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 b}' 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). 



