This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
The aloe plants are indigenous to eastern and southern Africa, but have been introduced into other tropical countries, as for instance the West Indies, and will flourish even in southern Europe. They produce spikes of yellow or red flowers and large fleshy leaves resembling those of the American agave, an ornamental plant commonly grown in this country and often erroneously called an aloe.
The leaf which is fleshy and mucilaginous in its interior, contains near the epidermis a row of isolated fibrovascular bundles each of which is surrounded by an endodermis. The cells of the pericycle and sometimes of the neighbouring parenchyma are unusually large, and filled with a viscous, yellow liquid, the aloetic juice. When the leaves are cut from the plant this juice slowly drains from the cells containing it. Probably the transverse walls of the elongated and axially arranged cells give way under the pressure exerted upon them by the surrounding tissue, and thus the aloetic juice is drained from cells situated at a considerable distance from the incision.
Little or nothing flows from the mucilaginous tissue in the centre of the leaf, and as it contains no aloetic juice no effort is or should be made to obtain it; in fact, its presence would lower the quality of the aloes.
The drug is obtained chiefly from Eastern and Southern Africa and from the West Indies. The means by which it is prepared are probably practically identical in all three countries. The leaves are cut from the plant and placed so that the juice that drains from the aloetic cells may be received in any convenient vessel. This juice is then concentrated, either (probably) by allowing it to evaporate spontaneously (Socotrine aloes) or by boiling it until it has reached the desired consistence, when it is poured into boxes or gourds (West Indian aloes) or transferred it to kegs (Socotrine aloes) or skins (Zanzibar aloes), or cases or barrels (Cape aloes) and allowed to solidify.
The manner in which this operation is performed has a remarkable influence on the physical characters of the aloes. If the juice is rapidly concentrated, the concentration carried as far as practicable, and the resulting aloes quickly cooled, a drug is obtained that breaks with a vitreous or glassy fracture, and in small splinters is quite homogeneous and transparent even when examined under the microscope. Such an aloes is termed a ' vitreous,' ' lucid,' or ' glassy ' aloes. But if the evaporation is carried on slowly and not quite so far as in the preceding case, a drug is obtained that is opaque and exhibits when examined under the microscope minute prismatic crystals embedded in a transparent resinous mass. Such an aloes is termed ' hepatic ' or ' livery.' The crystals that it contains are aloin, and the reason why in this case the aloin crystallises, whilst in the vitreous aloes it does not, is probably to be found partly in the fact that the conditions under which evaporation takes place are more favourable to crystallisation, and partly in the conversion of the crystalline aloinin to an amorphous modification (see below). Nevertheless every variety of aloes not only may, but actually does, occur in both the vitreous and the opaque modification.
Fig. 239. - Transverse section near the margin of an Aloe leaf, ep, epidermis; gfb, vascular bundle, the pericyclic cells, a, of which are much enlarged and contain a yellow secretion (aloes); p, palisade; g, parenchyma; sp, stoma; cr, calcium oxalate crystals; m, mucilaginous parenchyma. (Magnified.) (Tschirch).