The Electrical Effects of Light upon Green Leaves. 129' 



" The Electrical Effects of Light upon Green Leaves." (Prelimi- 

 nary Communication.) By AUGUSTUS D. WALLER, M.D., 

 F.RS. Beceived June 6, Eead June 14, 1900. 



In connection with an investigation of electrical effects of light 

 upon the retina,* I have examined vegetable protoplasm (green leaves) 

 with reference to electrical effects that might be expected to occur in 

 connection with the chemical changes produced by light. 



Under certain favourable conditions that I hope to determine 

 further, a true electrical response to light is obtained, consisting in 

 the establishment of a potential difference between illuminated and 

 non-illuminated half of a leaf, amounting to 0*02 volt. 



Among ordinary garden leaves, I have found to be well adapted to 

 demonstration those of young Iris plants, about 6 inches high, and of 

 " ten-week Stocks " in active growth. The former, tested by Sachs r 

 method, exhibited no evidence of starch activity in consequence of 

 insolation ; the latter in favourable instances exhibited marked deposit 

 limited to illuminated parts. Leaves of Tropaeolum, of Begonia, and 

 of Nicotiana have also proved to be suitable objects of study. 



Most of the following description refers to young Iris leaves in the 

 first half of the month of May. 



The method of observation is as follows : 



FIG. 1. Normal Response to Light. Iris leaf . Primary " negative 5 

 during illumination. 



effect 



A freshly cut but otherwise uninjured leaf is laid upon a glass plate 

 and connected with a recording galvanometer by means of two unpolaris- 

 ablef electrodes A and B. One half of the leaf is shaded by a piece 



* ' Proc. Roy. Soc.,' March 29 ; and ' Phil. Trans.,' B, 1900 (in the press). 



f Du Bois-Reymond's electrodes of the usual type, not D'Arsonval's, which 

 are rendered electromotive by light. Illumination of a chloride of silver electrode 

 by a " 16-C.P." lamp at 10 cro. distance gave a reaction amounting to between 

 0'002 and O'OOS volt. A zinc electrode under similar conditions gave a reaction 

 of about O'OOOl volt, in which, however. I did not attempt to distinguish separate 

 effects of light and heat. 



