50 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



real fibres will depend not only on the kinds of incrusting substances 

 present but, even after these have been removed, on the relative propor- 

 tion of the "crystalline" to the "non-crystalline" material. For 

 obviously the lack of regularity in the packing of the chains in non- 

 crystalline regions can lead only to a decrease in density. This effect 

 in turn may well depend on the degree of orientation of the micelles, 

 and so it is understandable that the density may vary widely and in a 

 manner dependent in quite a complicated way upon structure. Density 

 determinations do, however, yield a rough idea of the ratio of crystal- 

 line to non-crystalline cellulose, a figure as low as 1-53 for instance, 

 implying almost 100% non-crystalline material, and a figure of 1-59 

 or 1-60 purely crystalline. There are, naturally, many other ways of 

 determining the crystalline/non-crystalline ratio, both physical (18) and 

 chemical (19) but these will not be discussed here. Just as, however, in 

 technology the amount of non-crystalline material present in a fibre is 

 of paramount importance to the properties of the fibre, so also here the 

 amount of non-crystalline material may be of very considerable impor- 

 tance and it will be found necessary to refer to it again and again in the 

 following pages. 



Analysis of structure by optical methods 



The method of X-ray analysis thus briefly described has, unfortu- 

 nately, certain rather severe limitations as applied to botanical objects. 

 As normally used it is necessary to work with material a few millimetres 

 long and about one millimetre thick in the direction of the X-ray beam, 

 and therefore with a whole tissue. Since even the simplest tissue can be 

 composed of more than one cell type, and since the wall structure of 

 each single cell may differ from point to point in the wall, it is sometimes 

 a matter of difficulty to give a correct interpretation in terms of single 

 cell walls — with which the botanist is most concerned — and the way is 

 open dangerously wide for serious misinterpretations. These have, in 

 fact, occurred in the literature. Certainly there is no theoretical reason 

 why spectrometers should not be designed to work with the smallest 

 pieces of plant cell wall which could be handled, but in practice there 

 are limits below which it is not at the moment feasible to go. For 

 instance, using the normal sealed-olf types of X-ray tube and with a 

 specimen-object distance of 3 cm., then a piece of wood as thick as a 

 match-stick will give a reasonably intense diagram in about two hours. If 

 one needs to investigate, however, a single cell, then the exposure time 

 is increased enormously. Firstly, the diameter of the slit through which 

 the X-rays impinge on the specimen must be reduced from the customary 



