Abstract
The confident identification to species rank of fragmentary and sterile plant material is often challenged by the absence of diagnostic characters, which are present in intact specimens, reproductive parts, and plants in habitat. Here, we consider leaf surface micromorphology for the identification of the genus Aloe in the Horn of Africa region. Primary and secondary sculpturing of the leaf epidermis and stomata were characterised from SEM micrographs of 35 taxa representing 31 species of Aloe (Asphodelaceae subfam. Alooideae). Detailed comparison revealed that leaf surface characters are conserved between species and within-species variation is modest. Closely related taxa in the Aloe adigratana—A. camperi—A. sinana species complex could be distinguished using leaf surface micromorphology alone. These characters also guide species delimitation; in the species complex including A. schoelleri and A. steudneri, a narrow circumscription is supported, whereas with
A. ankoberensis and A. pulcherrima, a wider circumscription merits consideration. The observed trait combinations are characteristic of plants in xeric environments, with the most notable feature being stomata that are most deeply sunken in species in more arid habitats. Our findings support the use of comparative study of micromorphological leaf surface characters for species identification and taxonomy in the genus Aloe.
References
<p>Bachman, S.P., Wilkin, P., Reader, T., Field, R., Weber, O., Nordal, I. & Demissew, S. (2020) Extinction risk and conservation gaps for <em>Aloe</em> (Asphodelaceae) in the Horn of Africa. <em>Biodiversity and Conservation</em> 29: 77–98. https://doi.org/10.1007/s10531-019-01870-0</p>
<p>Baker, J.G. (1898) Uliaceae. <em>In</em>: Thiselton-Dyer, W.T. (ed.) <em>Flora of Tropical Africa </em>7. Reeve, London, pp. 421–569.</p>
<p>Barthlott, W. (1990) Scanning electron microscopy of the epidermal surface in plants. <em>In</em>: Claugher, D. (ed.) <em>Scanning electron microscopy in taxonomy and functional morphology</em>. Systematic Association Special, vol. 41. Clarendon Oxford, Oxford, pp. 69–94.</p>
<p>Barthlott, W., Neinhuis, C., Cutler, D., Ditsch, F., Meusel, I., Theisen, I. & Wilhelmi, H. (1998) Classification and terminology of plant epicuticular waxes. <em>Botanical Journal of the Linnean Society</em> 12 (3): 237–260. [https://www.tandfonline.com/doi/abs/10.1006/bojl.1997.0137]</p>
<p>Beaumont, J., Cutler, D.F., Reynolds, T. & Vaughan, J.G. (1985) The secretory tissue of aloes and their allies.<em> Israel Journal of Botany </em>34: 265–282. [https://www.tandfonline.com/doi/abs/10.1080/0021213X.1985.10677026]</p>
<p>Berger, A. (1905) Über die systematische Gliederung der Gattung <em>Aloë</em>. Botanische Jahrbücher fur Systematik, Pflanzengeschichte und Pflanzengeographie 36: 42–68. [https://www.biodiversitylibrary.org/page/66#page/49/mode/1up]</p>
<p>Brandham, P.E. & Cutler, D.F. (1978) Influence of chromosome variation on the organisation of the leaf epidermis in a hybrid <em>Aloe </em>(Liliaceae). <em>Botanical Journal of the Linnean Society</em> 77: 1–16. https://doi.org/10.1111/j.1095-8339.1978.tb01368.x</p>
<p>Carter, S. (1994) The family Aloaceae. <em>In</em>: Polhill, R.M. (ed.) <em>Flora of Tropical East Africa</em>. Balkema, Rotterdam and Brookfield, 328 pp.</p>
<p>Carter, S. & Brandham, P. (1983) New species of <em>Aloe</em> from Somalia. <em>Bradleya</em> 1: 17–24.</p>
<p>Carter, S., Cutler, D.F., Reynolds, T. & Brandham, P.E. (1984) A multidisciplinary approach to a revision of the <em>Aloe somaliensis</em> complex (Liliaceae). <em>Kew Bulletin</em> 39: 611–633. https://doi.org/10.2307/4108600</p>
<p>Carter, S., Lavranos, J.J., Newton, L.E. & Walker, C.C. (2011) <em>Aloes. The definitive guide.</em> 1st ed. Kew Publishing, Richmond, 719 pp.</p>
<p>Chase, M.W. & Fay, M.F. (2009) Barcoding of plants and fungi. <em>Science</em> 325: 682–683. https://doi.org/10.1126/science.1176906</p>
