The significance of mitochondria in plant cells is a subject to which a great deal of attention has been paid in past years, and from the studies of many cytologists it has become apparent that these bodies are fundamentally concerned in the formation of many different substances in the cell. Fat droplets, anthocyanins, essential oils (28) and protein grains may all be quoted as examples of such substances (Cowdry, 5). Starch and other plastid products also appear to owe their origin indirectly to mitochondria; for it has been shown by various observers that plastids are in reality enlarged and transformed mitochondria, which take on the varying functions of the production of starch, chlorophyll, anthocyanins, fat, etc., according to whether they become amyloplasts, chloroplasts, chromoplasts, elaioplasts or other such protoplasmic structures. It seems, moreover, that these substances are originally formed within the mitochondria, which later enlarge to form the body of the plastids (Cowdry, 6). A suggested explanation of this productive activity of mitochondria is found in the eclectosome theory of Regaud (32), according to which mitochondria play the rôle of plasts, selecting materials from the cytoplasm, and fabricating them in their interior into various products. More in accord with known physicochemical processes, however, is the now generally accepted interpretation recently expressed by Cowdry (7), wherein the phase-boundary of the mitochondria and the surrounding cytoplasm is regarded as the seat of processes of elaboration, beginning with adsorption of the molecules of certain solutes, and ending with a series of chemical or physical interactions between the mitochondrial material and the incoming substances—such interactions leading to the building up of new compounds of widely different character.
