4 BULLETIN 839, U. S. DEPARTMENT OF AGRICULTURE. 
starch grains are dissolved, or the preparation "cleared," as is com- 
monly stated. Vigorous heating of the slide is to be avoided in order 
to prevent burning of the material before the preparation has been 
sufficiently cleared. After gentle heating, the slide is quickly trans- 
ferred to the stage of the microscope, where it is allowed to remain a 
short time before counting is begun. The cold stage causes the larger 
part of the air bubbles that may be present in the preparation to dis- 
appear, the very few that remain not hindering in the enumeration 
of the bran particles and hairs. 
Careful adherence to the details of this technique is necessary to 
insure a suitable slide for counting. If a slide is improperly prepared, 
the resulting count probably will not be representative of the flour 
under consideration. 
COUNTING BRAN PARTICLES AND HAIRS. 
A thorough acquaintance with the histology of the wheat grain is 
essential before attempting an examination of flours. Any standard 
work on microscopy or plant anatomy of the common food products 
contains adequate descriptions of the tissues of the wheat berry in 
various sections. The following brief description of the anatomy of 
the wheat berry 1 is given for the purpose of indicating the tissues 
which are depended upon for judging a flour with respect to its offal 
content. 
The wheat grain is, botanically, the fruit of various subspecies and 
varieties of the genus Triticum. This grain or fruit consists of a 
series of tissue systems, the outermost of which is the pericarp which 
is composed of three layers, the epicarp, mesocarp, and endocarp. 
The pericarp is essentially the fruit coat or matured ovary wall. 
Within the pericarp is the testa (or spermoderm), rather yellowish- 
brown in color, and easily distinguished in either cross or surface 
sections under the microscope. Within the testa is a layer of rectan- 
gular cells (in transverse section) known as the aleurone layer, contain- 
ing protein material but no starch. This is essentially the outer layer 
of the endosperm or albumen of the seed. The remainder of the grain 
within the aleurone layer consists of very thin-walled parenchymatous 
cells packed full of starch grains. The small embryo, or germ, is 
located at the end opposite the bearded apex. A crease or groove 
passes longitudinally from the base of the grain to the apex. 
The essential purpose of milling is to produce the finely ground 
endosperm or starchy portion of the wheat grain as free as possible 
from bran particles, hairs, and germ tissues. These bran particles, 
hairs, and germ tissues are known as offal in milling terminology. The 
wheat offal, therefore, consists primarily of all the tissue elements of 
i A. L. Winton. The Microscopy of Vegetable Foods, 2d ed., pp. 65-73. 1916. 
