All mature members ofPlatanus are tall, reaching 30–50 m (98–164 ft) in height. Thetype species of the genus is the Oriental planePlatanus orientalis. All except forP. kerrii aredeciduous, and most are found inriparian or otherwetlandhabitats in the wild, though provingdrought-tolerant in cultivation. The hybridLondon plane (Platanus × hispanica) has proved particularly tolerant of urban conditions, and has been widely planted in London and elsewhere across the temperate world.
Trunk of an agedPlatanus orientalis, in Trsteno, nearDubrovnik, CroatiaRipe plane tree fruit
The flowers are reduced and are borne in balls (globose heads); 3–7 hairysepals may be fused at the base, and the petals are 3–7 and arespatulate. Male and female flowers are separate, but borne 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 3–8stamens; the female has a superior ovary with 3–7carpels. Plane trees are wind-pollinated. Male flower-heads fall off after shedding their pollen.[citation needed]
After being pollinated, the female flowers becomeachenes that form an aggregate ball. The fruit is a multiple of achenes (plant systematics, Simpson M. G., 2006). 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 thedandelion.[citation needed]
The leaves aresimple and alternate. In the subgenusPlatanus they have a palmate outline. The base of the leaf stalk (petiole) is enlarged and completely wraps around the young stem bud in its axil. The axillary bud is exposed only after the leaf falls off.[citation needed]
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.[citation needed]
Platanus palmeri (=P. occidentalisvar.palmeri) – forming the southwesternmost populations ofP. occidentalis s.l.[6][7] – carries nuclearintron sequences (second intron of theLeafy gene) of PNA-E origin.[6] It lacks the plastid haplotype specific for the northeastern populations (P. occidentalis s.str.)[7]
Theinternal transcribed spacers of the nuclear-encodedrRNA genes ofP. occidentalis s.l. andP. rzedowskii include ANA-specific variants with functional5.8S rDNA as well as PNA-E-specific variants showing signs ofpseudogeny.[9] The latter are shared withP. gentryi, the PNA-E species closest to the ANA clade area and the northern/ interior populations ofP. mexicana s.l. This indicates that already thecommon ancestor ofP. rzedowskii andP. occidentalis s.l. had been in contact with a member of the PNA-E clade.
Likewise,P. rzedowskii fromNuevo León is a genetic mosaic, and may have originated from earlier hybridization within the ANA clade, between southernmostP. occidentalis s.l. (P. palmeri) andP. mexicana s.l., or their ancestors.[6] Today the ranges ofP. occidentalis s.l. andP. mexicana s.l. are mutually exclusive.Platanus rzedowskii is geographically and morphologically intermediate betweenP. occidentalis s.l. andP. mexicana s.l.[5][7]
Morphological reinvestigation including the originally collected material revealed that the interior populations ofP. mexicana (northernQuerétaro and northernHidalgo;P. mexicana var.interior according Nixon & Poole) mark thehybrid zone betweenP. rzedowskii andP. mexicana s.l. and the (former) contact zone to the species of the PNA-E clade (P. gentryi,P. wrightii). Since theholotype ofP. mexicana is from this zone and shows the characteristical intermediate morphology,P. mexicana s.str. would represent anothospecies:P. × mexicana.[10] The remaining populations ofP. mexicana s.l.,P. lindeniana, show no sign of introgression from eitherP. rzedowskii, P. occidentalis-palmeri or the Western North American species (P. racemosa species aggregate),[6][7] with the exception of oneheterozygoticP. oaxacana population from northcentralOaxaca.[7]
Sorting and evolutionary history of three differentnoncoding nuclearloci contributing to thegene pool of modern-day species ofPlatanus (after[6])
The genusPlatanus exemplarily illustrates the concept of aCoral of Life, a species network. Its modern-day species are not only the product of evolutionarydichotomies (cladogenesis), the splitting of an ancestral lineage into two (Tree of Life metaphor) but also evolutionaryanastomoses: hybridization and introgression.