<p>Christian, H.B. (1951) <em>Aloe mcloughlinii</em>. <em>The Flowering Plants of South Africa</em> 28: t. 1112.</p>
<p>Coopoosamy, R.M. & Naidoo, K.K. (2011) Anatomical features of the medicinal importance of <em>Aloe excelsa</em> (Berger). <em>African Journal of Biotechnology</em> 10: 7622–7632. https://doi.org/10.5897/AJB11.113</p>
<p>Crang, R., Lyons-Sobaski, S. & Wise, R. (2018) Plant anatomy. A concept-based approach to the structure of seed plants. Springer Nature, Switzerland, pp. 423–430.</p>
<p>Cutler, D.F. (1969) Cuticular markings and other epidermal features in <em>Aloe</em> leaves. <em>Notes from the Jodrell Laboratory</em> 6: 21–24.</p>
<p>Cutler, D.F. (1972) Leaf anatomy of certain <em>Aloe</em> and <em>Gasteria</em> species and their hybrids.<em> In: </em>Ghouse, A.K.M. & Yunus, M. (eds.) <em>Research Trends in Plant Anatomy</em>. Tata McGraw Hill, Bombay, pp. 103–122.</p>
<p>Cutler, D.F. (1979) Leaf surface studies in Aloe and Haworthia species (Liliaceae): taxonomic implications. Tropische & Subtropische Pflanzenwelt 28: 449–471.</p>
<p>Cutler, D.F. (1982) Cuticular sculpturing and habitat in certain <em>Aloe</em> species (Liliaceae) from southern Africa. <em>In</em>: Cutler, D.F., Alvin, K.L. & Price, C.E. (eds.) <em>The plant cuticle</em>. Academic Press, London, pp. 425–444.</p>
<p>Cutler, D.F. (2004) <em>Aloe</em> leaf anatomy. <em>In</em>: Reynolds, T. (ed.) <em>The genus Aloe</em>. CRC Press, Boca Raton, pp. 372–377.</p>
<p>Cutler, D.F., Botha, T. & Stevenson, D.W. (2007) The epidermis. <em>In</em>: Cutler, D.F., Botha, T. & Stevenson, D.W. (eds.) <em>Plant anatomy</em>: <em>an applied approach</em>. Blackwell Publishing Ltd, Oxford, pp. 76–93.</p>
<p>Cutler, D.F. & Brandham, P.E. (1977) Experimental evidence for the genetic control of leaf surface characters in hybrid Aloineae (Liliaceae). <em>Kew Bulletin</em> 32: 23–32. https://doi.org/10.2307/4117256</p>
<p>Cutler, D.F., Brandham, P.E., Carter, S. & Harris, S.J. (1980) Morphological, anatomical, cytological and biochemical aspects of evolution in East African shrubby species of <em>Aloë</em> L. (Liliaceae). <em>Botanical Journal of the Linnean Society</em> 80: 293–317. https://doi.org/10.1111/j.1095-8339.1980.tb01666.x</p>
<p>Engler, A. (1895) <em>Die Pflanzenwelt Ost-Afrikas und der Nachbargebiete</em>, Vol. 2. D. Reimer, Berlin, 433 pp. https://doi.org/10.5962/bhl.title.587</p>
<p>Fortini, P., Antonecchia, G., Di Marzio, P., Maiuro, L. & Viscosi, V. (2015) Role of micromorphological leaf traits and molecular data in taxonomy of three sympatric white oak species and their hybrids (<em>Quercus</em> L.). <em>Plant Biosystems </em>149: 546–558. https://doi.org/10.1080/11263504.2013.868374</p>
<p>Fradera-Soler, M., Rudall, P.J., Prychid, C.J. & Grace, O.M. (2021) Evolutionary success in arid habitats: morpho-anatomy of succulent leaves of Crassula species from southern Africa.<em> Journal of Arid Environments</em> 185: 104319.</p>
<p>Gilbert, M.G. & Demissew, S. (1992) Notes on the genus <em>Aloe</em> in Ethiopia: misinterpreted taxa.<em> Kew Bulletin</em> 47: 647–653.</p>
<p>Gilbert, M.G. & Sebsebe Demissew. (1997) Further notes on the genus <em>Aloe</em> in Ethiopia and Eritrea.<em> Kew Bulletin</em> 52: 139–152. https://doi.org/10.2307/4117846</p>
<p>Glen, H.F. & Hardy, D.S. (1995) <em>Aloe</em> section <em>Anguialoe</em> and the problem of <em>Aloe spicata</em> L.f. (Aloaceae). <em>Haseltonia</em> 3: 92–103.</p>
<p>Grace, O.M. (2011) Current perspectives on the economic botany of the genus <em>Aloe</em> L. (Xanthorrhoeaceae). <em>South African Journal of Botany</em> 77: 980–987. https://doi.org/10.1016/j.sajb.2011.07.002</p>
