RESUMO
We reconstruct the evolutionary changes in different anatomical markers in order to understand the evolution and functional aspects of growth rings during the diversification of seed plants (spermatophytes), one of the largest and most diverse lineages of the tree of life. We carried out a wide revision of the anatomy of secondary xylem in spermatophytes and reconstructed the evolution of the different anatomical markers in a time-calibrated phylogeny. By embodying a functionally and evolutionarily significant concept in growth rings we reveal a new panorama for their frequency and show how common they are in diverse lineages of tropical plants. In this context, the principal anatomical markers of growth rings are identified in the evolutionary history of plants and their association with climate-related ecological characteristics. We discuss the function of these anatomical markers, especially for thick-walled and/or radially flattened latewood fibres, fibre zone and dilated rays. Despite the high evolutionary lability of the anatomical markers evidenced by our analyses, they appear to represent deep homologies.
Assuntos
Clima , Xilema , Filogenia , Plantas/genética , Sementes/genéticaRESUMO
In the stems of terrestrial vascular plants studied to date, the diameter of xylem water-conducting conduits D widens predictably with distance from the stem tip L approximating D â Lb , with b ≈ 0.2. Because conduit diameter is central for conductance, it is essential to understand the cause of this remarkably pervasive pattern. We give reason to suspect that tip-to-base conduit widening is an adaptation, favored by natural selection because widening helps minimize the increase in hydraulic resistance that would otherwise occur as an individual stem grows longer and conductive path length increases. Evidence consistent with adaptation includes optimality models that predict the 0.2 exponent. The fact that this prediction can be made with a simple model of a single capillary, omitting much biological detail, itself makes numerous important predictions, e.g. that pit resistance must scale isometrically with conduit resistance. The idea that tip-to-base conduit widening has a nonadaptive cause, with temperature, drought, or turgor limiting the conduit diameters that plants are able to produce, is less consistent with the data than an adaptive explanation. We identify empirical priorities for testing the cause of tip-to-base conduit widening and underscore the need to study plant hydraulic systems leaf to root as integrated wholes.