“Primitive” Wood Characters are Adaptive: Examples from Paracryphiaceae

Abstract

Why plants rich in plesiomorphic (“primitive”) features are alive today is a question that receives little comment. Apomorphies in angiosperms are often interpreted as valuable adaptations. However, both apomorphies and plesiomorphies can be keyed to ecological and physiological features. If a particular habitat remains little modified for long periods of geological time, plesiomorphic features should theoretically persist. The Bailey-Frost-Kribs correlations (usually between tracheary element length and character states in other wood features), deemed useful in their day, did not include adaptation to ecology, nor did they have the advantages that molecular-based phylogenies bring to us today. Montane cloud forests or cool wet forests in geologically older areas (New Zealand, New Caledonia, New Guinea, northern Queensland), especially closer to the equator, contain many species rich in wood plesiomorphies. Wood plesiomorphies can also occur in areas frozen in winter but with moderate transpiration rates during the growing season. Wood plesiomorphies that correlate with these conditions include long and narrow vessel elements with scalariform perforation plates that have wider bars and narrower perforations sometimes occluded by pit membrane remnants. These remnants often take the form of axially-oriented strands, the retention of which correlates with the nature of flow in vessels. The direction of these strands contrasts with that in circular bordered pits of conifers, and is basic to the differences in wood of the two groups. Other plesiomorphic features form a coherent grouping of characters related to low conductivity and moderate peak tensions in water columns: scalariform lateral wall pitting in vessels, tracheids as the imperforate tracheary element, thin porose membranes in bordered pits of tracheids and vessels, rays numerous per mm, abundant upright cells in rays, and diffuse axial parenchyma. Details about the woods and other features of Paracryphiaceae are offered as examples of these tendencies. Flower and fruit characters in Paracryphiaceae show clear apomorphies. Scalariform perforation plates are not acquired secondarily; once extinguished in a clade, the genetic basis for their formation cannot be completely restored, and other features (tracheids, vessel grouping, etc.) provide better methods for achieving conductive safety. Woody plants in drier areas, as well as annuals, caudex perennials, and drought-deciduous trees and shrubs have fewer plesiomorphic features.