WALL STRUCTURE IN THICK CELL WALLS 143 



of two layers, one in which the microfibrils are arranged almost 

 transversely and the other almost longitudinally. Both sets of microfibrils 

 show marked angular dispersion. Similarly the walls of sieve tubes are 

 almost isotropic, and again little is known concerning them though 

 recent electron micrographs are rather illuminating (8). Two cell 

 types do, however, show structural orientations of an interesting type — 

 the vessels, and the cells of the coUenchyma. 



Vessel elements 



The walls of vessel elements are commonly heavily pitted and this 

 naturally leads to such a disturbance of structure that the wall becomes 

 very complicated indeed. The cellulose micelles in all pits, whether the 

 large simple pits of parenchyma cells, the bordered pits of tracheids or 

 the slit pits of fibres, tend to lie parallel to the nearest edge of the pit. 

 This is shown most strikingly in the heavily bordered pits of tracheids 

 (Plate III, Fig. 4) where the border shows a clear Maltese cross. Never- 

 theless it can be said that in vessels the micelles of cellulose lie almost 

 transversely, though there are some exceptions (50(6)). This has been 

 shown both by examination under the polarizing microscope of wall 

 areas free from pits, and by the X-ray photograph of single vessel walls 

 (Plate VIII, Fig. 4). The general run of the cellulose chains can, indeed, 

 be seen in isolated vessels by the run of the mouths of slit pits on those 

 parts of the walls which have been in contact with elongated cells. Thus 

 in Fig. 53 are given a few examples of vessels in which the run of these 

 slit pits is marked by a series of short lines. It will be obvious that while 

 in general the pits lie transversely there are exceptions. The wall 

 structure is subject to sudden and abrupt changes which may perhaps 

 be associated with the very large diameter of these cells. 



In transverse section under the polarizing microscope, the wall of 

 vessels is often homogeneous, or very nearly so, and it may be concluded 

 that in these vessels, therefore, the wall is homogeneous in cellulose 

 chain direction. In some cases, however, notably in some vessels of 

 Fraxinus americana, almost all vessels in Sassafras officinale and 

 vessels of Castanea dentata a lamellation can be observed (50(/j)) which 

 recalls strongly the layering in bamboo fibres and presumably has the 

 same origin. This can be confirmed in Fraxinus since those vessels 

 showing this optical heterogeneity also show sUt pits whose mouths 

 twist in the wall— the so-called "spiral" pits. These vessels also show, 

 where the vessel element is in contact with another vessel, a series of 

 scalariform pits whose mouths lie transversely, breaking up nearer the 

 lumen into a number of slit pits with tilted slit mouths. All this suggests 



