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Journal of Agricultural Research voi. xxvn, no 
the available data on the effect of light, and more specifically the duration 
of the daily illumination period, on the active acidity of the cell sap. 
This problem is complicated by the marked differences in acidity 
which are found in the different organs and tissues of the plant under 
varying environmental conditions. An additional complication is the 
progressive change in acidity relations with increase in age of the in¬ 
dividual organs and of the plant as a whole. In this connection, it is 
necessary also to keep in mind the fact that the “age” of the organ or 
organism, as indicated by the stage of development attained, can not be 
measured solely by lapse of time but is dependent largely on the par¬ 
ticular environment to which the plant happens to be exposed. To give 
a concrete illustration, when late-flowering cosmos is exposed to a daily 
illumination period of, say, 15 hours, the apex of the stem shows a pro¬ 
gressive and marked increase in hydrogen-ion concentration as growth 
proceeds, maximum acidity being attained only after the lapse of several 
weeks. In this case vegetative development of the stem continues for a 
more or less indefinite period. On the other hand, if exposed to 10 hours 
of light daily, the apex of the stem shows no such marked increase in 
active acidity until after the flower bud has appeared. Under the 
longer illumination period the active acidity of the apex is much greater 
than that of the base of the stem, while under the shorter illumination 
period these relations may be actually reversed. Proper sampling of the 
material for study offers considerable difficulty, since for practical 
purposes it is often impossible to separate completely tissues, or organs, 
even, especially in the earlier stages of development. In most cases, 
however, the differences in acidity are so well defined as to leave no 
doubt as to the adequacy of the methods of sampling followed, where 
only comparative results are required. 
In these experiments, except in special cases, no effort was made to 
follow out in detail the change in acidity through the 24 hours of the 
day, for such a procedure is out of the question where it is necessary to 
conduct a large number of separate tests. This difficulty was overcome 
as far as possible by choosing different hours of the day for making the 
observations in the course of a particular series, always collecting samples 
at as nearly the same hour as possible in comparing material exposed to 
two different illumination periods. In certain cases, moreover, com¬ 
parative readings were made in the morning and afternoon hours, as a 
further check on the results obtained. In all cases the hour at which 
sampling was done was recorded and the material was used for observa¬ 
tion as soon as possible after it had been collected. 
In carrying out the measurements the plant material was crushed in a 
porcelain mortar or in a tinned food chopper and the crushed material 
was strained through several thicknesses of cheesecloth. The hydrogen- 
ion concentration was determined electrometrically, the potential being 
measured with either a Northrup “ millivolt er” or a Northrup hydrogen- 
ion “pvrovolter” and a Leeds and Northrup small, portable galvanometer. 
The saturated KCl-Calomel electrode was used and the hydrogen elec¬ 
trode was made and “platinized” essentially as directed by Bovie (j). 
The platinum coat on the wire, however, was always deposited just be¬ 
fore being used. Electrolytic hydrogen compressed in cylinders was 
caused to bubble through the sample of sap and the potential was first 
observed after a period of 10 minutes. Further observations were made 
at intervals of 5 to 10 minutes until a constant reading was obtained, 
