The chemistry of the aloe plant has been studied for many years from a number of viewpoints. The leaf, the most frequently studied organ, can be divided into the outer green mesophyll, including the vascular bundles and the inner colourless parenchyma containing, to various degrees, the well known aloe gel. When a typical aloe leaf is cut there appears on the cut surface an exudate arising from cells adjacent to the vascular bundles (Beaumont etal, 1985), which is usually yellow-brown, which in a few species, eg. A. confusa Engl. can change to a deep blood red. This exudate from certain species, when dried, is the bitter aloes of commerce, used as a bittering agent or as a somewhat violent purgative. Early work identified the bitter, purgative factor as an anthrone-C-glucoside, barbaloin (Birch and Donovan, 1965; Hay and Haynes, 1956). Then, other compounds in the product were recognized and eventually characterized (reviewed Reynolds, 1985a). Subsequently, when a wider range of species was examined using thin-layer chromatography, a larger number more than 80-chromatographic zones were revealed, representing mostly unidentified compounds (reviewed Reynolds, 1985b). These zones were given code numbers to distinguish them and to aid further discussion. Since then many of these constituents have been isolated and characterized (reviewed Dagne, 1996). The distribution of these entities, either as chromatographic zones or as characterized compounds has been used for chemotaxonomic discussions. Their biological activities remain largely unknown but might be expected to yield interesting results.
Interest in the parenchyma gel has centered on its well-known therapeutic properties, although there is some evidence of chemotaxonomic variation in the polysaccharides and no doubt in the lectins. A certain amount of research has established the characteristics of some of these gel components but they are largely unknown in species away from the few well-known, widely distributed ones.
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