WALL STRUCTURE IN THICK CELL WALLS 



131 



elongated cells (p. 152) introducing any complications. Secondly, the 

 birefringence (n^' — nj of the outer layer in transverse section is the 

 same in immature and in mature fibres; this makes it rather certain that 

 the outer layers in immature and mature fibres are essentially similar in 

 general physical make-up (Table VIIIa). We can safely conclude, there- 

 fore, that the diagram of immature fibres represents the condition in the 

 outer layer both of immature and of mature fibres, and that the steeper 

 spiral deducible from the diagram of the mature fibres refers to the 

 thick central layer only. There can thus be not the slightest doubt here 

 but that the outer layer is composed of cellulose chains lying in a fairly 

 flat spiral, the angle to cell length being of the order of 40°, while the 

 central layer later deposited upon it is composed of chains lying much 

 more steeply, with an average inclination to cell length of about 20°. 



It is therefore again very curious that there is, in the diagram of the 

 mature fibres, no record of the orientation which we know must be 

 present in the outer layers; the "mature" diagram is not merely a super- 

 position of the diagrams of the outer and the central layers but corres- 

 ponds to the central layer only. This is exactly the condition we have 

 met in conifer tracheids. There for the time being we may leave the 

 sisal fibres, though we shall have cause to return to them again. 



Turning next back to the 

 tracheids, it has been found 

 possible to establish more or 

 less the same type of structure 

 even though mature cells only 

 are available; and in fact to 

 add something to the sisal 

 story which may help to ex- 

 plain the curious anomaly just 

 mentioned. The method used 

 takes advantage of the fact 

 that conifer tracheids are 

 approximately rectangular in 

 transverse section, with two 

 opposite walls lying tangentially in the stem and two radially. In any 

 small section (containing nevertheless hundreds of cells) the tangential 

 walls are almost exactly coplanar — sufiiciently nearly so, at any rate, 

 for present purposes. Consider one such cell (Fig. 47), In transverse 

 section the chains (marked by dotted lines) will be seen in the same 

 "perspective" in all four walls and these are equally birefringent. When, 

 however, the cell is cut in some other plane XX YY then one wall AB 



Fig. 47. For explanation, see text. 



