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FIG. 34. A, inner surface of a stoma of Equi- 

 setum isevigatum, showing the silicious bars 

 (X 250). JB, surface of a large internodal cell 

 from a young plant of Chara sp., showing 

 masses of carbonate of lime (X 250). 



by the introduction of new particles between the old ones ; i.e. by 

 Intussusception. It seems probable, however, that both processes 

 are active in its growth. 



Incrustation. Mineral substances often occur in the cell-wall, 

 sometimes to such an extent as to render the tissues of stony hard- 

 ness. This reaches its 

 jP 4 g j maximum in certain Algae, 



particularly the marine 

 Corallines and Siphoneae, 

 which live especially about 

 coral reefs, to the build- 

 ing of which they often 

 largely contribute. Here 

 the incrustation is car- 

 bonate of lime, as it is in 

 the tissues of the true 

 Corals. Of fresh-water 

 Algae, the Stoneworts, 

 species of Chara, are the 

 best-known examples of 

 calcareous incrustation. 

 Silica is also a common incrusting agent, and is abundant in the 

 epidermal tissues of many land plants e.g. Grasses, Equisetum 

 whose epidermis is either hard and polished as is the stems of 

 Bamboo, or rough like sandpaper, as in Equisetum and the harsh 

 cutting margins of many Grass leaves. Beautiful examples of the 

 impregnation of the cell-wall with silica are offered by the shells 

 of Diatoms. By burning away the organic matter, the silicious 

 skeleton may be obtained. 



Chemical Changes of the Cell-wall. Of the various chemical changes 

 which the cell-wall may undergo, lignification and suberization are 

 perhaps the most familiar, although the exact nature of these 

 changes is not entirely understood. Lignification is the change into 

 wood found especially in the water-conducting tissues of the higher 

 plants. It seems likely that the change into wood involves a chemi- 

 cal change in the constitution of the original cellulose membrane, 

 combined with an infiltration of various substances, including gum, 

 mineral constituents, etc. The woody membranes are good conduc- 

 tors of water, especially when young, and are of special importance 

 to the plant in this connection. Their firmness also makes the wood- 

 cells the most important of the skeletal elements of the higher 

 plants. 



The suberized or corky cell-walls, unlike the woody walls, are 

 impervious to water, and are especially developed where it is desir- 

 able to protect the tissues against loss of water. Cork-cells are 



