1917] 
FREIBERG—MOSAIC DISEASES 177 
judgment and the uniformity of results obtained from an ex- 
tensive number of tests. 
On account of the delicacy of the tests employed, all glass- 
ware and instruments used were cleaned with the greatest 
care. Куеп the pith used in sectioning the tissue was soaked 
and rinsed in alcohol and distilled water, and the sections 
were rinsed before the application of reagents, in order to 
insure the removal of all superficial and extraneous salts or 
foreign matter. The best reagents obtainable were used 
throughout the work. The results obtained with diseased 
tissue were always contrasted with those obtained with 
healthy tissue. The texts of Molisch (713) and Tunmann 
(713) not only served as ап outline for the methods employed, 
but also as an index to the extensive literature on micro- 
chemical reactions. 
NITROGEN 
The detection of inorganic nitrogen in plants was first at- 
tempted in the work by Molisch (’83), and it is to these re- 
searches that all subsequent work owes its foundation. The 
test for nitrogen is based upon the fact that nitrates and 
nitrites give a blue color with diphenylamine, while a red color 
results upon the application of brucine. 
Reaction with diphenylamine.— The most reliable of all 
tests for inorganie nitrogen is the one based on the reaction 
of nitrogen with diphenylamine. The reagent is applied in 
the form of .01-.1 gram of diphenylamine in 10 сс. reagent 
sulphurie acid. Since diphenylamine is insoluble, or only 
very slightly soluble, in water, it is necessary to apply the 
reagent to dry sections. When applied to wet sections, the 
diphenylamine will be precipitated and thus be unable to re- 
act with the nitrogenous compounds. Sections are therefore 
placed on the slide and allowed to dry, after which enough 
of the reagent is applied adequately to cover the mount. In 
the presence of nitrates a blue color results which gradually 
fades, but almost invariably shades into a light brown. The 
test may be negative or the reaction almost imperceptible in 
the presence of very minute quantities of nitrogen. In this 
