624 SECTIONAL TRANSACTIONS.— K. 



Mr. J. C. Waller. — Towards an Interpretation of Photo-electric Currents in 

 Leaves. 

 Oxidation is generally regarded as an increase of positive charge, reduction as the 

 converse. 



From this point of view the fluctuations of alternating positive and negative 

 electrical potential which occur in leaves as a result of illumination may be regarded 

 as indicating a predominance of reduction or of oxidation respectively. 



In the following schema the photo-electric currents of the leaf fall into their 

 place as an incident or phase characteristic alike of photosynthetio and respiratory 

 processes. 



Oxidation, towards COj, Reduction towards carbo- 



accompanied by negative -<— (Aoid)^- hydrate, accompanied by 

 photo-electric current. positive photo-electric current. 



The subsidence of the fluctuations of potential during prolonged illumination i* 

 explicable by the establishment of a regime in which reduction and oxidation of 

 acid equalise each other. The subsidence of the after-effect of illumination may be 

 similarly explained. 



The following observed facts are in consonance with the theory : 



1. In absence of oxygen the negative current is rapidly abolished. 



2. A leaf tested in the morning (or after being kept in darkness for some time), 

 i.e. a leaf which may be regarded as charged with acid, shows a greatly augmented 

 positive current. 



3. The presence of atmospheric COj tends to act in the same manner as previous 

 keeping of the leaf in darkness, but the effect is less marked. 



Prof, H. H. Dixon, F.R.S., and Mr. T. A. Bennett Clark.— T^e Influence 

 of Temperature on Response to Electrical Stimulation. 



The evidence is discussed for the belief that a sudden change in the electrical 

 conductivity of a tissue is due to a proportionate change in the permeability of the 

 constituent protoplasts to ions. The passage of an alternating current of sufficient 

 voltage through a tissue causes a change in permeability which is the resultant of two 

 reactions which tend to raise and lower the permeability respectively ; positive 

 reactions exceed the negative when the duration is short ; when the duration is long 

 the negative are greater. The position of the neutral point where both reactions are 

 equal in magnitude affords a measure of the sensitivity. 



Results are given which show the enormous decrease in sensitivity during the 

 winter months ; during a period of rapid change the sensitivity may be halved in 

 one week. 



This change is independent of the effect of temperature on the sensitivity which 

 may be illustrated by one from many results : 



Stimuli of 200 volts applied for 0*15 sees, on January 1-7 gave the following 

 responses: at 0° C. + 14 percent. ; at 8"5°C. percent. ; at 20° C— 13 per cent. ; at 

 30° C— 48 per cent. ; at 40° C— 78 per cent. Similar stimuli for 0'05 sees, gave 

 responses at 0°C.-|-5 percent.; at 8'5° C.+ 6 per cent.; at 20° C.-|-7 per cent. ; at 

 30° C — 1 per cent.; at 40° C— 32 per cent. Raising the temperature from 0° C. 

 causes a given stimulus to evoke less negative responses, the maximum positivity 

 (of which the numerical value is determined also by the duration of stimulus) being 

 attained between 8° C. and 20° C. ; further rise in temperature causes the same 

 stimulus to evoke larger and larger negative responses. It is suggested that the 

 hyper-sensitive state obtaining above 45° C. is the primary cause of the breakdown 

 of the Bemi-permeabiUty at these temperatures. 



It is seen that the temperature has as great a controlling influence on the 

 magnitude of the responses as have the duration and intensity of the stimulus. 

 Stimuli at the same voltage, temperature, season and duration are found to evoke 

 the same response from Hedera leaves of high and low resistance ; the relation between 

 the effective energy and the expended energy is discussed. 



Afternoon. 

 Mr. C. T. Ingold. — The pH and Buffers of the Potato Tuber. 



The p'B. of the potato sap varies from pH 5-6 to pK 6-2. 



