The Scale of Structural Units in Biopoesis 



391 



will be formed which, if the transverse direction is limited, will have the struc- 

 ture of a roving or thread according to the degree of twist. 



The case of long particles of equal length occurs predominantly in organized 

 nature for derivatives of long-carbon-chain compovmds, fatty acids, phospho- 

 lipids, lipoproteins. These seem to be the basis of the double layers found in 

 almost all cells from which, by a spiral rolling up, are formed the myelin sheaths 

 of nerves. With them are associated sterols and other more complex elongated 

 molecules of approximately the same length, 20 Â-25 Â, as the predominant 

 C16-C20 straight-chain hydrocarbons. 



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(>) (c) (b) 



Fig. 5. Modes of regular packing of elongated particles. 



(a) — when these are of equal length leading to multiple sheet structure ; (b) — when 



these are of unequal length leading to tactoid or fibre structure; (c) — illustrates 



the hexagonal net packing common to (a) and (b). 



The packing of hehcal particles offers more complexities. Close-coiled heUces 

 approximate to cylinders and pack accordingly in hexagonal close packing. If, 

 however, the helices are of the same sense their coils may interlock as appears 

 to be the case for tobacco mosaic virus [7] (Fig. 6). This process may go further 

 still in more loosely coiled helices. Where the pitch is equal to twice or more of 

 the diameter of the hehcal particle, more than one helix may share a common 

 axis, leading to the two- or three-fold twined heUces such as are found in deoxy- 

 ribosenucleic acid (DNA) and in collagen. Such compound hehces may them- 

 selves be straight or may have an axis forming another hehx of different pitch 

 (coüed coil). This may lead in turn to further coihng and so to cable-Hke struc- 

 tures of almost indefinite complexity (Fig. 6). 



These considerations are all of an extremely general kind, involving hardly 

 more than the geometrical consequences of the arrangements of impenetrable 

 soUds whose shape can in turn be evolved from the aggregation of spherical 

 particles. Such a phenomenon as crystallization, in the sense of indefinite regular 

 aggregations, can be exemplified by structures on a variety of scales from atoms, 

 viruses, and bacteria [8] up to matches and cannon balls. To correspond with an 

 actual scale of structures other more physical conditions have to be invoked 

 and filled in to the more abstract geometrical ones already described. If we 

 confine ourselves to structures stable in water and ionic solutions, and this is 



