18 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



kind that the investigations described in the following pages are largely 

 concerned. 



The secondary wall which thus surrounds the lumen in this condition 

 is usually rather complicated in structure. It can, however, vary from 

 the apparently simple and homogeneous (as in some vessels) to a com- 

 plex of several concentric lamellae alternating either in chemical make- 

 up or in physical constitution or both. Normally these layers are 

 distinguishable under the ordinary microscope even without pre-treat- 

 ment, but in many cases special procedures have to be adopted, such as 

 staining and the like, in order to make them out at all. Invariably the 

 visible differences arise through a complex of many underlying chemical 

 and physical factors and it will be a large part of the endeavours here 

 to sort out the various factors which impart these differences to the 

 layers and therefore to the cells as a whole. In addition to this com- 

 plexity in the fundamental building material of the wall layers there are 

 other disturbances of structure which, although perhaps not of such 

 immediate importance to an understanding of the fundamentals of 

 wall structure, must nevertheless be taken into account, particularly 

 when attempting to define the properties of single walls from those of a 

 whole tissue, however homogeneous. In the majority of cell types the 

 secondary wall is not uniformly thick over the whole surface. Here and 

 there, arranged sometimes at random but more often in some remark- 

 ably uniform pattern, there occur thin places in the wall where secondary 

 deposition has never taken place. A further remarkable thing is that 

 wherever one cell has such a thin place, the adjacent cell in contact with 

 it has a similar thin place at the corresponding point in the wall. These 



are spoken of as pit pairs. In 

 parenchyma cells this leads to the 

 development of cylindrical canals 

 in the wall which are closed off 

 merely by the two primary walls of 

 each cell and the tenuous middle 

 lamella between them — the simple 

 pits (Fig. 4). Such pits have a most 

 profound influence on the structure 

 of the wall area immediately ad- 

 jacent to them. Of even more 

 consequence are the bordered pits 

 typical of, for instance, the tra- 

 cheids of conifer stems. Here, as 

 the secondary wall becomes thicker 



MIDDLE LAMELLA 



SECONDARY WALL 



PRIMARV WALLS 



Fig. 4. Part of the walls of two adjacent 

 parenchyma cells showing the form of 

 simple pits. The section at the right of 

 the figure shows that the pit membrane 

 is three layered. No attempt is made to 

 show protoplasmic connections. 



