat and of surface 

 evaporation that necessarily accompany illumination. 



These effects are small in comparison with the true response, and of 

 opposite sign. Illumination of a dead leaf gives little or no effect, 

 and what little effect there is, is directed in the leaf towards the 

 illuminated half, where heating and evaporation are provoked. 



The true response to light varies with varying physiological states 

 of the leaf and of its parent plant. 



Not every leaf gives response, nor is the response of equal magni- 

 tude in different leaves to luminous stimulation (arc light) of con- 

 stant intensity and duration. 



The external condition by which the state of leaf is most obviously 

 governed is temperature. 



My first experiments were made upon Iris leaves taken almost at 

 random from young plants (old roots) about 6 inches high at the end 

 of March (temperature not noted, but presumably below 15). The 

 response to light was between 0*001 and 0'002 volt. 



The next set of experiments commenced on May 8th on young 

 leaves of similar plants. 



The responses then observed were 



Warm { 



8 0-005 



10 0-008,0-025 



11... 0-005 



Cold, 



10 i 12 ml 



13... nil 



