Plastids, differentiated portions of the protoplasm in a cell (q.v.). Like the nucleus (q.v.), they are not formed de novo, but multiply by division. Though various in form, they seem uniformly to consist of a ground-substance of the proteid ch'loroplastin, with imbedded fibrils of the colourless proteid metawin. The plastids themselves maybe colourless (leucoplastids), green (chloroplastids or chlorophyll-granules), or otherwise coloured, generally red or yellow (chromoplastids); but under certain conditions these forms are interchangeable.