168 THE CELL 



bundles of white fibres which cross one another at right angles in 

 alternate lamineB. 



Not infrequently cellulose membranes show delicate sculptur- 

 ings, especially upon the inner surface. Thus thickenings may 

 originate in the interior ; these may run into each other to form 

 a spiral, or may be arranged in large numbers transversely to 

 the long axis of the cell, or finally, may be united together in an 

 irregular fashion to form a network. On the other hand, the 

 thickenings may be absent at various places, where neighbouring 

 cells touch, and thus pits or perforations are produced (Fig. 72 A), 

 by means of which neighbouring cells can interchange nutrient 

 substances with greater ease. 



Moreover, as regards its composition, the cell-wall can alter its 

 character in various ways soon after its original formation ; this 

 may be produced by the deposition of various substances upon it, 

 or by its transformation into wood or cork. 



Lime salts or siliceous substances are not infrequently deposited 

 in the cellulose, thus producing greater solidity and hardness of 

 the walls. When portions of such plants are burnt, the cellulose 

 is destroyed and a more or less perfect skeleton of lime or silica 

 remains in the place of the framework of the cell. Lime is 

 deposited in Corallinece, in Characece, and in CucurbitacecB ; and 

 silica in Diatomacece, Equisitacece, Grasses, etc. 



Similarly the cell- wall obtains very great strength through the 

 formation of wood. Here the cellulose becomes mingled with 

 another substance, woody substance (lignin and vanillin), this 

 may be dissolved away by means of potassic hydrate, or with a 

 mixture of nitric acid and chlorate of potash, after which a frame- 

 work, which gives the reaction, of cellulose remains. 



In the formation of cork the cellulose becomes united in larger 

 or smaller quantities with corky substance or suberin. In this case, 

 also, the physical properties of the cell-wall are altered, it being 

 no longer permeable to water. Thus cork cells are formed on the 

 surface of many parts of plants in order to prevent evaporation. 



Whilst it is evident, that in the deposition of lime and silica, the 

 particles of these substances must be conveyed by the protoplasm 

 to the place where they are required, and where they are de- 

 posited between the particles of cellulose, whereupon molecular 

 combinations are again called into play, two explanations may 

 be given concerning the formation of wood and cork. Either the 

 wood and cork substances are constructed in a soluble form, by 



