184 HISTOLOGY OF MEDICINAL PLANTS 
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 grains 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 con- 
ditions 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 com- 
pound 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). 
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