WALL STRUCTURE IN THICK CELL WALLS HI 



prominent in these diagrams. In addition, however, there is often 

 an arc corresponding to planes spaced 12-5 A. apart and oriented 

 parallel to the 7-4 A. planes. This possibly implies the presence of a 

 second substance in crystalline form, and was interpreted in this way 

 by Sisson. Since that time, however, long spacings of this kind have 

 repeatedly been found even in native cellulose and it is not now clear 

 just what such spacings mean. Superposed over the whole pattern 

 there is a diffuse scattering indicating non-crystalline substances such 

 as often appear, however, also in native celluloses. As regards the 

 organization of the cellulose in the wall, mercerized cellulose of course 

 is still composed of molecular chains, and these are again united into 

 micelles in the same sense as used for native cellulose. Here, however, 

 any resemblance to Valonia ceases. In diagrams obtained by passing 

 an X-ray beam normal to the wall surface, a number of complete rings 

 are observed (Plate V, Fig. 2) quite unlike the sharp arcs in Valonia 

 and the 7-4 A. arc is missing. This means that the molecular chains 

 are oriented in the surface at random. If the beam is passed parallel to 

 the wall surface, however, the 7-4 A. arc is now very strong. The planes 

 of this spacing are therefore parallel to the wall surface, just like the 

 6-1 A. planes in Valonia; and again these are the planes richest in — OH 

 groups. It seems therefore to be a general rule, in the algae at least, 

 that planes rich in — OH groups tend to lie parallel to the wall surface. 



In spite of the similarity in shape, therefore, and of the fairly close 

 relationship, the cells of Halicystis and Valonia are very different. 

 Nevertheless it will be clear that the architecture in Halicystis is still 

 such that the wall is isotropic in its physical properties, so that a uniform 

 pressure within the cell will cause the wall to stretch uniformly in all 

 directions. If a piece of wall is stretched mechanically in one direction, 

 then the molecular chains tend to align themselves in the direction of 

 stretching, just as they do in the case of cellophane. During growth, 

 however, the tension in the wall will be approximately the same in all 

 directions and it would therefore be expected that the cell would remain 

 spherical. 



So far, so good. When the filamentous forms in this group are con- 

 sidered, however, difficulties immediately again appear. Problems of 

 growth are never so easy as that! 



Let us take as an example Hydrodictyon. Although this is not a 

 typical filament, the constituent cells are rather long cylinders and are 

 more easily handled than the smaller cells of the other species. The 

 plant takes the form of a net of cells, each cell steadily growing larger 

 as time goes on, and therefore making the net larger. Even at their 



