Groover points to wood, surely a defining feature of trees, as a case in point. “True” trees (we’ll get to that later) make wood through what scientists call secondary growth; this allows trees to grow out (thicken), in addition to growing up. Secondary growth emerges from a ring of specialized cells that encircle the stem. Called the vascular cambium, these cells divide in two directions: toward the outside of the tree, yielding bark, and toward the center of the tree, yielding wood. Year after year, this wood is deposited in new inner rings of growth that are doped with cellulose and the long, rigid polymer called lignin. After this cellular stiffening, the wood cells are killed and dismantled, for the most part, until nothing but their rigid walls remain.
In plants that exist today, secondary growth probably had a single evolutionary origin, although the now diminutive club mosses and horsetails invented their own version some 300 million years ago, enabling the extinct Lepidodendron, for example, to grow more than 100 feet tall. But secondary growth doesn’t automatically lead to tree-ness: Despite that single origin, woodiness pops up scattershot across the plant family tree. Some groups of plants have lost the ability to form wood; woodiness has reappeared in lineages where it had vanished. It seems to evolve fairly quickly after plants colonize islands. Hawaii, for example, has woody violets, and the Canary Islands have dandelion trees.
The very concept of woodiness is quite flexible, belying its literal robustness—think of the stiff stems of garden salvia or lavender. It’s not a matter of present or absent, but a matter of degree. “Nonwoody herbs and large woody trees can be thought to represent two ends of a continuum, and the degree of woodiness expressed by a given plant can be influenced by environmental conditions,” Groover and a colleague write in a 2010 review in New Phytologist. “Indeed, the terms ‘herbaceous’ and ‘woody,’ while practical, do not acknowledge the vast anatomical variation and degrees of woodiness among plants variously assigned to these classes.”
Molecular biology offers some insights into why the ability to make wood is maintained and reappears so often in plant evolution. Genes that are involved in regulating the growing shoot—the upward, “primary” growth of trees and non-trees alike—are also active during the secondary growth that yields wood. This suggests that these already-existing and essential shoot-growth genes were co-opted during the evolution of woodiness. And it might explain why the ability to become woody is maintained in nonwoody plants and why it’s relatively easy, from an evolutionary standpoint, to dial woodiness back up.
That said, you don’t need wood to be a tree. Monocots, an enormous group of plants that lost the ability to undergo secondary growth, have several arborescent members that aren’t “true” trees but sure look like them. Bananas grow tall with what appears to be a trunk but is really a “pseudo-stem” mass of tightly packed, overlapping leaf bases or sheaths. The true stem of a banana plant emerges only when it’s time to flower, pushing itself up and out through the leaf sheaths. Yet banana trees can be more than 10 feet tall. The family of palms, also monocots, grow tall by extending their initial, fat shoot topped by an enormous bud (note that palm stems don’t widen as they grow tall).