The Electrical Effects of Light upon Green Leaves. 133 



Response before COo = 0-008 volt. 



during and after COo = nil. 



subsequently = 0'013 ,, 



during and after COo = nil. 



subsequently = O'OIO ,, 



The second experiment was made upon a rather "old" leaf on 

 May 21st, the test being applied at intervals of 40 minutes, and the 

 leaf chamber being at 25. 



1. Normal response = 0'004 to 0*005 



2. After chloroform = 0-003,0-002,0-005 



3. After more chloroform = 0'005, 0*008 



4. After carbon dioxide ... = 0-002, nil, O'OOl, 012, 0-005 



Upon other leaves (Mathiola, Tropaeolum) I have witnessed 



1. Augmentation of response in consequence of an air-supply con- 

 taining 1 to 3 per 100 of C0,>. 



2. Prompt abolition of response when a full stream of CO^ is run 

 through the leaf-chamber. 



3. Gradual abolition of response when the air-supply to the leaf- 

 chamber has been kept clear of CO^ ; followed by gradual recovery on 

 the readmission of a small amount of COo. 



The action of ether upon a leaf of Nicotiana Tabaeum was as 

 follows : 



Time Normal = -0-0016 



15 mins. ^Etherisation^ -0-0016 

 30 J L - 0-0004 



45 -0-0008 



60 -0-0008 



90 -0-0016 



150 -0-0020 



each period of illumination lasting for 2 minutes. 



Nature of the Normal Response (Iris Leaves). 



Direction. The accidental or " normal " leaf currents observed 

 when the electrodes are first applied to a leaf are of no significance, as 

 regards the response to light. Such " normal " current may be due to 

 accidental injury or to physiological inequality or to unequal imbibi- 

 tion of contacts, and necessarily includes the small amount of current 

 that may arise from the unpolarisable electrodes. It may be positive 

 negative, or non-existent. 



The regular and normal response to light is independent of such 

 VOL. LXVII. L 



