32 Alfred J. Etoart : 



may be converted into gallic acid. There is no evidence to show 

 that it is present as a glucoside. Since tannin vacuoles must have 

 an osmotic pressure corresponding to that of the cell in "which they 

 lie, with their surface tension pressure added (possibly more than 

 1 atmosphere), the tannin in them must be more concentrated than 

 in the cell-sap. 



Exact estimates of the amount of tannin present in the cell are 

 impossible, owing to the difficulty of extracting the whole of it. If 

 any oxidation occurs the brown oxy-tannin combines very firmly 

 with the protoplasm. Although traces of brown colouration may 

 Ije imparted to the sap at first, once the browning is complete, it i» 

 not removed by 1 per cent, hydrochloric or sulphuric acids, darkens 

 in stronger acid or in dilute sodium hydrate, and is not removed 

 by ether, alcohol or peroxide of hydrogen. In a saturated solution 

 of sulphurous acid the pulp rapidly becomes a lighter yellowish 

 brown, but retains this colour even after two weeks without furthei- 

 fading. On washing, the pulp turned to the same shade of black 

 with Fe Cls ;is before. After three weeks in hydrogen peroxide the 

 brown pulp was distinctly paler, and gave a fainter tannin reaction 

 with Fe CI3. 



Estimations of the tannic acid in a mixed solution are difficult 

 to carry out accurately. In contact with granulated zinc or zinc 

 foil tannic acid is slov.ly precipitated as a white tannate soluble in 

 HCl, and a slow evolution of hydrogen is shown. With magnesium 

 the evolution of gas is somewhat more rapid, but the white precipi- 

 tate largely adheres to the magnesium, and becomes browned after 

 a time. The first method, however, can be used even when the 

 tannin is in an acid cell sap, and it appears to be capable of quan- 

 titative use, but it is so far not possible to devise a method which 

 will determine the exact amounts of tannic acid originally present 

 ill the acid pulp, and apparent differences in the tannin contents of 

 dead and living parts may be merely the result of unequal extrac- 

 tion. 



The influence of oxygen on browning. 



Preliminary trials showed that both peeled and unpeeled apples 

 c«iuld stand repeated evacuations and replacement of an atmosphere 

 of hydrogen or carbon dioxide for three days without the pulp cells 

 being affected. Slices were then floated on glass boats on a one per 

 1000 solution of copper sulphate, and the air removed by repeated 

 evacuation and replacement with hydrogen or carbon dioxide. 

 They were then shaken into the liquid, and all gas drawn out by 

 evacuation. After one dav all tlie slices were quite unbrowned. 



