38 MUCILAGE AND GUM 



The walls of mature cells often undergo chemical changes 

 other than lignification, some of which (e.g. the change affecting 

 the cells of the cork, p. 136) will be described later, but a few 

 special cases may conclude the present chapter. In a number 

 of plants the thickening of the walls of certain cells takes the 

 form of layers of mucilage, whose exact chemical constitution is 

 not known. These, in the dry condition, are hard and horny, 

 but in the presence of moisture they soften and swell up con- 

 siderably. Indeed, such mucilaginous walls possess a remarkable 



FIG. 19. Transverse section of the leaf of a Nettle (Urtica) showing a 

 cystolith (C.). CM., Cuticle; Ep., epidermis ; A, hair; Pa., palisade 

 tissue ; Sp., spongy parenchyma. 



power of absorbing and holding water, and are consequently 

 particularly prevalent in plants characteristic of dry situations. 

 The slimy character of many Seaweeds (e.g. the common Bladder- 

 wrack) is due to the mucilaginous nature of nearly all the cell- 

 walls, and here the change in bulk, as between the dry and wet 

 condition, is especially evident. Mucilaginous walls stain deeply 

 and rapidly with aniline blue. 



The gums formed by certain plants are probably very similar 

 chemically to the mucilages, and, like them, appear, in many 

 cases at least, to arise as a result of chemical alteration of the 



