The demarcation and definition of the genera within the family Aloaceae have been debated since 1753, when Swedish biologist Carl Linnaeus classified these ‘lilies’ under the Hexandria Monogynia in his book Species Plantarum. Linnaeus’ system divides plants into 24 classes, according to the number and arrangement of their stamens, and then into orders according to the number of pistils.
Class VI Order I: Hexandria Monogynia comprises those species (including aloes) with six stamens and a single ovary. Linnaeus divided the orders into genera, and the genera into species. This early classification system is artificial in the sense that members of groups formed on the basis of a few characters only are often not closely related in an evolutionary sense. Modern systems of classification, on the other hand, strive to demarcate groups based on assumed shared common ancestry.
Even at the rank of family, consensus is yet to be reached regarding the best placement of the group of related alooid genera (Aloe and relatives), members of which have been variously classified in the broadly defined Liliaceae, the rather heterogeneous Asphodelaceae, the considerably expanded Xanthorrhoeaceae or, as we prefer here, the narrowly defined family Aloaceae.
A. comptonii is a typical cliff-dweller in its natural habitat.
According to the biological species concept, a species is a sexually reproducing population of individuals. Hence, aberrant (but often horticulturally improved) individuals resulting from hybridization between two aloe species in nature do not qualify for species status, especially if such hybrids are sterile. Genetic (or intrinsic) variation among individuals is a common feature of sexually reproducing organisms and an integral property of a species. Genetically distinct local forms within a species, especially those with a geographical bias, are referred to as ecotypes.
Intrinsic variation sometimes manifests at the physiological or chemical level only. For example, populations of Aloe ferox in the southern Cape are rich in the chemical compound aloin (with purgative properties), whereas populations further north in the Eastern Cape contain less aloin. Morphologically, however, plants from the two regions appear similar. In such instances the different chemical forms are referred to as chemotypes.
The differences between ecotypes and chemotypes found in the same species are retained even if the different forms are grown together in a garden. Given enough time, ecotypes may eventually develop into separate species. If differences between ecotypes (more rarely chemotypes) are marked and there is some discontinuity in variation, they may be formally recognized as different subspecies, varieties or forms of a species.
Horticulturallty superior clones of genetically identical variants are regularly encountered in cultivation where they are being maintained deliberately by vegetative rather than sexual reproduction. Such forms are technically ‘cultivars’, not species. In some circles, cultivars are often incorrectly referred to as ‘varieties’. Over the years, plant breeders have produced numerous aloe cultivars with outstanding horticultural properties.
From earliest times humans have been classifying objects naturally and almost instinctively. In order to understand the bewildering diversity of plants and animals, modern biologists have developed means of classifying organisms into groups or clusters (‘taxa’) according to shared features and assumed common descent. Thus, taxonomy is concerned with classification.
Two steps are involved in the practice of taxonomy. The first is the discrimination of the species – most popularly accepted as a reproductively isolated aggregate of populations that can interbreed with one another because they share isolating mechanisms that, in nature, prevent them from interbreeding with other species. The second step is the classification of related species into a group of a higher ranking. Thus, related species are grouped into ‘genera’ (singular = genus). Similarly, related genera are grouped into families, and families into orders, orders into classes, and so on to create a hierarchy that in plants ends with the all-embracing ‘kingdom’ of plants (Plantae). Each plant is allocated a two-part species name that comprises the genus name followed by the specific epithet (for example, Aloe ferox).
The family Aloaceae can be divided into five related genera – Aloe, Astroloba, Chortolirion, Gasteria and Haworthia. Each genus is described and illustrated on the following pages, and a key is provided that will enable easy identification of these genera in the field (see pages 26–27). Bear in mind, though, that in attempting to provide a bird’s eye view of the genera of the Aloaceae, some generalizations are unavoidable.
The natural hybrid between Aloe arborescens and Aloe ferox can grow as a scraggly shrub or as a small tree of over a metre tall. However, its single or multi-branched inflorescences are always fairly long, and the flowers are mostly densely collected into bright orange or red inverted cone-shaped clusters.