<p>Grace, O.M., Buerki, S., Symonds, M., Forest, F., van Wyk, A.E., Smith, G.F., Klopper, R.R., Bjorå, C.S., Neale, S., Sebsebe Demissew, Simmonds, M. & Rønsted, N. (2015) Evolutionary history and leaf succulence as explanations for medicinal use in aloes and the global popularity of <em>Aloe vera</em>. <em>BMC Evolutionary Biology</em> 15: a29. https://doi.org/10.1186/s12862-015-0291-7</p>
<p>Grace, O.M., Klopper, R.R., Figueiredo, E. & Smith, G.F. (2011) <em>The aloe names book. Strelitzia 28.</em> South African National Biodiversity Institute and the Royal Botanic Gardens, Pretoria and Kew, pp. 1–3.</p>
<p>Grace, O.M., Klopper, R.R., Smith, G.F., Crouch, N.R., Figueiredo, E., Rønsted, N. & van Wyk, A.E. (2013) A revised generic classification for <em>Aloe</em> (Xanthorrhoeaceae subfam. Asphodeloideae). <em>Phytotaxa</em> 76: 7–14. https://doi.org/10.11646/phytotaxa.76.1.2</p>
<p>Grace, O.M., Simmonds, M.S.J., Smith, G.F. & Van Wyk, A.E. (2009) Taxonomic significance of leaf surface morphology in <em>Aloe</em> section Pictae (Xanthorrhoeaceae). <em>Botanical Journal of the Linnean Society</em> 160: 418–428. https://doi.org/10.1111/j.1095-8339.2009.00982.x</p>
<p>Ickert-Bond, S.M., Harris, A.J., Lutz, S. & Wen, J. (2018) A detailed study of leaf micromorphology and anatomy of New World <em>Vitis</em> L. subgenus <em>Vitis</em> within a phylogenetic and ecological framework reveals evolutionary convergence. <em>Journal of Systematics and Evolution</em> 56: 309–330. https://doi.org/10.1111/jse.12313</p>
<p>Jayeola, A.A. & Thorpe, J.R. (2000) A scanning electron microscope study of the genus <em>Calyptrochilum</em> Kraenzl. (Orchidaceae) in West Africa. <em>Feddes Repertorium</em> 111 (5–6): 315–320. https://doi.org/10.1002/fedr.20001110506</p>
<p>Klimko, M. & Wiland-Szymanska, J. (2008) Scanning electron microscopic studies of leaf surface of taxa of genus <em>Dracaena</em> L. (Dracaenaceae). <em>Botanika—Steciana</em> 12: 117–127. https://doi.org/10.1021/1a9017322</p>
<p>Koch, K., Bohn, H.F. & Barthlott, W. (2009) Hierarchically sculptured plant surfaces and superhydrophobicity. <em>Langmuir</em> 25: 14116–14120. https://doi.org/10.1021/la9017322</p>
<p>Li, X., Yang, Y., Henry, R.J., Rossetto, M., Wang, Y. & Chen, S. (2015) Plant DNA barcoding: from gene to genome. <em>Biological Reviews of the Cambridge Philosophical Society</em> 90: 157–166. https://doi.org/10.1111/brv.12104</p>
<p>Linnaeus, C. (1753) <em>Species Plantarum</em>, Vol. 1. Impensis Laurentii Salvii, Stockholm, 560 pp. https://doi.org/10.5962/bhl.title.37656</p>
<p>Lubbinge, J. (1971) ’n Studie van die anatomiese bou van twee stingels en vier bloei-asse van die genus <em>Aloe</em>. <em>Aloe</em> 4: 13–28.</p>
<p>Medhanie, G. & Dioli, M. (2006) Observations on three poorly known aloes from Eritrea. <em>Cactus & Succulent Journal</em> 78: 36–41. https://doi.org/10.2985/0007-9367(2006)78[36:ootpka]2.0.co;2</p>
<p>Newton, E. (1972) Taxonomic use of the cuticular features in the genus <em>Aloe</em> (Liliaceae). <em>Botanical Journal of the Linnean Society</em> 65: 335–339. https://doi.org/10.1111/j.1095-8339.1972.tb00123.x</p>
<p>Newton, L.E. (2020) <em>Aloe</em> ASPHODELACEAE. <em>In</em>: Eggli, I. & Nyffeler, R. (eds.) <em>Illistrated handbook of succulent plants: Monocotyledons</em>. Springer Verlag, Berlin, Heidelberg, pp. 485–696. [http://link.springer.com/10.1007/978-3-662-56486-8_35]</p>
<p>Newton, L.E. & Rowley, G.D. (2001) <em>CITES Aloe and Pachypodium Checklist.</em> U. Eggli (ed.) The Trustees of the Royal Botanic Gardens Kew, Kew, pp. 1–156.</p>
<p>Pathan, A.K., Bond, J. & Gaskin, R.E. (2009) Sample preparation for SEM of plant surfaces. <em>Materials Today</em> 12: 32–43. https://doi.org/10.1016/S1369-7021(10)70143-7</p>
<p>Pierce, A.R. & Laird, S.A. (2003) In search of comprehensive standards for non-timber forest products in the botanicals trade. <em>International Forestry Review</em> 5: 138–147. https://doi.org/10.1505/IFOR.5.2138.17418</p>
