Hedera is a taxonomically difficult genus in which species delimitation is very controversial. About 14 species are currently recognised but many more have been described. Valcárcel & Vargas (2010) – in a multivariate statistical analysis – roughly confirmed the recognition of 12 “good” species, including those accepted in the present treatment. All species are native to Europe, Macaronesia, temperate Asia and northern Africa. Several have a very restricted native distribution area but have become more widespread as a result of cultivation. Only one species, Hedera helix L., is native in Belgium. Almost all known species of Hedera are grown as ornamentals.
Most diacritic features (indumentum type, leaf shape,…) are fairly variable and sometimes difficult to appreciate. In most species leaf shape differs much in flowering and juvenile branches. As a rule and unless otherwise stated, leaf shape in the present treatment always refers to leaves of non-flowering branches. Indumentum is best seen on (very) young leaves or young branches. Older leaves and branches are soon glabrescent. Colour and shape of trichomes is an important character as well (but only seen under high magnification). Odour also is a reliable feature for the distinction of species but this requires fresh material.
Hedera helix L. is a common native species but at least part of the Belgian populations are probably of non-native origin (dispersal by birds or from discarded garden waste of several cultivars). The cultivars ‘Arborescens’, ‘Palmata’ and ‘Pedata’ are sometimes seen in woodlands or on old walls. Green & al. (2011) demonstrated that Hedera helix itself is polyphyletic and that some of its infraspecific taxa are in fact more closely related to other species than to H. helix. In addition and apparently as a result of global warming (laurophyllisation) Hedera helix has considerably enlarged its original native distribution area in large parts of Europe (Dierschke 2005).
Belgian records of Hedera algeriensis Hibberd (see for instance Verloove 2006) are possibly erroneous. At least part of the collections in fact belong to Hedera colchica. Pending further studies it is here excluded.
1. Trichomes orange or reddish, almost peltate with rays fused for ca. ½-1/4 their length === 2
1. Trichomes whitish or pale yellowish-brown, stellate with rays only fused at extreme bases === 3
2. Leaves of non-flowering shoots wider than long, distinctly 3-(5) lobed (the middle lobe largest). Trichomes usually with ca. 4-9 rays === 3. Hedera maroccana
2. Leaves of non-flowering shoots longer than wide, usually unlobed (heartshaped or bluntly 3-lobed). Trichomes usually with ca. 15-25 rays === 1. Hedera colchica
3. Rays of the trichomes lying +/- parallel to the leaf surface, often (at least in part) pale yellowish-brown. Largest leaves up to 20 cm across, usually lobed less than ½ way to base === 2. H. hibernica
3. Rays of the trichomes projecting at various angles from the leaf surface, greyish-white. Largest leaves usually less than 10 cm across, often lobed more than ½ way to base (native) === H. helix
Lum & Maze (1989) and Ackerfield (2001) emphasise on trichome morphology. Older revisions or treatments of the genus Hedera have become obsolete since several taxa have only been described in recent years (see for instance Rutherford & al. 1993).
Additional information on the genus in the Americas is available with The American Ivy Society at www.ivy.org.
Ackerfield J. (2001) Trichome morphology in Hedera (Araliaceae). Edinburgh J. Bot. 58(2): 259-267.
Ackerfield J. & Wen J. (2002) A morphometric analysis of Hedera L. (the ivy genus, Araliaceae) and its taxonomic implications. Adansonia 24(2): 197-212.
Dierschke H. (2005) Laurophyllisation – auch eine Erscheinung im nördlichen Mitteleuropa? Zur aktuellen Ausbreitung von Hedera helix in sommergrünen Laubwäldern. Berichte der Reinhold-Tüxen-Gesellschaft 17: 151-168.
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