Aloeae

Aloeae leaves are more or less succulent and arranged in strongly tufted terminal rosettes (in arborescent species, as in Aloidendron barberae) or in basal rosettes, and are organized in distinct ranks. Leaves are succulent and have distinctive white or other uniformly coloured tubercules. This is hypothesized to be a derived condition, possibly as a mechanical defensive mechanism to make the leaf less palatable, or to prevent heat damage in arid conditions. The cross-section of the leaves is distinctly boat or crescent shaped, which represents a synapomorphy for all Aloeae taxa.[2]

The inflorescence is compacted into a many-flowered spike, or a simple or branched raceme, and is apical, although it can seem axillary. Stems are monopodial (unbranched) until an inflorescence is formed, and then sympodial (potentially branched); this prevents the rosette from dying as in Agave. Flowers of all taxa within Aloeae are tubular and have some fusion of petal-like tepals, although the amount of fusion varies among genera.[2] Differences in flower structure and colour among genera are considered to represent pollination syndromes, particularly shifts between insect and bird pollination, and so are less reliable indicators of relationships among species than was once thought. Thus Astroloba corrugata, like most species of Astroloba, has small, pale, horizontally held flowers that are insect-pollinated, whereas A. rubriflora has larger, red, vertically held flowers that are bird-pollinated.[3]

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  Astroloba corrugata – insect-pollinated flowers
• 
  Astroloba rubriflora – bird-pollinated flowers

Within the subfamily Asphodeloideae, Aloeae can be diagnosed by their succulent foliage, a basic chromosome number of x = 7, and the presence of specific glycosides in the leaves and anthraquinones in the roots.[3]

The unity of this group of plants, the "aloes", has long been recognized, although its classification and nomenclature have varied considerably. In 1753, Carl Linnaeus followed authors before him in using a single genus, Aloe, for all the "aloes" known to him.[4] The genus was subsequently split several times; for example, Kumara was separated out in 1786, Haworthia and Gasteria in 1809.[3] Regardless of the number of genera, the "aloes" were placed in the family Liliaceae, formally described by Antoine Laurent de Jussieu in 1789. The Liliaceae sensu lato grew in scope until it included most of the lilioid monocots.[5] Attempts were made to carve out separate families. In 1802, August Batsch placed the "aloes" in the family Aloaceae rather than the Liliaceae. However, his family was rarely adopted by other botanists.[6][7]

The Dahlgren system of 1975 onwards was one of the first to produce a classification more in line with current understanding; it placed the "aloes" in the family Asphodelaceae, with the subfamily Alooideae more-or-less corresponding to Batsch's Aloaceae.[6][5] This sense of the Asphodelaceae was supported by the first APG system of 1998. However, the Angiosperm Phylogeny Group later adopted a broader circumscription, so that in the APG IV system of 2016, the Asphodelaceae sensu stricto becomes the subfamily Asphodeloideae of the Asphodelaceae sensu lato.[8] In this system, the "aloes" form the tribe Aloeae. However, the APG's approach has not been followed by many botanists, who prefer to maintain the older, narrower circumscription of the Asphodelaceae, with the "aloes" forming the subfamily Alooideae.[3]

The largest genus, Aloe, with about 400 species, is natively distributed in Africa (including Madagascar), and in the Arabian Peninsula. Aloidendron, with six or seven species, also has a quite wide native distribution in southern and east Africa extending northwards to the Arabian Peninsula. The remaining genera are native only to southern Africa, some being restricted to a few locations in South Africa. Kumara, Haworthia, Astroloba, Tulista and Haworthiopsis are found in winter rainfall areas of southern Africa. Gonialoe species are found in arid regions of South Africa and Namibia.[12] Many species are cultivated as ornamental plants, and Aloe species in particular have become widely naturalized.[13]

Many species are cultivated by succulent plant enthusiasts; Aloe species in particular are used in temperate climates as ornamental garden plants.[14] Aloe vera and Aloe ferox are used in traditional medicine and cosmetics for their moisturizing and emollient effects.[15][14] Industries based on these two species were reported to be worth "millions of dollars per year" in South Africa alone.[14] Injured leaves of many species of Aloe exude a gel that contains aloin (also known as barbaloin). Aloin has been used as a laxative,[16][17] and also to add a bitter taste to foods.[18] Some species of Haworthia and Gasteria are also used in traditional medicine.[14]