THE ASH OF PLAKTS. 



305 



species of cactus {Cactus senilis) 80% of the dry 

 matter consists of crystals of calcium oxalate and phos- 

 phate. 



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 observed 

 by Nobbe & Siegert, who remark that the two portions 

 of buckwheat, cultivated by them in solutions and in gar- 

 den-soil respectively (p. 203), both contained crystals 

 and globular crystalline masses, consisting probably of 

 calcium and magnesium oxalates, and phosphates, depos- 

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

 abundant in the water-plants whose ash-percentage was 

 twice as great as that of the garden-plants. 



These insoluble substances may be either entirely unes- 

 sential, or, having once served the wants of the plant, may 

 be rejected as no longer useful, and by assuming the in- 

 soluble form, are removed from the sphere of vital action, 

 and become in reality dead matter. They are, in fact, 

 excreted, though not, in general, 

 formally expelled beyond 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 encysted in the living 

 cells. 



The occurrence of crystallized salts 

 thus segregated in the cells of plants 

 is illustrated by the following cuts. 

 Pig. 23 represents a crystallized con- 

 cretion of calcium oxalate, having a basis or skeleton of 

 cellulose, from a leaf of the walnut. (Payen, Ghimie In- 

 dustrielle, PI. XII. ) Fig. 24 shows a mass of crystals of the 

 same salt, from the leaf stem of rhubarb. Fig. 25 illus- 

 trates similar ciystals from the beet root. In the. root of 

 the young bean, Sachs found a ring of cells, containing 



Fig. as. 



