STRUCTURAL VARIATIONS IN HOMOLOGOUS CELLS 153 



longer a cell has become during development the more nearly do its 

 cellulose chains lie longitudinally. Certainly even at that time there 

 were obvious exceptions, the outstanding example being the cotton 

 hairs. As we have seen, these cells, though extremely long, nevertheless 

 persist in laying down cellulose chains at an angle of some 30° to cell 

 length. Cotton hairs are, however, exceptional in other ways too, the 

 most notable being the frequent reversals of the spiral at intervals along 

 the length of individual hairs, so that it seemed safe to leave such 

 exceptional cases for the time being out of account. In any case, once 

 the possibility was grasped that there might be a correlation between 

 cell length and wall organization then it became clear at once that 

 correlations could only be expected among individual cells of similar 

 type, and that other factors might intervene to render comparisons 

 between cells of different types invalid. We shall, in fact, see that when 

 cotton hairs are considered by themselves they form no exception. 



It seemed therefore worth while to investigate the possibility of a 

 connection between cell dimensions and wall structure in a population 

 of cells of similar type, within which the only major differences would be 

 those of cell dimensions. The investigation would obviously call for the 

 observation of some thousands of cells, so that an optical method had 

 to be employed rather than the (considerably more intricate and time 

 consuming) X-ray analysis of single cells. This restricted attention 

 immediately to tissues within which the cells were all of the same type, 

 in order to provide large numbers of similar cells without the necessity 

 of selection, and in which the m.e.p. could be relied upon to give a 

 reasonably accurate idea of chain orientation. For these reasons, and 

 for others which will appear as the argument proceeds, the wood of 

 conifers was chosen. With the exception of ray tissues, wood of this 

 kind consists only of tracheids all of which have been developed in the 

 same way, from identical cells, and have, if not identical, at least very 

 similar chemical composition. There are, in addition, features of 

 development in these woody stems which make the selection of length 

 classes among the cells very easy indeed. 



The results of the investigation thus initiated, covering the examina- 

 tion of some 60,000 individual tracheids, first appeared in 1934 (47(c)), 

 and it at once became evident that a relation of the type suspected did 

 in fact obtain. Although these earliest results led to an error in inter- 

 pretation, it is nevertheless interesting to follow their development into 

 the later generalizations to cover cells of other types and into the more 

 modern interpretation of the correlations thus revealed. In order to 

 obtain a clear insight into these matters we must first make a small 



