144 PLANT ANATOMY. 
which are soluble in slightly alkaline water aud in alcohol. 
Crystals of this character have also been detected by Adolph 
Meyer * in the leaves of Coniwm maculatum. 
The peculiar bitter substances of Aloé leaves, aloes, are con- 
tained in special cells, which are located directly in front of the 
vascular bundles (Fig. 63 @), and are confined toward the exterior 
by a nucleus-sheath in a single row and with bitter contents. The 
before-mentioned cells are short, and occasionally their contents. 
are crystalline. The entire remaining tissue of Aloé leaves con- 
tains an abundance of mucilage, but no bitter substances. 
The kino, from species of Pterocarpus, also occurs as a con- 
stituent of longitudinally extended cells.’ 
When a part of a plant is incinerated, there remains in the cru- 
cible a white residue—the ash. Since by careful ignition, in 
very many cases, the general outlines of the consumed portion 
of the plant remain preserved (leaves of the Graminex, hemp 
leaves, the shells of diatoms), it follows that the inorganic con- 
stituents of the membrane which resist the action of heat (at least 
in part) are so finely deposited that the molecule of the mem- 
brane can be removed therefrom by incineration, while the direct: 
connection of fhe inorganic particles is not destroyed thereby. 
Not only in the membrane, however, do we meet with deposits 
of mineral constituents, but the contents of the cells are also 
abundantly provided therewith. It has already been shown that. 
protoplasm contains an abundance of salts, that crystals of 
inorganic bases occur in the cell-sap, and also that the globoids 
(p. 98) consist of inorganic double salts. The cell-sap, more- 
over, also contains not inconsiderable amounts of such mineral 
constituents of plants as are soluble in water, and these are in 
fact the most important, namely, nitrates, phosphates, and 
salts of potassium and calcium. 
Accordingly, the ash of plants contains all those substances 
which are known to, be the necessary nutritive materials, name- 
ly: potassium, magnesium, calcium, iron,’ phosphoric, sul- 
' Compare also Fliickiger, ‘‘ Pharmakognosie,” 1883, p, 663. 
* See the subsequent chapter on Receptacles for Secretions. 
* A deficiency of iron is shown in leaves by their becoming yellow 
(chlorosis). 
