scopic structures revealed with the electron microscope and to determine 

 the wavelength of radiations. The dimensions of various molecules, 

 particles, and cells, in microns, as compared with the wavelengths of differ- 

 ent radiations measured in angstrom units are shown in Table 2-1. 



TABLE 2-1. Comparison Between the Diameters of Molecules, Viruses, Bacteria, and Cells. 

 The lower limits of resolution are shown for microscopes employing visible light, ultraviolet 



light, and electron beams.* 



Lower limit of resolution 

 practical in electron 

 microscopical work 



r 



Lower limit of resolution of ultraviolet microscope 

 Lower limit of resolution of light microscope 



Molecules Viruses Bacteria Cells 



* Data from Windle, W. F., 1960. "Textbook of Histology," 3rd ed., McGraw- 

 Hill Book Co., New York, N. Y., Fig. 1-1, p. 2. 



Shape, like size, is highly variable, ranging from spherical to columnar 

 and including amorphous types which have no specific geometrical for- 

 mula (Figure 2-1). Most of this variation can be assigned more or less 

 adequately to extraneous factors such as mechanical pressure and surface 

 tension. A naked protoplast left to itself tends to approach the spherical 

 as an ideal rarely realized in nature except to a certain extent in the case 

 of gamete mother cells and possibly certain blood cells. That there is a 

 close correlation between shape and function is generally admitted, but 

 again the question has received relatively little attention. A few such 

 associations appear obvious, as, for example, the varying shape of an 

 ameba with motility, the spindle shape of a smooth muscle cell with 

 undirectional contraction and elongation, and the columnar shape of a 

 vascular element in plants with transport of sap. How such shapes be- 

 come established is, however, an unsolved problem bound up with the 

 whole question of growth and differentiation. In his delightful book, 

 "Growth and Form," D'Arcy Thompson discusses the problem of cell 

 shape particularly in relation to equilibrium figures arising from surface 

 tension phenomena. From this discussion it would appear likely that 

 internal stresses, as well as external pressures, dictate shape. 



GENERAL MORPHOLOGY AND CHEMISTRY OF THE CELL / 9 



