270 
Journal of Agricultural Research 
Vol. VII, No. 6 
With two notable exceptions the grain particles are entirely embedded 
in the tissue of the leaf, being distributed more or less evenly in the web 
and along the veins. One of the superficial types appears as raised 
black dots, which are found chiefly on dark-colored leaves of a heavy 
texture. The other is composed of those which give somewhat the 
appearance of minute disks situated on or immediately beneath the 
surface on either side of the leaf. These latter seem most conspicuous 
on leaves of a medium or light color. 
MICROSCOPIC CHARACTERS OF THE GRAIN 
When a cased leaf of tobacco containing grain is stretched over the 
stage of the microscope and examined in ordinary transmitted light with 
a low magnification, most of the larger grain particles appear as more 
or less rounded, highly refractive bodies of compact structure and bright 
reddish brown color. Indeed, the ground tissue appears uniformly light 
in color in comparison with that of the leaf observed macroscopically, 
and it is seen that by far the larger part of the brown color of the leaf is 
localized in the grain and in some of the small veins. Even when one 
looks with a hand lens through a moist leaf against a strong source of 
light, this localization of color in the grain is evident; the effect is that 
of minute garnets embedded in a yellowish brown matrix. 
In material mounted in Canada balsam, after having been dehydrated 
in alcohol and cleared in xylol, the grain bodies become less conspicuous 
in ordinary transmitted light, inasmuch as their refractive index more 
nearly approaches that of the balsam than that of the air. The bodies 
may be located, however, because of their difference in color from the 
surrounding ground tissue, owing to the concentration in them of the 
brown coloring matter and their slight power of refraction of light in 
balsam (PI. 16, fig. A). 
It is in polarized light that the grain bodies become most conspicuous, 
since, as stated before, they are of crystalline structure. Favorable ma¬ 
terial mounted in balsam then exhibits clearly their size, shape, and 
structure, as well as showing the degree to which the grain material has 
been brought together into definite aggregates, a point which will be 
mentioned later in this paper. 
VARIOUS FORMS OF GRAIN 
There are five general types of grain which merge insensibly into one 
another and which may be briefly described in their order of abundance, 
as follows: The first and most common type consists of more or less 
spherical masses each composed chiefly of a group of palisade cells dis¬ 
tended with minute, radiating, needle-shaped crystals of a brown color 
(PI. 16, fig. B). The second type includes flat or roughly hemispherical 
bodies composed of cells of the mesophyll and epidermis, which are filled 
with light brown to almost colorless, comparatively large plates ar¬ 
ranged somewhat regularly (PL 16, fig. C). The third is similar to the 
