Pittermann, Jarmila, Watkins, James E., Cary, Katharine L., Schuettpelz, Eric, Brodersen, Craig, Smith, Alan R., and Baer, Alex. 2015. "The Structure and Function of Xylem in Seed-Free Vascular Plants: An Evolutionary Perspective." In Functional and Ecological Xylem Anatomy. Hacke, Uwe, editor. 1–37. Springer International Publishing. https://doi.org/10.1007/978-3-319-15783-2_1.
Water transport in seed-free vascular plants such as ferns and lycopods occurs through strands of primary xylem. In roots, rhizomes and fronds, the xylem and phloem are packaged in steles comprising either a single vascular bundle (protostele) or many such bundles (dictyosteles) that are variously arranged throughout the segment. Seed-free vascular plants, ferns in particular, are ecologically and morphologically diverse yet very little is known about the structure and function of their xylem. With few exceptions, the xylem of seed-free vascular plants consists of tracheids that are longer and wider than those of conifers, with abundant scalariform pitting present on conduit walls. Recent work indicates that the xylem of seed-free vascular plants may be at least as efficient as that of conifers and angiosperms due to large tracheids, tight conduit packing and permeable pit membranes. Hydraulic benefits may be amplified in some polyploids which exhibit significantly longer tracheids. However, the presence of adaptive xylem does not fully compensate for the absence of a cambial layer, secondary xylem and vessels. Indeed, developmental and in part vascular limits constrain the diversity of seed-free plant morphospace, especially as compared to angiosperms. The fossil record indicates that extinct lineages of seed-free vascular plants had more diverse xylem structures as well as secondary xylem. Few studies have addressed the resistance of seed-free vascular plants to drought-induced cavitation but given the recent evolution of fern epiphytes, ferns may be well adapted to episodic water deficit. Available data indicate that in ferns and lycophytes, narrow tracheids are less vulnerable to air entry than large ones; by extension, species with smaller conduits, such as epiphytes, may be less susceptible to cavitation. The functional significance of stelar arrangements of fern petioles is largely unexplored but mapping stelar patterns onto the fern phylogeny reveals that in the more derived eupolypod lineages, selection has favoured the evolution of simpler, less divided vascular networks.
