AQ) 
the thicker part of the veins the tissue remains green longest. Gradu- 
ally a stage is reached in which only the midrib and basal parts of the 
lateral ribs still have their normal color, being fringed by but narrow 
strips of yellow mesophyll, the chief mass of the leaf having turned 
brown.! Thus, the layman is inclined to infer that the substance 
which turns yellow is also the same that turns brown later on. This, 
however, is not correct. The yellow color is due to a chemical change 
of the cllorophyll green, while the brown color is due to the oxidation 
of several different compounds contained in the cell sap. The brown 
color shows also in the veins of the leaf, but not the yellow, since there 
is too little chlorophyll in the veins. It sometimes happens, however, 
that leaves in the barn die quickly by drying up instead of slowly by 
starvation, thus not allowing sufficient time for development of the 
yellow color, and in this case the brown color develops directly upon 
the still green leaf. Such leaves show a distinctly green coloration when 
held between the eye and the bright daylight, and on treatment with 
strong alcohol they yield a green solution of chlorophyll, showing that 
the green is not destroyed, but merely concealed by the brown. The 
chlorophyll, therefore, is not the source of the browncoloration. Further- 
more, when afresh leaf is dried at a moderate temperature in an air bath 
it will no longer turn yellow when exposed for a considerable time to 
a moist atmosphere, showing that the change from green to yellow in 
the curing barn is still a process of life, although a pathologic one. 
As to the brown color, it can easily be shown that it is produced by 
the action of oxidase and not by the action of peroxidase, since after 
“killing” the oxidase the brown color fails to develop. Freshly 
expressed juice of the midrib was heated for four minutes to 75° C. 
(167° F.) and left with a little chloroform in a flask plugged with 
cotton. After ten days this liquid was still colorless, although con- 
taining peroxidase, while the control liquid became light brown within 
one day.? The juice of the lamina turns dark brown much sooner. 
WHY DOES THE OXIDASE NOT PRODUCE THE BROWN COLOR WHILE 
THE LEAF IS ALIVE? 
The oxidizing enzyms are evidently contained in the plasmatic living 
part of the cell, and not in the cell sap which fills the vacuoles. On the 
other hand, the matters easily oxidized by them and representing 
products of metabolism or by-products of certain synthetical operations 
are mostly contained in the cell sap. These matters, often of a chromo- 
1The brown color darkens still more during the sweating process. When the 
expressed juice of green tobacco leaves, after filtering, is exposed to air it gradually 
forms an insoluble deposit, the greater part of which consists of a phlobaphen-like, 
dark-brown substance, and the smaller part of a colorless flocculent compound. The 
former is easily soluble in dilute ammonia; the latter is not. 
*It may be mentioned here that, according to A. Mayer, a dark-colored tobacco is 
obtained by a rich nitrogenous manuring, and that, according to Kosutany, barn- 
yard manure with ammonium salts yields a product of a reddish brown tint, while 
nitrate manuring yields a more greenish-brown one. 