<p>Rendle, A.B., Baker, E.G. & Moore, S.L.M. (1908) An account of the plants collected on Mt. Ruwenzori by Dr. A.G.F. Wollaston. <em>Journal of the Linnean Society of London, Botany</em> 38: 228–279. https://doi.org/10.1111/j.1095-8339.1908.tb00851.x</p>
<p>Reynolds, G.W. (1953a) Four new aloes from Kenya Colony. <em>Journal of South African Botany</em> 19: 9–11. [https://archive.org/details/journalofsouthaf19unse/page/n9/mode/2up]</p>
<p>Reynolds, G.W. (1953b) A new <em>Aloe</em> from Somaliland Protectorate. <em>Journal of South African Botany</em> 19: 21–23. [https://archive.org/details/journalofsouthaf19unse/page/n41/mode/2up]</p>
<p>Reynolds, G.W. (1954a) Two new <em>Aloe</em> species from Southern Ethiopia. <em>Journal of South African Botany</em> 20: 25–30. [https://archive.org/details/journalofsouthaf20unse/page/n37/mode/2up]</p>
<p>Reynolds, G.W. (1954b) Notes on <em>Aloe angiensis</em> De Wild., with a new variety from Kenya Colony. <em>Journal of South African Botany</em> 20: 179–183. [https://archive.org/details/journalofsouthaf20unse/page/n211/mode/2up]</p>
<p>Reynolds, G.W. (1955) A new <em>Aloe</em> from Ethiopia.<em> Journal of South African Botany</em> 21: 59–61, plate, X. [https://archive.org/details/journalofsouthaf21unse/page/n75/mode/2up]</p>
<p>Reynolds, G.W. (1957) Six new species of <em>Aloe</em> from Ethiopia. <em>Journal of South African Botany</em> 23: 1–12. [https://archive.org/details/journalofsouthaf23unse/page/n11/mode/2up]</p>
<p>Reynolds, G.W. (1966) <em>The aloes of Tropical Africa and Madagascar.</em> The Aloes Book Fund, Mbabane, 537 pp.</p>
<p>Salm-Reifferscheidt-Dyck, J.F.M.A.H.I. (1837) <em>Monographia generum aloes et mesembryanthemi</em>, Fasc. 1. Maximillian Cohen, Bonn, 60 plates, 72 pp.</p>
<p>Salmeri, C. (2019) Plant morphology: Outdated or advanced discipline in modern plant sciences? <em>Flora Mediterranea</em> 29: 163–180. https://doi.org/10.7320/FlMedit29.163</p>
<p>Schönenberger, J. & von Balthazar, M. (2012) Modern plant morphological studies. <em>Botanical Journal of the Linnean Society</em> 169: 565–568. https://doi.org/10.1111/j.1095-8339.2012.01287.x</p>
<p>Schweinfurth, G. (1894) Sammlung Arabisch-Aethiopischer Pflanzen. <em>Bulletin Herbier Boissier, Append</em> 2: 62–107.</p>
<p>Sebsebe, D. & Brandham, P.E. (1992) A new species of <em>Aloe</em> from the Ethiopian Rift Valley. <em>Kew Bulletin </em>47: 509–512. https://doi.org/10.2307/4110578</p>
<p>Sebsebe, D. & Gilbert, M.G. (1997) Aloaceae. <em>In</em>: Edwards, S., Sebsebe, D. & Hedberg, I. (eds.) <em>Flora of Ethiopia and Eritrea</em>, Vol. 6 - <em>Hydrocharitaceae to Arecaceae</em>. The National Herbarium, Biology Department, Science Faculty, Addis Ababa University, Ethiopia, and The Department of Systematic Botany, Uppsala University, Sweden, pp. 117–135.</p>
<p>Sebsebe, D. & Nordal, I. (2010) <em>Aloes and lilies of Ethiopia and Eritrea.</em> Shama Books, Addis Adaba, pp. 42–110.</p>
<p>Simpson, M.G. (2019) Plant morphology. <em>In</em>: Simpson, M.G. <em>Plant systematics. Elsevier</em>, Oxford, pp. 469–535.</p>
<p>Smith, G.F. & van Wyk, A.E. (1992) Systematic leaf anatomy of selected genera of southern African Alooideae (Asphodelaceae). <em>South African Journal of Botany</em> 58: 349–357. https://doi.org/10.1016/s0254-6299(16)30821-3</p>
<p>Thiers, B. (2021 [continuously updated]) Index herbariorum: a global directory of public herbaria and associated staff. New York Botanical Garden’s Virtual Herbarium. Available from: http://sweetgum.nybg.org/ih/ (accessed August 2021)</p>
<p>Todaro, A. (1876) <em>Aloe macrocarpa</em>. <em>Hortus Botanicus Panormitanus</em> 1: 36, t. 9. Available from: https://www.biodiversitylibrary.org/item/105101#page/46/mode/1up (accessed August 2021)</p>
<p>Todaro, A. (1882) <em>Hortus Botanicus Panormitanus</em> 2: 25–28, t. 29. [https://www.biodiversitylibrary.org/item/105101#page/159/mode/1up]</p>