The open flowers of A. speciosa are bright white, which contrasts sharply with the red colour of the buds.
The derivation of the word ‘aloe’ is uncertain, but the following has been suggested:
1. From the Arabic alloch or alloeh, referring to species used medicinally; a vernacular name for such members of the genus.
2. From the Greek aloë, the dried juice of aloe leaves, akin to or derived from earlier Semitic (alloeh), Hebrew (ahalim or allal, i.e. bitter) and Sanskrit words.
The genus Aloe comprises more than 550 species of fat-leaved plants that range from trees over 20 m tall to miniatures reaching only a few centimetres above the ground. Most aloes have their succulent leaves arranged into rosettes that are borne at the tips of thin or robust stems. The small, tube-shaped flowers are usually brightly coloured and are carried, either loosely or densely, on single or branched candles that extend well beyond the leaf rosette. The dry leaves of aloe plants are usually persistent on the stems.
Recently, the genus Lomatophyllum, originally with about 20 species from Madagascar and the Mascarene Islands, was placed in Aloe as a specialized group. This treatment is now widely accepted. In its days of acceptance as a genus, the argument would have been that the berry-like (but still late-dehiscent like a capsule) fruit distinguishes Lomatophyllum from Aloe, which has capsular fruits that rapidly become woody when they dry out. In general, the leaves of the species formerly included in Lomatophyllum are not as succulent as those traditionally included in Aloe.
The leaves of Astroloba spiralis are often grazed off near ground level by domestic stock and game.
In some places, Astroloba foliolosa leaves are red.
Derivation: From the Greek aster; astros, meaning star; and Greek lobos, meaning lobe, for the stellately spreading perianth lobes.
The species included in Astroloba all have fairly short and thin, but distinct, stems that are borne erectly or sprawl along the ground with age. The leaves of astrolobas are invariably small, triangular and closely packed on the stems. Because of the small size of the sharp-tipped leaves, the plants are widely regarded as a hazard for grazing livestock as the leaves easily get stuck in the animals’ throat. The main difference between representatives of the genera Astroloba and Haworthia lies in the shape of their flowers. The flowers of astrolobas are not as distinctly two-lipped as those of haworthias. Species of Astroloba favour, and are restricted to, the arid interior of the southern karroid parts of South Africa.
Like Lomatophyllum, Poellnitzia is another genus that is no longer recognized by most botanists. It was included in Astroloba a few years ago, after making its presence felt in virtually all the other Aloe-like genera. It was once treated as a monotypic genus, with Poellnitzia rubriflora its only species. Its leaves are triangular and closely resemble those of species of Astroloba, but when it flowers there can be no doubt as to why it was regarded as a separate genus for many decades after its discovery. The flowers are pencil-shaped, bright red, and clearly adapted to bird pollination. Astroloba rubriflora, as it is now known, is strictly confined (endemic) to the Worcester-Robertson Karoo in the southwestern Cape.
Chortolirion angolense has an onion-like underground bulb; it survives the cold, dry winters of its grassland habitat by shedding its leaves. In the summer-rainfall season, new grass-like leaves (circled) grow above the ground.
Derivation: From the Greek chortos, feeding place, and leirion, lily, for its preferred habitat in grassland.
This genus consists of a single species, in other words it is monotypic. The species, Chortolirion angolense, varies greatly throughout a very large geographical distribution range that crosses the southern African winter- and summer-rainfall regions, something that very few Aloe species do. In addition, it occurs in desert areas along the west coast, and in tropical habitats in Zimbabwe. It is one of the few bulbous alooids that have a small whorl of deciduous leaves above ground. Its flowers look uncannily like those of some species of Haworthia. Two distinct flowering times have been observed for this species: some specimens flower in early spring, usually before the first summer rains have arrived, while others flower in autumn.
The flowers of Gasteria species have a basal swelling ranging from inconspicuous to pronounced. The flowers of summer-flowering Gasteria acinacifolia, a coastal species from the Eastern Cape, are shown here.
