Being familiar with and understanding the parts that make up plants in the aloe family will help in identification and also cultivation of the species, and will foster an appreciation of their intricate biology.
The thin, rather insignificant looking leaves of this grass aloe, Aloe micracantha, belie the fact that it carries thick roots underground. This species occurs in the Eastern Cape in grassy fynbos, which is prone to fire and burns easily.
Aloes and their relatives have an adventitious root system – a network of roots that grow only a few centimetres below the soil surface. This allows the plants to benefit from even relatively low amounts of rainfall. Aloe roots are comparatively soft, which makes them ideal for planting next to man-made structures such as walls and swimming pools. Aloe roots usually do not increase in girth with age and are therefore not strong enough to break concrete. Broken or damaged aloe roots are a bright yellow colour.
Top and above: The leaves of most aloes are boat-shaped in cross-section, as in Aloe globuligemma (top); in contrast, leaves of the red-hot pokers (Kniphofia species) are distinctly keeled, as in the leaves of Kniphofia ensifolia subsp. ensifolia (above).
Although an amazing variety of leaf shapes are found in plants of Aloe and its generic relatives, a basic underlying architecture is readily discernible among nearly all the species. Aloe leaves are succulent (filled with juice), boat-shaped in cross-section and arranged in terminal clusters, called rosettes.
Water that accumulates in succulent leaves enables the plants to survive during the dry season. Although the degree of succulence differs vastly among alooid species, most are significantly succulent.
The leaves of most aloes are not fibrous and can easily be broken by hand or cut from a rosette using a knife. In contrast, the leaves of agaves are extremely tough and fibrous, cannot easily be broken off by hand and require a very sharp knife to sever from a rosette.
Aloe leaves are designed to dry out on the plants, often forming a skirt of dried leaves that remains attached to the stems or basal rosettes. In very few cases, as in the tree aloes (for example, A. barberae and A. dichotoma), old leaves at the base of rosettes are regularly shed as new ones develop, resulting in neat, clean-looking trunks, branches and rosettes.
Although the inflorescences of Kniphofia species, such as those of Kniphofia ensifolia subsp. ensifolia, closely resemble those of aloes, their leaves are consistently less succulent.
Leaves of species in the aloe family are mostly boat-shaped in cross-section. By contrast, the leaves of Kniphofia species (red-hot pokers) – often mistaken for aloes – are V-shaped in cross-section. Kniphofias have many characters in common with aloes but, with the exception of one species, Kniphofia typhoides, they do not have succulent leaves.
One of the most obvious benefits of having boat-shaped leaves arranged in a rosette is that the leaves act as a very effective funnel through which water and moisture are collected and channelled to the plant’s roots.
The fan-shaped (distichous) arrangement of leaves, as seen in Aloe plicatilis and A. haemanthifolia, is commonly found only in the seedling stage of aloes and most of their relatives. But the vast majority of these species lose this character as they mature, with the leaves taking on a distinctly rosette-shaped arrangement. Retaining a juvenile character into the mature stage is referred to as neoteny. Neoteny is seen in some representatives of the related genus Gasteria, such as Gasteria disticha from the Worcester-Robertson Karoo, some forms of G. maculata from the succulent thickets of the Eastern Cape, and the strikingly beautiful miniature G. armstrongii, which occurs in grassy fynbos vegetation near Jeffrey’s Bay on the southeast coast.
The best-known example of an Aloe species making a transition from a fan-leaved juvenile to a rosette-shaped adult is A. suprafoliata. The common name of this species is ‘book aloe’, referring to its leaves that in young plants are closely adpressed to one another in two rows, the leaves of the plants looking uncannily like the pages of a book opened in the middle. Plants of A. suprafoliata retain this character for longer than most aloes and will even flower while the leaves are still arranged in a fan-shape, but eventually they invariably take on a rosulate arrangement. A fan-shaped leaf arrangement is also found in the miniature Aloe compressa from Madagascar, but they retain this leaf arrangement into adulthood.
Leaf margins of several Agave species, including Agave xylonacantha, are adorned with decorative teeth.
The massive, fibrous leaves of century plants (e.g. Agave americana subsp. americana, above) were produced to last many years. Agaves store reserves in their leaves to enable the production of a massive tree-like flowering pole rising several metres above the ground-level rosette, such as produced by Agave gigantensis (above).