<p>Ugochukwu, A.I., Hobbs, J.E., Phillips, P.W.B. & Kerr, W.A. (2018) Technological solutions to authenticity issues in international trade: The case of CITES listed endangered species. <em>Ecological Economics</em> 146: 730–739. https://doi.org/10.1016/j.ecolecon.2017.12.021</p>
<p>Verdoorn, I.C. & Christian, H.B. (1947) <em>Aloe debrana</em>. <em>The Flowering Plants of South Africa</em> 26: t. 1016. [https://archive.org/details/floweringplantso26unse/page/n73/mode/2up]</p>
<p>Viljoen, A.M., van Wyk, B.-E. & Newton, L.E. (1999) Plicataloside in <em>Aloe</em>–a chemotaxonomic appraisal. <em>Biochemical Systematics & Ecology</em> 27: 507–517. https://doi.org/10.1016/S0305-1978(98)00111-2</p>
<p>Young, A.J., Suarez, L.P., Kapralov, M. & Opel, M.R. (2017) Leaf epidermal structure in the dwarf succulent genus <em>Conophytum</em> N.E.Br. (Aizoaceae). <em>Bradleya</em> 35: 217–237. https://doi.org/https://doi.org/10.25223/brad.n35.2017.a25</p>
<p>Baker, J.G. (1898) Uliaceae. <em>In</em>: Thiselton-Dyer, W.T. (ed.) <em>Flora of Tropical Africa </em>7. Reeve, London, pp. 421–569.</p>
<p>Barthlott, W. (1990) Scanning electron microscopy of the epidermal surface in plants. <em>In</em>: Claugher, D. (ed.) <em>Scanning electron microscopy in taxonomy and functional morphology</em>. Systematic Association Special, vol. 41. Clarendon Oxford, Oxford, pp. 69–94.</p>
<p>Barthlott, W., Neinhuis, C., Cutler, D., Ditsch, F., Meusel, I., Theisen, I. & Wilhelmi, H. (1998) Classification and terminology of plant epicuticular waxes. <em>Botanical Journal of the Linnean Society</em> 12 (3): 237–260. [https://www.tandfonline.com/doi/abs/10.1006/bojl.1997.0137]</p>
<p>Beaumont, J., Cutler, D.F., Reynolds, T. & Vaughan, J.G. (1985) The secretory tissue of aloes and their allies.<em> Israel Journal of Botany </em>34: 265–282. [https://www.tandfonline.com/doi/abs/10.1080/0021213X.1985.10677026]</p>
<p>Berger, A. (1905) Über die systematische Gliederung der Gattung <em>Aloë</em>. Botanische Jahrbücher fur Systematik, Pflanzengeschichte und Pflanzengeographie 36: 42–68. [https://www.biodiversitylibrary.org/page/66#page/49/mode/1up]</p>
<p>Brandham, P.E. & Cutler, D.F. (1978) Influence of chromosome variation on the organisation of the leaf epidermis in a hybrid <em>Aloe </em>(Liliaceae). <em>Botanical Journal of the Linnean Society</em> 77: 1–16. https://doi.org/10.1111/j.1095-8339.1978.tb01368.x</p>
<p>Carter, S. (1994) The family Aloaceae. <em>In</em>: Polhill, R.M. (ed.) <em>Flora of Tropical East Africa</em>. Balkema, Rotterdam and Brookfield, 328 pp.</p>
<p>Carter, S. & Brandham, P. (1983) New species of <em>Aloe</em> from Somalia. <em>Bradleya</em> 1: 17–24.</p>
<p>Carter, S., Cutler, D.F., Reynolds, T. & Brandham, P.E. (1984) A multidisciplinary approach to a revision of the <em>Aloe somaliensis</em> complex (Liliaceae). <em>Kew Bulletin</em> 39: 611–633. https://doi.org/10.2307/4108600</p>
<p>Carter, S., Lavranos, J.J., Newton, L.E. & Walker, C.C. (2011) <em>Aloes. The definitive guide.</em> 1st ed. Kew Publishing, Richmond, 719 pp.</p>
<p>Chase, M.W. & Fay, M.F. (2009) Barcoding of plants and fungi. <em>Science</em> 325: 682–683. https://doi.org/10.1126/science.1176906</p>
<p>Christian, H.B. (1951) <em>Aloe mcloughlinii</em>. <em>The Flowering Plants of South Africa</em> 28: t. 1112.</p>
<p>Coopoosamy, R.M. & Naidoo, K.K. (2011) Anatomical features of the medicinal importance of <em>Aloe excelsa</em> (Berger). <em>African Journal of Biotechnology</em> 10: 7622–7632. https://doi.org/10.5897/AJB11.113</p>
<p>Crang, R., Lyons-Sobaski, S. & Wise, R. (2018) Plant anatomy. A concept-based approach to the structure of seed plants. Springer Nature, Switzerland, pp. 423–430.</p>
<p>Cutler, D.F. (1969) Cuticular markings and other epidermal features in <em>Aloe</em> leaves. <em>Notes from the Jodrell Laboratory</em> 6: 21–24.</p>
<p>Cutler, D.F. (1972) Leaf anatomy of certain <em>Aloe</em> and <em>Gasteria</em> species and their hybrids.<em> In: </em>Ghouse, A.K.M. & Yunus, M. (eds.) <em>Research Trends in Plant Anatomy</em>. Tata McGraw Hill, Bombay, pp. 103–122.</p>
