[VoL. 7 
6 ANNALS OF THE MISSOURI BOTANICAL GARDEN 
was not constant in the different makes and bottles of the 
reagent necessarily used, and a closer examination of this point 
as the work progressed indicated clearly that we were not secur- 
ing the uniformity of Px in the solution which we felt that we 
might reasonably expect. It was not anticipated that the 
standard as indicated by Sörensen (’09-’10) and others would be 
attained unless the reagent were guaranteed free from phos- 
phorie or other acids. The variation in Pg is often too great 
to make it at all certain what is meant when this salt is desig- 
nated merely as acid potassium phosphate. One manufacturer 
who was appealed to on account of the variability referred to, 
assured the writer that it was not possible to furnish the salt 
free of phosphoric acid under the existing conditions. 
In this work the hydrogen ion concentration was invariably 
determined by the method of Clark and Lubs ('17), employing 
both their standard solutions and their indicators. The stand- 
ard solutions in this connection, however, were prepared with 
salts recrystallized two or three times, and the greatest care 
was given to every detail of the method. They were not con- 
trolled by the electrode method but by close comparison with 
the established effective ranges of the different indicators, espe- 
cially, also, indicators with overlapping ranges. When making 
comparisons there was arranged for each indicator a set of 
seasoned serological test-tubes each containing 5 ec. of any 
standard solution within the range of the particular indicator. 
The various sets were arranged on a rack with white paper 
background. From 3 to 5 drops of the indicator solutions were 
used. Inasmuch as many of the culture solutions employed 
were without color, sufficiently accurate determinations were 
readily made with the unaided eye. In the case of solution B, 
however, and likewise in certain cases referred to later, where a 
few algae appeared in the solutions, recourse was had to the 
method employing the colorimeter (Duggar and Dodge, '19) 
and later more especially to the micro-colorimeter (Duggar, '19). 
The seedlings used in various phases of the work were wheat 
(Triticum vulgare), corn (Zea Mays), and field peas (Pisum 
arvense). In all the work reported in tables r-vr, also xI-xIx, 
the wheat was of the variety Fultz, and that reported in tables 
vir-X was with a new variety, the Pacific Coast Blue Stem, 
