Plastids are major organelles (larger green, orange, red, or colorless organelles) found in most of the plant cells and euglenoids and occur in variety of shapes and sizes. As plastids bear some specific pigments, they impart specific colours to the plants. Due to their large size they are easily observed under the microscope.
Role of Plastids
Plastids of all types develop from proplastids and they are classified into (1) chloroplasts, (2) chromoplasts and (3) leucoplasts as the most noticeable one.
Chloroplasts - green plastids contain chlorophyll and carotenoid pigments, which trap light energy essential for photosynthesis.
Chromoplasts - fat soluble carotenoid pigments like carotene, xanthophylls are present which gives the part of the plant a yellow, orange or red colour.
Leucoplasts - common to cells of higher plants, colorless plastids of varied shapes and sizes with stored nutrients:
(a) Amyloplasts - store carbohydrates (starch)
(b) Elaioplasts - store oils and fats
(c) Aleuroplasts - store proteins