THE ASH OF PLANTS. 



191 



exists in comparatively large quantity. In tiie dense teak 

 wood, concretions of phosphate of lime have been noticed. 

 Of a certain species of cactus, {Cactus senilis,) 80" I,, of 

 the dry matter consists of crystals, probably a lime salt. 



That the quantity of matters thus segregated is in some 

 degree proportionate to the excess of them in the nourish- 

 ing medium in which the plant grows has been observ- 

 ed by ISTobbe & Siegert, who remark that the two por- 

 tions of buckwheat, cultivated by them in solutions and 

 in garden soil respectively, (p. 188,) both contained crys- 

 tals and globular crystalline masses, consisting probably 

 of oxalates and phosphates of lime and magnesia, depos- 

 ited in the rind and pith ; but that these were by far most 

 abundant in the water-iolants, whose ash-percentage loas 

 twice as great as that of the land-plants. 



These insoluble substances may either be entirely unes- 

 sential, as appears to be the case with silica, or, having 

 once served the wants of the plant, may be rejected as no 

 longer useful, and by assmning the insoluble form, are re- 

 moved from the sphere of vital action, and become as good 

 as dead matter. They are, in fact, excreted, though not, 

 in general, formally expelled be- 

 yond the limits of the plant. They 

 are, to some extent, thrown off into 

 the bark, or into the older wood 

 or pith," or else are virtually en- 

 cysted in the living cells. 



The occurrence of crystallized 

 salts thus segregated in the cells 

 of plants is illustrated by the 

 following cuts. Fig. 23 represents 

 a crystallized concretion of oxalate 

 of lime, having a basis or skeleton of cellulose, from a leaf 

 of the walnut. (Payen, Chimie Industrielle PI. XII.) Fig. 

 24 is a mass of crystals of a lime salt, from th^ leaf stem 

 of rhubarb. Fig. 25, similar crystals from the beet root. 



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