Platanus

Platanus is a genus consisting of a small number of tree species. They are the sole living members of the family Platanaceae.
All mature members of Platanus are tall, reaching in height. The type species of the genus is the Oriental plane Platanus orientalis. All except for P. kerrii are deciduous, and most are found in riparian or other wetland habitats in the wild, though proving drought-tolerant in cultivation. The hybrid London plane has proved particularly tolerant of urban conditions, and has been widely planted in London and elsewhere across the temperate world.
They are often known in English as planes or plane trees. A formerly used name that is now rare is plantain tree. Some North American species are called sycamores, although the term is also used for several unrelated species of trees. The genus name Platanus comes from Ancient Greek πλάτανος, which referred to Platanus orientalis.

Botany

The flowers are reduced and are borne in balls ; 3–7 hairy sepals may be fused at the base, and the petals are 3–7 and are spatulate. Male and female flowers are separate, but borne on the same plant. The number of heads in one cluster is indicative of the species. The male flower has 3–8 stamens; the female has a superior ovary with 3–7 carpels. Plane trees are wind-pollinated. Male flower-heads fall off after shedding their pollen.
After being pollinated, the female flowers become achenes that form an aggregate ball. The fruit is a multiple of achenes . 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 as in the dandelion.
The leaves are simple and alternate. In the subgenus Platanus they have a palmate outline. The base of the leaf stalk is enlarged and completely wraps around the young stem bud in its axil. The axillary bud is exposed only after the leaf falls off.
The mature bark peels off or exfoliates easily in irregularly shaped patches, producing a mottled, scaly appearance. On old trunks, bark may not flake off, but thickens and cracks instead.

Phylogeny

There are two subgenera, subgenus Castaneophyllum containing the anomalous P. kerrii, and subgenus Platanus, with all the others; recent studies in Mexico have increased the number of accepted species in this subgenus. Within subgenus Platanus, evidence from both chloroplast and nuclear gene sequences suggests that the P. racemosa species complex in Western North America is more closely related to the Eurasian P. orientalis than it is to the other North American species. The two groups form genetically and morphologically distinct evolutionary lineages, informally called the “ANA clade” and “PNA-E clade”. Both lineages have been affected by reticulate evolutionary processes in the past :

Platanus palmeri – forming the southwesternmost populations of P. occidentalis s.l. – carries nuclear intron sequences of PNA-E origin. It lacks the plastid haplotype specific for the northeastern populations
The internal transcribed spacers of the nuclear-encoded rRNA genes of P. occidentalis s.l. and P. rzedowskii include ANA-specific variants with functional 5.8S rDNA as well as PNA-E-specific variants showing signs of pseudogeny. The latter are shared with P. gentryi, the PNA-E species closest to the ANA clade area and the northern/ interior populations of P. mexicana s.l. This indicates that already the common ancestor of P. rzedowskii and P. occidentalis s.l. had been in contact with a member of the PNA-E clade.
Likewise, P. rzedowskii from Nuevo León is a genetic mosaic, and may have originated from earlier hybridization within the ANA clade, between southernmost P. occidentalis s.l. and P. mexicana s.l., or their ancestors. Today the ranges of P. occidentalis s.l. and P. mexicana s.l. are mutually exclusive. Platanus rzedowskii is geographically and morphologically intermediate between P. occidentalis s.l. and P. mexicana s.l.
Morphological reinvestigation including the originally collected material revealed that the interior populations of P. mexicana mark the hybrid zone between P. rzedowskii and P. mexicana s.l. and the contact zone to the species of the PNA-E clade. Since the holotype of P. mexicana is from this zone and shows the characteristical intermediate morphology, P. mexicana s.str. would represent a nothospecies: P. × mexicana. The remaining populations of P. mexicana s.l., P. lindeniana, show no sign of introgression from either P. rzedowskii, P. occidentalis-palmeri or the Western North American species, with the exception of one heterozygotic P. oaxacana population from northcentral Oaxaca.

