THE ROLES OF PRIMARY VERSUS SECONDARY GROWTH DURING THE EVOLUTION OF INCREASED SIZE IN VASCULAR PLANTS

Maximum land plant height increased by three orders of magnitude over the mid Paleozoic. Dramatic increases in size allowed for greater surface area of leaves and other tissues dedicated to photosynthesis, greater spore production and dispersal capacity, and deeper rooting, but also involved important tradeoffs. Increases in plant height require increased investment in axial diameter for adequate vascular transport and biomechanical support. In modern seed plants, large axial diameter often is accomplished over time via secondary growth of wood, thus allowing exploration of the environment with thin branching stems and subsequent production of a significant amount of wood only going to a small subset of the axes. However, during the early evolution of vascular plant form, increases in plant size could be entirely dependent on primary growth—requiring upfront investment in tissue production and slower rates of axial extension—even in plants that did have the capacity of wood production. Understanding the evolution of the distinct but related traits of primary axial diameter and secondary wood production is complicated by the tendency of wood production to obliterate external primary tissues and thereby prevent direct documentation of the original primary diameter of the axis. However, primary xylem and pith can still be preserved intact since internal to any secondary xylem production and can serve as a minimum bound on axial thickness—just the pith of many Carboniferous plants can be wider than the full axis of most Devonian plants. Here, we use the diameter of these internal tissues to trace dependence on primary versus secondary growth over the evolution of plant form through time.