Conifer tracheids resolve conflicting structural requirements: Data, hypotheses, questions

The nature of conduction involves movement of a liquid (under tension or pressure) through a solid (cell walls necessary to direct the liquid and provide mechanical strength). The numerous consequences of the liquid/solid nature of the conductive interface in plants can be viewed as a series of conflicting requirements that are resolved by various mechanisms. For example, the types of mechanical strength conferred by thicker cell walls (latewood) run counter to optimal conduction (earlywood). Conflict resolution situations are examined with light microscopy and SEM to show in detail not merely conflicting requirements but the various types of resolution in various conifers. Abies is presented as exemplary of a cool temperate conifer with numerous aspects to earlywood/latewood structure. Tropical conifers (Araucaria) present different compromises; the riparian conifer Dacrydium guillauminii has only earlywood; the parasitic conifer Parasitaxus has only latewood. Particular conifers have only some of the features by which latewood differs from earlywood. Cell dimorphism is only one aspect of resolution of conflicting requirements; others include modifications in pit size, shape, and density; the nature of the pit membrane; the nature of the pit cavity, pit border and pit aperture; and surface relief (warty layer) of the tracheid wall. The invention of coniferous bordered pits involves a circular shape, so that tension on the margo strands is equal, and thus the pit can be closed. These factors and margo pore maximization necessitate expending a large amount of space to pits in earlywood, the strength of which is thereby lessened and must be compensated by greater wall strength in latewood. The paper concludes with a series of twenty features which represent resolutions of conflicting requirements in terms of anatomical structure. Wood physiological literature is integrated with the anatomical observations.