426 Scientific Proceedings, Royal Dublin Society. 



many advantages, when dealing with clear solutions especially. However, 

 with plant sections and drops of liquid a somewhat larger selection of 

 indicators is very useful. For example, though the range from pH 5'4 to 6'0 

 is covered both by methyl red and brom cresol purple, yet for the greater 

 portion the tints are yellowish. This is not readily observed in tissues, hence 

 the value of di-ethyl red introduced and found reliable by Lubs and Clark 

 (1915), but excluded later as unnecessary. This changes from yellow at 

 pH 6'0 to red at 5'8, the colour increasing in depth with increase of acidity. 

 Furthermore, it is readily taken up by those plant tissues with which it gives 

 a red colour, whereas brom cresol purple is not well retained. On the more 

 acid side the useful range of methyl red ends at pH 4*4, at which point brom 

 phenol blue is blue. Methyl orange is often of service for greater acidity, 

 especially as brom phenol blue is dichroic. The former is yellow at pH 4'6, 

 and varying shades of orange to 4'0 ; at 3'8 and 3-6 the red predominates, and 

 beyond 3'4 it is clear red. This is just the region in which the dichroic blue 

 has become a yellow, and so difficult to observe in a section. The colours 

 and ranges mentioned apply only to the dilutions found suitable for 

 colorimetric work. 



Summary. 



1. Plant cells are rarely alkaline, andpHS is not surpassed in them. On 

 the acid side pH 1-4 has been observed. 



2. By a microchemical method it is possible to determine the pH values 

 of the cells and tissues. It has been found that the xylem is more acid than 

 the pith and medullary rays, and the midrib of a leaf more acid than the 

 parenchyma. Parenchymatous tissue is often in the neighbourhood of pH 6 ; 

 woody tissue nearer pH 5, or more acid. When grown in neutral or alkaline 

 soil, the root is usually less acid than the other portions of the plant. The 

 influence of soil reaction is reserved for consideration in another paper. 



3. The transpiration stream in Coloeasia antiquorum is almost neutral, as 

 is also the liquid in the leaf -base troughs of Dipsacus laciniatvs. The pitchers 

 of Sarracenia sj^p. may be as acid as pH 5, and the glandular secretion of 

 Drosera rotundi/olia may be even more acid. 



4. It has been pointed out that the pH value met with in a tissue is 

 usually near, but slightly less than, the optimum for the activity of the 

 characteristic enzyme at ordinary air temperature. This ensures that the 

 acidity does not destroy the enzyme at such higher temperatures as may be 

 experienced by the plant under natural conditions. 



5. Attention is drawn to the usefulness of di-ethyl red as a reagent for 

 microchemical work, as it gives a red in a region where methyl red and brom 

 cresol purple are yellowish ; in this range many plant tissues are found to lie. 