<p>Cutler, D.F. (1979) Leaf surface studies in Aloe and Haworthia species (Liliaceae): taxonomic implications. Tropische & Subtropische Pflanzenwelt 28: 449–471.</p>
<p>Cutler, D.F. (1982) Cuticular sculpturing and habitat in certain <em>Aloe</em> species (Liliaceae) from southern Africa. <em>In</em>: Cutler, D.F., Alvin, K.L. & Price, C.E. (eds.) <em>The plant cuticle</em>. Academic Press, London, pp. 425–444.</p>
<p>Cutler, D.F. (2004) <em>Aloe</em> leaf anatomy. <em>In</em>: Reynolds, T. (ed.) <em>The genus Aloe</em>. CRC Press, Boca Raton, pp. 372–377.</p>
<p>Cutler, D.F., Botha, T. & Stevenson, D.W. (2007) The epidermis. <em>In</em>: Cutler, D.F., Botha, T. & Stevenson, D.W. (eds.) <em>Plant anatomy</em>: <em>an applied approach</em>. Blackwell Publishing Ltd, Oxford, pp. 76–93.</p>
<p>Cutler, D.F. & Brandham, P.E. (1977) Experimental evidence for the genetic control of leaf surface characters in hybrid Aloineae (Liliaceae). <em>Kew Bulletin</em> 32: 23–32. https://doi.org/10.2307/4117256</p>
<p>Cutler, D.F., Brandham, P.E., Carter, S. & Harris, S.J. (1980) Morphological, anatomical, cytological and biochemical aspects of evolution in East African shrubby species of <em>Aloë</em> L. (Liliaceae). <em>Botanical Journal of the Linnean Society</em> 80: 293–317. https://doi.org/10.1111/j.1095-8339.1980.tb01666.x</p>
<p>Engler, A. (1895) <em>Die Pflanzenwelt Ost-Afrikas und der Nachbargebiete</em>, Vol. 2. D. Reimer, Berlin, 433 pp. https://doi.org/10.5962/bhl.title.587</p>
<p>Fortini, P., Antonecchia, G., Di Marzio, P., Maiuro, L. & Viscosi, V. (2015) Role of micromorphological leaf traits and molecular data in taxonomy of three sympatric white oak species and their hybrids (<em>Quercus</em> L.). <em>Plant Biosystems </em>149: 546–558. https://doi.org/10.1080/11263504.2013.868374</p>
<p>Fradera-Soler, M., Rudall, P.J., Prychid, C.J. & Grace, O.M. (2021) Evolutionary success in arid habitats: morpho-anatomy of succulent leaves of Crassula species from southern Africa.<em> Journal of Arid Environments</em> 185: 104319.</p>
<p>Gilbert, M.G. & Demissew, S. (1992) Notes on the genus <em>Aloe</em> in Ethiopia: misinterpreted taxa.<em> Kew Bulletin</em> 47: 647–653.</p>
<p>Gilbert, M.G. & Sebsebe Demissew. (1997) Further notes on the genus <em>Aloe</em> in Ethiopia and Eritrea.<em> Kew Bulletin</em> 52: 139–152. https://doi.org/10.2307/4117846</p>
<p>Glen, H.F. & Hardy, D.S. (1995) <em>Aloe</em> section <em>Anguialoe</em> and the problem of <em>Aloe spicata</em> L.f. (Aloaceae). <em>Haseltonia</em> 3: 92–103.</p>
<p>Grace, O.M. (2011) Current perspectives on the economic botany of the genus <em>Aloe</em> L. (Xanthorrhoeaceae). <em>South African Journal of Botany</em> 77: 980–987. https://doi.org/10.1016/j.sajb.2011.07.002</p>
<p>Grace, O.M., Buerki, S., Symonds, M., Forest, F., van Wyk, A.E., Smith, G.F., Klopper, R.R., Bjorå, C.S., Neale, S., Sebsebe Demissew, Simmonds, M. & Rønsted, N. (2015) Evolutionary history and leaf succulence as explanations for medicinal use in aloes and the global popularity of <em>Aloe vera</em>. <em>BMC Evolutionary Biology</em> 15: a29. https://doi.org/10.1186/s12862-015-0291-7</p>
<p>Grace, O.M., Klopper, R.R., Figueiredo, E. & Smith, G.F. (2011) <em>The aloe names book. Strelitzia 28.</em> South African National Biodiversity Institute and the Royal Botanic Gardens, Pretoria and Kew, pp. 1–3.</p>
<p>Grace, O.M., Klopper, R.R., Smith, G.F., Crouch, N.R., Figueiredo, E., Rønsted, N. & van Wyk, A.E. (2013) A revised generic classification for <em>Aloe</em> (Xanthorrhoeaceae subfam. Asphodeloideae). <em>Phytotaxa</em> 76: 7–14. https://doi.org/10.11646/phytotaxa.76.1.2</p>
