1191. 'When we examine the ashes of plants' says Schleiden, 'we find in particular the four following constituents, giving them their characteristics: readily soluble alkaline salts; earths, especially lime and magnesia; phosphoric acid, and silicic acid or silex. Sometimes one, sometimes two of these substances predominate in the ashes of the plant. According to this, Liebig divides the cultivated vegetables into: 1. Alkali plants; to which belong potatoes and beets. 2. Lime plants; clover, peas, &c. 3. Silex plants; the grasses. 4. Phosphorus plants; comprehending rye and wheat.' But besides these, which are only the general outline, plants have been found to contain various other earths or salts, sometimes to the number of five or six different kinds, in very small proportions.
