48 THE CELL 



by caustic potash and yellowish brown by chlor-zinc-iodine. Mucilaginous 

 walls are formed by a special modification of cellulose or pectic 

 membranes; their characteristic chemical features are, however, often 

 apparent when the walls are first laid down. While all cell-walls 

 contain a certain amount of mineral matter, the proportion of inorganic 

 substance present is greatest in the case of old membranes. 

 Silica and salts of calcium (calcium carbonate and calcium oxalate) 

 are among the commonest mineral constituents of the cell-wall, and 

 are sometimes present in large quantities. 



Both the chemical changes which cell-walls undergo, and the im- 

 pregnation with certain organic or inorganic substances to which they 

 are subjected, are frequently correlated with special physiological or 

 ecological relations. Suberisation, for instance, renders a membrane 

 impervious, or at any rate much less pervious, to water and gases ; 

 hence the frequent occurrence of suberised walls in dermal tissues. 

 Certain cell-walls, however, present the reactions of cutinised mem- 

 branes, and are nevertheless readily permeable by water ; the outer 

 walls of epidermal hydathodes illustrate this latter condition. The 

 physiological importance of lignification still remains unexplained. 

 This chemical modification cannot have any mechanical significance, 

 since fibres may be quite unlignified and yet display great tensile 

 strength. Sachs believed that lignification endows a membrane with a 

 special aptitude for conducting water ; but apart from other difficulties, 

 Sachs' view is inadmissible, because it is founded on the assump- 

 tion that the transpiration-current travels in the walls, and not, as is 

 now generally held to be the case, in the lumina of the conducting 

 elements. Mucilaginous walls often serve as a means of water storage, 

 especially when they occur in foliage leaves ; in the case of seeds and 

 fruits, on the other hand, mucilaginous membranes frequently assist in 

 fixing the seed to the substratum during the first stages of germination. 

 In a number of Leguminosae, the mucilaginous walls of the endosperm- 

 cells represent a store of plastic material. The mucilage which so 

 frequently occurs in submerged plants, finally, should perhaps be regarded 

 as a means of protection against animal foes. The function of silicified 

 and calcified walls is frequently mechanical, especially in the case of 

 epidermal cells and hairs ; impregnation with silica or lime renders the 

 membrane harder and at the same time more brittle, and may thus 

 become a very important feature in the case of organs which are 

 specialised as instruments of protection against animals. The brittle 

 condition which ensures the breaking off of the point of a stinging hair 

 at the right moment, results from the combined effects of calcification 

 and silicification. 



Both the first formation of the cell-membrane and its sub- 



