THE GROSS LAYERS OF THE CELL WALL i) 



which, unless examined with special technique, appear more or 

 less homogeneous in structure. As a cell of this kind enlarges 

 and becomes mature, its wall naturally increases in surface. In 

 addition, the wall may become much thicker, apparently as the 

 result of (1) deposition of new particles of wall substance among 

 those present, a process termed ^^ intussusception^' by Nageli, 

 and/or (2) the deposition of successive plates or lamellae of wall 

 material, a process termed "apposition" by von Mohl. The 

 process of ai)position usually occurs in a centripetal direction 

 with reference to the first or original wall and may result in the 

 almost complete obliteration or occlusion of the cell lumen, as is 

 typical of certain fibers and sclereides (cf. Exercises VIII and 

 IX). 



Much confusion exists in textbooks and in the specialized 

 research literature on cell walls with respect to the suhdivision of 

 the wall into major regions or layers. Distinctions between the 

 successive layers are based upon such criteria as (1) origin, (2) 

 appearance when viewed under polarized light, and (3) chemical 

 and physical structure. Very recently Kerr and Bailey (1934), 

 as a result of an extensive study of material with modern tech- 

 nique as well as a critical survey of the literature, proposed a 

 terminology which has been adopted by Anderson (1935). Ilay- 

 ward and others and which will be followed throughout this 

 book. According to Kerr and Bailey, three main categories of 

 wall layers exist : 



1. The intercellular "layer" or substance which is composed 

 largely of prjlyuronides and is isotropic (i.e. dark or non-re- 

 fringent when viewed under polarized light). Further in- 

 formation on the precise origin of the intercellular layer is ur- 

 gently needed but provisionally it may be regarded as being 

 derived from the cell plate which is produced following mitosis. 

 In trachearA' elements, the intercellular layer may be more or less 

 lignified. 



2. The primary wall which consists largely of cellulose and 

 polyuronides and is anisotropic (i.e., bright or refringent when 

 viewed under polarized light). Kerr and Bailey emphasize that 

 the term "primary wall" should be restricted to the original 

 cambial wall (in the cells of xylem and phloem) and its homo- 



