Protoplasm, ihe Cell, and the Organism - 17 



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Fig. 2-1. Photomicrograph of typical cells in a stained 

 section of the root of an onion plant. Note that each 

 nucleus appears as a dark round body surrounded by 

 the lighter cytoplasm. In a few cells the nuclei are not 

 in focus. Also some of the cells are dividing, in which 

 case the chromosomes (p. 41) are visible. (Copyright, 

 General Biological Supply House, Inc.) 



but no nuclear membrane segregates this 

 from the cytoplasm. Some caution is neces- 

 sary, however, because these cells are ex- 

 tremely small and the details of their struc- 

 ture are difficult to observe. Until about 10 

 years ago it was generally believed that bac- 

 terial cells possessed such a chromidial struc- 

 ture, but now it is recognized that most, if 

 not all, bacteria do have definitive nuclei. 



Another relatively rare condition is dis- 

 played by the slime molds (p. 602), certain 

 green algae (p. 596), and the fibers of the skel- 

 etal type of muscle (Fig. 15-14). In these cases, 

 no limiting membranes are found separating 

 the individual "cells." Rather, several nuclei 

 are enveloped within a common mass of cyto- 

 plasm. Such an atypical arrangement is called 

 a syncytium (Fig. 2-4). Some biologists have 

 designated both of these atypical conditions 

 — whether chromidial or syncytial in nature 

 — as "noncellular." But it seems more logical 

 to regard them as variations of the normal 



protoplasmic pattern, differing from the 

 usual mainly in the lack of limiting mem- 

 branes — between the nucleus and cytoplasm 

 in the first case and between adjacent cells 

 in the second. In any event, extensive evi- 

 dence is available (p. 34) showing that no 

 "cell" can long endure or carry on all of its 

 vital activities in the complete absence of 

 either its nuclear or cytoplasmic components 

 (p. 35). The nucleus and cytoplasm, there- 

 fore, must be regarded as complementary, 

 mutually dependent parts of one protoplas- 

 mic unit, namely the cell. 



The Size of Cells. A great majority of cells 

 are too small to be seen with the naked eye, 

 which can resolve an object only if the diam- 

 eter is not less than about 0.1 millimeter 

 (mm). However, most cells can be seen plainly 

 with the microscope, which extends the range 

 of vision a thousandfold — down to diameters 

 of about 0.1 micron (/i). Many bacteria — 

 which are among the smallest cells — lie at the 

 very lowest limit of microscopic visibility 

 (Figs. 2-5 and 2-6). At the other extreme, the 

 largest single cells are the egg cells of birds. 

 This kind of cell — which popularly is desig- 

 nated as the "yolk of the egg" — may measure 

 more than 3 centimeters (cm) in diameter, as 

 in the case of the ostrich egg. But such exam- 

 ples are quite rare. Most cells, in both plants 

 and animals, have dimensions between 1 and 

 100 microns, and thus most cells lie definitely 

 within the range of the compound micro- 

 scope. 



There is a natural limit to the growth of 

 any cell. To support metabolism the cell 

 must be able to obtain an adequate supply 

 of oxygen and other foods, and to give off 

 carbon dioxide and other wastes fast enough 

 to avoid accumulation. These necessary ex- 

 changes between the cell and the environ- 

 ment can occur only at the surface of the cell, 

 whereas metabolic activity pervades the en- 

 tire protoplasmic mass. Consequently, the 

 surface of the cell must be kept adequately 

 large in proportion to the protoplasmic vol- 

 ume. But as a cell grows larger, particularly 

 if its shape is compact and rounded, the 



