Speaking the Plane Truth

Sycamores, also called plane trees, are among the most common shade trees planted in the United States and Europe. My guess is that any typical U. S. town having public parks or official street trees and a civil center likely has planted at least one specimen-if not a line or grove-of these trees. I would even wager that anybody reading this article has eaten lunch, at least once, beneath the canopy of a sycamore. Among the streets and avenues named after plants, Sycamore has to be one of the most common, behind Maple or Elm or Walnut. Don't you think that it is time to learn "the plane truth" about these strong and durable deciduous trees?

Plane trees are members of Platanus, the only genus in the plane tree family (Platanaceae). At present experts recognize about eight species plus an interspecific hybrid, although the number of published scientific names of extant species exceeds seventy. In our country there are three native species: P. occidentalis, called the sycamore, buttonball tree, or the American plane tree; P. wrightii, the Arizona sycamore; and P. racemosa, the California or western sycamore. Forms of all of these species also occur in Mexico, as do several other currently recognized species, P. chiapensis, P. oaxacana, and P. rzedowskii. At least two other species are indigenous to Eurasia, including the highly variable oriental plane tree, P. orientalis, and a very bizarre, fully evergreen species with narrowly elliptic leaves, P. kerrii, discovered in tropical forests of Laos and Vietnam. Platanus orientalis is a native of southern Eurasia from west central Italy eastward to Persia.

The London plane tree is the popular interspecific hybrid, widely cultivated from the Avenue des Champs-Elysées to Bruin Walk. Platanus x acerifolia-elsewhere called P. hybrida, P. hispanica, and P. intermedia-was long suspected, and is now confirmed with DNA analysis, to have arisen as a cross between P. orientalis of Eurasia and P. occidentalis of eastern North America. Allegedly from a fruit obtained from Montpellier, the original parent was probably grown at the Oxford Botanic Garden in England during the 18th century, and was first named by Aiton in 1789. Other interspecific hybrids have been created within the genus.

The temperate species are winter deciduous, although around Westwood you will find California sycamores that retain some leaves throughout the year. Nevertheless, growth of new shoots on California sycamores is one of the first indicators that spring has sprung on campus, beginning in mid-February. At least I treat it-el aliso-as UCLA's harbinger of spring when the velveteen leaves first emerge from dormant buds.

Species of Platanus have alternate, simple and palmately lobed leaves, and the stem is often somewhat zigzag, bending slightly to the right, then the left, then the right, and so forth, as each leaf is formed. Each node is noticeably swollen just below the petiole. Remarkably in plane trees, the petiolar base is hollow and dilated (expanded), completely covering and hiding the axillary bud. The conical bud is eventually revealed when the leaf abscises and falls off, leaving a circular scar. Above each node occurs a prominent pair of fused (connate) stipules, which encircle the stem, and when the stipules fall off ringlike (annular) scars remain on the twig. The petioles (2.5 to 8 centimeter long) become bent, thereby changing the permanent orientation of the leaf blades and repositioning them to receive the highest possible lighting.

Excepting P. kerrii, which belongs to its own subgenus, the other plane trees have lobed leaves matching the sizes of human hands. The occidentalis forms, including London plane trees, have large, broad teeth and three shallow lobes, whereas blades of P. racemosa, P. wrightii, and P. orientalis have very distinct, long triangular lobes, with or without irregular teeth. On the leaf underside, you can see that three prominent veins arise at or just above the base of the blade, diverging toward the tip and two major lobes. Where there are five lobes, as commonly seen in California sycamore, or seven lobes, as commonly seen in Arizona sycamore, the primary veins for the additional lobes diverge from the outer veins toward the lower lobes. This type of venation is termed pseudopalmate, and really shows nicely in some of the ancient fossil leaves of the plane tree family that had as many as nine distinct lobes.

