PROBLEM I . The Couiposition of Lhing Things 



Cell membrane 



107 



•Cell body (cyfop/asm)- 



Nucleus {nucleoplasm)- 



Animal 



Fig. 140 (above) All cells have three d'niicn- 

 sions. In the animal cell as in the plant cell there 

 are two kinds of protoplasju. What other parts 

 does the plant cell have? 



Fig. 141 (right) Cell from root tip of Trades- 

 cantia plant. What are the parts of this plant 

 cell? Which parts are livijig? Which are life- 

 less? Conipare this typical plant cell with the 

 mouth lining cells of Figure 1^9. 



The nucleus of the cell. Cell bodies 

 are of many different shapes but nuclei 

 are all much alike in shape and structure. 

 In many cells they lie near the center 

 of the cell with cytoplasm all around. 

 Every nucleus has its own nuclear mem- 

 brane which separates it from the cell 

 body. The protoplasm of the nucleus is 

 denser and less liquid than the protoplasm 

 of the cell body. By means of micro- 

 needles used with the aid of a powerful 

 microscope the nucleus can be pulled 

 out of a cell. This shows that it is of 



Plant 



Vacuole 

 Cell wa 



Cell body 

 (cyfoplasmj 



Cell membranes 



leolus 



Vacuole 



firmer consistency than the cytoplasm. 

 All nuclei contain a special substance 

 that differs from other substances in the 

 cell in that it stains deeply with certain 

 dyes. Because of this substance a stained 

 nucleus shows up clearly under the mi- 

 croscope. The living unstained nucleus 

 is difficult to see. The material that takes 

 the stain is present in a network or as 

 scattered granules; it is called chro- 

 matin (crow'mat-in). You will read 

 much more about chromatin later, for 

 the nucleus with its chromatin plays a 



