14 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



whole, therefore, has only twofold symmetry about its longitudinal 

 axis (i.e. the cell will be coincident with itself only twice per revolution 

 about the longitudinal axis) but even this implies that opposite walls 

 should be similar in all respects though adjacent walls need not be. 

 The argument here would be that since the opposite walls presumably 

 began alike and have gone through the same deformations then the 

 final structures must be alike. It is, of course, a natural consequence of 

 growth that as the cell elongates it may develop new contacts and there- 

 fore new facets, and some of these can be seen in the beautiful plaster 

 casts of elongated cells prepared by Lewis (2); and in any case as cells 

 vacuolate they develop between themselves intercellular spaces — 

 usually rather small in a tissue of elongated cells — but neither of these 

 minor changes should be allowed to mask the fundamental underlying 

 symmetry. In many elongated cells, in fact, the cells are so long that 

 if their tips be ignored then the longitudinal axis is a sixfold axis, 

 since the shape of the faces is masked, or even more when intercellular 

 spaces develop so that the cells become almost cylindrical; and we should 

 expect in these cases that the longitudinal walls would be constructed on 

 some uniform plan. This is actually realized in some wood fibres and in 

 phloemfibres. In other cases lateral extension of an opposite pair of walls, 

 or some other effects, accentuate the twofold symmetry and we might 

 expect in this case adjacent longitudinal walls to differ in structure even 

 though opposite walls are identical. Both these conceptions are realized, 

 the latter more particularly in the wood tracheids of conifer stems. 



During this extension, the cell remains clothed in the original thin 

 membrane which must therefore in some way be able to accommodate 

 itself to change in dimension as the protoplasm increases in volume. 

 The relationship between this primary wall and the protoplasm is, in 

 fact, highly complex and we shall have occasion to examine it in much 

 more detail later on. At the moment we may merely note that up to this 

 time the wall and the protoplasm adhere together rather strongly. 

 This is evidenced by the fact that if young growing cells are placed in a 

 strong sugar solution then, as the solution draws water from the cell 

 by osmosis, the whole cell often crumples without any cleavage between 

 wall and protoplasm; whereas in an adult cell a similar procedure 

 causes the protoplasm and the wall to separate in the phenomenon 

 known as plasmolysis. There is much more evidence than this, con- 

 sideration of which may be left until later when the organization of 

 these walls has been considered. 



During the elongation period the cell wall has not, as far as one can 

 tell under the microscope, become any thinner. At this stage the 



