42 
SUNBURNED TOBACCO LEAVES. 
When the tobacco plants are harvested it often oceurs that the cut 
plants are left for several hours in the field, exposed to a scorching 
sun, and many of the leaves are killed either in part or in isolated 
spots. These prematurely killed portions remain green and never 
change color in the curing barn. There are, indeed, not infrequently 
found among the cured leaves of commerce some with green spots on 
them, due to this circumstance. The microscope reveals the fact that 
the fine veins of such spots are only of a faint brown color, compared 
with the well-cured portions. 
It appears to be of some interest to decide whether it is only the 
effect of the intense light which causes such a change in the properties 
of the leaf, or whether a partial drying up in the sun forms one of the 
conditions. Possibly the light may kill the enzyms as soon as the 
cells themselves are killed by the heat. The gathered ieaves warm up 
with the sun’s rays considerably more than the leaves of plants stand- 
ing in the field, where the transpiration current provides moisture 
which lowers the temperature by continuous evaporation. 
A piece of ripe leaf was heated in a closed test tube in a water bath 
for five minutes to from 60 to 61° C., which caused the loss of the nat- 
ural stiffness (turgor), the change being caused by the loss of the 
osmotic properties of the plasma sack when the leaf dies. Another 
piece of the same leaf was heated to 60° C. for five minutes, and then 
for five minutes longer to a temperature of from 70 to 71° C. Both of 
these pieces were hung up in a covered beaker containing some water, | 
and compared with another piece of the same leaf not so treated. 
After eighteen days the pieces were moistened and spread on a glass 
plate, when it was easily recognized that the color of the piece heated 
to a temperature of from 60 to 61° C. was almost the same as that of 
the control piece, while the color of that heated finally to a tempera- 
ture of from 70 to 71° C. had a far different look, only the larger veins 
having become brown,! while the lamina was of a weak_ yellowish 
green. This shows that the production of color is due to the action of 
the oxidase (not yet killed at 60° C.), and not to the peroxidase (not 
yet killed at 70° C.), since a special test proved that the piece heated 
for five minutes to a temperature of from 70 to 71° C. contained con- 
siderable peroxidase, but no longer any oxidase. Still the analogy to 
the sunburned leaves was not fully established by this test, since the 
sunburned leaves retain their full natural green, while with the samples 
mentioned the original green was somewhat changed. Tests in which the 
pieces of the leaf were not, however, treated in closed test tubes, but 
were left to dry openly in an air-bath at 58° C., gave a different result. 
In these the green was preserved almost in its natural condition, and 
‘This browning was probably developed between 40 and 66°, after the death of 
the cell and before the death point of the oxidase was reached. 
