Non-random vessel distribution in woods: patterns, modes, diversity, correlations.

Abstract

Vessel grouping is a form of non-random distribution that becomes functionally valuable when the background consists of non-conductive imperforate tracheary elements (fiber-tracheids and libriform fibers); ungrouped vessels, randomly placed, often occur in an all-tracheid background. Types of vessel grouping are described and illustrated: diagonal, tangential, radial, median radial bands, and growth rings. Other non-random distributions considered include degrees and kinds of cable construction, patchy vessel distributions, vessel displacement related to succulence, and patterns involving successive cambia. Non-random vessel distributions inevitably involve non-random placement of imperforate tracheary elements, so that a parallel set of mechanical adaptations is often simultaneously achieved. Correlations between various types of non-random vessel patterns and possible physiological factors are hypothesized. Most correlations involve enhanced conductive safety, but vessel distribution related to water and photosynthate storage, resistance to torsion, and increased longevity of xylem are cited. Non-randomness of vessels is a source of diversity in wood structure that can be achieved readily (as growth rings show) and polyphyletically. These modifications offer numerous ways in which wood histology can be repatterned for probable adaptations in conductive physiology, mechanical strength, and storage capability, perhaps by means of regulatory genes. Grouping of vessels into vascular bundles in primary xylem of stems and leaves in dicots is a form of non-randomness, and the significance of vascular bundles (as opposed to steles) as adaptive forms of organization is considered briefly. Monocots differ from dicots in rarely having division of labor in tracheary elements within an organ, but monocots exhibit tradeoffs in which conductive efficiency (vessel presence in an organ) and conductive safety (tracheids but no vessels in an organ) can be achieved within a single plant.