UNMASKING OF THE A EFFECT 75 



of Musa was taken and two pieces of thin muslin were spread 

 over two areas, A and b (fig. 18, left-hand fig.). When 

 moistened with normal saline solution, the muslin becomes 

 transparent, and therefore does not obstruct the light. The 

 moist pieces of muslin were led to two non-polarisable 

 electrodes, a sensitive galvanometer being interposed in 

 the circuit. The electric response of A and b was obtained 

 by alternately throwing a beam of light on A or b. The 

 response of the leaf of Musa was found to be negative, the 

 D effect masking the A. I will presently describe means 

 for unmasking the A reaction. 



The Photo-Electric Cell 



The use of non-polarisable electrodes has, however, 

 certain drawbacks : (i) the electrodes offer a very great 

 electric resistance ; (2) the zinc sulphate solution in the 

 U-tube may leak into the plant-tissue and exert a poisonous 

 action. These difficulties I have been able to avoid by the 

 device of a photo-electric cell, dispensing with non-polar- 

 isable electrodes. The two halves of the leaf are suspended 

 in water in a glass vessel, the leaf having been divided along 

 the midrib, and each half is connected with the galvano- 

 meter by means of a gold wire inserted into the midrib. 

 The two halves correspond to the two metallic plates in a 

 galvanic cell, the conducting electrolyte being the water. 

 Stimulation of one half of the leaf by light gives rise to a re- 

 sponsive current flowing in one direction, while stimulation of 

 the other half gives rise to a current in the opposite direction. 

 The electric response is abolished on the death of the leaf. 



Unmasking of the A Effect on Sudden Stoppage 



of Light 



It was stated that the positive A effect under light is 

 often masked by the D effect. I have, however, succeeded 

 in unmasking A by the employment of three independent 

 methods, one of which is mechanical, the other two electrical. 



