Patterns of segregation and convergence in the evolution of fern and seed plant leaf morphologies

Abstract

Global information on Paleozoic, Mesozoic, and extant non-angiosperm leaf morphol ogies has been gathered to investigate morphological diversity in leaves consistent with marginal growth and to identify likely departures from such development. Two patterns emerge from the principal coordinates analysis of this data set: (1) the loss of morphological diversity associated with marginal leaf growth among seed plants after sharing the complete Paleozoic range of such morphologies with ferns and (2) the repeated evolution of more complex, angiosperm-like leaf traits among both ferns and seed plants. With regard to the first pattern, morphological divergence of fern and seed plant leaf morphologies, indirectly recognized as part of the Paleophytic-Mesophytic transition, likely reflects reproductive and ecological divergence. The leaf-borne reproduc tive structures that are common to the ferns and Paleozoic seed plants may promote leaf morpho logical diversity, whereas the separation of vegetative and reproductive roles into distinct organs in later seed plant groups may have allowed greater functional specialization-and thereby mor phological simplification-as the seed plants came to be dominated by groups originating in more arid environments. With regard to the second pattern, the environmental and ecological distri bution of angiosperm-like leaf traits among fossil and extant plants suggests that these traits pref erentially evolve in herbaceous to understory plants of warm, humid environments, thus sup porting inferences concerning angiosperm origins based upon the ecophysiology of basal extant taxa.

Angiosperm leaf vein evolution was physiologically and environmentally transformative

“…Each of these lineages also represents a convergence upon angiospermlike venation consisting of a reticulate hierarchical network with internal vein endings (Boyce 2005). However, other examples of convergence upon angiosperm venation patterns possess much lower densities-Ophioglossum (1.4-2.0, mean of 1.7 mm mm…”

Viewing leaf structure and evolution from a hydraulic perspective

“…The first multi-vein leaves likely possessed open dichotomising venation systems (Boyce 2005) that allowed leaf size to increase substantially beyond the limits of needle and scale morphologies. Open dichotomous venation is not a complete solution to leaf water supply however, because vein density gradients in the leaf are inevitable, leading to nonhomogeneous water transport and leaf water potential during transpiration (cf.…”

“…Reticulate vein systems evolved early in the diversification of land plants and are found in most major clades of plants both living and extinct (Boyce 2005). The advantage of reticulate plumbing in the leaf is that multiple orders of veins can be developed without generating a gradient in D v across the leaf.…”

Plant Paleoecology in Deep Time1

“…Similar to studies of allometry within a phylogenetic framework are recent studies of physiology linking fossil and modern plants by Boyce andcolleagues (Boyce et al, 2003, 2004;Boyce, 2005), discussed above. This work has permitted convergent patterns in morphology to be identified and linked to underlying physiological controls, revealing certain patterns of trade-off in the basic plant functions, support versus hydraulics.…”

“…In the deeper past, Phillips and DiMichele (1992) considered the consequences for arborescent isoetalean lycopsids of CAM metabolic pathways, which have been found in modern isoetaleans (Keeley & Busch, 1984) and also inferred from carbon isotopic analysis (Raven & Spicer, 1996). Boyce and colleagues (Boyce & Knoll, 2002;Zwieniecki et al, 2004;Boyce, 2005) examined the physiological attributes of leaves and their xylary support systems in modern seed plants and pteridophytes as indicators of possible constraints on the evolution of leaves in primitive gymnosperms and ferns. As part of this, Boyce et al (2004) examined the physiology of tracheid lignification in a phylogenetically diverse sample of extant vascular plants.…”