Gasteria acinacifolia leaves are typically densely mottled. These plants usually grow in the shade of nurse plants.
Derivation: From the Greek gaster meaning belly or stomach, alluding to the swollen base of the floral tube.
Species of Gasteria are characterized by their curved, mostly tricoloured flowers with pronounced bulbous swellings. The leaves of most species are picturesquely spotted, with distinct light green or whitish flecks and dots, and they are usually armed with sharp, bony edges rather than with marginal teeth.
It is possible to construct a strong infrageneric classification for Gasteria based on the morphology of the flowers, as this group of species is very distinct, especially when in flower, and cannot be confused with any other alooid genera. Indeed, it is probably the only genus in the group for which a strong case can be made for it not to be split or combined with other genera. In habitat, species of Gasteria are mostly confined to the shade of sparsely or even densely branched nurse plants. In nature, very few species venture into fully exposed growing positions. Gasteria species favour mild coastal habitats but also occur in some adjacent, climatically less severe inland areas, such as parts of KwaZulu-Natal and the Eastern Cape hinterland.
Haworthia fasciata is called ‘small aloe’ because its growth form resembles that of its larger kin in Aloe.
Haworthia glabrata flowers are small and dull-coloured. The free tips are bent backwards like smiling lips.
Derivation: Named for Adrian Hardy Haworth (1768–1833), English botanist, entomologist, gardener and writer on succulent plants.
Species of the genus Haworthia can be identified primarily by means of their rather drab, bilabiate (two-lipped) flowers and small rosettes of highly succulent leaves. The species are mostly small in stature; in fact, with the exception of H. maxima and a few species with snakelike, leafy stems, such as H. coarctata and H. viscosa, most species are less than 100 mm tall when not in flower.
To date three subgenera have been recognized in the genus. The type subgenus, Haworthia, consists mostly of the so-called soft-leaved species, many of which have a decidedly blue-green leaf colour. Species of this subgenus have the largest, most beautiful flowers of the three subgenera and they are the only ones whose flowers tend to have distinct tinges of yellow or pink.
In contrast, those species normally included in the subgenus Hexangulares are rather stiff-leaved and have less significant flowers. Species of this subgenus tend to be larger than those included in Haworthia subg. Haworthia.
The third group, Haworthia subg. Robustipedunculares, includes the largest of the species of Haworthia. With the exception of H. marginata, the leaves of all the species are adorned, to a lesser or greater extent, with white or concolorous tubercles. The flowers of this group of rather robust plants are the smallest of all the species of Haworthia, but are mostly borne on multibranched inflorescences.
Many species of Haworthia subg. Haworthia (and even H. bruynsii of the subgenus Hexangulares) have windowed leaves of which the tips are sometimes apically flattened or bent backwards. These species typically grow flush with the ground.
Southern Africa has become home to several groups of exotic aloe look-alike plants, most of which belong to the family Agavaceae, which is native to the Americas and some Caribbean islands. Horticulturally, the most popular of these leaf succulents are the frost-hardy yuccas and the massive agaves, or century plants, which live for 15–20 years or more before flowering once and then dying.
Agaves, or century plants, are monocarpic – that is, they die after having flowered. Some species are perennial through side shoots, such as this specimen of the miniature Agave nizandensis.
The large, dull green, clustered flowers of Agave nizandensis are borne on a tall, thin inflorescence.
Derivation: From the Greek agavos for noble, referring to their imposing stature, especially of the large-growing species when they flower.
The best known of the Agavaceae is undoubtedly Agave, a genus of about 250 species that is centred in Mexico. In contrast to Aloe species, most members of Agave are stemless, although they can grow to enormous proportions. A primary difference between Aloe and Agave is that species of the latter are exclusively monocarpic – that is, they flower only once before dying. The formation of plantlets on the inflorescences of agaves is a common occurrence, but is rather rare in representatives of Aloe.
