5 - Evolution and Diversity of Woody and Seed Plants

Abstract

The lignophytes, or woody plants (also called Lignophyta), are a monophyletic lineage of euphyllous vascular plants that share the derived features of a vascular cambium, which gives rise to wood, and a cork cambium, which produces derived independently in these taxa, being unifacial. Growth of the vascular and cork cambia is called secondary growth because it initiates after the vertical extension of stems and roots due to cell expansion (primary growth). A vascular cambium is a sheath, or hollow cylinder, of cells that develops within the stems and roots as a continuous layer, between the xylem and phloem in extant, eustelic spermatophytes. The cells of the vascular cambium divide mostly tangentially (parallel to a tangential plane), resulting initially in two concentric layers of cells. One of these layers remains as the vascular cambium and continues to divide indefinitely; the other layer eventually differentiates into either secondary xylem = wood, if produced to the inside of the cambium, or secondary phloem, if produced to the outside. Because layers of cells are produced both to the inside and outside of a continuously generated cambium, this type of growth is termed bifacial. Generally, much more secondary xylem is produced than secondary phloem.[Note that a secondary cambium independently evolved in fossil lineages within the lycophytes (e.g., Lepidodendron) and equisetophytes (e.g., Calamites), but this cambium was unifacial, producing secondary xylem (wood) to the inside but no outer secondary phloem, likely limiting in terms of an adaptive feature.