The etymology of Platanus is not entirely clear, but is from the Greek platanos or perhaps platys, for broad, referring to the wide leaves. Leaves of the oriental plane tree and the American sycamore are so distinctive that Linnaeus, when he recognized the genus Platanus in 1753 (borrowed from earlier workers), also named Acer pseudoplatanus for the so-called sycamore maple (Greek, pseudes, false), a tree having leaves like plane trees. Some have suggested that the vernacular name sycamore for American species may have been borrowed from the European sycamore maple, not the reverse. There is also a linguistic relationship to the sycomore fig, Ficus sycomorus, from the eastern Mediterranean region, from the Greek word sykomoros for mulberry. Another example with sycamorelike leaves is the Norway maple, Acer platanoides, which I learned to recognize growing up in Ohio as the maple that bleeds white latex when a fresh leaf is plucked from a tree. In the tropics there are several other plants with specific epithets referring to Platanus-like leaves, because the taxonomist learned their botany in northern latitudes where plane trees were cultivated.

Leaf blades, petioles, and young stems of Platanus are initially soft to the touch because they are covered with dense, branched hairs that look like microscopic trees. These diagnostic, nonglandular, "dendritic" hairs die and may become rubbed or broken off, but persist especially on the blade underside. Leaves on the same or neighboring trees can vary greatly in hairiness. Stems most easily shed the hairs as the first layer of bark (periderm) forms, so that on the chestnut-brown internodes hairs are gone and tiny grayish white speckles-the gas-exchanging lenticels-form as minute blisters in the young bark.

The nature of older bark is another distinguishing feature of plane trees. What begins as a tight, continuous bark cover with lenticels, on large branches and the trunk becomes thicker bark that eventually flakes off (exfoliates) in plates of various sizes. The thin bark plates are hard but not tough, being composed largely of thick-walled, lignified stone cells. Exposed patches of younger inner bark are chalky white or yellowish, but as they age patches become progressively darker via chemical changes of the bark cells. This produces the characteristic mottled appearance of the tree trunks and large branches. On very old trunks, bark may persist for years, so that it becomes thick, brown-gray, and cracked.

All sycamore species are monoecious, meaning that they produce separate male and female flowers on the same plant, although individual plants occasionally are mostly either male or female. These unisexual flowers are produced in conspicuous, balllike clusters (globose capitula or heads), with tiny, nonshowy flowers densely packed around a solid core. Male (staminate) flowers each bear three or more stamens with short filaments; female (pistillate) flowers each bear three or more pistils. Authors have sometimes observed stamens or staminodes on female flowers. The parts of the perianths, i.e., sepals or petals, are indistinct and vestigial, whereas the stigmas and pollen-packed anthers are long. Plane trees are thereby designed for wind pollination, in which copious, lightweight pollen is released and the stigmas are adapted to provide maximal exposure to intercept the pollen grains being carried by the wind.

The unisexual heads of flowers are produced on pendulous stalks (peduncles) and thereby hang below the leaves. During early spring, an axillary bud will begin growth by forming a relatively short shoot with two to several foliage leaves and terminating in the inflorescence. Although I have not investigated this, it is likely that the inflorescences and buds of the flowers are already formed late in the growing season of the previous year and remain preformed within the bud during the inactive fall and winter months. Walnuts and pecans, as well as other wind-pollinated deciduous trees, have the same method of preforming their flowers.

One way to tell the species apart is to count the number of female heads per inflorescence. The highest number-10 to 12-occurs in P. kerrii, and some of the oldest fossils of Platanaceae also had many heads. California and Arizona sycamores tend to have typically four to six heads (three to seven, but inflorescences rarely can have nine); P. orientalis has three, rarely two; P. occidentalis has one, rarely two; and the interspecific hybrid Platanus x acerifolia typically is intermediate between its parents with two, rarely three. When three or more female heads occur per stalk, the peduncle is zigzag, and each head may be borne on a short stalk. Male heads are 8 to 15 mm in diameter and less than half that of the female heads when the flowers are just opening. Male inflorescences shed pollen and then fall off the branches while the female inflorescences remain firmly attached.

