STRUCTURAL VARIATIONS IN HOMOLOGOUS CELLS 163 



it is possible to give an answer for the outer layer only of the wall, and 

 even that is an incomplete one. The difficulty here is that striations 

 corresponding to the orientation in this layer are seldom observed 

 unless the tracheids are treated somewhat roughly; in which case it is 

 impossible to ensure that the wall structure has not been deformed. 

 Recourse has therefore to be made to more indirect optical methods. 



If thin transverse sections of a piece of wood, including the first few 

 annual rings, are observed under a polarizing microscope then it is 

 found that the birefringence of the bright outer layers of the tracheids, 

 as measured by the method described earlier (p. 69), steadily decreases 

 outwards from one annual ring to the next (provided of course that the 

 equivalent regions are compared from each annual ring). Figure 57 

 gives a good example of this phenomenon. Now, provided that other 

 structural features remain constant, this must mean that the molecular 

 spirals are becoming steeper on passing from the pith outwards. There 

 is at present no reason to suspect any variation other than in orientation 

 so that we have here at any rate qualitative evidence of the length/angle 

 relationship suspected to hold. It is impossible to be precise about the 

 relationship, but a rough approximation may be achieved in the follow- 

 ing way. If we assume that the highest birefringence observed in these 

 sections corresponds to quite transverse cellulose chains (and the close 

 correspondence between this maximum figure and that observed in 

 sections cut parallel to the cellulose chains in this layer, see p. 131, 

 suggests that this is not far from being true) then the spiral angle can 

 be calculated for any other value of birefringence at any other point 

 in the section. This has been done and the calculated angles are included 

 in Fig. 57. These are naturally liable to considerable error, but they do 

 show in a striking way that the type of length/angle relationship already 

 found for the central layer holds also for the outer layer. It seems 

 reasonable to expect that a similar statement will eventually be possible 

 for the innermost layer, so that it can be said even now that the chain 

 orientation in the whole wall is conditioned by cell length. 



This is a point of very considerable importance in so far as the 

 physical properties of timber depend on chain orientation, but before 

 considering its implications it will be as well to glance, if only rather 

 briefly, at the other cell types where a similar relation is known, or 

 suspected, to exist. 



Dimensional relationships in bamboo fibres 



While the later part of this work was still in progress the opportunity 

 presented itself of making a somewhat similar series of observations on 



