There are at least 600 known species of Aloe (Family Liliaceae) (Kawai etal, 1993), many of which have been used as botanical medicines in many countries for thousands of years (Grindlay and Reynolds, 1986; Gjerstad and Riner, 1968; Reynolds and Dweck, 1999; Swanson, 1995). Species of the genus Aloe are indigenous to Africa (A. ferox Miller, A. africana Miller, A. spicata Baker, A. platylepis Baker, A. candelabrum Berger) and Socotra (A. perryi Baker, A. forbesii Balf.fil.). Some have been introduced in Asia (A. chinensis Baker), the Barbados Islands in Central America (A. barbadensis Miller, otherwise known as A. vera [L.] Burm. or A. vulgaris Lamarck) and Europe (A. arborescens Miller). Aloe ferox, known in commerce as 'Cape aloe,' is easy to hybridise and cultivate in Africa. The term 'Cape aloe,' for accuracy, refers to the dried latex of the leaves of several species of the Aloe genus, especially A. ferox, and the hybrids and preparations made from them (Blumenthal, 1998). Aloe barbadensis, known in commerce as 'Curacao aloe,' was said to be native to northern Africa but was introduced into the Barbados Islands in the seventeenth century and is now cultivated in Florida, USA (Bruneton, 1999). A. chinensis, a variety of A. vera, was introduced into Curacao from China in 1817 by Anderson. The plant was cultivated in the Barbados Islands until the end of the nineteenth century. Curacao aloe is often called Barbados aloe. Other varieties are cultivated throughout India while some grow wild on the coasts of Bombay, Gujarat, southern Arabia, Madagascar and areas surrounding the Red Sea and the Mediterranean Sea (Morton, 1961; Kapoor, 1990). At the present time the principal areas for production of aloes are South Africa, Venezuela, Haiti, Florida and the Dutch islands of Aruba and Bonaire. The plant grows very well if adequately protected from cold weather; aloes are injured at 2 °C and generally killed at —1 °C.
The genus Aloe includes trees (e.g. A. ferox: Figure 9.1) of variable height (from 2 to 15 metres), shrubs and herbs (A. barbadensis). They are succulent plants with perennial, strong and fibrous roots and numerous (15—30) large, fleshly leaves, carrying spines at the margin. In some species the leaves form a rosette at ground level (A. perryi). The flowers are grouped in erect, terminal spikes, and are borne by a floral stalk, which is either unique (A. vera) or ramified (A. ferox); the corolla is tubular, divided into narrow segments (six) at the mouth and of a red (A. perryi), yellow (A. vera) or white (A. speciosa Baker) colour (Dezani and Guidetti, 1953). In some aloes (A. arborescens) the vascular cambium develops with age, initiating extensive secondary growth and allowing considerable lateral expansion (Cotton, 1997).
Aloes resemble to some extent the agave or century plant (Agave americana L., Family Amaryllidaceae); however, the aloe plant is in flower during the greater part of the year while the agave plant is remarkable for the long interval between its periods of flowering.
Aloe leaves in section show from outer to inner: (i) a cuticularized epidermis; (ii) a parenchyma containing chlorophyll, starch and bundles of needles of calcium oxalate; (iii) large peryciclic cells; and, (iv) a central region (3/5 of the diameter of the leaf) consisting of large parenchymatous cells.
Aloe is from the Arabic alloeh or the Hebrew halal and means a shining, bitter substance (Tyler, 1993); ferox is from the Latin and means ferocious or wild; vera is from the Latin verus, meaning true; barbadensis refers to the Barbados Islands; africana, or chinensis, refers to the habitat of the plant; spicata refers to the flowers in spikes. However, the word aloe in pharmacopoeias and formularies means a drug derived from the dried leaf juice. This has always created confusion because the leaves of the genus Aloe are the source of two products that are quite different in their chemical composition and their therapeutic properties, aloe latex and aloe gel (Capasso etal., 1998). These two products are obtained from two different specialised cells, latex from pericyclic cells and gel from parenchymatous cells. Therefore the term juice must be avoided, as it could mean either the latex from the pericyclic cells, or the gel after extraction from the leaf. There is also a preparation obtained from the whole leaf (total extract) which contains all the components present in aloe leaves. Aloes contain another medically important part, the leaf epidermis (Imanishi etal, 1981). There are also the aloe wood and the aloe root. The first preparation, so called lignaloe or aloe of the Bible, is a fragrant wood obtained from an entirely different plant that was once used as an incense (Tyler etal, 1993). The second is a synonym of unicort root, a preparation that when dried becomes a valuable bitter tonic (Grieve, 1998).
Apart from aloe wood and aloe root which have nothing to do with the genus Aloe, the other preparations have very similar names that are sometimes interchanged. Aloe in one form or another is a common domestic medicine and is the basis of most pharmaceutical preparations.
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