Hello Internet! I’m writing again for all of you curious tree people after the holidays. When last I left, you were in your living rooms examining your Christmas/non-denominational holiday trees as a guide for how tree anatomy works. Toward the end I pulled the rug out from under our tropical and sub-tropical readers when I revealed that palms aren’t actually true trees at all.
Palm trees are pretty sneaky. They have a number of tree-like characteristics. They’re tall and have a main stem resembling a trunk. The stem is woody and tough. Put enough palms together and you can have lovely palm forests. If you look down at the roots or take a close look at a palm’s leaf-like fronds or cut at a palm with a sharp object you’ll notice some distinct differences.
Look up at a palm tree and you’ll see that the growth isn’t like other trees. The canopy of the palm is restricted to the crown of frond leaves surrounding the apical meristem. Lateral growth and meristems don’t emerge. If a second apical meristem emerges it leads another trunk with another crown of fronds. This gives palms a distinctive, paint-brush shape. The single growing, meristem is surrounded by overlapping leaf bases.
Think back to the previous article where I told you to risk certain poking at the needles of your Christmas tree to reveal the green, cambium layer under the bark. If you did this with a palm tree you wouldn’t find a green layer. Palm trees do not produce cambium. In fact, palms are incapable of the ring-shaped secondary growth seen in other trees. Cut a palm tree down and you’ll see tiny circular vessels distributed evenly throughout the trunk. These are vascular tissues, xylem and phloem. Stem cells lining these vessels produce “anomalous secondary growth” to thicken young trunks but once the trunk reaches its maximum diameter this no longer occurs. This keeps palm trunks narrow and stiff, perfect for supporting their frond crowns.
The drawback is that injuries to palm tree trunks can never heal. Without a cambium to direct closure over a wound, fungi, insects and other pests can easily penetrate wounded trees. The “bark” of the palm tree is not bark at all; it is made of “sclerified” (hardened) cells left over from the bases of previously shed fronds. This makes a palm not unlike a column of reinforced concrete with the vessels acting as rebar.
Palm leaves are also different. They emerge as primary growth from the meristem. The youngest leaves are at the top of the leaf crown. As the palm grows taller and older more leaves are added to the top and older leaves grow larger until they hit their maximum size. When the leaves reach the base of the crown they are cut off from the vascular system, “abscising” their bases into new bark. Some palms don’t drop their leaves cleanly, resulting in “skirts” of dead leaves that dangle below the crown. Growing like this means that you can predict which leaves will drop from a palm tree, prune them, and collect them before they fall.
The beautiful strangeness of palms can be traced back to the way palm seedlings grow. When a palm first starts growing it doesn’t have a full set of leaves. Roots grow out from the tiny, seedling trunk. Each root adds new vascular bundles, widening the trunk and providing more vascular tissue to support leaves. These roots form a shallow, fibrous network, not unlike the roots of grasses, onions and bamboo.
Unlike other trees which grow the root system and the shoot system at the same rate, a palm tree rapidly expands the root system so that it can expand its trunk and begin to grow upward. In essence, a palm tree grows in the same way that you might build a house. It establishes the foundation and then builds the upper stories.
Why are palms this weird? As it turns out, palm trees belong to a family of group of plants called monocots. Monocots are one of the eight main groups of plants within the angiosperms, the flowering plants. In an earlier blog post I briefly mentioned that angiosperms began to dominate the earth’s plant species at the end of the age of the dinosaurs, replacing conifers as the largest plant group. Monocots are the second largest group by species, containing 70,000 members including grasses, orchids and palms. The largest group, eudicot, contains 175,000 species including oaks, apples, maples, sunflowers and nightshades. Most “true” trees are eudicots, the rest are conifers ( ie: pines and their cousins).
Palm trees are preserved in some of the earliest flowering plant fossils that we have discovered. Palm trees emerged 94 million years ago when dinosaurs walked the earth. Some species, such as Nypa fruticans and Acrocomia aculeata have been identified in fossil pollen, making the modern species living fossils. This means that palm trees diverged early from other monocot species. Their global spread can be attributed to the breakup of the continent of Gondwana into Africa, South America, Antarctica, India and Australia. The distinctive strangeness of palm trees is tied up in their early divergence from other flowering plants and their rapid (by the standards of geologic time) spread over newly formed continents. Hopefully the next time you pass a palm tree you give it a nod to honor its pioneering, almost-grassy heritage.
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