After a pistil is pollinated, a single seed develops within the ovary, which becomes a skinny dry fruit termed an achene. From the bases of the ovaries grow long, cinnamon-colored hairs. Fruits elongate and fattened, forming a dense, hard head with perhaps 1000 achenes. Fruiting heads in the various species are 20 to 40 mm across. As the fruits in the head finally ripen, the achenes are pushed outward when their bases become separated from the core, the long hairs are exposed, and the lightweight packages are then wind-dispersed via these parachutes.

So commonly cultivated in Southern California are Platanus x acerifolia and P. racemosa that anyone interested in horticulture should be trained to recognize each type. California sycamore is most easily identified when its leaves have five long, distinct lobes extending more than half the length of the blade; blades are 10 to 20 centimeters long and slightly wider, although occasionally you may find a leaf greater than 30 centimeters across. The fruit heads of P. racemosa are formed most often in zigzag chains of fours, fives, and sixes-the specific epithet describes this chain of heads but, in reality, is a misnomer because the structure is not a true raceme. The fused stipules, which generally persist during the growing season at each node, look like miniature leaves several centimeters wide, often with teeth like the foliage leaves, but I have seen these up to six centimeters wide. Their bark characteristically is chalky white to dark gray and exfoliates in fairly large patches, but bark features vary too much for generalizations. A small minority of the specimens tends to have leaves with mostly three lobes that are indented less than half the length of the blade. MEMBG has one California sycamore specimen having leaves with only three lobes and pendent inflorescences with only three fruit heads. Platanus racemosa often grows taller than 25 meters. Landscape architects sometimes favor planting this species in informal woodlands and to lean slightly, because, when it forms its spreading lower branches, the trees thus create a more natural ambiance, resembling the sycamore stands along streams and in canyon bottoms. In the Santa Monica Mountains, you could see such a stand on a hike, say in Big Sycamore Canyon or Sycamore Creek campground!

The London plane tree is grown as a smaller and always upright tree. Its leaves are broader than long (only 5 to 10 centimeters in length), with only three shallow lobes and scalloped with broad teeth, and the stipules are not long persistent and not very leaf like. There are only two spherical fruit heads on the majority of inflorescences. This interspecific hybrid has a more formal look with a canopy that can be perfectly conical in outline, which is why it can be used as a very uniform street tree, and why it works well in formal alley landscapes. The bark tends to be drab and more olive and purple-gray than in P. racemosa, and flakes off in smaller pieces. The leaves and twigs of Platanus x acerifolia are more highly resistant to diseases than are those of P. racemosa, but neither requires regular pruning to keep them neat and healthy, unlike figs and eucalypts.

Sycamores grow rapidly and can live a very long time. Horticulturists have documented cultivated specimens of both the American and oriental plane trees living 400 years. The maximum diameter of P. occidentalis is listed as 3.5 meters, and individuals 50 meters tall have been recorded, making this the tallest indigenous deciduous tree species of temperate North America. In Southern California, well-watered specimens of our native species can achieve a height of 30 meters in less than a quarter of a century. The tallest giants have straight trunks and hard but relatively flexible wood, whereas California sycamore tends to have slightly leaning trunks and spreading branches. The term "plane" may refer to "timber" and the use for flexible lumber, although no plane tree has been really important for building material and more frequently was used for veneer (e.g., lacewood).

A few words should be said about Platanaceae, for which we have a rich fossil record dating back about 115 million years (Aptian of the Lower Cretaceous). Leaves, fruit heads, and pollen grains of this family, strikingly similar to living plane trees, have been preserved in sedimentary rocks around the Northern Hemisphere. These fossils demonstrate that the forests covering North America were formerly continuous in both directions, to Asia and Europe. But the North American species have been geographically isolated from those is Eurasia for tens of millions of years, so it is remarkable that U. S. species are still capable of producing perfectly fertile hybrids when crossed with Eurasian species. This is why the origin of Platanus x acerifolia is so widely publicized in textbooks on evolution. And that's the plane truth.


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