Evolution of trees

A fascinating article came into my mailbox this morning, about the evolution of the first trees. It is fascinating to me (no surprise) because it is relevant to the message of Purpose and Desire: that evolution is a process driven more by homeostasis than gene selectionism.

The paper is by Hong-He Xu and several colleagues, and it is titled Unique growth strategy in the Earth’s first trees revealed in silicified fossil trunks from China. Here is the landing page for the article at the Proceedings of the National Academy of Sciences (USA), which published the article. Here is a news article by Shawna Williams from The Scientist describing the work.

In a nutshell, the paper analyzes the vascular tissues of primitive “woodless” trees. These began to appear in the fossil record about 400-350 million years ago, during the Devonian period. Most people may have learned that the Devonian was the so-called “age of amphibians”, although botanists would label this period the “age of vascular plants.” The vascular plants include all modern plants, which are “vascular” by virtue of the extensive network of tiny tubes that transport materials between leaves and roots. Plants with sap are vascular plants. They, too, came into being during the Devonian. It is then that we saw the first forests.

Without vascular tissue, large plants are simply not possible. Without the associated ability of vascular plants to construct wood around their vascular tissue, tall plants like trees would not be possible. The fossil trees Xu and his colleagues were studying lived at a time when the vascular tissues were developing, but wood had apparently not yet appeared. They studied the vascular tissues of a particular species of a fossilized woodless tree (Cladoxylopsida, if you’re interested).

A couple of noteworthy points. These trees formed short conical trunks. Growth occurred at the periphery of the trunk (as in modern trees). Unlike modern trees (except those hollowed out by wood rot), the Cladoxylopsida trunk was hollow.

Now, I look at a hollow tubular structure, and I think of flexural stiffness (I know …).

Hollow tubes are interesting because they are exceptionally good at supporting heavy loads with little material. This makes them both strong and light. Think of a bicycle frame. The bicycle frame is light because its elements are not solid metal cylinders, but hollow tubes. The frame is strong because most of the load is borne anyway by the metal at the margins of the cylinder—the material that makes up the wall of the tube. Metal in the center bears little load, and contributes nothing but weight—parasite weight, as engineers call it.

Hollow cylinders are also common elements of living load-bearing structures. Think of hollow bones. These are favored for the same reason bicycle frames favor the hollow tube: they are strong and light. They are also favored because they are economical in their use of material. This can confer advantage if there is competition with others. Whichever organism uses material the most economically will have a leg (a twig) up over others that use material less economically.

OK, back to trees.

What Xu and colleagues found in their fossils was evidence of extensive remodeling of the primitive vascular tissues in these trees. The stresses in the trunk were apparently always breaking tubes and forcing the tree to remodel them. This underscores an important point: the vascular network is not a structure but a process. Something similar happens in bones: a bone is not a structure, but an intelligent process (a cognitive process, really), always remodeling itself to bring its shape into conformity with the loads it must bear. This happens in modern trees as well: trunks also remodel themselves according to the loads they must bear. (If you want to learn more, I wrote about this extensively in The Tinkerer’s Accomplice). The extensive remodeling going on in the vascular tissue of Cladoxylopsida suggests a similar process happening there.

If that is the case, it suggests that trees develop the ability to stand tall through a cognitive intentional process: physiology, not gene selection. The interesting unanswered question (and unaddressed by Xu et al.) is whether there is an epigenetic feedback onto the genome from the tree’s lived experience. As such feedbacks are beginning to turn up everywhere, I suspect they will be discovered in the evolution of trees as well.

Bottom line? Trees may indeed have evolved to stand tall because of a literal striving to be tall: purpose and desire, even in plants.

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