160 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



now we know that in fact it corresponds rather closely to the chain 

 direction in the central layer only of the wall. 



The steepening of the spiral is always much more marked in the 

 radial walls than in the tangential walls and this, taken with the fact 

 that there is a reasonably close parallel between the average length and 

 the average cotangent of the spiral angle (Table XIII) led to the sugges- 

 tion that the relation between spiral angle and length is of the form 



L=K. coi d 



for the radial walls only. This is the relation to be expected of a spiral 

 elongating at constant girth and it was in these terms that an interpre- 

 tation of this relation was first attempted. Since the only cells which 

 elongate here are the cambial cells, this involved the assumption that 

 the primary wall of these cells contained cellulose chains oriented in 

 directions similar to those in the tracheids which they produced. Now 

 that we know this to be untrue (p. 177) this tentative interpretation falls 

 to the ground. There was in any case always the difficulty that, if the 

 elongation of the cambial cells causes a steepening of the spiral then, the 

 radial expansion should cause a much more pronounced flattening 

 unless other collateral assumptions are made. 



That the steepening of the spiral in passing from inner to outer 

 annual rings is nevertheless a real one, and not associated with any 

 peculiarities involved in the determination of the m.e.p., is shown most 

 effectively by the X-ray diagram of wood. A series of three such 

 diagrams is illustrated in Plate V, Fig. 3, and it will be clear by com- 

 parison of these diagrams with those of the model spirals in Plate VI, 

 Fig. 1, that a steepening of the spiral does in fact occur. This has now 

 been verified abundantly with many species both in the author's 

 laboratory and elsewhere. There is therefore no doubt as to the reaUty 

 of the phenomenon. 



In seeking an alternative explanation it was decided to investigate the 

 possibiUty that a relation of this kind existed, not only among tracheids 

 cut off from the same cambial cell but also among tracheids generally 

 in a tree irrespective of the cambial cell from which they were derived. 

 The determinations were therefore repeated on a series of tracheids 

 chosen at random from a piece of wood. Now, however, that it was 

 necessary to determine chain direction and length in one and the same 

 cell it was no longer possible to make use of the m.e.p. Instead, tracheids 

 were chosen showing striations, which we now know to correspond in 

 direction to that of the cellulose chains in the central layer, and attention 

 was confined therefore to late wood tracheids where this layer is well 



