| Platanus × acerifolia | |
|---|---|
| London Plane seed ball | |
| Scientific classification | |
| Kingdom: | Plantae |
| (unranked): | Angiosperms |
| (unranked): | Eudicots |
| Order: | Proteales |
| Family: | Platanaceae |
| Genus: | Platanus |
| Species: | P. × acerifolia |
| Binomial name | |
| Platanus × acerifolia (Aiton) Willd. |
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| Synonyms[1] | |
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Platanus × acerifolia, the London plane, London planetree, or hybrid plane, is a tree in the genus Platanus. It is usually thought to be a hybrid of Platanus orientalis (oriental plane) and the Platanus occidentalis (American sycamore). Some authorities think that it may be a cultivar of P. orientalis.
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The London Plane is a large deciduous tree growing to 20–35 m (exceptionally over 40 m) tall, with a trunk up to 3 m or more in circumference. The bark is usually pale grey-green, smooth and exfoliating, or buff-brown and not exfoliating. The leaves are thick and stiff-textured, broad, palmately lobed, superficially maple-like, the leaf blade 10–20 cm long and 12–25 cm broad, with a petiole 3–10 cm long. The young leaves in spring are coated with minute, fine, stiff hairs at first, but these wear off and by late summer the leaves are hairless or nearly so. The flowers are borne in one to three (most often two) dense spherical inflorescences on a pendulous stem, with male and female flowers on separate stems. The fruit matures in about 6 months, to 2–3 cm diameter, and comprises a dense spherical cluster of achenes with numerous stiff hairs which aid wind dispersal; the cluster breaks up slowly over the winter to release the numerous 2–3 mm seeds.
It was first recorded as occurring in Spain in the 17th century, where the Oriental Plane and the American Plane had been planted in proximity to one another. The leaf and flower characteristics are intermediate between the two parent species, the leaf being more deeply lobed than P. occidentalis but less so than P. orientalis, and the seed balls typically two per stem (one in P. occidentalis, 3-6 in P. orientalis). The hybrid is fertile, and seedlings are occasionally found near mature trees.
Controlled reciprocal pollinations between P. occidentalis and P. orientalis resulted in good yields of germinable seed and true hybrid seedlings. Crosses of both species, as females, with P. racemosa and P. wrightii produced extremely low yields of germinable seed, but true hybrids were obtained from all interspecific combinations. Apomixis (asexual reproduction from non-fertilized seeds) appeared common in P. orientalis.[2]
In 1968 and 1970, Frank S. Santamour, Jr., recreated the P. orientalis, P. occidentalis cross using a P. orientalis of Turkish origin with American sycamores (P. occidentalis). The offspring were evaluated following several years of exposure to anthracnose infection. Two selections'Columbia' and 'Liberty' were released in August, 1984.[2][3]
Platanus × acerifolia was first formally described in the botanical literature by the Scottish botanist William Aiton in his 1789 work Hortus Kewensis as a variety of Platanus orientalis.[4] Aiton described this variety with a two-word Latin diagnosis, "foliis tranſverſis" (using the long s or "foliis transversis" without it), and called it the Spanish Plane Tree.[5] In 1805, Carl Ludwig Willdenow chose to elevate Aiton's variety to species rank, publishing the new species Platanus acerifolia in the fourth edition of Species Plantarum.[6][7] The species name was then modified to include the multiplication symbol to indicate its suspected hybrid parentage. The other name commonly used for this taxon, Platanus × hispanica auct. non Mill. ex Münchh., is a nomen dubium based on an uncertain description.[8][9]
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| Avenue of London plane trees on Jesus Green. |
The London plane is very tolerant of atmospheric pollution and root compaction, and for this reason it is a popular urban roadside tree. It is now extensively cultivated in most temperate latitudes as an ornamental and parkland tree, and is a commonly planted tree in cities throughout the temperate regions of the world, not just London but Buenos Aires, Curitiba, New York City, Paris, Madrid, Melbourne, Mannheim, Shanghai, Nanjing[12], Chicago, Sydney, Rybnik and many others. It has a greater degree of winter cold tolerance than the Oriental Plane, and is less susceptible to anthracnose disease than the American Plane. The seeds are used as a food source by some finches and squirrels.
The tree is fairly wind-resistant. However, it has a number of problems in urban use, most notably the short, stiff hairs shed by the young leaves and the dispersing seeds; these are an irritant if breathed in, and can exacerbate breathing difficulties for people with asthma. The large leaves can create a disposal problem in cities. These leaves are tough and sometimes can take more than one year to break down if they remain whole.
London Planes are often pruned by a technique called pollarding. A pollarded tree has a drastically different appearance than an unpruned tree, being much shorter with stunted, club-like branches. Although pollarding requires frequent maintenance (the trees must usually be repruned every year), it creates a distinctive shape that is often sought after in plazas, main streets, and other urban areas.
According to Lois Nam of the New York City Department of Parks and Recreation the symbol of that organisation is a cross between the leaf of the London plane and a maple leaf. It is prominently featured on signs and buildings in public parks across the city. The tree is today on the NYC Parks Department's list of restricted use trees for street tree planting.