14 LECTURE II. 



The Cell-wall. 



The wall of a parenchymatous cell such as that shewn in 

 Fig. 3 will be seen to be a thin and apparently homogeneous 

 membrane. If it be treated with solution of iodine it will 

 assume a yellow colour, and if a drop of strong sulphuric acid 

 be added the yellow will be replaced by a deep-blue colour. 

 This reaction is characteristic of cellulose, and we may there- 

 fore conclude that the cell-wall consists, principally at least, 

 of this substance which belongs to the group of the carbohy- 

 drates and to which the formula xC 6 H 10 O 5 has been assigned. 



In the cell-wall, as in all organised structures structures, 

 that is, which have been formed by living organisms a 

 certain proportion of water is contained holding small quan- 

 tities of various substances in solution. It has been found 

 that the proportion of this water may be made to vary within 

 certain limits without injury, but if these limits be overstepped 

 disintegration of the cell-wall is the result. The variation in 

 the amount of water present produces a corresponding varia- 

 tion in the volume of the cell-wall ; hence the absorption of 



FIG. 4 (after Dippel). Cell of Equisetum isolated and in transverse section, viewed 

 with crossed Nicols. 



