Aloe

Aloe is a large genus of succulent, flowering plants comprising more than 650 recognised species. Best known for Aloe vera, the genus is widely cultivated for its ornamental, medicinal, and cosmetic applications. Aloe plants are adapted to arid environments through specialised water-storing tissues in their leaves, stems, or roots, allowing them to survive prolonged drought. The genus is native primarily to tropical and southern Africa, as well as Madagascar, the Arabian Peninsula, and islands of the Indian Ocean, though many species have become naturalised far beyond their original range.

Etymology

The name Aloe derives either from the Arabic al-ulūh (“bitter, shiny substance”) or the Hebrew ahal (plural ahalim), both references to the plant’s resinous sap. The bitter exudate of many species historically contributed to their use in traditional medicines.

Description

Aloe species are characterised by:

• Rosettes of thick, fleshy leaves, often toothed along the margins.
• Leaf colours ranging from pale grey-green to deep bright green, sometimes mottled or striped.
• Tubular flowers in shades of yellow, orange, pink, or red, borne in dense clusters on leafless, sometimes branched inflorescences.
• Growth habits that vary widely: some species remain stemless at ground level, others develop tall, tree-like trunks as seen in several South African species.

Fleshy leaves store water in enlarged parenchyma, enabling survival in harsh, arid climates and making the genus a model example of xerophytic adaptation.

Systematics and Classification

According to the APG IV system (2016), Aloe belongs to the family Asphodelaceae, subfamily Asphodeloideae. Historically the genus was placed in Aloaceae or the broadly circumscribed Liliaceae, but molecular phylogenetic studies from the early 2000s onward have resulted in major taxonomic revisions.
Genetic evidence has shown that the formerly broad genus Aloe was not monophyletic. In 2014, a major molecular study by Manning and collaborators divided the traditional assemblage into six more coherent genera:

• Aloe (in the strict sense)
• Aloiampelos
• Aloidendron
• Aristaloe
• Gonialoe
• Kumara

Some genera historically separated from Aloe, such as Lomatophyllum, have been absorbed back into the lineage. Plants commonly mistaken for aloes—such as Agave americana (“American aloe”)—belong to entirely different families, including Asparagaceae.

Distribution

Native populations occur across:

• Southern and eastern Africa
• The Horn of Africa and Arabian Peninsula
• Madagascar and neighbouring Indian Ocean islands
• Various arid and semi-arid zones extending northwards into Jordan

Aloe species have been introduced or naturalised in:

• The Mediterranean
• India
• Australia
• The Americas, including the Hawaiian Islands

Diversity and Species

More than 600 species are accepted, with many more hybrids and unresolved taxa. Representative species include:

• Aloe vera – the best known medicinal aloe
• Aloe ferox – widely harvested for pharmacological use
• Aloe arborescens – a striking, shrubby species used ornamentally
• Aloe marlothii, A. dichotoma (now Aloidendron dichotomum) – tree-like forms
• Aloe polyphylla – the iconic spiral aloe from Lesotho
• Aloe maculata, A. humilis, A. capitata – common garden species
• Aloe jucunda, A. peglerae, A. suzannae – highly localised or threatened species

Aloes hybridise readily in cultivation, resulting in horticultural groups such as Gasteraloe (Aloe × Gasteria) and Aloloba (Aloe × Astroloba).

Uses

1. Ornamental UseAloes are popular succulents in gardens and containers, valued for their sculptural rosettes and vibrant winter-to-spring blooms. Numerous garden cultivars are available, including Aloe ‘Lizard Lips’, recipient of the Royal Horticultural Society’s Award of Garden Merit.
2. Medicinal and Cosmetic UseAloe vera is cultivated globally for:

• Topical gels used for burns, skin irritation, and cosmetic formulations
• Processed juices marketed for digestive applications (though clinical evidence is limited)

Approximately 75% of Aloe species are used locally in traditional medicine.
Historical usage extends back to Greco-Roman antiquity. Medieval texts describe the use of yellow aloe latex as a purgative; this sap contains aloin, an anthraquinone derivative with strong laxative properties. Due to safety concerns, the U.S. FDA banned aloin-containing aloe extracts from over-the-counter laxatives in 2002. Modern aloe beverages typically remove aloin during processing.
3. Hazardous or Misused ProductsCancer Research UK has cautioned against unproven products, including concentrated aloe preparations falsely marketed as cancer cures. No clinical evidence supports aloe as an anticancer treatment.

Chemical Properties

The main biologically active components include:

• Aloins (anthraquinone glycosides), divided into:
  • Nataloins – yield picric and oxalic acids with nitric acid; do not redden
  • Barbaloins – produce aloetic acid, chrysammic acid, picric and oxalic acids; redden with nitric acid

Barbaloins are further subclassified based on their reaction to cold or warmed nitric acid, differing between aloes of Barbados, Socotra, and Zanzibar.
These compounds occur primarily in the bitter, yellow exudate found between the leaf rind and inner gel.

Cultivation and Ecology

Aloes thrive in:

• Full sun to light shade
• Well-drained, sandy soils
• Warm, dry climates with minimal frost

Their drought tolerance and architectural forms make them important components of xeriscaping, and in their native ecosystems they support pollinator networks centred on sunbirds and insects attracted to their nectar-rich inflorescences.