WALL STRUCTURE IN THICK CELL WALLS 105 



spacing first, the arc on the equator obviously arises from both the 

 longitudinal set of chains (positions A, A') and the transverse set 

 (positions C, C). The equatorial 5-4 A. arc, on the other hand, can 

 arise only from the longitudinal set (positions B, B'), and the meridional 

 arcs only from the transverse set (positions D and the one on the other 

 side of the set and not shown). The 3-9 A. reflections arise from both 

 the longitudinal set (positions intermediate between A and B) and, 

 diagonally, from the transverse set (positions C, C) and are therefore 

 multiple. The arrangement of the unit cells illustrated diagrammatically 

 in Fig. 31(a), therefore, explains completely the diagram actually ob- 

 tained and it may be concluded that all filaments of Cladophora pre- 

 senting this diagram are wound with two sets of ceHulose chains, and 

 that the planes of 6-1 A. spacing again tend to be parallel to the wall 

 surface. This is fufly supported by observation of striation directions, 

 though naturally no complete check can here be made that striation 

 direction and chain direction are quantitatively the same. Further, the 

 walls observed in face view between crossed Nicols are uniformly almost 

 dark, as a rule, as could be expected from this type of structure. 



The position is somewhat different towards the base of a plant. Here 

 both sets of chains form spirals round the cell the "steep" set being less 

 steep than in the upper parts of the plant and the "flat" set more steep. 

 A further peculiarity lies in the fact that, in any individual cell, the 

 flatter spiral becomes flatter and the steeper spiral steeper towards the 

 end of the cell nearer the tip cell (Fig. 38); and the flatter spiral at any 

 point in a cell becomes flatter as we pass from the base of the filament 

 towards the tip and at the same time the steep set becomes steeper. At 

 any cross wall, therefore, there is a sudden change in striation direction 

 which is less marked towards the apices of a filament. It should be par- 

 ticularly noted that, since these spirals are of opposite sign, then these 

 changes are such as to maintain the angle between their windings 

 constant throughout a filament. 



It becomes a little difficult, therefore, to understand the very different 

 morphology of Valonia and Cladophora in terms of changes in shape due 

 to interaction of internal hydrostatic pressure and the elastic properties 

 of the walls. In the basal regions it is understandable that, since both 

 sets of chains constitute rather flat spirals, the ceU will blow out into a 

 cylinder rather than a sphere; but in the upper regions there seems to 

 be no such direct connection between wall structure and growth form. 

 It should particularly be noted that growth continues at the tip long 

 after the steep set of striations has become longitudinal, and from this 

 point on changes in cell dimensions involve no further change in chain 



