(a) The term plastids was introduced by E. Haeckel in 1866.
(b) Plastids are semi-autonomuos organelles having DNA and double membrane envelope which store or synthesize various types of organic compounds Depending upon their colour, plastids are of three types:
(1) Leucoplast : They are colourless plastids which occurs .near the nucleus and possess internal lamellae. Leucoplast are of three types : starch containing amyloplast, fats storing elaioplast and proteins bearing proteinoplast. Grana and photosynthetic pigments are absent in leucoplast.
(2) Chromoplast : These are coloured plastids with reddish and yellow colours because of the presence of carotenoid pigments. The pigments are often found in crystallised state so that the shape of the plastids can be like needles, spindles or irregular.
These provide colour to many flowers for attracting pollinating insects. They are also the site of synthesis of membrane lipids.
(3) Chloroplast : They are green plastids which possess photosynthetic pigments, chlorophyll and carotenoids, and take part in the synthesis of food from inorganic raw materials in the presence of radiation energy.