192 



BOTANY 



organic substances, it remains as a firm siliceous skeleton, preserving 

 the structure of the cell walls. The hardness and firmness of the 

 cell walls are greatly increased by these siliceous deposits; they 



themselves have about the de- 

 gree of hardness 2, but the 

 silicified epidermis of Equisetum 

 has the hardness 4 ( = Fluor- 

 spar), and that of the fruits of 

 Coix 7 ( = Quartz). Some of 

 the Equisetaceae are even used 

 for polishing and scouring. 

 The silicified cell walls of 

 Diatoms occur as fossils, and 

 form deposits of SILICEOUS 

 EARTH (Kieselguhr) in some 

 geological formations. The 

 value of the siliceous concre- 

 tions, termed " tabasheer," 

 that are found within the 

 joints of the Bamboo has not, 

 as yet, been satisfactorily ex- 

 plained. ALUMINIUM, although 

 like silica everywhere present 

 in the soil, is only in excep- 

 tional instances taken up by 

 plants (Lycopodiaceae, Lichens, 

 Vitaceae, Leguminosae). Some 

 species of Lycopodium contain 

 a sufficient quantity of acetate 

 of aluminium to render the sap 

 useful as a mordant. The 

 same salt is found also in 

 Grapes. On the other hand, 

 although scarcely a trace of 

 iodine can be detected by an 

 analysis of sea -water, -it is 

 found, nevertheless, in large 

 quantities in seaweeds, so much 

 so that at one time they formed 

 the principal source of our 

 supplies of this substance ( 15 ). 



The substances which, as culture experiments show, are not indispensable for 

 the life of the plant are, however, of use in so far as they can replace for some 

 purposes (such as the neutralisation of free acids, etc.) essential elements of plant 

 food. The latter are thus available for the special purposes for which they are 

 indispensable. Thus K can be partially replaced by Na, and Mg by Ca. 



Flo. 184. Salii-ofiiiii herliucec, a. characteristic halo- 

 phyte. (From SCHIMI-BB'S riant-Geography.) 



