More Woodiness/Less Woodiness: Evolutionary Avenues, Ontogenetic Mechanisms

Early angiosperms were minimally woody; increase in woodiness and changes in wood histology yielded trees, lianas, and shrubs in various clades. Many eudicot herbs have been derived from variously woody ancestors. Some of those derivatives have, at various stages, evolved secondary woodiness to various degrees. Categories of information by which we can trace these progressions are presented: length-on-age curves for vessel elements, perforation plate morphology, ray histology, DNA-based phylogenies, geological and ecological factors, dispersal capabilities, and speciation ability. Trajectories that angiosperms have followed are analyzed in terms of growth forms: sympodial habits, cane shrubs, lianas, trees, various herb-related forms, stem succulents, and plants with successive cambia. Phylogenetic modalities that are related to degree of woodiness are discussed: retention of and departure from juvenile wood features in basal angiosperms, overlay effects (additive or modifying effects of factors on woodiness), character independence and interdependence, and degrees and types of transitions between more woodiness and less woodiness. Production of procumbent ray cells (which excel at radial conduction) is the result of not just subdivision of ray initials but also infrequent tangential divisions in ray initial derivatives. In juvenilistic woods, this process runs in parallel with shortening of fusiform cambial initials, but in woodier species, fusiform cambial initials become longer over time whereas ray initials become vertically shorter. Examples and original information on eudicot woods are mostly from orders and families of the campanulid clade. Juvenile features are multiple, with each capable of being retained, modified, or lost independently. This article takes the form of an eclectic essay that includes original data and observations, hypotheses, and critiques as well as presenting questions and syntheses, and it supplements previous articles by the author.