File:PlatanusNuclearEvolution.png|thumb|Sorting and evolutionary history of three different noncoding nuclear loci contributing to the gene pool of modern-day species of Platanus
The genus Platanus exemplarily illustrates the concept of a Coral of Life, a species network. Its modern-day species are not only the product of evolutionary dichotomies, the splitting of an ancestral lineage into two but also evolutionary anastomoses: hybridization and introgression.
The fossil record of leaves and fruit identifiable to Platanus begins in the Paleocene. Despite the geographic separation between North America and Old World, species from these continents will cross readily resulting in fertile hybrids such as the London plane, which is an anthropogenic hybrid between the North American P. occidentalis sensu stricto and the Mediterranean P. orientalis. Widely used as a park tree across the Northern Hemisphere, it frequently backcrosses with both its parents.

Species

The following are named species of Platanus; not all are accepted by all authorities:

Botanical name	Common names	Distribution and taxonomic notes	Flowerheads	Systematics
Platanus chiapensis Standl.	Chiapas plane	Mexico ; part of P. mexicana species aggregate, probably a junior synonym of P. lindeniana	2–7	Subgenus Platanus, ANA clade
Platanus gentryi &	Gentry's plane	Mexico ; part of the P. racemosa species aggregate	?	Subgenus Platanus, PNA-E clade
Platanus kerrii Gagnep.	Kerr's plane	Laos, Vietnam	10–12	Subgenus Castaneophyllum J.-F.Leroy
Platanus mexicana sensu lato	Mexican sycamore, Mexican plane	Mexico, Guatemala; in a strict sense synonymous with P. mexicana var. interior Nixon & Poole, restricted to Guanajuato, Hidalgo, Querétaro and San Luis Potosí, and of hybrid origin	s.l.: 1–7; s.str: 1–3	Subgenus Platanus, ANA clade
Platanus lindeniana M.Martens & Galeotti syn Platanus occidentalis var. lindeniana	Linden's plane tree, Linden's sycamore	Mexico, Guatemala; part of the P. mexicana species aggregate, synonymous with P. mexicana var. mexicana according Nixon & Poole	2–5	Subgenus Platanus, ANA clade
Platanus oaxacana Standl.	Oaxaca plane	Mexico ; part of the P. mexicana species aggregate, junior synonym of P. lindeniana or distinct species	2–4	Subgenus Platanus, ANA clade
Platanus occidentalis L.	American sycamore, American plane, buttonwood, occidental plane, water beech	Canada, United States	1–2	Subgenus Platanus, ANA clade
Platanus palmeri ined.	Plateau sycamore, Palmer's sycamore	Mexico and United States	1–2	Subgenus Platanus, ANA clade
Platanus rzedowskii Nixon & J.M.Poole	Rzedowski's plane, Rzedowskii's sycamore	Mexico	1–2	Subgenus Platanus, ANA clade
Platanus orientalis L.	Oriental plane	Eurasia	3–6	Subgenus Platanus, PNA-E clade
Platanus racemosa Nutt.	California sycamore, western sycamore, aliso	United States, Mexico	3–7	Subgenus Platanus, PNA-E clade
Platanus wrightii S.Watson	Arizona sycamore	United States, Mexico ; part of the P. racemosa species aggregate	2–4	Subgenus Platanus, PNA-E clade
Platanus × hispanica Mill. ex Muenchh.	London plane, hybrid plane	Worldwide, cultivated origin; hybrid of P. occidentalis and P. orientalis	1–4	Subgenus Platanus; interlineage hybrid

Diseases

Planes are susceptible to plane anthracnose, a fungal disease that can defoliate the trees in some years. The most severe 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.
Ceratocystis platani, a wilt disease, has become a significant problem in recent years in much of Europe. The North American species are mostly resistant to the disease, with which they probably coevolved, while the Old World species are highly sensitive.
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.
In the 21st century a disease, commonly known as Massaria disease, has attacked plane trees across Europe. It is caused by the fungus Splanchnonema platani, and causes large lesions on the upper sides of branches.