THE STRUCTURAL COMPONENTS OF PROTOPLASTS 



31 



Plastids with no color are known as leucoplasts. Ordinarily they are 

 small and occur in considerable numbers in meristematic plant cells and 

 in parts not exposed to light. Under appropriate conditions some of 

 them enlarge and develop into green chloroplasts or plastids of other 

 colors. Under other conditions, notably in roots and other storage 

 organs, they remain colorless but become active in the conversion of 

 soluble carbohydrates into granules of storage starch; 

 such leucoplasts are therefore known as aniyloplasts 

 (Figs. 20, 31). Hence the carbohydrate appearing as 

 starch in a potato tuber has been through two plastids. 

 It was first elaborated in chloroplasts in the leaf as a 

 soluble sugar; if not removed at once, it was converted 

 by enzymes into visible starch granules. Later this 

 starch was reconverted to sugar and transported to the 

 tuber, whore, within the amyloplasts, it was again trans- 

 formed into granules of starch. In some plants, e.g., 

 the yellow-green algae, diatoms, and the onion plant, 

 the visible products of synthetic activity are fats. 



Of the greatest interest is the recent announcement 

 that the cellulose of plant cell w-alls is elaborated in 

 minute colorless plastids in higher plants and in 

 chloroplasts in an alga. We shall revert to this impor- 

 tant subject in Chap. VI. 



Plastids of higher plants are often csiWed chromoplnsts 

 when they show some color other than green; in literal 

 terms, however, a chloroplast is also a chromoplast, or 

 chromato}:)hore. The ordinary tomato fruit is red 

 l)ecause lycopene, related to carotene, appears in the 

 chloroplasts during ripening. Nasturtiums owe their 

 yellow color, though not their red, to their chromo- 

 plasts. Such special pigments may develop in leuco- 

 plasts or in chloroplasts. The red, light-sensitive 

 eyespots of certain flagellate and algal cells seem to be 

 plastid-like differentiations. Other eyespots have a 

 different origin. 



Structure of the CJdoroplast. — The fact that a chloroplast sw"ells in 

 distilled water or a hypotonic solution and the manner in which its 

 boundary often appears to separate from the green substance under such 

 conditions indicate the presence of a limiting membrane with osmotic 

 properties. The ground substance (stroma) of the chloroplast appears as 

 colorless cytoplasm. B}^ grinding and centrifugation it is possible to 

 separate the plastids from other cell components and to show that 50 

 l)er cent of the i)rotein in tobacco leaves is in the plastids. The (;hloro- 



FiG. 20.— Cells 

 of pea root tip. 

 The elongate leu- 

 coplasts contain 

 starch grains. 

 Note their ar- 

 rangement near 

 poles of dividing 

 nuclei. (After R. 

 H. Bowcn.) 



