THE FORM OF THE PLANT CELL 



19 



by the continual depositions of new layers within the old, the new layers 

 encroach over the thin region so that this becomes over-arched and, by 

 the time that secondary deposition ceases, the "hole" in the wall has 

 become much smaller (Fig. 5). This leaves scattered over the wall a 

 series of over-arched regions within which, and to some less extent 

 surrounding which, the wall structure is quite different from elsewhere. 

 This is of particular importance in, for instance, 

 the walls of the vessels in Quercus alba and in 

 many other ring-porous dicotyledonous trees 

 (see Fig. 53), where the pitting is crowded over 

 almost the whole wall surface and is bordered, 

 narrowly in contact with ray cells and wood 

 parenchyma and more widely in contact with 

 other vessels. 



There can be no wonder, then, that study 

 of structural details in botanical material by 

 physical methods is fraught with difficulties 

 and beset with pitfalls. Although the bulk of 

 the material which the biophysicist is called 

 upon to handle here is composed of secondary 

 walls and can therefore be examined in dead 

 tissue, the morphological complexities alone 

 are still so enormous that progress is of 

 necessity rather slow. Naturally, therefore, it 

 is desirable to begin with the simplest possible 

 case. To those who are interested primarily in 

 the more academic aspects of plant behaviour, 

 this would lead naturally to a study of those 

 cells which grow unimpeded by the obstacles to growth, or the 

 control of growth, consequent on tissue formation, e.g. to the algae 

 where the reactions of cells could be investigated independently. 

 Historically, however, the cells which took precedence in the modern 

 investigations of submicroscopic structure in plant cells were the 

 phloem fibres, and in particular ramie fibres, for several reasons, not the 

 least of which is the economic importance of such cells. These are, in 

 fact, rather satisfactory, since they are composed of cells all of one type 

 and approximately of the same size; long thin cells, some one hundred 

 or more times longer than wide, with long, tapering ends and with thick 

 cell walls. These fibres are, moreover, arranged quite parallel to each 

 other, or are so long that cells separated chemically from the tissue can 

 be laid strictly parallel to form a bundle, the properties of which reflect 



Fig. 5. For explanation, 

 see text. (Reproduced by 

 permission from A Text- 

 book of General Botany 

 by HolmanandRobbins. 

 Published by John Wiley 

 & Sons, Inc., 1938.) 



