76 



PART II. ANATOMY AND HISTOLOGY. 



in the form of delicate spirally-wound bands with their edges in 

 contact. The lines of the striation are the planes of contact of the 

 edges of these spiral bands. A well-marked illustration of the 

 spiral mode of deposition of cell-wall by protoplasm is afforded by 

 the spiral vessels already mentioned (Fig. 44 s). 



3. The Chemical Composition of the Cell-ivall. As a rule, the 

 organic constituent of the newly formed cell- wall is cellulose 

 (C 6 H ]0 5 ), a carbohydrate, the characteristic reaction of which 

 is that it turns blue when treated with sulphuric acid and iodine, 

 or with a mixture of iodine, iodide of potassium, and chloride of 

 zinc (chlor-zinc-iod). 



It is, however, commonly the case that when a cell-wall has 



undergone thickening, some 

 at least of its constituent 

 layers do not consist of 

 cellulose. The chemical 

 changes which are pre- 

 sented by cell- walls may be 

 distinguished as follows : 

 a. The cell- wall may un- 

 dergo cuticularisation: e.g. 

 walls of epidermal cells or 

 cork-cells, of spores. The 

 cuticularised or corky cell- 

 wall contains a substance 

 termed cutin. It is but 

 slightly permeable to water ; 

 FIG. so.-surface view of the wall of a ceil it is extensible and highly 



showing striation, from the pith of Dahlia varia- elastic ; it turns yellow 

 btlw. ( x 249: after Strasburger.) / 



when treated with sul- 

 phuric acid and iodine, or with iodised chloride of zinc. The cuti- 

 cularisation of the cell- wall is most marked in the external layers ; 

 in fact the external layer consists entirely of cutin, whilst the 

 internal layers (of which there may be several, as the cuticularised 

 wall is often much thickened) consist more and more largely of 

 cellulose, the innermost layer consisting frequently of pure cellu- 

 lose, though it is sometimes more or less lignified (cork). This can 

 be shown by treating the cuticularised tissue with strong chromic 

 acid for some time, or by warming it in a mixture of nitric acid 

 and chlorate of potash, when the cutin is removed, and the re- 

 maining tissue gives the characteristic cellulose-reactions. 



