Platanus

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iPlatanus
Leaves and fruit of a London Plane
Leaves and fruit of a London Plane
Scientific classification
Kingdom: Plantae
Division: Magnoliophyta
Class: Magnoliopsida
Order: Proteales
Family: Platanaceae
Genus: Platanus
Species

See text

The genus 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.

They are known as planes in the Old World, and as sycamores in North America. Elsewhere, the name "sycamore" refers to either the fig Ficus sycomorus (the species originally so named), or the Sycamore Maple Acer pseudoplatanus.

The reduced flowers, borne in balls (globose head), 3–7 hairy sepals (may be fused at base), and 3–7 (or no) spathulate petals. The 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 superior ovary with 3–7 carpels. Plane trees are wind-pollinated. Male balls fall off 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 the 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 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.

There are two subgenera, subgenus Castaneophyllum containing the anomalous P. kerrii, and subgenus Platanus, with all the others; recent studies in Mexico (Nixon & Poole 2003) 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 (Feng et al. 2005). 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.

The following are recognized species of plane trees:

Scientific name Common name Distribution flowerheads Notes
Platanus kerrii Kerr's Plane Laos, Vietnam 10-12 Subgenus Castaneophyllum
Platanus chiapensis Chiapas Plane southeast Mexico  ? Subgenus Platanus
Platanus gentryi Gentry's Plane western Mexico  ? Subgenus Platanus
Platanus mexicana Mexican Plane northeast and central Mexico 2-4 Subgenus Platanus
Platanus oaxacana Oaxaca Plane southern Mexico  ? Subgenus Platanus
Platanus orientalis Oriental Plane southeast Europe, southwest Asia 3-6 Subgenus Platanus
Platanus occidentalis American Sycamore, American Plane or Buttonwood eastern North America 1-2 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
Platanus × hispanica
(P. occidentalis × P. orientalis;
syn. P. × acerifolia)		 London Plane Cultivated origin 1-6 Subgenus Platanus

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. Platanus species are used as food plants by the larvae of some Lepidoptera species including Setaceous Hebrew Character.

[edit] References

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