Plastids

Plastids are cellular organelles that become specialized to serve different purposes. Surely the most significant plastids are the chloroplasts, which were discussed separately. Two other types of plastids are leucoplasts and chromoplasts. Leucoplasts are colorless and function in the storage of starch and oil. Chromoplasts are colored plastids, having various pigments primarily xanthophylls and carotenes. Anthocyanins are water soluble and found in vacuoles. Xanthophylls and carotenes are fat soluble and are found in plastids. Chromoplasts tend to be yellow, orange, or red. Tomatoes, oranges, and carrots owe their colors to chromoplasts.

Plastids are commonly disk shaped, sphere shaped or shaped like a double convex lens. Shape as well as color varies with the tissue and species. Plastids are interchangeable in form. Leucoplasts, such as those in a potato, will become chloroplasts if exposed to light. Chloroplasts can become chromoplasts. Granules of chromonucleoprotein have been found in plastids, suggesting that plastids have their own genetic mechanism.

Plastids frequently have their beginnings as proplastids in meristematic cells. These proplastids appear as minute bodies scarcely visible through a light microscope. They may divide by simple constriction, resulting in large numbers of them. They grow into mature plastids as the cells mature.