<p>Grace, O.M., Simmonds, M.S.J., Smith, G.F. & Van Wyk, A.E. (2009) Taxonomic significance of leaf surface morphology in <em>Aloe</em> section Pictae (Xanthorrhoeaceae). <em>Botanical Journal of the Linnean Society</em> 160: 418–428. https://doi.org/10.1111/j.1095-8339.2009.00982.x</p>
<p>Ickert-Bond, S.M., Harris, A.J., Lutz, S. & Wen, J. (2018) A detailed study of leaf micromorphology and anatomy of New World <em>Vitis</em> L. subgenus <em>Vitis</em> within a phylogenetic and ecological framework reveals evolutionary convergence. <em>Journal of Systematics and Evolution</em> 56: 309–330. https://doi.org/10.1111/jse.12313</p>
<p>Jayeola, A.A. & Thorpe, J.R. (2000) A scanning electron microscope study of the genus <em>Calyptrochilum</em> Kraenzl. (Orchidaceae) in West Africa. <em>Feddes Repertorium</em> 111 (5–6): 315–320. https://doi.org/10.1002/fedr.20001110506</p>
<p>Klimko, M. & Wiland-Szymanska, J. (2008) Scanning electron microscopic studies of leaf surface of taxa of genus <em>Dracaena</em> L. (Dracaenaceae). <em>Botanika—Steciana</em> 12: 117–127. https://doi.org/10.1021/1a9017322</p>
<p>Koch, K., Bohn, H.F. & Barthlott, W. (2009) Hierarchically sculptured plant surfaces and superhydrophobicity. <em>Langmuir</em> 25: 14116–14120. https://doi.org/10.1021/la9017322</p>
<p>Li, X., Yang, Y., Henry, R.J., Rossetto, M., Wang, Y. & Chen, S. (2015) Plant DNA barcoding: from gene to genome. <em>Biological Reviews of the Cambridge Philosophical Society</em> 90: 157–166. https://doi.org/10.1111/brv.12104</p>
<p>Linnaeus, C. (1753) <em>Species Plantarum</em>, Vol. 1. Impensis Laurentii Salvii, Stockholm, 560 pp. https://doi.org/10.5962/bhl.title.37656</p>
<p>Lubbinge, J. (1971) ’n Studie van die anatomiese bou van twee stingels en vier bloei-asse van die genus <em>Aloe</em>. <em>Aloe</em> 4: 13–28.</p>
<p>Medhanie, G. & Dioli, M. (2006) Observations on three poorly known aloes from Eritrea. <em>Cactus & Succulent Journal</em> 78: 36–41. https://doi.org/10.2985/0007-9367(2006)78[36:ootpka]2.0.co;2</p>
<p>Newton, E. (1972) Taxonomic use of the cuticular features in the genus <em>Aloe</em> (Liliaceae). <em>Botanical Journal of the Linnean Society</em> 65: 335–339. https://doi.org/10.1111/j.1095-8339.1972.tb00123.x</p>
<p>Newton, L.E. (2020) <em>Aloe</em> ASPHODELACEAE. <em>In</em>: Eggli, I. & Nyffeler, R. (eds.) <em>Illistrated handbook of succulent plants: Monocotyledons</em>. Springer Verlag, Berlin, Heidelberg, pp. 485–696. [http://link.springer.com/10.1007/978-3-662-56486-8_35]</p>
<p>Newton, L.E. & Rowley, G.D. (2001) <em>CITES Aloe and Pachypodium Checklist.</em> U. Eggli (ed.) The Trustees of the Royal Botanic Gardens Kew, Kew, pp. 1–156.</p>
<p>Pathan, A.K., Bond, J. & Gaskin, R.E. (2009) Sample preparation for SEM of plant surfaces. <em>Materials Today</em> 12: 32–43. https://doi.org/10.1016/S1369-7021(10)70143-7</p>
<p>Pierce, A.R. & Laird, S.A. (2003) In search of comprehensive standards for non-timber forest products in the botanicals trade. <em>International Forestry Review</em> 5: 138–147. https://doi.org/10.1505/IFOR.5.2138.17418</p>
<p>Rendle, A.B., Baker, E.G. & Moore, S.L.M. (1908) An account of the plants collected on Mt. Ruwenzori by Dr. A.G.F. Wollaston. <em>Journal of the Linnean Society of London, Botany</em> 38: 228–279. https://doi.org/10.1111/j.1095-8339.1908.tb00851.x</p>
<p>Reynolds, G.W. (1953a) Four new aloes from Kenya Colony. <em>Journal of South African Botany</em> 19: 9–11. [https://archive.org/details/journalofsouthaf19unse/page/n9/mode/2up]</p>
<p>Reynolds, G.W. (1953b) A new <em>Aloe</em> from Somaliland Protectorate. <em>Journal of South African Botany</em> 19: 21–23. [https://archive.org/details/journalofsouthaf19unse/page/n41/mode/2up]</p>
<p>Reynolds, G.W. (1954a) Two new <em>Aloe</em> species from Southern Ethiopia. <em>Journal of South African Botany</em> 20: 25–30. [https://archive.org/details/journalofsouthaf20unse/page/n37/mode/2up]</p>
