Aspects of the Taxonomy of Aloe Arborescens Mill. (Asphodelaceae: Alooideae)

Bradleya 30/2012 pages 127 – 137 Aspects of the taxonomy of Aloe arborescens Mill. (Asphodelaceae: Alooideae) Gideon F. Smith1, Ronell R. Klopper2, Estrela Figueiredo3 & Neil R. Crouch4 1 Office of the Chief Director: Biosystematics Research & Biodiversity Collections, South African National Biodiversity Institute, Private Bag X101, 0001 Pretoria, South Africa / Acocks Chair, H.G.W.J. Schweickerdt Herbarium, Department of Plant Science, University of Pretoria, 0002 Pretoria, South Africa / Centre for Functional Ecology, Departamento de Ciências da Vida, Universidade de Coimbra, 3001-455 Coimbra, Portugal. (email: [email protected]). 2 Biosystematics Research & Biodiversity Collections Division, South African National Biodiversity Institute, Private Bag X101, 0001 Pretoria, South Africa / Department of Plant Science, University of Pretoria, 0002 Pretoria, South Africa. (email: [email protected]). 3 Department of Botany, P.O. Box 77000, Nelson Mandela Metropolitan University, 6031 Port Elizabeth, South Africa / Centre for Functional Ecology, Departamento de Ciências da Vida, Universidade de Coimbra, 3001-455 Coimbra, Portugal. (email: [email protected]). 4 Ethnobotany Unit, South African National Biodiversity Institute, P.O. Box 52099, 4007 Berea Road, South Africa / School of Chemistry and Physics, University of KwaZulu-Natal, 4041 Durban, South Africa. (email: [email protected]). Summary: Taxonomic concepts in the morpholog- Cultivare behandelt werden sollten, statt als for- ically variable Aloe arborescens Mill. are dis- melle taxonomische Einheiten auf Ebenen unter- cussed. Previous approaches to the taxonomy of halb der Art. Formelle Unterscheidungen auf this species varied from describing aberrant Ebenen unterhalb der Art können erst gemacht material as new entities, using formal taxonomic werden, wenn die Art aufgrund intensiver For- categories, to selecting forms with horticulturally schungen über das gesamte Verbreitungsgebiet desirable characters and naming these as culti- im Rahmen von Feldarbeit unter Einbezug aller vars. We propose that in the case of this species, möglichen Formen von Daten (morphologisch, the preferred approach to recognise variation anatomisch, chemisch, molekular, etc.) unter- among entities in horticulture should be at the sucht worden ist. level of cultivars, rather than formal taxonomic entities at infraspecific ranks. It is only after ex- Introduction tensive fieldwork throughout the complete distri- Geographically, Aloe arborescens Mill. is one bution range of this species and intensive of the most wide-ranging species in the genus research, including all possible forms of data Aloe L. It occurs from the Cape Peninsula (where (morphological, anatomical, chemical, molecular, it is arguably naturalised), through the southern etc.), that any formal distinctions at infraspecific and eastern regions of South Africa, and further level can be made. north to Mozambique and the eastern mountains of Zimbabwe and Malawi (Figure 1). The very Zusammenfassung: Die taxonomischen Konzepte broad geographical amplitude of A. arborescens is innerhalb der morphologisch variablen Aloe arbo- mirrored in a similarly broad range of variation rescens Mill. werden diskutiert. Frühere Versu- in most of its vegetative and reproductive charac- che bezüglich der Taxonomie dieser Art ters. Adding to this variation is the ease with variierten, und abweichendes Material wurde ent- which A. arborescens hybridises with a variety of weder unter Nutzung formeller taxonomischer other aloes (Reynolds, 1950). Kategorien als neue Taxa beschrieben, oder aus- Aloe arborescens has been reported to have gewählte Formen mit gärtnerisch erwünschten several medicinal uses, including significant wound- Merkmalen wurden als Cultivare behandelt. Wir healing, anti-bacterial, anti-ulcer, anti-inflamma- schlagen vor, dass für diese Art gärtnerisch wich- tory, anti-carcinogenic, alopoeic and anti-fungal tige taxonomische Einheiten bevorzugt als activity (Lane, 2004; Bosch, 2008; references cited Bradleya 30/2012 127 Figure 1. Geographical distribution map of A. arborescens. by Smith et al., 2008). However, probably its most The krantz aloe readily grows from cuttings popular use is in the treatment of burns and wounds and seed. It has been in cultivation at the Dutch (Reynolds, 1950; Lane, 2004; Bosch, 2008). It East India Company’s Gardens in Cape Town, received worldwide attention after the second World South Africa, since at least 1695 (Reynolds, 1950; War, when the value of a gel prepared from its West, 1974) and also in Japan since the 17th cen- leaves was demonstrated, following its application tury (Bosch, 2008). Today it is widely cultivated to skin burns of victims of the nuclear bombs in the