London plane

The London plane is a large deciduous tree growing , exceptionally to tall, with a trunk up to 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 long and broad, with a petiole 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 (monoecy). The fruit matures in about 6 months, to 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 seeds. The London Plane is one of the most efficient trees in removing small particulate pollutants in urban areas. It shares many visual similarities with American sycamore (Platanus occidentalis), from which it is derived; however, the two species are relatively easy to distinguish, considering the London plane is almost exclusively planted in urban habitats, while American sycamore is most commonly found growing in lowlands and alluvial soils along streams. ==Origin==

The species was formed by hybridization in the 17th century after P. orientalis and P. occidentalis had been planted in proximity to one another. It is most likely that the hybridization took place in Spain or southern France, as the P. occidentalis parent needs considerable summer heat to grow well and is near-impossible to grow successfully in cooler climates like Britain, but there is also speculation that it could have happened in Vauxhall Gardens in London where John Tradescant the Younger saw the tree in the mid-17th century. 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. In 1968 and 1970, Frank S. Santamour Jr. recreated the P. orientalis by 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. ==Taxonomy==

, Belgrade, was planted in 1834. Platanus × hispanica was first formally described in the botanical literature by the German botanist Otto von Münchhausen in his 1770 work der Hausvater. Later, it was described in the botanical literature by the Scottish botanist William Aiton in his 1789 work Hortus Kewensis as a variety of P. orientalis. Aiton described this variety with a two-word Latin diagnosis, "foliis transversis", and called it the Spanish plane tree. In 1805, Carl Ludwig Willdenow chose to elevate Aiton's variety to species rank, publishing the new species P. acerifolia in the fourth edition of Species Plantarum. The species name was then modified to include the multiplication symbol to indicate its suspected hybrid parentage. == Cultivation and uses ==

Two of 50 "Great British Trees" that the Tree Council selected in 2002 in honour of Queen Elizabeth II's Golden Jubilee are London planes. The list specifically mentions Britain's first London plane being in the city of Ely, Cambridgeshire. The London plane is very tolerant of atmospheric pollution and root compaction, and for this reason it is a popular urban roadside tree. It was planted extensively in Victorian times to weather the pollution of London. 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, in London and many other cities. 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. In China The London plane is cultivated in central, northeastern and southern China. ==Cultivars==

• 'Augustine Henry'. This is a tall growing cultivar, with very large, pale green leaves. It produces a strong leader and a cylindrical trunk. • 'Liberty'. A U.S. National Arboretum introduction, this pyramidal tree grows vigorously. It shows good tolerance for mildew, anthracnose, heat and drought. Image:cflps.jpg|A house finch eating London plane seeds in Seattle Image:Platanusxacerifolia04.jpg|Single seed ball per stem: similar to P. occidentalis, not found in all clones ==See also==