ELEMENTS OF OEGANIZED STRUCTURE. 249 



nnthickened portions of the cell-wall. The cells in fig. 32 exhibit each 

 a central nucleus surrounded by grains of aleuroiie. 



Cell Contents. Besides the protoplasm and nucleus, 

 the cell usually contains a variety of bodies, which have 

 been, indeed, noticed already as ingredients of the plant, 

 but which may be here recapitulated. Many cells are 

 altogether empty, and consist of nothing but the cell- 

 wall. Such are found in the bark or epidermis of most 

 plants, and often in the pith, and although they remain 

 connected with the actually living parts, they have no 

 longer any proper life in themselves. 



All living or active cells are distended with liquid. 

 This consists of water, which holds in solution gum, dex- 

 trin, inulin, the sugars, albuminoids, organic acids, and 

 other vegetable principles, together with various salts, 

 both of organic and mineral acids, and constitutes the 

 sap of the plant. In oil-plants, droplets of oil occupy 

 certain cells, Fig. 17, p. 83; while in numerous kinds of 

 vegetation colored and milky juices are found in certain 

 spaces or channels between the cells. 



The water of the cell comes from the soil, or in some 

 cases from the air. The matters, which are dissolved in 

 the sap of the plant, together with the semi-solid proto- 

 plasm, undergo transformations resulting in the produc- 

 tion of various solid substances. By observing the sev- 

 eral parts of a plant at the successive stages of its devel- 

 opment, under the microscope, we are able to trace 

 within the cells the formation and growth of starch- 

 grains, of granular or crystalline bodies consisting chiefly 

 of albuminoids, and of the various matters which give 

 color to leaves and flowers. 



The circumstances under which a cell develops deter- 

 mine the character of its contents. The outer cells of 

 the potato-tuber are incrusted with corky matter, the 

 inner ones are for the most part filled with starch. 



In oats, wheat, and other cereals, we find, just within 



