106 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[24. 



larger or smaller angle to the long axis of the cell, sometimes even 

 'transversely. The planes of striation are commonly different in 

 the different layers constituting the thickness of the wall, and these 

 seem in the surface-view to cross each other (Fig. 64). The cause 

 of striation appears to be this, that when a considerable area of 

 cell-wall has to be formed, it is deposited by the protoplasm not as 

 one continuous sheet, but 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. 58 s). 



3. The Chemical Composi- 

 tion of the Cell-wall. As a 

 rule, the organic constituent 

 of the newly formed cell- 

 wall is cellulose (C 6 H 10 0-), 

 a carbohydrate, the charac- 

 teristic reaction of which, 

 is that it turns blue when 

 treated with sulphuric acid 

 and iodine, or with a mix- 

 ture of iodine, iodide of 

 potassium, and chloride of 

 zinc (chlor-zmc-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 presented by cell- walls may be distinguished as follows: 



a. The cell-wall may undergo cutlcularisation : e.g. walls of 

 epidermal cells, of cork-cells, of spores. The cuticularised or 

 corky cell-wall, contains a substance termed cutin. It is but 

 slightly permeable to water ; it is extensible and highly elastic ; 

 it turns yellow when treated with sulphuric acid and iodine, 

 or with iodised chloride of zinc. The cuticularisation 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 they may be several, as the cuticularised wall is often 

 much thickened) consist more and more largely of cellulose, the 



Frs. 6t. Surface view of the wall of a cell, 

 showing sbriatioii, from the pith of Dahlia varia- 

 bilis. (x 240: after Strasburger.) 