The margins of aloe leaves are often armed with spines. These vary considerably from small, soft, harmless and hair-like to large, sharp, pungent and vicious protuberances that can cause serious bodily harm. The upper and lower leaf surfaces can also be variously armed, but this is less common. Leaves of Aloe marlothii, in particular, are usually densely covered with spines, but completely smooth-leaved forms of this species are found near Malkerns in Swaziland, among other places. The species with the most attractive leaf spines is undoubtedly A. aculeata, where the marginal and surface spines are situated on a prominent white protrusion. With few exceptions, such as the ferociously spined Aloe ferox, the leaf marginal spines of Aloe species are less harmful than those of Agave species, which will easily pierce and tear human skin.
Many aloes and their relatives have spotted leaves. The marks may vary from very large, white to creamy, H-shaped spots, arranged in merging bands transversely on the leaves, as encountered among the maculate aloes, to minute white spots that are densely and irregularly scattered across both, or only the upper, leaf surfaces.
Leaf spots are believed to make it more difficult for grazing and browsing animals to see the plants. This well-known camouflage mechanism occurs in many other species, including aloe relatives. The genus in which it is most effective is Gasteria. Both Gasteria maculata and G. acinacifolia, two common species in Eastern Cape thickets, grow in the rather dense, dappled shade of thorn bush and other shrubs, where the plants perfectly blend with their surroundings.
The considerably smaller leaves of Haworthia species are variations of the generally much larger leaves of their bigger counterparts; in all cases, the leaves are more or less triangular or lance-shaped, or thin and grass-like.
It is not only aloes that have leaves that are variously mottled with irregular white spots or sections. This specimen of Sanseveria hyacinthoides also has leaves with dense, confluent white bands.
In some species of Aloe, the leaves of young plants are spotted with white flecks while those of mature specimens lack spots. Young (top) and mature (above) specimens of Aloe littoralis are shown here.
The Haworthia species with arguably the most beautiful leaves is the form of Haworthia maxima with doughnut-shaped ‘pearls’ on both leaf surfaces. These bright white structures sometimes have a minute fringe attached to them. Other species, such as H. attenuata, H. glabrata and H. radula, have a multitude of tiny white warts, or tubercles, scattered on both surfaces, which may play a role in camouflaging the plants.
Visual signals, such as those that occur during pollination and seed dispersal, are used by many plants to communicate with and advertise to animals. The white spots or stripes that are associated with species bearing spines on their leaves and stems may perform a similar function in plants, as does the warning coloration found in some dangerous or unpalatable animals. It is possible that once plant-eating animals learn to associate white markings with unpleasant qualities (such as sharp spines or an unpleasant taste), they will avoid those plants displaying them. Conspicuously coloured spines (as occur in many aloes and agaves) may also help to protect the plants by acting as a danger signal to herbivores.
White markings on leaves could also serve as a form of mimicry. The marks may mimic damage done to the leaves by insect herbivores, thus signalling to other insects that the plant has already been under attack. It is known that when herbivores feed on plants, the damaged leaves, as well as adjacent undamaged leaves, become less palatable. Such wound-induced reactions may affect the egg-laying and/or feeding behaviour of leaf-eating insects, causing them to avoid plants, or parts of plants, displaying increased concentrations of deterrent chemical substances. Thus damage mimicry could function as a false-warning signal, provided that the plant is capable of inducing defensive responses to actual leaf damage. Mimicry damage caused by feeding can also function as a potential anti-herbivore adaptation if mimic leaves attract predators and parasitoids that prey on herbivores attacking these plants.
Frithia pulchra, a member of the vygie family, has cylindrical leaves with flat, windowed tips.
A window-leaf is one where part of the leaf blade is transparent because the chloroplasts (microscopic organelles that make a leaf appear green) are lacking. Leaf-windows act as effective, if somewhat opaque, filters through which light can reach the inner parts of the leaves where life-supporting photosynthesis takes place. This character is especially useful for plants that grow with their leaves partly sunken into the ground, as it allows a plant to contract into the soil, exposing very little transpiration surface to the atmosphere, yet still carry on with normal, albeit slow, growth. The presence of a window on a leaf may protect its inner tissues against damage caused by the intense light found in arid habitats.
