INTRODUCTION. 



5 



green by a substance known as chlorophyll. These are con- 

 sequently known as chloroplasts or chlorophyll-bodies (figs. 

 3, 4). In other cells, particularly those for the storage of 

 food, they may develop into smaller, denser, flattened or 

 roundish, uncolored bodies, called leucoplasts (figs. 5, 6, 7). 

 These may act either as starch-accumulators, or in case 



FIG. 6. 



FIG. 7. 



F IG g Part of the cell-contents of an inner cell of white potato, z, nucleus; j, 

 starch-grains, each having been formed by a leucoplast, /, which is still attached to 

 one side of the grain; , crystalloid. Magnified abont 1000 diam. After Zimmer- 

 mann. 



FIG. 7. Leucoplasts in place in a young cell of a leaf of vanilla. /, leucoplasts; z, 

 nucleus; e. an oil-former or elaioplast. The unshaded spaces surrounded by proto- 

 plasm are vacuoles. Magnified about 1000 diam. After Wakker. 



of need, in young cells, may even be converted into chloro- 

 plasts. In other cells, particularly in highly colored parts, 

 the plastids may become of most diverse form and size, and 

 colored red or yellow, whence they are called chromoplasts 

 or color-bodies (figs. 8, 9). 



9. Wall.- -The cell-wall is formed by the protoplasm. 

 In green plants when first formed it consists chiefly of cell- 

 ulose, with which, as it grows older, various other substances 

 may be mixed.' Some of these, such as pectin, are present 

 even in the young wall, and may increase with age ; others 



