114 



CELLS AND TISSUES 



when under the microscope, can be seen streaming around the cell 

 wall or across the cell from side to side or end to end. 



The protoplasm consists of a number of structures which differ in 

 organization, and each of which has one or more special functions. 

 (Fig. 106.) One of the most conspicuous of these structures is the 

 nucleus, which is a comparatively compact protoplasmic body, 

 usually spherical in shape. Although usually centrally located 

 in actively growing cells, the nucleus commonly has a lateral posi- 

 tion in old cells. The nucleus is enclosed by a membrane, called 



the nuclear membrane, and is 

 filled with a liquid known as 

 nuclear sap, which consists of 

 water and dissolved substances. 

 However, nuclear sap is usually 

 colorless and, therefore, not vis- 

 ible. Within the nucleus also 

 occur one or more small globu- 

 lar bodies known as nucleoli 

 (singular nucleolus) and much 

 chunky or granular material 

 known as chromatin, which is 

 regarded as the most impor- 

 tant part of the nucleus and is 

 so named because it stains so 

 readily when stains are applied 

 to the cell. Around the nu- 

 cleus and filling up the general 

 cavity within the cell wall is 

 that portion of the protoplasm, 

 known as cytoplasm, which is a loose spongy structure full of many 

 cavities called vacuoles. The vacuoles are filled with a liquid called 

 cell sap, which like the nuclear sap consists of water containing 

 dissolved sugars, salts, and other substances. The border of 

 the cytoplasm is in contact with the cell wall and is modified 

 into a membrane known as the cell membrane, which, since it is 

 closely applied to the cell wall, can not ordinarily be seen until 

 the cell is bathed in salt water or some other solution strong 

 enough to shrink the protoplasm, so that the cell membrane is 

 drawn away from the wall where it can be seen. (Fig. 107.) 

 Within the cytoplasm commonly occur a number of small bodies 



FIG. 106. A growing cell, w, cell 

 wall; c, cytoplasm; v, vacuoles filled 

 with cell sap; n, nucleus; a, nucleoli; 

 m, nuclear membrane; g, chromatin 

 granules. Enlarged about five hun- 

 dred times. 