The best known window-leaved species in the aloe family are Haworthia truncata and H. maughanii; closely related species that occur in the Little Karoo, where they hide in the shade of medium-high shrubs, with only their leaf tips exposed at ground level. These species therefore follow a two-pronged approach to prevent damage to the light-sensitive inner tissues of their leaves.
A number of Haworthia species have leaves that are apically flattened and windowed; although a few species of South American cacti grow completely below-ground, none of the Haworthia species does so. The occurrence of windows at or near the tips or margins of succulent leaves is not uncommon, and is found among representatives of families as unrelated as Piperaceae, the pepper family (Peperomia dolabriforme), the Mesembryanthemaceae or vygie family (Frithia pulchra and Fenestraria aurantiaca) and Asphodelaceae/Asphodeloideae, the red-hot poker family (Bulbine haworthioides).
The youngest leaves in seedlings and old plants of Aloe and their kin are often borne upright, whereas mature leaves tend to grow more horizontally. Having young leaves pointing more or less skywards exposes a smaller surface area of the leaf to direct and scattered sunlight. One consequence is that these leaves receive a lower heat load and so are exposed to less potentially harmful radiation, which in turn promotes leaf development, and therefore plant growth and development (see Seeds and seedling survival, page 43). Once leaves begin to mature, they can more readily tolerate higher heat loads and ultraviolet radiation, resulting in their orientation changing from vertical to more horizontal.
Vertical leaf orientation is also believed to minimize heat loss to the night sky through radiation, thereby protecting sensitive young leaves from nocturnal chilling. Although temperature differences between vertical and horizontal leaves are quite small at night (usually less than one degree), it has been shown in bean seedlings, the leaves of which change orientation from horizontal during the day to vertical at night, that leaf growth can be significantly retarded when the nocturnal orientation of leaves is mechanically constrained to stay horizontal.
The leaves of Aloe dorotheae, from Tanzania, turn bright red under conditions of environmental stress.
Kalanchoe sexangularis var. sexangularis’ leaves are known to turn red under environmental stress.
The leaves of some deciduous trees change in autumn from green to shades of yellow, orange and red. This happens when chlorophyll, the green pigment in leaves, decomposes to reveal yellow and orange colours (both present in leaves throughout the year but masked by the chlorophyll). At about the same time, various chemical processes in the leaves produce red pigments (known as anthocyanins) that accumulate in the cell sap and cause the leaves to turn varying shades of red. The pinks, reds and blues of petals in most plants (excluding, for example, vygies/mesembs and cacti), the red colours of ripe fruit and the deep red skins of wine grapes are due to the presence of these pigments.
Even if it is grown in cultivation, Aloe vanbalenii leaves can turn bright red if the plant is subjected to stress.
Aloe leaves are not shed in the same way as those of deciduous trees preparing for their winter sleep, and their leaves can turn a deep brown or red at any time of the year, regardless of season. Frequently, the onset of red in aloe leaves coincides with stress being imposed on the plants, such as by drought or cold, re-potting or re-planting. (The leaves of some aloes are virtually always red; Aloe dorotheae from Tanzania and A. vanbalenii from the northeastern savannas of southern Africa are two examples.)
Many aloes from summer-rainfall areas experience their greatest environmental stress during the cold, dry winter season, when most of them flower. High levels of irradiance also contribute substantially towards aloe leaves changing colour from green to brownish red. Anthocyanins are known to absorb light selectively, for example, ultraviolet and green wavelengths, both of which might be harmful to young, sensitive leaf tissues, so these red pigments act as a protective light shield. This explains why the young growth of many plants, especially trees, is often bright red.
In addition to their role as a light-shield, anthocyanins are powerful antioxidants. During stressful periods, plant tissues are subjected to damage by so-called free radicals and reactive forms of oxygen which can damage cell membranes and DNA, often to the extent of causing the death of cells. It is now believed that one of the principal functions of anthocyanins in plants is to remove these free radicals. As human cells can also be damaged by free radicals, it is not surprising that anthocyanins derived from plants are available in certain health foods and medicines.
The leaves of Agave species are fibrous, strong and sturdy and last for the plant’s lifetime. These ‘hard fibres’ are actually bundles of water-conducting cells (xylem) surrounded by a sheath of individual fibre cells. Most fibre bundles, such as in the sisal leaf (Agave sisalana), are located just below the leaf surface and provide structural support that enables the leaf to remain upright.