Furcraea species, such as Furcraea foetida var. mediopicta, look very much like agaves, but their flowers hang down and the plantlets formed on the inflorescences tend to be more globular than those of agaves.
Derivation: Named for Antoine F. de Fourcroy (1755–1809), a French politician and chemist.
Furcraea is another New World genus that includes several aloe-like plants. Like the agaves, it occurs mainly in Mexico and northern South America. It contains a number of species that can attain tree-like dimensions, but several remain essentially trunkless. Like agaves, they flower only once before dying. In contrast to the agaves, which have erect flowers, the bell-shaped flowers of Furcraea hang down from drooping inflorescence branches. In addition, the plantlets produced on the inflorescences tend to be globular, like marbles, rather than distinctly recognizable as immature plantlets ready for the planting.
Flowers of Hesperaloe parviflora are bright red and closely resemble those of some aloe species. Dasylirion wheeleri grows in the background.
Derivation: From Greek hespera for evening or night combined with ‘aloe’ (see p. 18), indicating the occurrence of these superficially aloe-like plants in the American west, where the sun sets.
Once you have seen flowers of Hesperaloe parviflora, one of half a dozen species today included in the genus, it is easy to understand why this native of Mexico and the southern USA was at first thought to be a true African aloe. The flowers are pinkish red, borne on tall, branched inflorescences and, at first glance, look almost exactly like those of aloes in terms of architecture. However, the leaves of the species are thin, longitudinally strongly incurved, and adorned with beautifully curled marginal leaf fibres, a character that is absent from Aloe species.
Yucca desmetiana leaves are leathery rather than succulent. Interestingly, this species of Yucca has never been seen to flower. It grows well in both pot and open-ground cultivation.
Derivation: Yuca, with one ‘c’, is a name used for edible roots of cassava, and it could have been misapplied to some yucca species, which have edible flowers.
Members of the genus Yucca, although less aloe-like than agaves and furcraeas, can also be confused with aloes. However, their leaves are consistently flatter and thinner than those of aloes. The plants are mostly shrubby, while a few, such as the well-known Joshua tree (Yucca brevifolia), can attain tree-like dimensions. Yucca flowers are lantern-shaped and they tend to be adapted for pollination at night. The classic example of mutualism, a type of symbiotic relationship, is that between the yucca plant and its highly specialized moth pollinator (see box below).
• When species share the same habitat, competition for resources and space can be fierce, but it is not uncommon to find varying degrees of collaboration between different species. Symbiosis refers to an intimate relationship between two or more organisms of different species. Depending on whether the association is advantageous to one or both partners, three broad types of symbiotic relationship are distinguished:
Although they are sharp-tipped, the leaves of the tall-growing Mexican Yucca elephantipes are harmless. The inflorescence, with its lantern-shaped, uniformly creamy white flowers, hardly extends beyond the erect young leaves at the tip of the leafy stem.
• Mutualism: a relationship in which both partners benefit from the association. An example is the close relationship between flowers and specialized pollinators; in the case of aloes, mainly honey bees and/or sunbirds. A particularly close form of mutualism, in which one partner can no longer exist without the other, is the relationship between the yucca plant (right) and the yucca moth.
• Parasitism: a relationship in which one member (the parasite) benefits and the other (the host) is adversely affected. Many harmful organisms, including species of insects, mites and fungi, are parasites on aloes.
• Commensalism: a relationship in which one organism benefits and the other one is neither harmed nor helped. Examples include epiphytic orchids in the canopy of tree aloes, lichens on the bark of old aloe stems, and birds, lizards, spiders and insects that nest among the dry, spiny and protective leaf remains on the stems of some aloes. In all these examples, the aloe plant is unaffected, while the associated partner derives benefit.
