CHAPTER VIII 



Structural Variations in Homologous Cells 



Dimensional relationships in tracheids and fibres 



THE RATHER prccisc picturc of wall structure in elongated cells 

 described hitherto, nevertheless leaves a good deal to be desired. 

 Among the outstanding features which still await explanation, the 

 remarkably wide variation in spiral angle between individual cells from 

 the same sample calls attention to itself most forcefully. The question 

 arises as to whether the variation is quite random or if something 

 systematic can be described about it. Together with this variation in 

 structure, for instance, goes also a variation in cell length, in cell width 

 and in wall thickness as well as, probably, variations in chemical com- 

 position and in features of the cellulose complex other than orientation. 

 Is it possible that the variation in chain orientation is connected with 

 one or more of these coexistent variations? 



A glance over the earlier literature, prior to the year 1934, gives 

 immediate signs of a possible correlation particularly with cell dimen- 

 sions. Up to that time cellulose chain orientation in cell walls had been 

 investigated almost exclusively through observations of the m.e.p. of 

 walls in surface view, with the exception of fibres of which X-ray 

 diagrams had been used in the elucidation of the submicroscopic 

 structure of cellulose. As we have seen, there is always some doubt as 

 to the precise interpretation of m.e.p.s in terms of cellulose chains 

 except when supported by other evidence, and at that time this evidence 

 was lacking. Nevertheless it seemed rather safe to assume that, when- 

 ever wide differences in m.e.p.s were observed between cells, then that 

 in itself intimated correspondingly wide divergences in cellulose chain 

 direction. Taking that for granted, then the data in the literature were 

 most suggestive, particularly in the comparison afforded between 

 fibres, tracheids and vessels. In this order, these are long, thin cells; 

 shorter, wider cells; and short, fat cells. As we have seen already, the 

 general impression of structure is that the cellulose chains in fibres lie 

 relatively steeply, often almost longitudinally; in tracheids they lie in 

 spirals which may or may not be very steep; and in vessels they lie 

 almost transversely to cell length. In other words it appears that the 



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