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 Leaves and fruit of a London Plane
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Platanus is a small genus of trees native to the Northern Hemisphere. They are the sole members of the family Platanaceae.
They are all large trees to 30–50 m tall, deciduous (except for P. kerrii), and are mostly found in riparian or other wetland habitats in the wild, though proving drought tolerant in cultivation away from streams. The hybrid London Plane has proved particularly tolerant of urban conditions.
They are known as planes in Europe, and as sycamores in North America. (Outside North America, the name "sycamore" refers to either the fig Ficus sycomorus, the plant originally so named, or the Great, or Sycamore Maple, Acer pseudoplatanus.)
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Natural history
The flowers are reduced and are borne in balls (globose head); 3–7 hairy sepals may be fused at base, and the petals are 3–7 (or no) and spathulate. Male and female flowers are separate, but on the same plant (monoecious). The number of heads in one cluster (inflorescence) is indicative of the species (see table below). The male flower has with 3–8 stamens; the female has a superior ovary with 3–7 carpels. Plane trees are wind-pollinated. Male balls fall off the branch after shedding their pollen. The female flowers, on the other hand, remain attached to the branch firmly.
After being pollinated, the female flowers become achenes that aggregate on the ball. Typically, the core of the ball is 1 cm in diameter and is covered with a net of mesh 1 mm, which can be peeled off. The ball is 2.5–4 cm in diameter and contains several hundred achenes, each of which has a single seed and is conical, with the point attached downward to the net at the surface of the ball. There is also a tuft of many thin stiff yellow-green bristle fibers attached to the base of each achene. These bristles help in wind dispersion of the fruits like dandelion.
The mature bark peels (exfoliates) off easily in irregularly shaped patches, producing a mottled, scaly appearance. Very old bark may not flake off, but can crack instead. The base of the leaf stalk (petiole) is enlarged and completely wraps around the young stem bud in its axil. The bud will be exposed only after the leaf falls off.
Cultural history
For the cultural history, see Platanus orientalis.
Phylogeny
There are two subgenera, subgenus Castaneophyllum containing the anomalous P. kerrii, and subgenus Platanus, with all the others; recent studies in Mexico[1] have increased the number of accepted species in this subgenus. Within subgenus Platanus, genetic evidence suggests that P. racemosa is more closely related to P. orientalis than it is to the other North American species.[2] There are fossil records of plane trees as early as 115 million years (the Lower Cretaceous). Despite the geographic separation between North America and Europe, species from these continents will cross readily resulting in fertile hybrids such as the London Plane.
Species
The following are recognized species of plane trees:
| Scientific name | Common name | Distribution | flowerheads | Notes |
|---|---|---|---|---|
| Platanus chiapensis | Chiapas Plane | southeast Mexico | ? | Subgenus Platanus |
| Platanus gentryi | Gentry's Plane | western Mexico | ? | Subgenus Platanus |
| Platanus × hispanica (P. occidentalis × P. orientalis; syn. P. × acerifolia, P. × hybrida) |
London Plane | Cultivated origin | 1-6 | Subgenus Platanus |
| Platanus kerrii | Kerr's Plane | Laos, Vietnam | 10-12 | Subgenus Castaneophyllum |
| Platanus mexicana | Mexican Plane | northeast and central Mexico | 2-4 | Subgenus Platanus |
| Platanus oaxacana | Oaxaca Plane | southern Mexico | ? | Subgenus Platanus |
| Platanus occidentalis | American Sycamore, American Plane, or Buttonwood | eastern North America | 1-2 | Subgenus Platanus |
| Platanus orientalis | Oriental Plane | southeast Europe, southwest Asia | 3-6 | Subgenus Platanus |
| Platanus racemosa | California Sycamore | California | 3-7 | Subgenus Platanus |
| Platanus rzedowskii | Rzedowski's Plane | eastern Mexico | ? | Subgenus Platanus |
| Platanus wrightii | Arizona Sycamore | Arizona, New Mexico, northwest Mexico | 2-4 | Subgenus Platanus |
Diseases
Planes are susceptible to Plane Anthracnose (Apiognomonia veneta), a fungal disease that can defoliate the trees in some years. The worst infections are associated with cold, wet spring weather. P. occidentalis and the other American species are the most susceptible, with P. orientalis the most resistant. The hybrid London Plane is intermediate in resistance. Other diseases such as powdery mildew occur frequently, but are of lesser importance. Platanus species are used as food plants by the larvae of some Lepidoptera species including Phyllonorycter platani and Setaceous Hebrew Character.
Notes
References
- Feng, Y.; Oh, S.-H., & Manos, P. S. (2005). Phylogeny and Historical Biogeography of the Genus Platanus as Inferred From Nuclear and Chloroplast DNA. Syst. Bot. 30 (4): 786-799 abstract
- Nixon, K. C. & Poole, J. M. (2003). Revision of the Mexican and Guatemalan species of Platanus (Platanaceae). Lundellia 6: 103-137 abstract.
External links
- Botany of Plane trees
- Flora of North America: Platanus
- Photos with descriptions
- A developmental and evolutionary analysis of embryology in Platanus (platanaceae), abasal eudicot, abstract of article by Sandra K. Floyd et al. in American Journal of Botany, 1999;86:1523-1537.