<p>Reynolds, G.W. (1954b) Notes on <em>Aloe angiensis</em> De Wild., with a new variety from Kenya Colony. <em>Journal of South African Botany</em> 20: 179–183. [https://archive.org/details/journalofsouthaf20unse/page/n211/mode/2up]</p>
<p>Reynolds, G.W. (1955) A new <em>Aloe</em> from Ethiopia.<em> Journal of South African Botany</em> 21: 59–61, plate, X. [https://archive.org/details/journalofsouthaf21unse/page/n75/mode/2up]</p>
<p>Reynolds, G.W. (1957) Six new species of <em>Aloe</em> from Ethiopia. <em>Journal of South African Botany</em> 23: 1–12. [https://archive.org/details/journalofsouthaf23unse/page/n11/mode/2up]</p>
<p>Reynolds, G.W. (1966) <em>The aloes of Tropical Africa and Madagascar.</em> The Aloes Book Fund, Mbabane, 537 pp.</p>
<p>Salm-Reifferscheidt-Dyck, J.F.M.A.H.I. (1837) <em>Monographia generum aloes et mesembryanthemi</em>, Fasc. 1. Maximillian Cohen, Bonn, 60 plates, 72 pp.</p>
<p>Salmeri, C. (2019) Plant morphology: Outdated or advanced discipline in modern plant sciences? <em>Flora Mediterranea</em> 29: 163–180. https://doi.org/10.7320/FlMedit29.163</p>
<p>Schönenberger, J. & von Balthazar, M. (2012) Modern plant morphological studies. <em>Botanical Journal of the Linnean Society</em> 169: 565–568. https://doi.org/10.1111/j.1095-8339.2012.01287.x</p>
<p>Schweinfurth, G. (1894) Sammlung Arabisch-Aethiopischer Pflanzen. <em>Bulletin Herbier Boissier, Append</em> 2: 62–107.</p>
<p>Sebsebe, D. & Brandham, P.E. (1992) A new species of <em>Aloe</em> from the Ethiopian Rift Valley. <em>Kew Bulletin </em>47: 509–512. https://doi.org/10.2307/4110578</p>
<p>Sebsebe, D. & Gilbert, M.G. (1997) Aloaceae. <em>In</em>: Edwards, S., Sebsebe, D. & Hedberg, I. (eds.) <em>Flora of Ethiopia and Eritrea</em>, Vol. 6 - <em>Hydrocharitaceae to Arecaceae</em>. The National Herbarium, Biology Department, Science Faculty, Addis Ababa University, Ethiopia, and The Department of Systematic Botany, Uppsala University, Sweden, pp. 117–135.</p>
<p>Sebsebe, D. & Nordal, I. (2010) <em>Aloes and lilies of Ethiopia and Eritrea.</em> Shama Books, Addis Adaba, pp. 42–110.</p>
<p>Simpson, M.G. (2019) Plant morphology. <em>In</em>: Simpson, M.G. <em>Plant systematics. Elsevier</em>, Oxford, pp. 469–535.</p>
<p>Smith, G.F. & van Wyk, A.E. (1992) Systematic leaf anatomy of selected genera of southern African Alooideae (Asphodelaceae). <em>South African Journal of Botany</em> 58: 349–357. https://doi.org/10.1016/s0254-6299(16)30821-3</p>
<p>Thiers, B. (2021 [continuously updated]) Index herbariorum: a global directory of public herbaria and associated staff. New York Botanical Garden’s Virtual Herbarium. Available from: http://sweetgum.nybg.org/ih/ (accessed August 2021)</p>
<p>Todaro, A. (1876) <em>Aloe macrocarpa</em>. <em>Hortus Botanicus Panormitanus</em> 1: 36, t. 9. Available from: https://www.biodiversitylibrary.org/item/105101#page/46/mode/1up (accessed August 2021)</p>
<p>Todaro, A. (1882) <em>Hortus Botanicus Panormitanus</em> 2: 25–28, t. 29. [https://www.biodiversitylibrary.org/item/105101#page/159/mode/1up]</p>
<p>Ugochukwu, A.I., Hobbs, J.E., Phillips, P.W.B. & Kerr, W.A. (2018) Technological solutions to authenticity issues in international trade: The case of CITES listed endangered species. <em>Ecological Economics</em> 146: 730–739. https://doi.org/10.1016/j.ecolecon.2017.12.021</p>
<p>Verdoorn, I.C. & Christian, H.B. (1947) <em>Aloe debrana</em>. <em>The Flowering Plants of South Africa</em> 26: t. 1016. [https://archive.org/details/floweringplantso26unse/page/n73/mode/2up]</p>
<p>Viljoen, A.M., van Wyk, B.-E. & Newton, L.E. (1999) Plicataloside in <em>Aloe</em>–a chemotaxonomic appraisal. <em>Biochemical Systematics & Ecology</em> 27: 507–517. https://doi.org/10.1016/S0305-1978(98)00111-2</p>
<p>Young, A.J., Suarez, L.P., Kapralov, M. & Opel, M.R. (2017) Leaf epidermal structure in the dwarf succulent genus <em>Conophytum</em> N.E.Br. (Aizoaceae). <em>Bradleya</em> 35: 217–237. https://doi.org/https://doi.org/10.25223/brad.n35.2017.a25</p>