198 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



discouraged, for it seems unlikely in the extreme that the structural 

 features of plant protoplasm can be essentially different from that in 

 animals. By the time that the Cladophora study had been begun it was 

 already known that there were isolated examples of what looked like 

 protein orientation in the cytoplasm of the algae. Recently, however, 

 this possibility has been enormously strengthened through the investi- 

 gation by Picken(67), using optical and X-ray methods much as 

 described in this book, of the growth of Lepidoptera scales. In the 

 adult scales Picken has shown what Rudall(68) had already found for 

 insect cuticles, that protein and chitin chains in the chitin-protein 

 complex are similarly oriented to some morphological axis. Even more 

 important than this, however, Picken has obtained birefringence data 

 which suggest that, at an early period of growth when the structures 

 concerned are mainly protein, the protein chains are nevertheless 

 oriented parallel to the direction of growth. This lends further support 

 to the statements by Monne(69) and by Schmidt (70) that the fine 

 structure of the cytoplasm is a function of the form of the cell. The 

 growing protein-chitin complex resembles in some respects the cellulose- 

 protein complex which we have found to be present in the growing 

 plant cell wall. The earlier development of the protein suggests clearly 

 that this, as it were, forms the template upon which the subsequent 

 molecular chains, of chitin in the one case and cellulose in the other, 

 are oriented. This is, of course, as yet pure speculation but, if it turns 

 out to be true, then the evidence before us at the moment would 

 suggest that the spatial relationships between cellulose and protein 

 might be different from that between chitin and protein. For 

 whereas chitin chains like the proteins seem to be oriented parallel 

 to the direction of growth, cellulose chains are often laid down 

 perpendicular to this direction. The different association of chitin 

 and cellulose might well arise through the — NHCOCH3 groups of 

 the former. 



If our assessment of the present position is correct, then it seems 

 rather likely that the proteins of the cytoplasm form an organized 

 system which is responsible for the orientation, as well as the con- 

 struction, of the cellulose chains. This makes it further possible to 

 suggest that the proteins concerned, whether in the growing cell or in 

 the later development of a secondary wall, are of the nature of carbo- 

 hydrases. If this is so, then it becomes considerably easier to conceive 

 of the increase in area of the wall during growth without reorientation; 

 for the whole structure is then very labile and we could imagine the 

 breaking of bonds and the insertion of new material in such a way that 



