420 Scientific Proceedings^ Royal Dublin Society. 



the weight. It is necessary to test the bottle used for water to make sure 

 that it does not give off' an appreciable amount of alkali during the course 

 of the work. It is found that the ground-glass region of the stopper is 

 specially liable to cause trouble of this sort. It is, therefore, preferable to 

 remove water with a pipette and rubber teat rather than to pour it out. 

 With a little experience as to the tints given by the indicators it is possible 

 to work with approximate accuracy of pH 0-2 to 0"4 with the aid of the chart. 

 As a rule the determinations will be accurate to pH 0'2 or less, especially 

 if use is made of two or more indicators. If buffer tablets are carried, 

 the work can be as precise as in a laboratory. As an example of such deter- 

 minations, the following may be mentioned : — 



Anagallis arvensis, transverse and longitudinal sections of the stem 

 stained red in the vascular bundles and yellow in the pith with methyl red. 

 The former was, therefore, more acid than pH 5-2, and the latter less acid. 

 Di-ethyl red was not available at the time, so the lower limit was nob deter- 

 mined, but it was at least pH 5-8. 



Taraxacum officinale, transverse section of stem showed pH 46 ; latex 

 abundant; rootstock latex canals pH 4'4-4'8; vascular bundles much the 

 same, but not quite as acid as the latex, which gave a more purple tint 

 with methyl red; rootstock medulla pH 5 "4. Leaf parenchyma lay close to 

 pH 5 '8, as shown by brom cresol purple, and by its yellow colour to methyl 

 red, whereas the midrib was about pH 4-6. 



Cochlearia ccrmoracia, transverse section of stem was at pH 4'6 in vascular 

 bundles and sclerenchyma, pH 6'0, or slightly less acid, in parenchyma. 

 The leaf parenchyma was at pH 5 •4. 



These three plants were growing in soil which gave with water pH 7"6- 

 7'2, as shown by phenol red. 



In conclusion it may be mentioned that in eases in which the sap is 

 coloured, or the crushed material is turbid or milky, it is sometimes useful, 

 having made an approximate determination of the pH value with indicator 

 and drops of standard buffer solution, to try adding the buffer solution of the 

 nearest pH value to a portion of the crushed tissue. Since the buffer action 

 of the plant sap, whilst by no means negligible, as shown by Hempel (1917), 

 is not at all as great as that of the added solution, the alteration of the pH 

 value of the latter can be but slight with sap of approximately the same pH 

 value. The resulting colour of the true pH is, however, modified by the tint 

 or turbidity of the tissue, and thus a good match may be obtained with the 

 plant sap. This is especially useful with dichroic indicators, such as brom 

 phenol blue and brom cresol purple. 



