184 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



observational evidence to associate the two together; (b) the difficulties 

 encountered when attempting to interpret orientation in non-cylindrical 

 cells; and (c) the inadequacy of the hypothesis when secondary walls 

 are examined. 



As regards the first, the published support for the hypothesis in- 

 variably tries to associate orientation, not with stress but with the 

 resulting strain. Thus, if a series of squares are drawn on a partly in- 

 flated cylindrical rubber balloon, then they become rectangles with their 

 longer sides transverse when the balloon is further inflated. This, 

 however, is merely because in such a balloon expansion is taking place 

 more rapidly in the transverse direction than in the longitudinal on 

 account of the anisotropy of stress — and this is not the condition obtain- 

 ing in elongating cells, for the cells always increase their length /breadth 

 ratio. Again, to take one example of the evidence put forward by 

 observation of growing cells, let us consider the evidence brought 

 forward by Maas Geesteranus. The pith cells of Juncus develop hollow 

 cylindrical protrusions which elongate as the cell grows. These are 

 therefore quite separated from each other and from other cells and can 

 be studied individually in sections. Maas Geesteranus has measured the 

 phase difference (p. 69) of the (double, upper and lower) walls of these 

 cells and has found a relation between this phase difference and the 

 length of the arms as shown in Fig. 62. The increase in phase difference 

 (which is such that the m.e.p. is directed transversely) he attributes to 

 an increasing preference of the cellulose chains for the transverse 

 orientation (for which there is in any case no evidence since increase in 

 wall thickness, cellulose content, etc., would lead to the same result) 

 and this in turn to an increasing strain during growth. The observed 

 correlation, however, is with length and there is no evidence in the draw- 

 ings published by Maas Geesteranus that any appreciable transverse 

 strain occurs at all. If strain had any appreciable part to play in orienta- 

 tion, one would have expected the chains to become more nearly 

 longitudinal during growth. Indeed, it would seem much more reason- 

 able to conclude that the cell remained cylindrical because the chains 

 remain transverse than to attribute transverse orientation to the 

 cylindrical shape. Even this can be doubted, as will appear later. 



Under the second heading we may note that in Cladophora we have 

 transverse orientation par excellence although the growing tip of a 

 filament is dome-shaped and therefore not subject to extreme anisotropy 

 of stress and that, lower down in the tip cell, and in cells lower in the 

 filament, lamellae with transverse orientation are repeatedly sandwiched 

 between others with almost longitudinal orientation although the cell 



