58 



THE RESTING AND DIVIDING CELL 



growth, and reproduction. A cell without a nucleus (the nucleus 

 having been removed with a tiny knife) will not live. Without a 

 nucleus, although food is present, the cell cannot take in food, nor 



use food, and its respiration 

 becomes greatly reduced. 



The cytoplasm is the living 

 material of the cell, surround- 

 ncccleoKcs- 



cell 



pi CCS m.ct 



membrane- 



C^-toplccsm 

 vctcvrole.... 



A plant cell has a cell wall. When viewed 

 through a microscope large vacuoles are generally 

 seen in the cytoplasm. 



ing the nucleus. It is semi- 

 fluid, less dense than the 

 nucleoplasm, and is always 

 in motion throughout the cell. 

 The cell wall is found as 

 an additional bounding layer 

 around plant cells. It is com- 

 posed of a substance, cellulose, 

 which has the elements found 

 in starch ; namely, carbon, hydrogen, and oxygen. Cytoplasmic 

 threads penetrate into the cell wall, and cause it to grow in thick- 

 ness by depositing woody material. As the cell gets older, more 

 and more woody material is deposited. Within this thickened 

 wall of the plant cell is the plasma membrane. In the animal cell 

 the plasma membrane is the only bounding layer ; it lacks the 

 material which makes up the plant cell wall. The plasma mem- 

 brane is a very delicate layer of dense cytoplasm and has three 

 functions in plants and animals. It regulates osmosis by permit- 

 ting the entrance of needed materials. It determines the shape or 

 form of the cell. It affords protection against loss of water. 



A vacuole is a space in a plant cell which becomes filled with a 

 liquid called the cell sap. The cell sap contains a little dissolved 

 sugar, mineral salts, crystals (waste and storage products of the 

 plant cell), and a great deal of water. The presence of a vacuole 

 helps the plant cell to swell up and become very large even 

 though only a small amount of cytoplasm is present. This cyto- 



