There are white flowers blooming all over Philadelphia now. Some of them belong to the Amelanchier, also called shadbush, or serviceberry. Some of those white flowers are magnolias. Some are cherries, and there are others, too, but most of the ones that you’ll see at around this time are the blooms of the Callery pear (Pyrus calleryana), with their copious flush of white and their peculiar acrid smell.
If you look closely at them, one thing that you’ll notice is that nearly every set of flowers is accompanied by its own set of leaves. Look closely, and poke around in the inflorescence (a cluster of flowers, that is) and you’ll see that each individual flower is connected to a little stem, in a somewhat spiral arrangement, winding down the axis, or winding up it, depending on which direction you go. Each one of those flowers is accompanied by a little tiny leaf, or a leaf scar to show where a leaf once was – each one repeats the other, in modular form, winding around the stem, up and down the line. Except all the way down the line. At the lowermost position on the inflorescence, the one closest to main part of the tree, there is almost always a set of a leaves, a little tuft of green, instead of a flower.
Why are those leaves there and what are they doing? It could be that they provide a local food source for those flowers, early on in their development – those photosynthesizing leaves harvesting energy from the sun and gathering carbon dioxide from the atmosphere, twisting the two into sugar, and shipping that sugar right to the nearby flowers, flowers that need energy to grow and to breed, these leaves open up when the flowers do, since they are in there in the bud with them, and they pop out along with the flowers, and would provide sugars just a short distance away, and at the right time. It’s expensive to ship sugar around a tree, as it takes energy to move things, and so doing it locally, right next to the inflorescence, instead of having the leaves come up some distance away and transporting the food that some distance, it would just simply be more efficient to do it closeby. By having the leaves right next to the flowers, the source right next to the sink, this would save precious time and energy, always a valuable set of resources, but resources even more valuable in the spring, just after the trees have woken up and are drawing down their stored energy supplies, supplies that they packed away last year and have only in limited amounts. And by having those leaves right there in the bud, good timing is assured – no need to wait for the leaves farther away to open up (which they will later) when you have your own right there in the package with you. And so those little tufts of leaves at the bases of the inflorescences are quite likely feeding their neighboring buds and blooms.
And also, those leaves could be beneficial to the tree by drawing up water, pulling it towards the flowers, who need to drink, too. The way that water moves up a tree is, basically, that the water evaporates from the leaves through little holes, called stomata; this evaporation draws the water up through tiny pipes, called, collectively, xylem, in a somewhat similar way to how water goes up through a straw when you draw on it as it sits in your waterglass. In fact, you can see how this works by taking a drinking straw and putting it entirely underwater so it fills with water, then taking a piece of a cotton ball and jamming it tightly into one end of the straw and then taking the entire contraption up and out into the air. Then, place it, cottonball end up, into a glass with more water in it, but this time with water that has a little bit of food coloring in it. Then wait. Or just come back later. Either way, after a while, you’ll see the dyed water migrating up through the straw, ultimately dying the cotton ball whatever color you have chosen from your pack of food dye, that you used to stain the water in which the straw is sitting. This illustrates, essentially, how water moves through a tree – the water evaporates from the leaf (in our experiment, that’s analogized by the cotton ball), which pulls it through the xylem (the straw, in our table top set up), which in turn pulls the water out of the soil (which is represented by the glass that holds the tinted water). And so, if leaves are situated next to the flowers of the inflorescence, which they are in the case of the Callery pear, then they would be drawing up water, water that could be shared with the petals and the sepals, the anthers and the stigmata, the parts that are the hopes and dreams for the next generation of trees.
But whyever they are there, the fact remains that those leaves are there, and they are here and there throughout Philadelphia, along streets and in parks, pretty much in every neighborhood.
But how did they, and their accompanying trees, get here to Philadelphia? The Callery pear tree is originally from east Asia, and since trees can’t walk, and nor can they swim, they had to have had some help in getting to North America, and their main agent of dispersal, the organism who got the seeds of the Callery pear to the new world, was Frank Meyer.
