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 (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.