tropics and subtropics, and is an extremely dropped on Japan (Bosch, 2008). It is surprisingly popular garden plant in the Mediterranean region very little used for its medicinal properties in its (West, 1974; Lane, 2004; Bosch, 2008; Smith & native range (Smith et al., 2008), but is very highly Van Wyk, 2008). It is commercially grown in Italy esteemed in Asia and the Mediterranean (Bosch, (for its medicinal and cosmetic uses), Japan 2008). A leaf decoction is reportedly given to women (as medicine and food) and recently also in Israel to ease childbirth in South Africa (Reynolds, 1950). and China (Bosch, 2008). In Japan, leaves are widely used as a vegetable, a Not surprisingly, the wide ecological ampli- health food to ease constipation, and as a purgative tude of the species and its popularity in horticul- and for dermatological use (Bosch, 2008). Watt & ture, has also enabled it to successfully colonise Breyer-Brandwijk (1962) also reported the use of a localities remote from its natural habitat (Webb, cold infusion of A. arborescens as a drench in the 1980; Forster & Clifford, 1986; Reynolds & treatment of sick calves. Herring, 1991), becoming naturalised in Japan 128 Bradleya 30/2012 2 3 4 5 Figure 2. A. arborescens forms dense, much-branched shrubs with apical leaf rosettes and usually simple inflorescences (Kowyn’s Pass, Mpumalanga, South Africa). Photo: Neil R. Crouch. Figure 3. Leaves of A. arborescens are often greyish green with yellow marginal teeth (cultivated). Photo: Ronell R. Klopper. Figure 4. The characteristic elongated inverted-conical, dense raceme with cylindrical flowers of A. arborescens (cultivated). Photo: Gideon F. Smith. Figure 5. A. arborescens often grows on rocky slopes and outcrops (Kaapschehoop, Mpumalanga, South Africa). Photo: Neil R. Crouch. Bradleya 30/2012 129 6 7 8 9 Figure 6. Epiphytic A. arborescens plants are sometimes encountered (Graskop, Mpumalanga, South Africa). Photo: Neil R. Crouch. Figure 7. Leaves become reddish in winter in certain regions, for instance at Ongoya Forest, northern KwaZulu-Natal, South Africa. Photo: Arrie W. Klopper. Figure 8. Leaves can be a yellowish green, as in the western Soutpansberg, Limpopo, South Africa. Photo: Arrie W. Klopper. Figure 9. Racemes of A. arborescens can be narrowly conical or broadly conical as in this yellow form (cultivated). Photo: Arrie W. Klopper. 130 Bradleya 30/2012 (Bosch, 2008) and invasive in Portugal (Smith & while in others plants are low straggly bushes Figueiredo, 2009). with fairly thin stems (Figures 11 & 12). We consider here aspects of the morphological Aloe arborescens is known to hybridise regu- variation encountered in A. arborescens. It is pro- larly with a number of other aloes (Figure 13) posed that the most practical approach to the (Reynolds, 1950; Bosch, 2008) across many of taxonomy of A. arborescens is to recognize horti- the Sections established by Berger (1905), from cultural forms as cultivars. Establishing formal the tall tree-like Aloidendron to the relatively infraspecific names at the subspecific and varietal diminutive grass-like Leptoaloe. This is a conse- categories will lead to an inordinate and undesir- quence of both the widespread distribution of able proliferation of names. Such formal distinc- A. arborescens − usually locally abundant when tion will only be possible after extensive research. found, and highly floriferous − as well as its sym- patric occurrence with many other aloe species, Variation in Aloe arborescens most of which similarly have a diploid chromo- Although A. arborescens is morphologically ex- some number of 2n = 14 (Riley, 1959). This abil- tremely variable, it can nevertheless be diagnosed ity to hybridise occurs in spite of a somewhat by a suite of characters. It forms much-branched restrictive breeding system in A. arborescens, the shrubs up to 5 m tall, with the leaves in dense flowers of which are self-incompatible due to a rosettes at the branch apices (Figure 2). Leaves mechanism acting in the ovary (Hargreaves et al., are usually greyish green with yellowish teeth 2012). A total of 15 known natural crosses were (Figure 3). Inflorescences are usually simple to listed by Reynolds (1950) for A. arborescens, a once-branched with elongated inverted-conical, number only exceeded by A. marlothii crosses dense racemes, with large floral bracts and the (Reynolds, 1950). Several more hybrid combina- pedicels twice as long as the cylindrical flowers tions have been identified since, many arising in (Figure 4). Outer segments of the flowers are free cultivation (see Boxes 1 & 2). The result of these to the base. Its closest relatives are A. mutabilis hybrids breeding back into the original population Pillans from northwestern South Africa and east- generates yet

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