4 STUDIES IN GELS I 29 



to contradict the principle of short-range order, .,s the gel strands 

 show a criss-cross random arrangement, but we have to remember 

 that the picture represents a projection of the reticular texture, because 

 the great focal depth of the electron microscope causes gel strands 

 separated in space to be imaged in a single plane. It is likely that the 

 filaments crossing each other are not lying at the same depth in the gel, 

 but that the majority are oblique with respect to the image plane, as is 

 apparent from the faintness of outline of numerous strand "ends". 

 A stereoscopic view of Fig. 86a justifies the comparison of a gel with 

 a wad of cotton wool. At various points ramifications of the gel strings 

 are visible, showing that, notwithstanding the apparent criss-cross 

 arrangement of the gel strands, there exists short-range order. The 

 figure further shows that, in the case of a gel thickness corresponding 

 to an ultrafilter, all possible orientations occur in spite of the short- 

 range order, so that there exists statistical isotropy, as indicated in 

 Fig. 5 3 (p. 69). The curved micellar strands which are visible in Fig. 

 86b are particularly interesting because they favour branching of the 

 strands (cf. Muhlethaler, 1949). 



As shown by Fig. 58 (blood fibrin, Wolpers and Ruska, 1939), the 

 reticular structure postulated has also been found in biological gels. 

 It can also be observed in gels of bacterial cellulose (Frey-Wyssling 

 and Muhlethaler, 1946), where we found cellulose strands of about 

 250 A diameter. Later the same strands were discovered in cell walls 

 (Frey-Wyssling, Muhlethaler and Wyckoff, 1948). Fig. 86b 

 shows the growing tip of the cellulose wall in the end cell of a thread 

 of the alga Spirog^ra. 



Whereas the gel strands of vanadium pentoxide (Fig. 86a), due 

 to the atomic number 23 of ^^V, produce sufficient contrast in the 

 electron microscope, the cellulose strands with ^^C must be shadowed 

 to produce distinct micrographs. Figs. 86b-d show how well high- 

 relief pictures of gels can be obtained if they are properly prepared and 

 shadowed. An important prerequisite to obtaining such results is the 

 complete removal of any incrusting material. In contrast to VgOg, 

 biological gels are not only full of water, but also incrusted with all 

 kinds of amorphous substances, such as hemicelluloses and lignins 

 in plant cell walls or proteins in animal skeleton materials. Thus Fig. 

 86d represents tunicin (Tunicate cellulose) from the mantle of Ciona. 

 All accompanying substances have been removed, so only the strands 



