46 THE CELL 



and the optical differences which are responsible for the stratified 

 appearance, are always due to differences in the water-content of suc- 

 cessive layers of the wall. This theory has been severely criticised ; 

 but Correns has shown that it certainly applies in the case of certain 

 bast-fibres, the walls of which lose their stratified appearance entirely, 

 or to a large extent, when they are thoroughly dried. There are, 

 however, undoubtedly also walls for example in the pith of 

 Podocarp us the stratification of which is not affected by removal of 

 the water of imbibition. In such cases the stratified appearance must 

 depend upon differences of a chemical nature. It is quite conceivable 

 that in yet other instances stratification is produced by a combination 

 of physical and chemical factors. 



Sometimes the thickening layers of a cell-wall, when examined in 

 surface view, exhibit a system of delicate striations, which are generally 

 directed obliquely with reference to the longitudinal axis of the cell. 

 Occasionally two intersecting sets of striations are visible, in which case 

 each belongs to a different stratum of the wall. Like stratification, striation 

 may depend upon differences in water-content, upon chemical differences, 

 or, finally, upon a combination of the two factors. The first-mentioned 

 cause is responsible, as Correns has shown, for the transverse striation 

 of the epidermal walls of Hyacinthus and Omithogalum, and for the 

 oblique striation of fibres. According to Krieg, the striation of Coni- 

 ferous tracheides is similarly due to the alternation of relatively watery 

 lamellae with layers which contain less water. It is at present 

 unknown whether striation is ever actually produced by the other 

 causes which have been specified. 



Various investigators have attempted to demonstrate the presence, 

 in cell-walls, of an organisation which is even more minute than that 

 to which the phenomena of striation and stratification are partly 

 attributable. Nageli himself supposed that the walls of bast-fibres 

 and other prosenchymatous cells are composed of exceedingly minute 

 fibrils, and that the latter in their turn are built up by the 

 concrescence in rows of the ultimate particles of cell- wall substance, 

 or micellae. More recently Wiesner has devised a special method 

 whereby the cell-wall can be broken up into minute particles, which 

 he terms dermatosomes. This " atomisation " or " carbonisation " can 

 be effected by soaking the fibres or other tissues in dilute hydrochloric 

 acid for some hours and thereafter heating them to a temperature of 

 about 50 to 60 C; prolonged digestion with chlorine water produces 

 similar results. Treatment according to either of these methods, in 

 the majority of cases, causes the cell-wall to break up into a dust-like 

 mass of extremely minute particles or dermatosomes. It is of course 

 possible that these dermatosomes are mere artefacts ; on the other 





