Plants truly became trees when two innovations coincided: woodiness and axillary branching, a continuation of growth from buds that arise in the angle between leaves and stems, leaves being modified lateral branches anyways. Before axillary branching, branching took place only at the apex, and this very much restricts the forms that can result, but axillary branching allows new modules, iterative growth, and this and only this produces a true trees, although clubmosses and ferns produce, or did produce, a type of arborescent growth. Hmm, this is very hard to explain without a lot of diagrams, a lot of fossils, and quite a bit of knowledge of plant anatomy. But sometime in the late Devonian, true trees appeared, and they appeared before seeds!
Here's another way to think about it. If you cut the top out of a coleus, all the axillary buds begin growing and the plant bushes out as all the new modules grow and develop, with the modules producing new modules. But if you cut the top out of a tree fern, or a cycad, or some conifers, well, that's basically it. It cannot produce new modules because there are not lateral buds to produce new modules, no interative growth. Sorry, too tired to figure this out any better. But it was a good talk, although quite technical.
9 minutes ago in Variety of Life