SILICIFIED AND CALCIFIED WALLS 39 



cell-wall. Diverse species of Acacia yield gums of economic im- 

 portance, e.g. gum arabic (from A. Senegal), catechu used in 

 tanning (from A. catechu), etc.; gum tragacanth, which is far 

 less soluble, is obtained from species of Astragalus. 



Another frequent modification consists in the deposition of 

 mineral substances within the cell-wall. For example, in the 

 Horsetails and most Grasses the membranes of the outer cells 

 are impregnated with silica, to which many Grass-leaves owe 

 their sharp cutting edges. This silicification increases the sta- 

 bility of the plant and also serves as a protection against snails, 

 etc. A siliceous envelope is very characteristic of certain minute 

 unicellular Algae known as Diatoms (cf. p. 206), which often occur 

 in countless numbers in fresh and salt water. 



In a few cases the surface of the plant is encrusted with 

 carbonate of lime, as in some Stoneworts (Char a] and certain 

 other lowly members of the Vegetable Kingdom (cf. p. 204). 

 Local deposits of lime on special ingrowths of the walls are 

 not uncommon in the epidermal cells in certain groups of 

 Flowering Plants. The resulting structures are known as cysto- 

 liths, and are well seen in the leaves of the Stinging Nettle 

 (Fig. 19, C.) and of various kinds of Fig (e.g. in Ficus elastica). 

 If sections of the leaves are placed in weak acetic acid, the 

 carbonate of lime dissolves with effervescence and the framework 

 of the cystolith becomes visible. 