Hard fibres are used in the manufacture of cordage products (rope, twine, canvas), but can also be pulped for use in speciality papers, such as tissue paper, filter paper, tea bags, currency and security papers.
It is likely that the monocarpic habit of agaves requires that once carbon (the product of photosynthesis) has been expended on the production of structural material, such as fibres and leaf tissues, the plant should conserve its accumulated energy reserves until they are required for reproduction, sometimes up to a decade or more later. Producing the massive flowering pole or ‘mast’ (inflorescence) requires a lot of energy, structural material and water to be extracted from the fat leaves, the latter becoming deflated like elongated balloons when flowering starts. It is therefore likely that agave leaves are fibrous and strong as a result of the need to reproduce.
Although abundant fibres are not commonly found in aloe leaves, some unrelated species from widely differing areas have been shown to be fibrous. The best known is the aptly named Aloe fibrosa, which comes from the Machakos district in Kenya, while two fynbos aloes originating from the southwestern Cape, A. plicatilis and A. haemanthifolia, also have prominent fibres in their leaves. The functional significance of fibrous leaves in these aloes is not obvious. Elsewhere in the aloe family, fibres are also present in the leaves of Astroloba rubriflora, the rather odd species that is sometimes classified in a genus of its own, namely Poellnitzia.
Many plants form sharp-pointed hardened structures, supposedly as a defence against larger animals. Although the term ‘spine’ is generally used as a collective term for all sharp structures, there are subtle differences, depending on position, arrangement and morphology. In this book, we use ‘spine’ or ‘tooth’ to describe any sharp structure, including thorns and prickles.
Leaf margins and blades of the miniature Aloe brevifolia are armed with sharp teeth.
The leaf margins of Aloe forbesii, from the island of Socotra, are adorned with creamy white, pointed, but harmless, teeth.
• Spines are derived from stipules and leaves.
• Thorns are derived from stems. They are tough structures with their own vascular supply and are usually difficult to break off by hand. (‘Spine’ is sometimes used for straight structures and ‘thorn’ for hooked structures.)
• Prickles (also called emergences) occur on stems or leaves, usually not in fixed positions. They do not derive from any organ, but are structures that develop from superficially located tissues and are sometimes easily broken off. Typical prickles are found on roses, aloe leaves and leaves of members of the genus Agave. Technically, all spines or thorns on leaves of aloes and their relatives are prickles.
Aloe vossii flowers are vertically disposed in both the bud and wilted phases, an indication that animals should not visit them. The open flowers turn downwards, offering birds easy access to the nectar.
Flowers of this form of Aloe cryptopoda are a uniform bright red colour. The small flying insect (circled) destroys the exterior of the flower base to gain access to the nectar, but does not assist pollination.
Flower colour varies considerably among aloes. These flowers of the bulbous Aloe bulbicaulis, from Zambia, are a dull mustard yellow. In this species, the flowers, flower stalks and inflorescence branches are all the same rather unimpressive colour, a rare occurrence in Aloe.
In the aloe family, inflorescences can take the form of many-flowered spikes (inflorescences bearing stalkless flowers), simple racemes (bearing flowers that have stalks), or branched (compound) racemes, known as panicles.
Aloe plants or rosettes do not die after flowering, unlike the look-alike Agave species (see page 22). In most aloes, flowering progresses from the base of the inflorescence upwards. True aloes (Aloe species) tend to bear densely flowered, highly coloured inflorescences, while representatives of most of the other genera have laxly flowered inflorescences. During the flowering season, aloe plants can be seen from long distances and so are able to attract a variety of animals that can potentially contribute to successful pollination.
The flowers of all species included in the aloe family are tubular. The tubes are formed by six shortly elongated floral (perianth) segments that are variously fused longitudinally. In some species the segments are almost free to their bases (but the flowers are still more or less tubular), while in others they are fused virtually for their entire lengths. These tubes are excellent reservoirs for the copious amounts of nectar that most aloe species produce. Since most of the flowers point downwards when they open, the possibility of rain diluting or, worse, washing out the nectar is negligible. Furthermore, the desiccating and contracting floral tube serves to guide and concentrate nectar from the bottom of the flower to the tip where it often accumulates as a glistening droplet. This usually coincides with the female reproductive organs of the plants becoming receptive and possibly offers an additional attractant for pollinators.
