22 - The Cell 



CYTOPLASM- 



NUCLEUS 



Fig. 2-9. Epidermal cells of root showing develop- 

 ment of root hairs. Even the longest (uppermost) of 

 these root hairs is not fully mature. 



cell or tissue, using strong acids or toxic 

 metals, in order to stabilize the labile proto- 

 plasmic pattern and to obtain sharp resolu- 

 tion of structure. In addition, the cells are 

 frequently stained with special dyes, which 

 differentially color the various protoplasmic 

 parts. Tissues and other large masses of cells 

 are too opaque for direct microscopic study, 

 and such material must be cut into thin trans- 

 parent slices, or sections. Electron microscopy 

 not only requires that the cell be cut into 

 exceedingly thin (usually less than 1.0 mi- 

 cron) sections, but also that the entire water 

 content of the protoplasm be removed, in 

 order to achieve an adequate transmission of 

 the electron beam. Such drastic treatments, 

 especially complete desiccation in vacuum, 

 must alter the protoplasmic structure, leav- 

 ing only a derivative of the original pattern. 

 A study of these derivatives has proved most 

 fruitful and informative, but it is always im- 

 portant to check and recheck such observa- 

 tions, comparing them, whenever possible, 

 with studies on the living cell. 



The Nucleus. In the living cell, the nucleus 

 is hard to see, although usually a good micro- 

 scopist can bring it into view. The difficulty 

 arises from the fact that the nuclear and cy- 

 toplasmic parts of the protoplasm have many 

 common properties. Typically both are col- 

 orless, transparent, and fluid and both have a 

 broadly comparable chemical constitution 

 (Chap. 4). Consequently it is not surprising 

 to find that the optical differentiation be- 

 tween the nucleus and cytoplasm is often very 

 slight. 



When a nondividing cell is stained by any 

 of the conventional techniques of cytology, 

 the nucleus is plainly visible, since generally 

 it takes on a deeper color than the cytoplasm 

 (Fig. 2-1). Certain dyes, especially hema- 

 toxylin, display a distinct affinity for the 

 nuclear materials. However, such nuclear 

 dyes do not stain all parts of the nucleus with 

 equal intensity. There appears to be a net- 

 work of relatively more solid material that 

 stains very intensely. Collectively this densely 

 stainable material is referred to as chromatin. 

 The chromatin network, however, appears 

 to be suspended in a more fluid, nonstain- 

 able, achromatic material. One to several 

 larger masses of chromatin material, the nu- 

 cleoli, are usually found, closely associated 

 with the chromatin network (Fig. 2-1). 



The precise nature of the chromatin net- 

 work of the nondividing nucleus remains 

 somewhat debatable. However, considerable 

 evidence (Chap. 27) indicates that the chro- 

 matin network is made up of a specific num- 

 ber of elongate threads looped and massed 

 together in such a fashion that they give the 

 appearance of a network. These threads are 

 the chromosomes (p. 475). The chromatin ma- 

 terial of the chromosomes consists, essentially, 

 of nucleoproteins, especially DNA proteins, 

 although some RNA proteins are also repre- 

 sented in the nucleus. DNA proteins occur in 

 almost infinite variety, however, if we con- 

 sider the precise pattern of their chemical 

 structure (p. 521). In fact, the DNA proteins 

 of each organism are uniquely distinctive of 

 that organism. These, as we shall see, are the 



