THE SEED 5 



ing membrane which contains a living substance called 

 protoplasm. This is the essential part of the cell, and, so 

 far as we know at present, the physical basis of all Hfe. 

 Cells are commonly more or less rounded in shape, though 

 they take different forms according to the purpose they 

 serve. Sometimes, as in the fibers of cotton and the down 

 of young leaves, they are long and hairlike ; when closely 

 packed, they often become angular by pressure, like those 

 shown in Figs. 10, 11. The cells composing the thick body of 

 the bean are for the most part starch and other substances 

 stored up for food, which render observation difficult. It 

 will, therefore, be better to choose for a study of the indi- 

 vidual cell some kind that will show the essential parts more 

 distinctly. 



7. Microscopic examination of a cell. — Place under a high 

 power of the microscope a portion of fresh skin from one of 

 the inside scales of an onion, or a piece 

 of the root tip of a very young corn or oat 

 seedling, and fix your attention on one of 

 the individual cells. Notice (1) the cell -w 



wall or inclosing membrane, w (Fig. 11) ; ^ 



(2) the protoplasm, p, which may be 

 recognized by its granular appearance ; 



(3) thenwdeiis, n; and (4) thecellsap, s. "^i , . 



In very young cells the protoplasm will y^Sjl^ ^ 

 be seen to fill most of the interior; but p^^ 11— Typical cells: 

 in mature ones, like the large one on the «. nucleus ; p, protoplasm ; 

 right of the figure, it forms a thin lining ^' ^^ ^^ 

 around the wall, with the nucleus on one side, while the cell 

 sap, composed of various substances in solution, occupies the 

 central portion. Though there is generally an inclosing wall, 

 this is not essential, its office being to give strength and me- 

 chanical support by holding the contents together, as an 

 India-rubber bag holds water. It is the turgidity of the cell, 

 when distended with liquid, that gives firmness to herba- 

 ceous plants and the tender parts of woody ones. This 



