CELLS AND TISSUES 



35 



secreted by the protoplasm. It is pierced by fine holes, through which 

 substances may pass and the cytoplasm of one cell may connect with that 

 of adjacent cells. These tough, firm cell walls provide support to the 

 plant body. 



The nucleus of the cell is usually spherical or ovoid. It may have a 

 fixed position in the center of the cell or at one side, or it may be moved 

 around as the cell moves and changes shape. The nucleus is separated 

 from the cytoplasm by a nuclear membrane which controls the move- 

 ment of materials into and out of the nucleus (Fig. 3.2). Recent studies 

 with the electron microscope have shown that there are extremely fine 





Figure 3.2. -i, Electron micrograph of the 

 nucleus and surrounding cytoplasm of a frog 

 liver cell. The spaghetti-like strands of the 

 microsomes are visible in the lower right cor- 

 ner. Magnified 16,500 X. B, High power elec- 

 tron micrograph of mitochondria and micro- 

 somes within a rat liver cell. Granules of 

 ribonucleoprotein are seen on the strands of 

 microsomes, and structures with double mem- 

 branes are evident within the mitochondria 

 in the upper left corner and on the right. 

 Magnified 65,000 X. (Electron micrographs 

 courtesy Dr. Don Fawcett.) (Villee: Biology.) 



