Plane Family

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Plane Family

The Plane family is a family of trees and large shrubs known to botanists as the Platanaceae. This family has a single genus, Platanus, and seven to ten different species. The two most familiar species are the American sycamore (Platanus occidentalis ), which is native to eastern and central United States, and the London plane, a hybrid tree species, which is commonly planted as an ornamental in the United States and Europe. Both species have thick trunks at maturity, which have very characteristic scaly bark. The Platanaceae is probably closely related to the Hamamelidaceae, a plant family, which includes the witch hazels and sweet gums.

Botanical Characteristics

The leaves of all plants in the plane family are simple, deciduous, palmate, and somewhat maplelike in appearance. The leaves are palmately veined and have three to nine lobes, depending on the species. The leaves arise from a long petiole (stalk) that is swollen at its base on the twig. The leaves arise alternately on the stem (rather than opposite one another) and the twigs have a characteristic zig-zag appearance.

The flowers of all species are unisexual in that they contain either male organs or female organs, but not both. All species are monoecious, in that male and female flowers arise from the same individual tree. The flowers are minute and arise in large spherical clusters.

The fruit is a characteristic spherical cluster of small, one-seeded, dry, indehiscent fruits, referred to as achenes. Depending on the species, one to several of these spherical fruit clusters arises from a single long peduncle (stem) that is attached to the twig. The small seeds are wind dispersed.

The best known tree of this family is the American sycamore. Its fruit balls are about 1 in (2.5 cm) in diameter and consist of several hundred seeds densely packed together. Naturalist and writer Henry David Thoreau (1817–1862) eloquently described the seeds of this species as “standing on their points like pins closely packed in a globular pin-cushion, surrounded at the base by a bristly down of a tawny color, which answers the purpose of a parachute.”

Geographic Distribution

Of the seven to ten species in the plane family, all but two are native to North America. Three species are native to the United States. The well-known American sycamore grows in moist alluvial soils in central and eastern North America. The two other American species are small trees of western United States. The Arizona sycamore (Platanus wrightii ) grows along stream banks in Arizona and New Mexico. The California sycamore (Platanus racemosa ) grows along stream banks in the Sierra Nevada region.

Two species in the Plane family are from Europe and Asia. The Oriental planetree (Platanus orientalis ) is native to the Balkans and Himalayas, and Platanus kerrii is native to Indochina.

American Sycamore

The American sycamore is also referred to as the American planetree or the buttonwood. These trees grow in moist areas, such as along stream banks, in eastern and central United States. They can live for 500 years or more. At maturity, these trees can be over 100 ft (30.5 m) in height and have trunks up to 8 ft (2.4 m) in diameter. The American sycamore is the most massive tree species in eastern North America.

The bark of the American sycamore has a very characteristic mottled or scaly appearance. Its palmate leaves are 4 to 7 in (10.2 to 17.8 cm) in diameter and have three to five lobes each. The spherical fruit clusters are about 1 in (2.5 cm) in diameter and one fruit cluster arises from each stalk.

The wood of the American sycamore is very difficult to split. This property makes it ideal for construction of butcher’s blocks. The wood has also been used as a veneer for furniture.

Oriental Planetree

The Oriental planetree grows in alluvial soils in regions with a moderate climate in the Balkans (Greece, Turkey, elsewhere in the Mediterranean) and Himalayas of Asia. This species differs from the American sycamore in that it has several spherical clusters of fruits on each peduncle. This tree is often cultivated as an ornamental plant in the Mediterranean region of Europe.

London Planetree

In the early to mid 1600s, botanists grew the American sycamore and Oriental planetree close to one another at the well-known Oxford Botanical Gardens in England. Apparently, these two species spontaneously hybridized in the late 1600s and produced a new hybrid species, the London planetree (Platanus X hybrida, but also given other Latin names). Although Platanus occidentalis and Platanus orientalis are believed to have been separate species for at least 50 million years, their hybrid was fertile and produced its own seeds.

The London planetree combines some of the characteristics of each of its parent species, as is typical of hybrid species. The leaves of the American sycamore

KEY TERMS

Achene— A dry, indehiscent, one-seeded fruit, with the outer layer fused to the seed.

Hybrid— Offspring of the sexual union of two different species.

Peduncle— Stalk which bears a cluster of flowers.

have shallow lobes, the leaves of the Oriental planetree have deep lobes, and the leaves of the London planet-ree have lobes with intermediate depth. One fruit cluster is borne on each peduncle in the American sycamore, several fruit clusters are borne on each fruit cluster of the Oriental planetree, and two (or occasionally three) fruit clusters are borne on each peduncle of the London planetree.

Like the American sycamore, but unlike the Oriental planetree, the London planetree can endure cold climates. The London planetree can endure pollution and other environmental stresses better than either species. Thus, it is often cultivated as an ornamental tree and planted along streets in America and Britain. Moreover, the London planetree can grow up to 3 ft (0.9 m) per year, making it a very popular shade tree for homeowners.

In the 1920s, more than 60% of the trees planted along the streets of London, England, were London planetrees. They are also well known in the Kensington Gardens of London.