CELL-CONTENTS AND FORMS OF CELLS. 137 



phyll, they are known as CHLOROPLASTIDS or chloroplasts. (3) In 

 other cases, independently of the position of the cells as to light 

 or darkness, the plastids develop a yellowish or orange-colored 

 principle, which may be termed chromophyll, and are known as 

 CHROMOPLASTIDS. Chloroplastids are found in all plants except 

 Fungi and non-chlorophyllous flowering plants, and chromoplas- 

 tids in all plants except Fungi. Plastids vary in form from more 

 or less spherical to polygonal or irregular-shaped bodies, and 

 they increase in number by simple fission. They suffer decom- 

 position much more readily than the nucleus, and are found in 

 dried material in a more or less altered condition. 



Leucoplastids. The chief function of the leucoplastids is 

 that of building up reserve starches or those stored by the plant 

 for food, and they may be best studied in the common potato 

 tuber, rhizome of iris, and the overground tubers of Phaius (Fig. 

 2, b). The reserve starches are formed by the leucoplastids from 

 sugar and other soluble carbohydrates. 



The chloroplastids occur in all the green parts of plants 

 (see Frontispiece). They vary from 3 to n /*, in diameter and 

 are more or less spherical or lenticular in shape, except in the 

 Algae, where they are large and in the shape of bands or disks 

 (Figs. 8 and 9) , and generally spoken of as chromatophores. Chlo- 

 Toplastids are found in greater abundance in the cells near the 

 upper surface of the leaf than upon the under surface, the pro- 

 portion being about five to one. These grains, upon close exam- 

 ination, are found to consist of (i) a colorless stroma, or liquid, 

 in which are embedded (2) green granules; (3) colorless gran- 

 ules; (4) protein masses; (5) starch grains; and (6) a mem- 

 brane which surrounds the whole. The green granules are looked 

 upon as the photosynthetic bodies; the colorless grains are sup- 

 posed to assist in the storing of starch or in the production of 

 amylase, the conditions for these processes being directly opposite, 

 i.e., when photosynthesis is active, starch is stored, and when 

 this process is not going on, as at night, amylase is produced and 

 the starch is dissolved. The protein grains may be in the nature 

 of a reserve material of the plastid and probably are also formed 

 in connection with photosynthetic products. 



While the protoplasm has been termed by Huxley " The phys- 



