=Volatile oils= occur in cinnamon stem bark, sassafras root bark, flowers of cloves, and in the fruits of allspice, anise, fennel, caraway, coriander, and cumin. In none of these cases is the volatile oil diagnostic, but its presence must always be determined. When a powdered drug containing a volatile oil is placed in alcohol, glycerine, and water mixture the volatile oil contained in the tissues will accumulate at the broken end of the cells in the form of rounded globules, while the volatile oil adhering to the surface of the fragments will dissolve in the mixture and float in the solution near the under side of the cover glass. Volatile oil is of little importance in histological work. TANNIN =Tannin= masses are usually red or reddish brown. Tannin occurs in cork cells, medullary rays of white pine bark (Plate 48, Fig. B), stone cells, and in special tannin sacs. The stone cells of hemlock and tamarac bark and the medullary rays of white pine and hemlock bark contain tannin. Tannin associated with prisms occurs in tannin sacs in white pine and tamarac bark. These sacs are frequently several millimeters in length and contain a great number of crystals surrounded by tannin. Deposits of tannin are colored bluish black with a solution of ferric chloride. ALEURONE GRAINS =Aleurone grains= are small granules of variable structure, size, and form, and they are composed of reserve proteins. They occur in celery, fennel, coriander, and anise, fruits, in sesame, sunflower, curcas, castor oil, croton oil, bitter almond, and other oil seeds. In many of the seeds the aleurone grains completely fill the cells of the endosperm, embryo, and perisperm. In wheat, rye, barley, oats, and corn the aleurone grains occur only in the outer layer or layers of the endosperm, the remaining layers in these cases being filled with starch. In powdered drugs the aleurone grains occur in parenchyma cells or free in the field. STRUCTURE OF ALEURONE GRAINS Aleurone grains are very variable in structure. The simplest grains consist of an undifferentiated mass of proteid substance surrounded by a thin outer membrane. In other grains the proteid substance encloses one or more rounded denser proteid bodies known as globoids. In other grains a crystalloid--crystal-like proteid substance--is present in addition to the globoid. In some grains are crystals of calcium oxalate, which may occur as prisms or as rosettes. All the different parts, however, do not occur in any one grain. In castor-oil seed (Plate 77_a_, Fig. 8) are shown the membrane (_A_), the ground mass (_B_), the crystalloid (_C_), and the globoid (_D_). FORM OF ALEURONE GRAINS