THE ASH OF PLANTS. 191 
exists in comparatively large quantity. In the dense teak 
wood, concretions of phosphate of lime have been noticed. 
Of a certain species of cactus, (Cactus senilis,) 80°|, 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 Nobbe & 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-plants, whose ash-percentage was 
twice as great as that of the land-planis. 
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 assuming 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. XIL) Fig. 
24 is a mass of crystals of a lime salt, from the leaf stem 
of rhubarb. Fig. 25, similar crystals from the beet root. 
