Platanaceae
The Platanaceae are a family of flowering plants belonging to the order Proteales. This family has been recognized by almost all taxonomists, and is sometimes called the "plane-tree family". The family consists of only a single extant genus Platanus, with eight known species. The plants are tall trees, native to temperate and subtropical regions of the Northern Hemisphere. The hybrid London plane is widely planted in cities worldwide.
The plane-tree is referenced in Pliny the Younger's letter to Domitius Apollinaris as part of his description of his Tuscan Villa located somewhere in Tuscany in the first century.
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Description
- Large, sympodial, deciduous tree, speckled bark that sheds in large irregular sheets, leaving a smooth surface that is mottled and pale, persistent bark at the base of the trunk, indumentum with large glandular hairs, multicellular and uniserrate or short with uniserrate ramification, in stellate fascicles; glandular hairs with unicellular, globular capitulum, cuticular waxes without crystalloids, with rods and plates
- Leaves generally with very variable shapes and nervation, simple, alternate, more or less distichous, isobilateral palmate with three to seven lobes with whole edges or with glandular teeth, or penninerved and whole, this shape common in young, vernal leaves in other species, vernation folded, with the petiole usually sheathed, enclosing the axillary bud, stipules foliose, large, intrapetiolar, tubular, normally caduceus, in P. kerrii scarious, small, basally fused to the petiole, domatia present, stomata irregularly anomocytic
- Stems with aggregated rays in the xylem, with nodes septilacunar, cork cambium present and superficial, buds covered by single scale
- Plants monoecious, the flowers of each sex in separate inflorescences
- Inflorescences in large hanging peduncles, each one a unisexual, globular capitulum, pedunculate or seated, with numerous flowers, derived from the condensation of a panicle, with a circular bract at the base and bracteoles among the flowers
- Flowers small, inconspicuous, hypogynous, regular unisexual, receptacle short, smooth, hypogynous disk absent, perianth reduced, sepals number three to four, rarely eight, free or basally fused, shorter than the petals, triangular. Petals number three to four, rarely eight, truncated-spatulate or vestigial, scarious, frequently absent in the female flowers, male flowers with androecium haplostemonous, isostemonous, oppositisepal, with number three to four, rarely eight, stamens, gynostemium short or vestigial, anthers basifixed, not versatile, dithecous, tetrasporangiate, elongated, connectivum apically widened, peltate, dehiscence along longitudinal valves; pistillidium sometimes present, female flowers with superior gynoecium carpels apocarpous in two or three whorls, imperfectly closed apically, surrounded by large petals, linear stylodious, stigmas internal, decurrent in two ridges, more or less dry, two ovules per carpel but one nearly always aborts, orthotropous, bitegmic, crassinucellated, pendulous, apical to marginal placentation, three or four staminodes 3-4, no nectaries
- Fruits in achene, clavate, grouped in a globular capituliform infructescence, each fruit surrounded by long hairs
- Seeds small with thin testa with little endosperm, oily and proteinaceous, embryo thin and straight with two linear cotyledons, often uneven
- Pollen in subprolate monads, 16-22 μm in length, tricolpate, sometimes sextarugate, tectate-columellate, reticulated surface, the base layer as thick as the tectum
- Chromosomal number: 2n = 14, 16, 21, 42; x probably equal to 7 or 8
Ecology
The plants grow in cool situations in temperate climates and are frequently found on the banks of rivers and streams. They are totally absent from dry or excessively cold areas.
