20 ELEMENTS OF PLANT ANATOMY. 
behind, constituting the ash. These mineral matters are cal- 
cium, sodium, potassium, magnesium, iron, silicon, phosphorus, 
sulphur, chlorine, and some others. The exact nature of their 
union with the organic substances is not known. It is sup- 
posed, however, to resemble that of alloy among metals, rather 
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Fig. 14. 
Cells with bordered pores seen in long section to show how the plate prevents the contents 
of one cell from passing into the neighboring one. The dark portion represents 
contents colored with carmine. — (Th. Hurtig.) 
than a true chemical union. In some instances the mineral 
matter comprises a separate portion of the wall, as the silica in 
Equisetum, the calcium which forms part of the cystoliths found 
in certain cell walls, also that which occurs on the surface of 
some plants, as in the Characeae, one or two families of the 
Florideae, and different species of Potamogeton. 
