WALL STRUCTURE IN THICK CELL WALLS 129 



by A. B. Wardrop of the Forest Products Division, C.S.I.R.O., 

 Australia, in comparison with, for instance, those of vessels. We shall 

 not discuss these here; they will be found set out fully in the papers 

 which wiU be quoted later, but they did eventually stimulate the writer 

 in collaboration with Wardrop to a further optical analysis of tracheid 

 and wood fibres (48(a)), and at the same time to attempt to generahze 

 the investigation by looking also into the structure of sisal fibres (48(6)) 

 and bamboo (48(c)). Since all these elongated cells were found to tell 

 more or less the same story throughout, they may perhaps be dealt 

 with here together. In all major points which we shaU discuss in the 

 rest of this chapter any one of these fibres (and probably any fibrous 

 cell) may be chosen as example. 



Although it is the intention to follow through the history of conifer 

 tracheids since we do know now much more about them than about 

 any other single cell, we may begin with the simplest case analytically, 

 the case of sisal fibres. 



Sisal is simpler from the present point of view for this reason. 

 Normally, with fibres, we have to accept the mature fibre as the only 

 easily available material, and then to try to deduce the structure of the 

 various layers from the properties of the whole wall. With sisal, how- 

 ever, it is easily possible to obtain large quantities of fibres in which 

 the outer layer only is present and thus to examine this layer separately. 

 The sisal plant is monocotyledonous and the fibres occur in the leaves. 

 In common with other monocotyledons the leaves continue to grow at 

 the base throughout the life of the plant, so that in any one leaf we can 

 find all stages in fibre development from expanding fibres, at the base 

 of the leaf, with only the primary wall, through fully elongated fibres 

 with the outer layer only of the secondary waU, to fully mature fibres 

 farther towards the tip. It is therefore merely a matter of taking 

 material from the correct level above the base of the leaf to find fibres 

 in any desired condition. The results of examination of such material 

 are quite striking. The X-ray diagram of fibres carrying only the outer 

 layer of the secondary wall (Plate VIII, Fig. 1) is quite different from 

 that of fully mature fibres (Plate VIII, Fig. 2), and looking back at the 

 diagrams of model spirals we can see immediately that the spiral on the 

 immature fibres is much flatter than that on the mature fibres. The 

 spiral angles which may be derived from the diagrams are given in 

 Table VIII, together with a number of other features of these cells. The 

 following points should especially be noted. Firstly, the immature and 

 the mature fibres are of almost exactly the same (average) length; there 

 can be no question here of the length effect we shall find later in other 

 9 



