Nov. 6, 1916 
Grain of the Tobacco Leaf 
277 
be visible to the unaided eye. It may be mentioned, however, that 
occasionally a leaf was found which was classed among the “open¬ 
grained” the grain of which was not apparent on the surface of the leaf, 
though evidently the experienced hand could detect its presence and 
determine its character. 
The bum of the leaves thus selected was tested, always with the result 
predicted. A small piece was then cut from the unbumed portion of 
each leaf at about a quarter of an inch from the point at which the fire 
was extinguished. These pieces were dehydrated in absolute alcohol, 
cleared and mounted in cedar oil, and examined microscopically with polar¬ 
ized light. Without exception, the good-burning leaf showed that the 
grain material had become well aggregated into definite bodies separated 
one from the other by a band of tissue free from crystalline material, 
save for the scattered, single crystals and occasional idioblasts containing 
calcium oxalate. This condition was evident even in the open-grained 
leaves mentioned above in which no grain was macroscopieally visible on 
the leaf, for in those cases the bodies, though too small to cause a swell¬ 
ing on the surface, were definite in form and sufficiently separated to 
allow a zone of comparatively empty cells around each. 
In the pieces from the poor-burning (close-grained) tobacco, on the 
contrary, the crystalline grain material proved to be scattered more or 
less evenly throughout the tissue, without any considerable degree of 
aggregation into definite bodies large enough to leave an encircling zone 
of empty cells around each particle. A few poor-burning samples have 
been found, however, in which some aggregation of grain substance 
had occurred, but in these the intervening tissue was filled with a mass 
of grain material which, for some reason, had failed to form definite 
bodies, thereby producing the same condition found in the poor-burning 
samples which were without appreciable aggregation. 
From these facts it would seem that a certain degree of aggregation, 
with intervening tissue free from grain substance, is necessary in order 
that good fire-holding capacity may be assured. This suggests that the 
substance composing the grain bodies may have in reality a retarding 
effect upon the advance of fire in the tissue and that zones of grain-free 
cells must be present in order that a sufficient degree of heat may be 
generated, by their more rapid combustion, to ignite the solid grain 
particles. Verification of this suggestion has partially been obtained 
by watching through a binocular microscope the progress of the fire in 
the tissue. In a piece of leaf with prominent grain it was found that in 
the process of burning the fire line passed part way around and in some 
cases even beyond the centers of large grain bodies before they became 
ignited. It seems reasonable to suppose that the same process takes 
place in a leaf in which the grain particles are too small to admit of in¬ 
vestigation in this manner and, further, that in a close-grained leaf there 
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