SECTIONAL TRANSACTIONS.— K. 625 



The buffering of the sap is expressed by the buffer index, which is the number of 

 gram molecules of acid or alkali that must be added to a litre to give a shift of pH of 

 unity. 



The buffering increases markedly on the acid side of the pH of the expressed sap. 

 Thus in a particular case the bufFer index from ^H 4 to ^H 5 was 0'026 ; from pTL 5 

 to pB. G, 0-010, and from pK 6 to pK 7, 0-008. 



The buffer action in the potato sap is the result of several buffer systems. Those 

 so far isolated are : — 



1. Inorganic phosphates — active above pH 5-6. 



2. Citrate — -active below pH. 7 and increasingly effective up to pH. 4—5. 



3. Ether — soluble acids. 



The relative importance of these buffers in potato sap is roughly as follows : 



Range pH 6 to pH 7. Phosphates account for 30-40 per cent, of the buffer index ; 

 citrates for 20 per cent. 



Range pH 5 to pH 6. Phosphates account for 10 per cent. ; citrates for 40 per 

 cent. ; and ether soluble acids for 50 per cent. 



Range pK 4 to pH 5. Phosphates account for 1 per cent. ; citrates for 20 per cent. ; 

 and ether soluble acids for 50 per cent. 



The protein and asparagin in the sap have a negligible effect on the buffering. 



The j)H of the sap is not greatly affected by being in equilibrium with high con- 

 centrations of CO-2 such as may occur (20 per cent.) in the intercellular spaces of the 

 tuber, but this percentage does definitely alter the pH to the extent of 0-3 to 0-4. 



Miss M. T. Mabtin and Miss M. A. "Westbrook.— TAe Reaction of the 

 Epidennis of Pulmonaria Leaves to Ultra-violet Light. 



In the course of experiments on the effects of ultra-violet radiation on plants, it 

 was found that the plant surfaces exposed to the radiation frequently became browned, 

 the brown areas corresponding to regions where the epidermal cells were killed and 

 had collapsed. In the present investigation this epidermal collapse has been 

 investigated in some detail, using a variety of plants. Special attention has been 

 paid to the following points : 



1. The duration of the ' Latent period,' i.e. the time elapsing between the end of 

 the dose and the appearance of browning. 



2. The relation of the latent period to the dose given. 



3. The temperature relations of the reactions involved. 



Comparison is made with the sunburning of the human epidermis, where a reaction 

 is produced after a definite latent period varying with the dose given, and dependent 

 to a large extent on temperature. 



Dr. Winifred E. Brenchley. — The Phosphorus Requirements of Barley at 

 Different Stages of Growth. 



The requirement of plants for the various essential nutrient elements vary con- 

 siderably at different periods of growth, and it has been suggested that the absence of 

 certain nutrients during particular phases may be beneficial rather than detrimental. 

 The correlation between phosphate supply and the growth of barley is being worked 

 out in water culture, and it is clearly evident that the provision of phosphate during 

 the first few weeks is absolutely necessary for complete development. With late sown 

 barley the with-holding of phosphate for the first few weeks entirely inhibited ear 

 production, though tiller formation was not affected, and longer periods of initial 

 deprivation steadily depressed growth in all respects. On the other hand, the 

 provision of phosphate for the first six weeks only, during the period that tillering 

 became established, sufficed for maximum growth and yield, but there were no 

 indications of improved growth due to the absence of phosphate late in life. 



The amount of phosphate absorbed by the plant increased steadily in more or less 

 direct proportion to the length of time phosphate was given at the beginning of 

 growth, but sufficient was taken up in the first six weeks to enable the plant to make 

 its maximum dry weight. 



The absence of phosphate supply up to the first six weeks of growth caused an 

 extremely rapid drop in the amount ultimately taken up by the plant, after which a 

 more gradual decrease occurred with lengthening periods of phosphate deprivation. 



1928 S S 



