PLASTID. A plastid may be defined as a differentiated portion of the plant protoplasm which is not formed de novo, but arises from the division of a pre-existing plastid. Plastids are denser than the surrounding protoplasm and the interior is denser than the surface. There is no surrounding membrane. The shape is generally spherical or oval (Fig. 1), but many plants have very striking plastids, as in case of the alga, Spirogyra (Fig. 2). When plastids are colorless, they are called leucoplasts, or leucoplastids; when colored, they are called chromatophores. When chromatophores are green, they are called chloroplasts; when they have any color other than green, they are chro moptasts. However, most botanists do not use so many names, hut call all the colorless plas tids leucoplasts and all the colored ones chroma tophores. In all cases, the ground substance of the plastid is colorless, but it contains drop lets of an oily substance which is green, yellow, orange, red or brown, as the case may be. Plastids near the surface are likely to develop chlorophyll, a green oily substance of the utmost importance in plant life; while plastids more deeply placed are likely to remain as colorless leucoplasts whose principal function is to pro duce starch. However, a leucoplast may be come a chloroplast, as when potatoes are ex pcsed to the light and become green ; and they again become leucoplasts when the potatoes are kept in the dark. Chlorophyll-bearing plastids usually contain also some very small grains of starch. Such starch grains are the first visible
products of assimilation. Reserve starch, like the large starch grains of the potato, are formed inside the leucoplast and become so large that the leucoplast is stretched until it is only a very thin membrane surrounding the conspicuous starch grain. Botanists are not entirely agreed as to whether the plastid may originate de novo or not. The weight of authority is on the side of those who claim that the plastid is a perma nent organ which arises only by the division of a previous plastid, as already indicated in our definition. That plastids divide and that an unbroken series can be traced throughout the greater part of the life history no one disputes; hut in some phases of the life history, the most critical methods give no convincing demonstra tion of the presence of a plastid. In recent years, peculiar structures, called mitochondria, many of which resemble various forms of bac teria, have been claimed to develop into plastids (Fig. 3). This subject is still under vigorous investigation. Consult Mayer, A., 'Untersuch ungen fiber die Starkekorner); Schimper, A. F. W., Wher die Entwickelung der Chlorophyll kiirner and Farbkorper> (Botanische Zeitung, Vol. XLI, 1883, p. 105) ; Cavers, F., "Chondrio somes and Their Significance"' (in New Phy tologist,) Vol. XXXI, p. 170, 1914).