The dominant flower colours among aloes are red, yellow and orange. Inflorescences are usually monochromatic, but species with bi- or even tri-coloured inflorescences are also known. In a few species, flower colour can vary within the species, Aloe arborescens being one of the best known examples. With very few exceptions, the flowers of haworthias, astrolobas and the single chortolirion are dull-coloured, varying from white to drab brown, green, pink or yellow. Like the aloes, several gasterias have highly coloured red or orange flowers. However, their inflorescences are not as densely flowered as those of most aloes, making them less conspicuous in their natural habitats.
Once a flower has been successfully pollinated and fertilized, the fruits rapidly develop on the inflorescence, replacing the flowers. Those flowers that have not been fertilized dry out and are soon shed, indicating a lack of nectar or pollen rewards to visiting animals, particularly birds and bees.
The fruits of aloes (called capsules) become woody and dry when they ripen (those of the small group of species previously classified in the genus Lomatophyllum remain somewhat fleshy) and then release the usually black seeds. (There are exceptions to this, notably Aloe variegata, the seeds of which have prominent white wings.)
Flowering sequence of the registered cultivar Aloe ‘Spiraal’. The flowers are protrandric, meaning that the pollen matures before the stigma becomes receptive. Here, the stigma is exserted beyond the mouth of the flower in the bottom-most flower.
Aloe flowers don’t always open from the bottom of the inflorescence upwards. This specimen of Aloe speciosa seems unsure of what is expected of it when it comes to flowering sequence.
Aloe parvibracteata flowers often have a conspicuous, nectar-filled basal swelling.
Bees frequently visit Aloe marlothii flowers, possibly just to collect pollen rather than contribute to pollination.
The fruits of Aloe marlothii are light brown, tinged with green. The irregular bumps on the fruit surface are a sure sign that the capsules have been parasitised.
The spent flowers of Aloe porphyrostachys remain closely pressed to the inflorescence axis.
Fruits of Aloe broomii var. broomii are light green and so tightly packed that the dry floral remains are retained between them.
The closely packed orange-brown fruit capsules of Aloe ferox are almost as decorative as its flowers.
Seed of Aloe spicata germinate profusely under the protection of fallen tree leaves but, ultimately, some of these seedlings will succumb to predators, with only one or two reaching maturity.
Aloe seeds are almost invariably quite small and black. They are angled, slightly thickened in their centres, and have short or prominent wings which, although quite rudimentary in most species, are believed to facilitate wind dispersal of the seeds.
As seedlings, some species of Aloe often start life in the shade of ‘nurse’, or companion, plants. This is critically important as, during the first few months of their lives, the young plants are very tender with comparatively weak root systems, making them prone to a multitude of setbacks. These include being grazed by game or domestic livestock, or being attacked by parasites such as red spider mites (a type of arachnid) and sap-sucking insects. Most aloes soon outgrow their nurse plants and tower over them, making it difficult to imagine that they initially depended on these companion plants for survival. But some, such as the grass aloes, never outgrow or overtop the plants with which they co-habit. Not only do their leaves resemble those of the grasses in which they grow, the aloe plants are also the same size as their companions. (See also page 77.)
Nurse plants provide the initial shelter required by young, tender seedlings of a variety of species, including succulents. In deserts in particular, succulents depend on nurse plants for survival during the establishment phase. The seeds of many aloes germinate underneath grass tussocks or low-growing shrublets, the latter serving as nurse plants which protect the young seedlings from excessive heat and radiation, desiccation, frost and plant-eating animals.
Individuals of many species of Aloe grow closely together in very dense stands, sometimes forming thickets. Examples of species that exhibit such gregariousness include A. africana, A. arborescens, A. ferox, A. marlothii, A. microstigma, A. spicata, and several others. In the case of many other plant species (with the exception of most wind-pollinated species), this is a rather uncommon phenomenon, as plants produce chemical signals that prevent the seeds of the same, or other, species from germinating in close proximity to them. Therefore, in terms of plant dynamics, spaces between plants of a certain species that remain unoccupied can be as informative as the occupied ones, regarding the structure of a plant population and the interaction among individuals in it.
