Living cells in wood. 2. Raylessness: histology and evolutionary significance

Abstract

Raylessness occurs in several hundred species belonging to about 40 families (fewer depending on taxonomic delineation). Fibre distribution (raylessness at first, followed by origin of rays), fibre wall thickness and sclerenchyma at pith margins support the idea that rapid acquisition of mechanical strength is basic to most instances of raylessness. Raylessness may be the most readily available process for achieving mechanical strength in ancestrally herbaceous groups lacking large amounts of phloem and cortical fibres. Raylessness is not a uniform phenomenon and a small number of instances suggest alternative causation, as in two lianas (Cobaea, Thunbergia). Raylessness occurs in only a small number of trees and annuals, but is found in woody herbs, subshrubs and some shrubs. It is indicative of secondary woodiness and wood paedomorphosis. Raylessness would seem to block the radial flow that rays typically provide, but a surprising number of rayless woods have moderately pitted fibres (indicative of flow) and septate or non-septate living fibres. Three-dimensional networks of conjunctive tissues in rayless species with successive cambia (Aizoaceae, Amaranthaceae, Nyctaginaceae) could also provide radial flow avenues. Ontogenetic changes from raylessness to ray presence within the stem of a given species are described and illustrated. Pseudo-raylessness, late-onset raylessness and early-onset raylessness are recognized. Systematic distribution and pertinent literature are given for known instances of raylessness and pseudo-raylessness. Raylessness shows that wood evolution involves not merely change in the abundance and position of cell types, but also redesign and diversification in cell types. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 178, 529–555.