All around Philadelphia, and in many other cities as well, the streets are lined with London Plane trees. These trees, with their trunks of exfoliating bark making them look to be covered in military camouflage, are recognizable from a hundred yards away. Or so I used to think, until I looked closely at the confusion and complexity that surrounds this seemingly simple and so common tree.
The London Plane is of hybrid origin – it is the offspring of two different species, the American sycamore (Platanus occidentalis) and the Oriental plane (Platanus orientalis), and it is a tree that did not exist prior to European colonization of the new world. Before then, American sycamores and Oriental planes were kept separate by an ocean (the Atlantic if you’re going east from the US, or the Pacific if you want to go all the way around the other way), and they didn’t come together until the 17th century, when John Tradescant the Younger, a botanist and gardener (as was his father – John Tradescant the Elder, that is), came to the colonies in the early part of that century and in 1636 took the American sycamore to England. There were also Oriental planes in England at that time (they’re originally from the more eastern parts of Europe), however though the geographic distances that had previously kept the orientalis and occidentalis separate were gone, still for some time after 1636 age differences would have maintained that separation, as it takes time for trees to grow, and to make flowers, and to shed pollen, and to make new seeds for new plants. And so for some time these different plants, though now together after having been so long separated, would still have remained distinct from one another.
By 1700, though, east had met west. We know this because in that year Leonard Plukenet, yet another English botanist, described an intermediate between the Platanus species that were already known (i.e., orientalis and occidentalis), and so from this we can infer that the London plane had arisen by then, an intermediate that looked like what it was, a hybrid cross of two morphologically distinct species. How, when, where and why this happened, we do not know exactly, except that it was in England in the latter part of the 17th century (the latter part because it would have had to have been long enough after 1636 to have allowed for the seeds Tradescant brought back to have grown into seed bearing trees themselves). Other than that, we can’t say with more detail, with any confidence at least, when this hybridization event occurred.
It also isn’t clear when this hybrid, the London plane, came to the US (or one could say semi came back to the US, since half its genetics is from the new world) and to understand why, you have to understand the plants.
London planes look quite a bit like their parent species, as you might expect, since that’s how genetics works (like begets like), and so it can be difficult to differentiate them. This is something I learned recently – I had thought for years it was easy to tell them apart, to differentiate the London plane from the American sycamore. But, as with so many things, I realized that the closer I looked, the more complicated it became.
There are, broadly speaking, four main characters that are useful for telling these trees apart, the London plane and the American sycamore. Two of those characters involve the bark. On London planes, the military camouflage appearance mentioned above, in the first paragraph, is found up and down the trunk of the tree – on American sycamores, the bark is thick and rough up most of the main trunk of the tree, only exfoliating (peeling away) further up. And American sycamores generally have a white, and commonly bone-white, tone to the underbark that is exposed from the peeling away of the outer bark layers, while the London plane’s underbark will have a yellow or green tinge (or sometimes even a salmon orange color, as is found on a tree in Pastorius Park in Germantown, here in Philadelphia).
The leaves also differ – the leaves of both trees are lobed, and in the London plane, the center lobe is longer that it is wide, whereas the sycamore’s is generally wider than it is long. A final character separates the two – the flowers (and then later, of course, the fruits) are borne on dangling peduncles (what we might call “stems” in normal English), and in American sycamores these are borne singly, while on the London plane they hang in twos and threes.
Something else also differentiates these trees – their habitats. If you’re in the floodplain of a river or creek and you see a tree that looks like an American sycamore, it probably is. If you’re on a city street, and you see a tree that looks like a London plane, it most likely is. London planes are phenomenally sturdy street trees – they’re called “London planes” because they are and were so common throughout London, and while yes this is due to their attractiveness, it’s also because they were able to grow in the soot filled air of 19th century cities, and so became exceedingly popular, especially in London with its thick industrial era atmosphere. Since its introduction to urban life, this tree has had times of peak interest, and times of reduced interest, as is noted in William Flemer III’s article “Island and Median-Strip Planting”, in Arnoldia [vol. 44(4), pp, 14-28 (Fall 1984)]: “The London plane tree has gone through several cycles of popularity and disapproval. Many years ago a few nurserymen grew the trees from seed that produced great variation in habit of growth and disease resistance and this may be one cause for the disapproval. Another may be the plane tree’s vulnerability to canker stain disease, a serious condition spread by pruning tools or other mechanical means. The severity of the disease once led the city of Philadelphia to enact ordinances that prohibited planting the tree.” But, I should add, even though the London plane has had its ups and downs in popularity, it still, generally, will do better in an urban environment than the American sycamore will. This habitat differentiation is noticeable if you go to the Powell house on 5th Street in downtown Philadelphia – there is a London plane planted right next to an American sycamore. The London plane is doing quite well, happy as can be – the sycamore, much less so.
