The chemical substance starch (C₆H₁₀O₅) is formed in chloroplasts. The starch thus formed is removed from the chloroplasts to other parts of the plant because it is the function of the chloroplasts to manufacture and not to store starch. The starch formed by the chloroplasts is acted upon by a ferment which adds one molecule of water to C₆H₁₀O₅, thus forming sugar C₆H₁₂O₆. This sugar is readily soluble in the cell sap, and is conducted to all parts of the plant. The sugar not utilized in cell metabolism is stored away in the form of reserve starch or starch grains by colorless plastids or amyloplasts. The amyloplasts change the sugar into starch by extracting a molecule of water. This structureless material (starch) is then formed by the amyloplast into starch grains having a definite and characteristic form and structure. Starch grams vary greatly in different species of plants, owing probably to the variation of the chemical composition, density, etc., of the protoplast, and to the environmental conditions under which the plant is growing. OCCURRENCE Starch grains are simple, compound, or aggregate. =Simple starch= grains may occur as isolated grains (Plates 70, 71, and 72), or they may be associated as in cardamon seed, white pepper, cubeb, and grains of paradise, where the simple grains stick together in masses, having the outline of the cells in which they occur. These masses are known as aggregate starch. =Aggregate starch= (Plate 76) varies greatly in size, form, and in the nature of the starch grains forming the aggregations. =Compound starch grains= may be composed of two or more parts, and they are designated as 2, 3, 4, 5, etc., compound (Plate 75). The parts of a compound grain may be of equal size (Plate 75, Fig. 4), or they may be of unequal size (Plate 75, Fig. 2). In most powders large numbers of the parts of the compound grains become separated. The part in contact with other grains shows plane surfaces, while the external part of the grain has a curved surface. There will be one plane and one curved surface if the grain is a half of a two-compound grain; two plane and one curved surface if the grain is a part of a three-compound grain, etc. The simple starch grains forming the aggregations become separated during the milling process and occur singly, so that in the drugs cited above the starch grains are solitary and aggregate. Many plants contain both simple and compound starch grains (Plate 74, Fig. 3). In some forms--_e.g._, belladonna root (Plate 75, Fig. 2) the compound grains are more numerous; while in sanguinaria the simple grains are more numerous, etc. OUTLINE The =outline= of starch grains is made up of (1) rounded, (2) angled, and (3) rounded and angled surfaces. Starch grains with rounded surfaces may be either spherical, as in Plate 74, Fig. 3, or oblong or elongated, as in Plate 71, Fig. 1.