Wood Anatomy of Brassicales: New Information, New Evolutionary Concepts

Abstract

Wood anatomical data for the 19 families of Brassicales are presented, based on light microscopy and scanning electron microscopy (SEM), arranged according to recent molecular phylogenetic evidence. Because of large species numbers and diversity in ecology and growth form, Brassicales are an ideal case study group for understanding wood evolution. Features newly reported include vestured pits in Cleomaceae, Koeberliniaceae, Pentadiplandraceae, Salvadoraceae, and Setchellanthaceae. Vesturing of primary xylem helices is shown for Raphanus (first report in angiosperms). Fiber dimorphism is newly reported in some genera of the crown group (Capparaceae + Cleomaceae + Brassicaceae). The fiber-tracheid is probably the ancestral imperforate tracheary element type for Brassicales, and from it, libriform fibers, living fibers (including septate fibers), and tracheids have likely been derived. The Baileyan concept of unidirectional evolution from tracheids to libriform fibers must have many exceptions in angiosperms, and tracheids are not uniform. Tracheids occur in Emblingiaceae, Koeberliniaceae, Pentadiplandraceae, Stixaceae, and Tropaeolaceae. Synapomorphies can be identified, as in the Akaniaceae—Tropaeolaceae clade (rays of two sizes, living fibers, scalariform perforation remnants) and the Moringaceae-Caricaceae clade (ground tissue of wood composed of thin-walled fibers or similar parenchymatous cells). Wood of Brassicales is mostly not paedomorphic, although paedomorphic characters suggesting secondary woodiness occur within the families Brassicaceae (abundance of upright ray cells, raylessness), Caricaceae, Cleomaceae, and Moringaceae. Brassicales are probably ancestrally woody, and wood of Sapindales and Malvales has a number of key character states (plesiomorphies) like those in Brassicales, as would be predicted by current molecular phylogenies. Surveys of large taxonomic groupings, such as Brassicales, tend to yield more examples of homoplasies and apomorphies that can be interpreted in terms of adaptation and functional interlinkage (e.g., ray evolution paralleling imperforate tracheary element evolution). In turn, these features can be interpreted in terms of ecology (e.g., xeric habitats) and growth forms (e.g., tree succulents). The assemblages of wood character information in a reasonably well known order of angiosperms permits hypotheses about wood evolution in angiosperms as a whole. Some of the more important hypotheses presented include: (1), that evolution of wood (and other) characters is always progressive, with gene overlays (silencing, modification, etc.) and simultaneous changes in multiple features, so that ancestral conditions are never truly re-attained. (2). Not all characters are of equal value in water economy of any given plant; some (presence of tracheids) may supersede others, and xeromorphic characters can be arranged relative to each other in tiers, although various taxonomic groups have different rosters of conductive safety features. (3). Heterochrony (protracted juvenilism, accelerated adulthood) is extensively represented in angiosperms, and acts as an overlay that is a source of diversity that angiosperms have drawn on since their inception (probably as minimally woody plants). (4). There may be no “purely taxonomic” characters, because genes of an organism relate primarily to changes, ancient and new, that are of adaptive significance, although we may not be able to detect selective value, past or present. Although many families of Brassicales are small and represent occupancy of specialized or extreme habitats (Batis, Koeberlinia, Moringa), active speciation in Brassicaceae and Capparaceae is related to tolerance of drought and cold with mechanisms such as vestured pits, narrow vessels, and abbreviation in life cycle length.