Abstract
Ferns are some of the oldest vascular plants in existence and they are the second most diverse lineage of tracheophytes next to angiosperms. Recent efforts to understand fern success have focused on the physiological capacity and stress tolerance of both the sporophyte and the gametophyte generations. In this review, we examine these insights through the lens of plant water relations, focusing primarily on the form and function of xylem tissue in the sporophyte, as well as the tolerance to and recovery from drought and desiccation stress in both stages of the fern life cycle. The absence of secondary xylem in ferns is compensated by selection for efficient primary xylem composed of large, closely arranged tracheids with permeable pit membranes. Protection from drought-induced hydraulic failure appears to arise from a combination of pit membrane traits and the arrangement of vascular bundles. Features such as tracheid-based xylem and variously sized megaphylls are shared between ferns and more derived lineages, and offer an opportunity to compare convergent and divergent hydraulic strategies critical to the success of xylem-bearing plants. Fern gametophytes show a high degree of desiccation tolerance but new evidence shows that morphological attributes in the gametophytes may facilitate water retention, though little work has addressed the ecological significance of this variation. We conclude with an emergent hypothesis that selection acted on the physiology of both the sporophyte and gametophyte generations in a synchronous manner that is consistent with selection for drought tolerance in the epiphytic niche, and the increasingly diverse habitats of the mid to late Cenozoic.
Highlights
Ferns are generally perceived as small-statured plants relegated to the forest understory due to limitations arising from a two-stage life cycle, but the morphology, habit and life-history of seedless vascular plants (SVPs) varies tremendously with respect to leaf shape, overall stature and leaf longevity (Ranker and Haufler, 2008; Mehltreter et al, 2010)
Efficient water transport was paramount to the successful colonization of land by plants and one implication is that the appearance of primary xylem in early-diverging SVPs was a key innovation that may have bridged the transition from simple non-vascular plants to the derived woody flora that soon followed (Kenrick and Crane, 1997; Sperry, 2003; Taylor et al, 2009; Pittermann, 2010)
DIRECTIONS There is good reason to suspect that physiological and morphological traits in both the sporophyte and gametophyte stages in a coordinated manner that was consistent with the Cenozoic diversification of the fern epiphytic flora, and possibly other fern radiations across the post-Eocene landscape
Summary
Ferns are generally perceived as small-statured plants relegated to the forest understory due to limitations arising from a two-stage life cycle, but the morphology, habit and life-history of seedless vascular plants (SVPs) varies tremendously with respect to leaf shape, overall stature and leaf longevity (Ranker and Haufler, 2008; Mehltreter et al, 2010). It is often overlooked that Pteridophytes (ferns) encompass at least 10,000 species making them the most diverse lineage next to angiosperms. In contrast to their bryophyte ancestors, the evolution of true vascular tissue allowed ferns and other SVPs to display a diversity of leaf shapes and sizes that include arborescent and viney ferns, but it is generally agreed that the period between the Late-Devonian and Carboniferous witnessed the peak of SVP diversity with the evolution of the now-extinct pro-gymnosperms and seed ferns (Taylor et al, 2009; Wilson and Knoll, 2010). Many factors have contributed to the continued success of Pteridophytes, but here we explore the water relations of sporophytes and gametophytes, focusing on the structure and function of the xylem tissue and drought-response patterns of the gametophyte stage
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