Phytochemistry
They contain cyanogenic glycosides derived from tyrosine, flavonoids belonging to the proanthocyanidins group and flavonols, in addition to triterpenols. They lack ellagic acid, saponins, and sapogenins.Cultivation
The main use for a number of the species is to provide shade in pedestrian areas in temperate regions, particularly the London plane-tree, which is widely distributed throughout Europe and North America. It is highly resistant, probably due to so-called hybrid vigour, although its use requires caution due to their allergy-producing thistledown. The parent species are also grown for the same effect, but with poorer results as they are less resistant to contamination, among other reasons. The wood is used in cabinetmaking, paneling, and other interior work, and is also prized for its long burn time.Fossils
A large number of fossils of this family have been recorded from the Lower Cretaceous. The examples from that time had very small pollen and a developed perianth and they lacked hairs at the base of the nucule. It is thought to have had entomophilous pollination. During the mid Cretaceous, the fossilized forms with platanoid leaves became mixed with pinnate leaves or pedatisect leaves, and these forms lasted until the Eocene. The leaves with typical stipules belonging to the sub-genus Platanus are very common in Palaeocene formations. It is thought that the only modern genus, Platanus, is a relict that can be considered a living fossil. It must have been polyploidy, during its evolution judging by the size of its stomata.Systematic position
The APG II system allows the option of including it in the family Proteaceae, or treating it as distinct as a segregate family. In as far as APG II accepts the family, it is placed in the order Proteales, in the clade eudicots. This represents a slight change from the APG system of 1998, which did accept this family. The Cronquist system of 1981 recognized the family and placed it in order Hamamelidales, in subclass Hamamelidae in class Magnoliopsida. The Dahlgren system and Thorne system also recognized this family and placed it in the order Hamamelidales in superorder Rosanae in subclass Magnoliidae sensu Dahlgren and Thorne. The Engler system, in its 1964 update, also recognized the family and placed it in the order Rosales in subclass Archichlamydeae of class Dicotyledoneae. The Wettstein system, last revised in 1935, also recognized the family and placed it in the order Hamamelidales in the Monochlamydeae in subclass Choripetalae of the class Dicotyledones. Based on molecular and morphological data the APW places the family in the order Proteales as a sister family to the Proteaceae, making them the Northern Hemisphere version of this family.Taxa included
The only extant genus, Platanus L., 1753, has the type species Platanus orientalis L., 1753. It is divided into two subgenera: the subgenus Castaneophyllum J.-F. Leroy, 1982, with elliptical, penninerved leaves with small scarious, stipules, that only includes Platanus kerrii Gagnep., 1939, an isolated relict species that represents the genus’ evolutionary basal branch and which is a sister genus and is the sister group of the other species, including the subgenus Platanus.| Scientific name | Common name | Distribution | Flowers | Notes |
| Platanus acerifolia Willd.—Sp. Pl., ed. 4 4: 474. 1805 | Maple-leaved plane | ? | ||
| Platanus algeriensis Hort. ex K.Koch—Dendrologie 2: 468. 1872 | - | ? | ||
| Platanus chiapensis | Chiapas plane | southeast Mexico | ? | Subgenus Platanus |
| Platanus gentryi Nixon & J.M. Poole, 2003 | Gentry’s plane | western Mexico | ? | Subgenus Platanus |
| Platanus × hispanica | London plane, hybrid plane | Origen cultivated | 1-6 | Subgenus Platanus |
| Platanus kerrii Gagnep., 1939. | Kerr’s plane | Laos, Vietnam, probably south China | 10-12 | Subgenus Castaneophyllum |
| Platanus mexicana Moric., 1837 | Mexican Plane | mexican sycamore northeast and central Mexico | 1-4 | Subgenus Platanus |
| Platanus oaxacana | Oaxaca plane | southern Mexico | ? | Subgenus Platanus |
| Platanus occidentalis L., 1753 | American sycamore, American plane, buttonwood, occidental plane Western plane Ghost tree water beech | Eastern North America | 1-2 | Subgenus Platanus |
| Platanus orientalis L., 1753 | Oriental Plane | Southeastern Europe, Near East, Iran, central Asia, Pakistan, India | 3-6 | Subgenus Platanus |
| Platanus racemosa Nutt., 1842 | California sycamore, western sycamore, aliso | California and Mexico | 3-7 | Subgenus Platanus |
| Platanus rzedowskii Nixon & J.M. Poole, 2003 | Rzedowski’s plane | eastern Mexico | 1-2 | Subgenus Platanus |
| Platanus wrightii S. Watson, 1875 | Wright’s plane, Arizona sycamore or alamo | Arizona, New Mexico, northwestern Mexico | 2-4 | Subgenus Platanus |
Hybrids
The London plane-tree or hybrid plane has long been considered a hybrid derived from the cross between P. occidentalis and P. orientalis, despite this its origin is not clear. Some experts think it originated in London and others in Spain or even in natural or cultivated hybrid form in Turkey. The question has not been investigated with modern molecular methods. As a consequence, even its nomenclature is hotly debated, to the extent that Anglo-Saxon authors deny the priority of the name used in Spain. The plant is not found in the wild, though it appears in a naturalized form along the banks of rivers and streams.Hybrid Platanus × hispanica Mill. ex Münchh., 1770 (= P. orientalis var. acerifolia Aiton, 1789; P. hybrida Brot., 1804; P. vulgaris Spach, 1841, nom. illeg.; P. × acerifolia.
Other names proposed for hybrids that are probably synonymous with the above, which is the only name in English, and which represent smaller minorities are:
- Hybrid Platanus × cantabrigensis A.Henry, 1919
- Hybrid Platanus × parviloba A.Henry, 1919