Mystery Plant! #756
By John Nelson
When clouds appear, wise men put on their cloaks;
When great leaves fall, the winter is at hand.
Shakespeare, Richard III: II, 3.
Can we talk about leaves?
Your botany professor probably waxed on and on about leaves, and how important they are for plants. He or she would have explained that leaves, as we commonly know them, are actually highly modified, flattened stems containing plenty of vascular tissue which is continuous with the interior of the branch upon which they are seated. Leaves have quite a number of functions, depending on the species involved, and the most important function, all around, is that of photosynthesis. Don’t worry: we aren’t going to get into the details of this complex set of physiological pathways. But it’s important to know that the process of producing carbohydrates, and oxygen as a by-product, takes place in leaves, at least with land plants. (Plenty of photosynthesis takes place in the oceans, of course, as a result of photosynthesis in algae. In fact, most of the oxygen that is present in the atmosphere comes actually from marine algae. And, all of the oxygen that we breathe comes from photosynthesis. Hooray for plants!)
Now, the leaves that we see on the trees and plants around us typically are composed of a single flattened blade, plus a stalk; the stalk is called a “petiole”, and it may be long and obvious, or very short and practically absent. Such a leaf, that is, one with a single blade, is what we call a “simple” leaf. Most of the leaves around us are simple, and for examples we have oak, magnolia, and holly. Or the blade may be variously divided into a number of divisions, or leaflets. The leaflets themselves may further subdivided. These leaves are said to be “compound”. A few examples of compound leaves would be those of hickory, horse-chestnut, or mimosa.
This week’s Mystery Plant produces a simple leaf with a short petiole. The tip of the blade is a bit pointed, and the base of the blade is a bit asymmetrical. The margins of the blade are characteristically toothy. (As you might expect, we botanists have come up with long lists of special words to describe these features. Never mind all that right now.)
All summer long, the leaves are bright green, happily doing their photosynthetic thing, until changes in day-length act as a signal for leaves to give it all up and drop off, but not before turning this beautiful gold color. This particular species is a small tree, native to China and Korea, commonly grown now along sidewalks in urban settings, providing attractive foliage and shade. Most people like it especially for its beautiful bark, which is delicately lacy, and quite attractive. The plants will produce plenty of one-seeded fruits, and these will sprout if given a chance. So, although beautiful, this tree is somewhat invasive. Where I live, we commonly see little seedlings growing here and there near the parent trees. And there are a few of these leaves left, the ones that have escaped the blare and racket of the insufferable leaf-blowers.
[Answer: “Chinese elm,” Ulmus parvifolia]
John Nelson is the retired curator of the Herbarium at the University of South Carolina, in the Department of Biological Sciences. As a public service, the Herbarium offers free plant identifications. For more information, visit www.herbarium.org or email email@example.com.
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