Frank Meyer was a Dutchman, and he was also a botanist. Towards the end of the 19th century, Meyer worked as the head gardener in the experimental garden at the Amsterdam Botanical Garden, but this wasn’t enough to satisfy his yearnings for travel and his love of plants, and so he came to America in 1901, where he soon began to work for the US Department of Agriculture as a plant explorer. One of his missions, towards the end of his career, was to collect the Callery pear.
Meyer wasn’t the discoverer of the Callery pear – that credit goes to the mid-19th century French missionary, Joseph Callery. Nor did Meyer introduce it into cultivation – that credit goes to E. H. Wilson of the Arnold Arboretum. However, what Meyer did was to send back bags of seeds, in 1917, from China, of the Callery pear, so that this plant could be introduced into cultivation by the USDA, thereby leading to its widespread dissemination. Why was he told to collect and send back those seeds? Well, at that time, the US pear industry was being decimated by fire blight, a bacterial disease, and it had been found that the Callery pear was resistant to this disease, and so could be used in breeding programs and for rootstock. This program was quite successful, and when it was also found that the Callery pear can live under some difficult conditions, like a fair bit of drought, the Callery pear became commonly used in orchard production. (fire blight is still a problem in pear orchards, by the way: http://www.ars.usda.gov/is/pr/2015/150130.htm )
And then, some decades after that initial delivery of seeds by Frank Meyer, USDA researchers in the 1950s noted that the Callery pear also had nice flowers and this, combined with its resistance to environmental stress, meant that the Callery pear was realized to be a tough and attractive street tree, and it rapidly became extremely popular, which is why we see it all over Philadelphia. But it most likely wasn’t just the tree itself that led it to be so widely planted. The timing of this work was probably key, also, to the widespread spread of the Callery pear.
These trees, the Callery pears, came in and replaced, in part, the American elm – the American elm was the premier tree of parks and streets and so many other landscapes throughout the US, up until the midpart of the 20th century. Its vaselike structure, with branches growing up and out, made for tremendous arboreal arches that gracefully covered many streets in small towns, and big cities, and other places, too, until the Dutch elm disease knocked out this elm, and many others like it, taking it out of commission as the street tree of choice, requiring a replacement, many replacements actually, to fill the elm’s ample metaphorical shoes. The Callery pear was one of those trees, and it became extremely popular starting in the 1960s as a tree for the street habitat, a niche that had been opened up by the Dutch elm disease.
If you see a row of Callery pears, what you’ll often notice (I know that I do) is that they frequently will all have a spraylike branching pattern, where the trunk will grow nice and straight up until about 4 or 5 or 6 feet or so, and then there will be a division of that upright axis, where branches will grow angled outwards, kind of like an upside down broom. You might think that this is very thoughtful of the tree, to make such an eyepleasing design at about eye level, reminiscent of the form of the American elm. However, plants don’t think, or at least I don’t think they do, and this shapely pattern is due to the hands of man.
If you cut the top off a tree, it generally will not die. It will however, grow quite differently than it would if it had not been decapitated. Branches that were formerly suppressed by hormonal signals sent out from that top leading growing tip can now grow, and they do – and sometimes all at once. This is what happens with the Callery pear. If the top of the tree is removed, you then get a spray like pattern to the branches that grow to replace that lost top.
And so, if you walk down a street, for example, 19th Street alongside the Academy of Natural Sciences, just below Logan Circle, you’ll see that at about shoulder height, the branches of the Callery pears begin to spread, and you’ll know that at some point in time, someone walked among those trees, cutting them at an easy height to reach, about shoulder height, to make those Callery pear branches angle like they do now, an angle reminiscent of an American elm.
People don’t plant Callery pears quite so often anymore – their branches tend to break, and there are concerns about its invasiveness. This tree, which is one of the few major introductions of the 20th century to our street tree flora has had its time in the sun. It was introduced, it had a population boom, and now we will see where it will go in the future, what part it will take among the street trees of Philadelphia and elsewhere.
Note: the Callery pears in Philadelphia are just now hitting there full bloom on the 13th of April 2014, and they are in fully bloom in NYC, too: