100 THE MOLiECULAR AlRCHITECTURE OF PLANT CELL WALLS 



mathematically exact since, among other things, the shape of the cell 

 cannot be defined with precision. Roughly speaking, however, the 

 spiral is formed in such a way that it makes a constant angle with the 

 meridians at each point of the wall. 



In Valonia therefore, we have now a remarkably clear picture of the 

 structure in the wall. The most important problem of all, however, and 

 one which strikes deep at the roots of life itself, still remains unsolved. 

 This problem will turn up again and again and under conditions where 

 an attack seems more feasible, but this first example cannot be allowed 

 to pass without some mention being made of it. At each point of the 

 wall here, and over the whole surface, the submicroscopic layers 

 alternate regularly in chain direction. As the wall is being deposited, 

 therefore, and after one such layer has been laid down, there must be a 

 sudden "switch" in some condition which involves the laying down of 

 further material with the chains oriented in a direction nearly at right 

 angles to that in the former layer. When this layer is completed, the 

 process is repeated; but now comes the crux of the whole matter. In 

 the third layer the chains, instead of being laid down in any direction, 

 are laid parallel to those in the last layer but one. Now once a set of 

 chains has been laid down it is quite conceivable that, unless something 

 catastrophic happens, chains will continue to be laid down in the same 

 direction by a sort of crystallization process. It is impossible to con- 

 ceive, however, of an orienting effect of one layer through another in 

 which the orientation is different. The conclusion seems inescapable 

 that the mechanism responsible for orientation resides, not in the wall, 

 but in the cytoplasm and probably at the very surface of the cytoplasm 

 which is in contact with the wall. This is, of course, self-evident when 

 a new cell wall is laid down either at a division or over the surface of a 

 naked egg. Here we see that, even when a well-oriented wall is present, 

 the orienting mechanism in the cytoplasm still takes precedence. 



The problem to be solved has therefore at least two aspects. We may 

 inquire, firstly, what changes either in the cytoplasm or in the sur- 

 roundings, or both, cause the chain direction in the wall to be changed; 

 and, secondly, why it is that alternate layers nevertheless have the same 

 direction, a direction maintained over some 400 such "switches". These 

 can be solved only through careful observations on material cultured 

 in the laboratory (since the observational apparatus is too cumbersome 

 to be carried to a remote sea coast). Unfortunately Valonia is difficult 

 to keep in culture for long periods and until this is done, use must be 

 made of other algae which show the same phenomenon and which can 

 be collected regularly from less remote habitats. In some ways these 



