v] OF ASYMMETRY AND ANISOTROPY 403 



membrane; and that this molecular constitution was anisotropous, 

 in such a way as to render extension easier in one direction than 

 another. Such a lack of homogeneity or of isotropy in the 

 cell-wall is often rendered visible, especially in plant-cells, in the 

 form of concentric lamellae, annular and spiral striations, and the 

 like. But there exists yet another heterogeneity, to help us account 

 for the long threads, hairs, fibres, cylinders, which are so often 

 formed. Carl NageH said many years ago that organised bodies, 

 starch-grains, cellulose and protoplasm itself, consisted of invisible 

 particles, each an aggregate of many molecules — he called them 

 micellae; and these were isolated, or "dispersed" as we should say, 

 in a watery medium. This theory was, to begin with, an attempt to 

 account for the colloid state; but at the same time, the particles 

 were supposed to be so ordered and arranged as to render the 

 substance anisotropic, to confer on it vectorial properties as we say 

 nowadays, and so to account for the polarisation of light by a starch- 

 grain or a hair. It was so criticised by Biitschli and von Ebner that 

 it fell into disrepute, if not oblivion; but a great part of it was true. 

 And the micellar structure of wool, cotton, silk and similar substances 

 is now rendered clearly visible by the same X-ray methods as 

 revealed the molecular orientation, or lattice-structure, of a crystal 

 to von Laue. 



It is now well known that the cell-wall has in many cases a definite structure 

 which depends on molecular assemblages in the material of which it is com- 

 posed, and is made visible by X-rays in the form of "diffraction patterns". 

 The green alga Valonia has very large bubbly cells, 2-3 centimetres long, with 

 cell-walls formed, as usual, of cellulose; this substance is a polysaccharide, 

 with long-chain molecules some 500 Angstrom-units, or say 0-05 /x long, 

 bound together sideways to form a multiple sheet or three-dimensional lattice. 

 In the cell-wall of Valonia one set of chains runs round in a left-handed 

 spiral, another forms meridians from pole to pole, and these two layers 

 are superposed alternately to build the wall. Hemp has two layers, both 

 running in right-handed spirals; flax two layers, crossing and recrossing in 

 spirals of opposite sign. Even the cytoplasm and its contents seem to be 

 influenced by molecular ''lignes directrices,'''' corresponding to the striae of 

 the cell-wall. Analogous but still more complicated results of molecular 

 structure are to be found in wool, cotton and other fibres*. 



* Cf. R. D. Preston, Phil. Trans. (B), ccxxiv, p. 131, 1934: Preston and Astbury, 

 Proc. R.S. (B), cxxii, pp. 76-97, 1937; and many other important papers by 

 Astbury, van Iterson, Heyn, and others. We are brought by them to a borderland 



