Atkins — The Hydrogen Ion Concentration of Plant Cells. 425 



interpolation between standard solutions at 2'60, 2'75, and 3'0 with thymol 

 blue. For the sulphuric acid pH 1'7 was obtained. Thus, at the tempera- 

 ture of the laboratory this degree of acidity had a very destructive action on 

 the enzyme. As pointed out, the pH values recorded here can only be 

 upper limits for the acidity actually in contact with the enzyme. In this 

 connexion the work of Talk (1915, 1917) is also of interest. He found 

 lipolytic enzyme from castor beans could be separated into an albumin-like 

 body more active towards ethyl butyrate than to glyceryl triacetate, and a 

 globulin-like body more active to the latter. It was ascertained that after 

 standing for a day at 0° C, in a liquid of acidity pH 3'5, the esterase had 

 almost become inactive when tested subsequently by its hydrolytic activity 

 at 38° C. At pH 4-5 one-third of its activity had been lost. The destruc- 

 tion of the lipase was not quite as great. It was further proved that after 

 standing for a day at 0° at pH 8 the esterase was slightly less active than at 

 pH 7, but at pH 11 it had lost 90 per cent, of its activity. 



Thus it is evident that the destructive action of an acidity of pH 4-5 

 at 0° upon lipase nmst stand in close relation to the fact that in the 

 seed the reaction of the stem and leaf, namely pH 4'6, is found to be 

 reduced to pH 5'4, a change of Ch 0-32 x 10'* to Ch0'04 x 10"*, namely to 

 one-eighth. 



Indicators for use ivitli plant tissues. 



Of the very numerous substances which act as indicators, Sorensen (1909) 

 selected a limited number as giving well-defined brilliant colours, and being 

 relatively uninfluenced by salt and protein errors, whereby a discrepancy 

 arises between the true hydrogen ion concentration as shown by the potential 

 difference method and the colorimetic method. Sorensen excluded cochineal, 

 Congo red, alizarin, and all litmus preparations. He included neutral red, 

 which is remarkably free from the errors, and is of great value by reason of 

 its penetration of the living cell without toxic effects when in moderate 

 concentration, for which purpose it was originally introduced by Ehrlich. 

 This indicator gives a good clear red from pH 7"0 to 6'6, and the more acid 

 region. It is reddish from 7'2 to 7'6, and orange beyond that. The stain is 

 of special use in testing the reaction of algal cells in an alkaline medium, as 

 will be described in detail elsewhere. 



Clark and Lubs (1915, 1917) introduced the phenol sulphone phthaleins 

 for use as indicators, and selected the best. All those shown in Clark's list 

 are of value, and by reason of their stability and ready solubility in water as 

 sodium salts, as well as their brilliancy, they are to be preferred to some on 

 Sorensen's list. They include methyl red, introduced by Palitzsch (1911), 

 which, though not readily soluble, covers a range much used. Clark and 

 Lubs aimed at reducing the number of indicators, and this has naturally 