The fossil record of leaves and fruit identifiable toPlatanus begins in thePaleocene.[11] Despite the geographic separation between North America and Old World, species from these continents will cross readily resulting in fertilehybrids such as the London plane, which is an anthropogenic hybrid (cultivar) between the North AmericanP. occidentalissensu stricto (ANA clade) and theMediterraneanP. orientalis (PNA-E clade). Widely used as a park tree across the Northern Hemisphere, it frequentlybackcrosses with both its parents.
Severe infections of anthracnose can sometimes defoliate large swaths of American sycamore forest during mid and late spring, but trees generally recover by mid-summer
Planes are susceptible to plane anthracnose (Apiognomonia veneta), afungal 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, withP. orientalis the most resistant. The hybrid London plane is intermediate in resistance.
Ceratocystis platani, awilt disease, has become a significant problem in recent years in much of Europe.[12] 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 aspowdery mildew occur frequently, but are of lesser importance.
In the 21st century a disease, commonly known as Massaria disease, has attacked plane trees across Europe. It is caused by the fungusSplanchnonema platani, and causes large lesions on the upper sides of branches.[13][14]
There have been cases of "platanus cough", symptoms of shortness of breath, coughing, and irritated eyes, which may affect several people in a place, and have led to initial suspicion of an attack with an irritant gas. After one such mass attack which affected schoolchildren in classrooms with open windows densely surrounded by plane tees, children had to be admitted to hospital, where they were treated and recovered without ill effects. It was found that the symptoms were due to the fine star-shapedtrichomes (hairs) on all parts of platanus trees, which are broken off by strong wind after a prolonged dry period. The dust created causes direct irritation and scratchiness in the eyes, throat, and nose, but not the runny nose and itching eyes and nose caused by anallergy. The school incident took place after a dry period, with a fairly high temperature of 29 °C (84 °F), and wind blowing at 50 km/h (31 mph).[15]
Protection against platanus cough is provided by avoiding contact and wearing protective glasses and masks under weather conditions promoting release of trichomes. When cleaning in an urban environment, sweeping up fallen leaves and branches can release hairs; cleaning by suction is preferred. It is not recommended that trees in cities be felled, as they are beneficial; in particular the platanus trichomes act as biofilters for air pollutants. Where there are urban concentrations presenting a risk, seasonal spraying of trees with a solution of apple pectin can prevent the star hair from breaking off.[15]
The principal use of these trees is as ornamental trees, especially in urban areas and by roadsides. TheLondon plane is particularly popular for this purpose. TheAmerican plane is cultivated sometimes for timber and investigations have been made into its use as abiomass crop. Theoriental plane is widely used as an ornamental tree, and also has a number of minor medicinal uses.
Most significant aspects of cultural history apply toPlatanus orientalis in the Old World. The tree is an important part of the literary scenery of Plato's dialoguePhaedrus. Because ofPlato, the tree also played an important role in the scenery of Cicero'sDe Oratore. The trees also provided the shade under whichAristotle and Plato's famed philosophical schools were held.[16]Handel's operaSerse has a famousaria, "Ombra mai fu", which the title character sings in praise of his favorite plane tree.
The plane tree has been a frequent motif featured inClassical Chinese poetry as an embodiment of sorrowful sentiments due to its autumnal shedding of leaves.[citation needed]
TheGerman camouflage patternPlatanenmuster ("plane tree pattern"), designed in 1937–1942 by Johann Georg Otto Schick, was the first dotted camouflage pattern.[17]
Naumann, Helmut (2007). "Die Platane von Gortyna". In Kämmerer., Thomas Richard (ed.).Studien zu Ritual und Sozialgeschichte im Alten [Orient / Studies on Ritual and Society in the Ancient Near East. Tartuer Symposien 1998–2004]. Berlin, de Gruyter. pp. 207–226.{{cite book}}: CS1 maint: location missing publisher (link)
Sunset Western Garden Book. 1995. pp. 606–607.
Journals
Feng, Y.; Oh, S.H.; Manos, P. S. (2005). "Phylogeny and Historical Biogeography of the Genus Platanus as Inferred From Nuclear and Chloroplast DNA".Systematic Botany.30 (4):786–799.doi:10.1600/036364405775097851.S2CID86021109.
