COMPOSITION OF LIVING THINGS 



29 



an onion shows well, as do thin sections of a young stem, as the bean or 

 pea. I have found one of the best places to study a tissue and the cells 

 of which it is composed in the leaf of a green water plant, Elodea. In this 

 plant the cells are large, and not only the outline of the cells, but the move- 

 ment of the living matter within the cells, may easily be seen, and most 

 of the parts described in the next paragraph can be demonstrated. 



Cells. A cell may be defined as a tiny mass of living matter, either 

 living alone or forming the building material of a living thing. The 

 living matter of which all cells are formed is known as protoplasm 

 (from two Greek works meaning first form). When viewed under a 

 high magnification of a compound microscope, it is a grayish, semi- 

 fluid mass, seemingly almost devoid of any structure. A careful 

 observer will find, however, that the material seems to be made of 

 a ground mass of fluid with 

 innumerable granules of 

 various size and form float- 

 ing in the fluid portion. 

 All plant and animal cells 

 appear to be alike in the 

 fact that every living cell 

 possesses a structure 

 known as the nucleus (pi. 

 nuclei}, which is found 

 within the body of the cell. 



The nucleus is composed of 

 living matter like the rest of 

 the cell, although it seems to 

 differ in some chemical way 

 from that part of the cell sur- 

 rounding it. This is seen 

 when a plant or animal is 

 placed in a liquid containing 

 some dye such as logwood. 

 Certain bodies in the nucleus 

 take up the stain much more 

 readily than the rest of the 



living matter of the cell, taking on a deep black color. They are thus 

 called the chromosomes (color-bearing bodies). 



The chromosomes, which are believed to be always definite in number for 

 every tissue cell, are of much interest to scientists. It is found that each 



Cf 



Diagram of a cell (after Wilson). The cell 

 protoplasm contains spaces to hold liquid cell 

 sap (G.s.) ; just above the nucleus (N.I.) is 

 a structure called the- centrosome (c), which 

 aids in cell division ; within the nucleus are 

 chromosomes (N.n.), which form a network; 

 t.n., nucleolus ; C.p., plastids; <?./., lifeless 

 material in the cell. 