An identification key is a device used to easily and quickly identify unknown plants or animals. One commonly used key – a dichotomous key – presents a series of paired statements that are sequentially numbered. Ideally, only one of the statements in a couplet must be applicable to the specimen that is being identified, while the other of course should not, the former then leading the reader to a further numbered couplet in the key. If a particular statement applicable to the unknown plant leads to a name, then the identification has been completed. A dichotomous key may be likened to travelling a well-marked road that forks repeatedly, each fork bearing directions. If the correct directions are taken, the traveller will arrive at his destination. However, it takes just one incorrect choice for the traveller to become lost, arriving at the wrong destination.
Most keys try to use characters that are readily accessible and easy to observe, either with the naked eye, or with a 10x hand lens or magnifying glass. Be sure to understand the meaning of the terms in the paired statements; do not guess. Always read both statements carefully – even if the first one seems to apply to your plant, the second one may be even better. Constructing an easy-to-use key is not a straightforward exercise; variation within some plant characters tends to be considerable, with characters and their states sometimes difficult to pin down with the required precision. Therefore, do not base a conclusion on a single observation, but arrive at an ‘average’ by studying several parts of a specimen.
1a. Flowers white, whitish or brownish; perianth segment tips distinctly flared open like a gaping mouth, often appearing somewhat irregular or 2-lipped. 
2a. Plants with a typical underground bulb. Leaves hardly succulent, grass-like, deciduous.
Chortolirion
2b. Plants without a bulb. Leaves distinctly succulent, not grass-like, persistent.
Haworthia
1b. Flowers usually shades of yellow, orange, pink or red, rarely white or green; perianth segments slightly or not at all flared at tips, more or less regular, not appearing distinctly 2-lipped. 
3a. Leaves lacking distinct spines and tubercles; margins white, bone-like, often armed with small semi-translucent, white tubercles. Inflorescences with flowers laxly arranged and usually on one side of the axis only. Flowers hanging down; perianth tube ± curved upwards, lower portion distinctly globosely swollen for up to two-thirds of the perianth length; swollen portion pale to dark pink, sometimes white.
Gasteria
3b. Leaves usually armed with spines or tubercles; margins green or brown, not bone-like; if without spines, then leaves usually lance-shaped with faint or prominent longitudinal lines (veins) (e.g. Aloe striata), or ± triangular (Astroloba). Inflorescences with flowers dense or lax, evenly arranged around axis or, if on one side of the axis only, then usually densely packed. Flowers variously arranged when open, ± straight or slightly curved downwards, lower portion of flower lacking a globose swelling or, if with a swelling (such as in the maculate aloes), then the swollen portion less than one quarter the length of the perianth and the three inner perianth segments fully fused to the three outer segments; perianth in shades of yellow, orange, pink or red, rarely white or green. 
4a. Flowers thin-textured, small, flimsy, inconspicuous, whitish or greenish. Leaves ± triangular, covering entire stem, dying back haphazardly along stem. Plants low-growing, rarely exceeding 0.3 m, or 0.7 m when in flower.
Astroloba
4b. Flowers thick-textured, large, fleshy, conspicuous, shades of yellow, orange, pink or red, rarely white or green. Leaves usually sword-shaped, clustered towards tips of stems, rarely ± triangular or covering entire stem, dying back from bottom of stem. Plants varying from very small to several metres tall and tree-like. 
5a. Flowers red to bright orange; erect; perianth tube apparently not opening at the tip; stamens included in perianth tube. Leaves ± triangular, covering entire stem. Plants low-growing, not exceeding 0.7 m when in flower.
Astroloba (One species; alternatively treated as Poellnitzia).
5b. Flowers in shades of yellow, orange, pink, red, rarely white or green; variously arranged; perianth tube with tip distinctly open when flowers are mature; stamens as long as/longer than perianth (i.e. often protruding from open flowers). Leaves sword-shaped, clustered towards tips of stems. Plants low- or tall-growing, occasionally reaching tree size. 
6a. Fruit a berry (fleshy, semi-dehiscent).
Aloe (± 20 species; alternatively treated as Lomatophyllum).
6b. Fruit a capsule (dry, dehiscent).
Aloe
The leaves of some aloes remain copiously white-spotted throughout. Aloe squarrosa, from Socotra, is a good example.
