86 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



the vaguely delimited micelles to which reference has so far been made 

 (40). Many of these represented little more than indulgence in mere 

 speculation but one of them, that cellulose consists of "fibrils" (i.e. 

 long slender threads) which can be made microscopically visible, has 

 persisted since the days of Grew and seems to be based on very firm 

 supporting evidence. A considerable number of investigators have 

 reported, from time to time and for a wide variety of cell types, that the 

 walls can be broken down mechanically into fine threads about 1 -4 fi 

 thick which again in turn can be shown to consist of finer threads some 

 0-1 to 0-4 fj, thick. This can readily be demonstrated with any elongated 

 cell particularly if it is first treated with sulphuric acid or caustic soda. 

 It has always seemed possible, however, that the development of such 

 fibrils provided evidence for nothing more than lines of weakness to 

 chemical and physical attack. Bailey and Kerr (47(a)) showed some time 

 ago that the interpenetrating matrices of cellulose and lignin in wood 

 cells grade down to the very limits of microscopic visibility and they 

 suggested that the so-called fibrils were nothing more than dissected 

 fragments of such a network. It is impossible by any ordinary method 

 to distinguish between these two possibilities. 



With the advent of the electron microscope, however, the possibility 

 of making a critical observation in this field became obvious. It is 

 assumed that the electron microscope has been so popularized as to 

 give readers suflficient acquaintance with electron microscopy to know 

 what it is all about, and it is not proposed here to attempt any descrip- 

 tion of the instrument or to enlarge upon the underlying theory. Per- 

 haps, however, a few words by way of introduction may be of service. 



Just as the light microscope is a device whereby optical magnification 

 may be achieved to a degree impossible to the unaided eye, so the 

 electron microscope is designed to achieve greater useful magnification 

 than can be attained in the light microscope. Useful magnification, 

 because there is, of course, no limit to the magnification which may be 

 made by optical means of a photograph taken under the light micro- 

 scope. Above about lOOOx, however, such magnification serves no 

 useful purpose; for clearly, and without going into the theory of light 

 too deeply, no optical instrument could distinguish detail smaller than 

 the wavelength of the light used, and once detail of this order is made 

 visible, no further magnification can bring out finer detail. More 

 formally, the diffraction theory shows that the resolving power (i.e. 

 the minimum distance whereby two small particles may be separated 

 and still remain separately visible) is given by 



o-eniN.A., 