Another habitat in which one seems to often encounter American sycamores is graveyards. There’s an enormous sycamore in Greenwood Cemetery, in Frankford, and another one in the churchyard/graveyard of St. James the Lesser, which is up near Laurel Hill Cemetery – there’s also some planted in the Palmer Burial Ground in Fishtown, and quite a few planted in West Laurel Hill, just over the city’s border, in Bala Cynwyd; there’s also an enormous one in the Germantown Friends burial ground that you can see easily and clearly from Germantown Ave, if you’re standing just a bit west of Coulter St.; there’s also one in the Friends Meeting House yard at 4th and Arch – there used to be a cemetery there, too. [there’s also a very large American sycamore at the Germantown Cricket Club, next to the parking lot] The symbolism of this makes sense, planting sycamores in Christian cemeteries. In the gospels, e.g., in Luke 19, when Jesus is going through Jericho, Zacheus, the chief tax collector in town, climbs what is called a sycamore to see Jesus as he is walking by – Jesus sees Zacheus up in the tree and calls to him, and Zacheus then “received him joyously”, as it says in the King James Version. Now, this tree from the bible is said to have most likely been a sycamore fig, and certainly was not the sycamore that we have in the US, but the symbolism, one might imagine, would still work and would be effective, because in a graveyard in which the dead await the return of the messiah, one would want a tree for them to climb up on Jesus’s return, so they can see him, and perhaps more importantly, be seen by him.
American sycamores can also grow to enormous size – in the early part of the 20th century, the American Genetic Association put out a call for photos of and associated data for the largest trees in America. They published results of this in 1919, in the Journal of Heredity, and the largest tree by far was an American sycamore in Worthington, Indiana – it was 42 1/4 feet in circumference. London planes can grow quite large, but I’m not aware of any that come close to that kind of size.
However, even though there are a number of differences between these plants, it can still be difficult to tell them apart. Why? Because even though the London plane is of hybrid origin, it still makes viable seeds, and those seeds are enormously variable, because they contain the genetics of both the parent species, in every combination available. Also, the London plane can back cross with its parent species, thereby further mixing up the characters. And so, many of the trees I see in Philadelphia that I once would have easily called “London plane”, I’ve come to notice often have a mix of the characters with those of American sycamore – and so separating them isn’t so easy.
This all helps to make it difficult to figure out when the London plane came to the US – if we find it difficult now to separate them, it’s not like it was any easier in previous decades or centuries, and trees identified as Oriental planes and American sycamores in old documents quite easily could have been misidentified London planes. William Hamilton, owner of the Woodlands and an avid plant collector who sent numerous plants over from England in the 1780s, when he wrote to his gardener in 1785, he mentioned he had sent over 12 plants of “platanus orientalis”. This may well have been London plane trees. Or it might not have been. We will most likely never know, because absent the plant itself, the name alone just doesn’t tell us for sure what it is. This is the case for listings in numerous nursery catalogs (including Meehan’s) – Platanus orientalis, for example, may be listed for sale, but we just can’t be sure if that was what they were selling, and so tracking the movement of the London plane is difficult, if not impossible, to do. An example of this confusion can be found in an article in the magazine “Park and Cemetery Landscape Gardening” from 1916, where, in an article enumerating “Trees for Adverse City Conditions” includes “Platanus orientalis (The Oriental or London Plane)”
And even the plant’s name is confused and confusing. The latin name most commonly used for the London plane is Platanus x acerifolia (the “x” indicates its hybrid status), however, another valid name is Platanus x hybrida – this confusion arises because we don’t know which name, hybrida or acerifolia, came first, we don’t know which was assigned before the other. This is important to botanical taxonomists (i.e., people who name plants) because sometimes a plant is named more than once; for example if someone thinks they’ve discovered a new species, and they describe it and name it, but it turns out that they just didn’t know that someone else had named it before, well that first name that was applied previously is the one that is supposed to be used, and not the later one. This often simplifies things because the choice is based simply and solely on date of publication of the name of the species. However, it’s not always easy to figure that out, and this is one of those cases – while we know that both of these names for the London plane were assigned in 1805 (hybrida by Félix Brotero, a Portuguese botanist, and acerifolia by Carl Willdenow, a botanist in Berin), we don’t know exactly when within that year these names were used, and so priority is confused, and so are we.
And there are further questions of nomenclature here – due to the backcrossing with the parent species, and because plants grown from seeds that are progeny of a hybrid are not, strictly speaking, hybrids themselves, it furthermore becomes complicated as to whether or not many of the London planes we see should truly be called hybrids, and therefore include that little “x” in there.
The confusion doesn’t end there. Due to that Brotero name, “hybrida“, one might think that the hybrid origin of the London plane was clear and understood early on, or at least by the earliest part of the 19th century. But even this wasn’t clarified until much later, in 1919, when Augustine Henry and Margaret Flood thoughtfully marshaled the evidence – intermediate morphology that is between the two parent species, highly variable seeds that show traits of both parent species in various combinations – in a paper in the Proceedings of the Irish Royal Academy that clearly indicate that yes, the London plane is a hybrid of the trees from two continents. So perhaps the confusion does end there, at least for that one question, and at least for now.
To read about the importance of cemeteries to urban planning, see here:
To read about some other local Platanus, see here:
To read about some other trees, see here: