418 Scientific Proceedings, Royal Dublin Society. 



These values are at least comparable ones, though on account of the 

 presence of proteins in the cells an error is introduced. The indicators 

 used, however, were selected by Clark and Lubs(1917) as having small errors 

 from this source, as has been further demonstrated by Homer (1917) in 

 serum studies. 



The application of the method to the mapping out of the acidity of tissues 

 may be illustrated by the following : — 



The Labiate Salvia Verhenaca grows plentifully near the Plymouth 

 laboratory, and affords convenient material. A fresh transverse section of 

 the stem was stained with various indicators, and it was seen that di-ethyl 

 red (Lubs and Clark, 1915) gave with the pith a yellowish colour, which was 

 also seen in the medullary rays and cambium. The wood was a good red in 

 the walls ; the bast fibres were deep red in the cells, as were also the cells of 

 the sclerenchyma in the four angles of the stem. The cortical parenchyma 

 between the bast fibres and assimilating cells was yellowish ; the assimilating 

 cells were apparently a very light yellow, as far as could be judged in the 

 presence of the chloroplasts. The epidermis was seen to be a deep red, but 

 this is its natural colour; to avoid error it is always necessary to examine 

 an unstained section. 



Since di-ethyl red changes from red to yellow between pH 5-8 and 6'0, it 

 is at once clear that the red-stained portions must be at least as acid as 

 pH 5"8, and the yellow equal to or less acid than pH 6-0. 



Using methyl red, the sclerenchyma and bast fibres give a salmon pink to 

 pink colour, corresponding to pH 5-4 to 5'2 ; the wood appears a faint pink, 

 not more acid than pH 5-4 to 5-6, and the medullary rays are yellow. The 

 upper limits of acidity are therefore fixed. 



It remains to determine the acidity of the portions appearing uniformly 

 yellow of the same tint with di-ethyl red. With phenol red the full yellow 

 colour appeared, showing an acidity of pH 6'6 or more. With brom thymol 

 blue a yellow with a green tinge was seen, matching the colour with pH 6-2. 

 A green tinge in the tissue may, however, make this acidity slightly too low. 

 Accordingly a small cube of the medulla was excised with a stainless steel 

 knife, which was found very useful for this work, and was crushed in a watch- 

 glass with an agate pestle, and to this one drop of the dilute brom cresol 

 purple was added. The tint was matched by taking two drops of standard 

 buffer solutions, pH 5-8, 6'0, and 6-2, with one drop of the reagent, since the 

 volume of the crushed tissue was closely the same as that of two drops. The 

 colours were pH 5-8 dirty greenish, 6-0 slaty blue, and 6-2 deep slate blue. 

 The medullary tissue was a very close match to 6-0. 



Tlius the various tissues seen in the cross-section of this stem with the 



