The cell contents of the plant are divided into two groups: first, organic cell contents; and secondly, inorganic cell contents.

The organic cell contents include plastids, starch grains, mucilage, inulin, sugar, hesperidin, alkaloids, glucocides, tannin, resin, and oils.

Chlorophyll

The chloroplasts of the higher plants are green, and they vary somewhat in size, but they have a similar structure and form.

Chloroplasts are mostly oval in longitudinal view and rounded in cross-section view. Each chlorophyll grain has an extremely thin outer wall, which encloses the protoplasmic substance, the green granules, a green pigment (chlorophyll), and a yellow pigment (xanthophyll). Frequently the wall includes starch, oil drops, and protein crystals.

Chloroplasts are arranged either in a regular peripheral manner along the walls, or they are diffused throughout the protoplast.

The palisade cells of most leaves are packed with chlorophyll grains. In the mesophyll cells the chlorophyll grains are not so numerous, and they are arranged peripherally around the innermost part of the wall.

Chloroplasts multiply by fission - that is, each chloroplast divides into two equal halves, each of which develops into a normal chloroplast.

Chlorophyll occurs in the palisade, spongy parenchyma, and guard cells of the leaf; in the collenchyma and parenchyma of the cortex of the stems of herbs and of young woody stems, and, under certain conditions, in rhizomes and roots exposed to light. Almost without exception young seeds and fruits have chlorophyll.

In powdered leaves, stems, etc., the chlorophyll grains occur in the cells as greenish, more or less structureless masses. Yet cells with chlorophyll are readily distinguished from cells with other cell contents. In witch-hazel leaf the chlorophyll grains appear brownish in color. Powdered leaves and herbs are readily distinguished from bark, wood, root, and flower powders.

Leaves and the stems of herbs are of a bright-green color. With the exception of the guard cells, the chloroplasts occur one or more layers below the epidermis; but, owing to the translucent nature of the outer walls of these cells, the outer cells of leaves and stems appear green.

Wild cherry, sweet birch, and, in fact, most trees with smooth barks have chloroplasts in several of the outer layers of the cortical parenchyma. When the thin outer bark is removed from these plants, the underlying layers are 'seen to be of a bright-green color.

Leucoplastids

Leucoplastids, or colorless plastids, occur in the underground portions of the plant; they may, when these organs in which they occur are exposed to light, change to chloroplastids.

Leucoplasts are the builders of starch grains. They take the chemical substance starch and build or mould it into starch grains, storage starch, or reserve starch.

Other characteristic chromoplasts found in plants are yellow and red. Yellow chromoplasts occur in carrot root and nasturtium flower petals. Red plastids occur in the ripe fruit of capsicum.

Starch Grains

The chemical substance starch (C6H20O5) 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 C6H20O5 thus forming sugar C6H22O6 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 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 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.