Atkins — The Hydrogen Ion Concentration of Plant Cells. 419 



aid of a hand lens or low-power (two-thirds inch) objective have been 

 mapped out with tolerable accuracy as lying betwen pH 5-2 and 6'0. The 

 results may be summed up as follows, the G^ values corresponding to the 

 pH values being also given, to emphasize the magnitude of the acidity 

 differences involved : — 



Thus it is seen that the acidity in the sclerenchyma is four to six times 

 as great as in the adjacent cortical cells, but, on account of the relatively 

 great bulk of the parenchyma, expressed sap would appear to be more nearly 

 of the acidity given by the latter. This relatively high acidity of the wood 

 and sclerenchyma seems to be quite a normal occurrence, and cannot fail to 

 have a physiological significance. 



As a further example of the difference in reaction between portions of a 

 plant in fairly close proximity may be mentioned the behaviour of wheat 

 seedlings (Pusa 4 variety). These were removed from soil v/ith a strongly 

 alkaline reaction, pH 8-8, and the roots were washed till the washing water 

 gave pH 7. The roots then were at pH 6'8, as shown by brom thymol blue 

 and phenol red. The white leaf bases and the young green leaves were both 

 at pH 5-4, as shown by methyl red and brom cresol purple. As already 

 mentioned, Kappen (1918) found cereal roots neutral, those of wheat even 

 being slightly alkaline. At Pusa the roots were slightly on the acid side of 

 neutrality for wheat, an identical value being given by those of both oats 

 and rice, viz. pH 6'8. The leaves of the oats were at pH 5 '4, while with the 

 rice the leaf gave pH 4'8, the first and third internodes of the stem being 

 at pH 5 0, intermediate with the value of the roots. 



With a very simple outfit it is possible to make determinations of the 

 acidity of plant tissues in the field. This is of value in work correlating the 

 acidity of the plant and the soil, and in cases where for any reason a labora- 

 tory is not to hand. A few small tubes of the necessary indicators are 

 required, one or more glass rods, and a few watch glasses. It is also well to 

 have a knife of stainless steel, a hand lens, and a bottle of distilled water, or 

 of rain water. On such expeditions a separate reprint of the coloured 

 chart of indicators at various pH values given by Clark is very useful, and 

 the buffer tablets already mentioned are a desirable item, and add little to 



S a2 



