Vessels in ferns: structural, ecological, and evolutionary significance

We have studied macerated xylem of ferns, supplemented by sections, by mean s of scanning electron microscopy (SEM) in a series of 20 papers, the resul ts of which are summarized and interpreted here. Studies were based mostly on macerations, but also on some sections; these methods should be suppleme nted by other methods to confirm or modify the findings presented. Guidelin es are cited for our interpretations of features of pit membranes. Fern xyl em offers many distinctive features: (1) presence of numerous vessels and v arious numbers of tracheids in most species; (2) presence of vessels in bot h roots and rhizomes in virtually all species; (3) presence of specialized end walls in vessels of only a few species; (4) multiple end-wall perforati on prates in numerous species; (5) lateral-wall perforation plates in numer ous species; (6) porose pit membranes associated with perforation plates in all species; and (7) pit dimorphism, yielding wide membrane-free perforati ons alternating with extremely narrow pits. Multiple end wall perforation p lates and lateral wall perforation plates are associated with the packing o f tracheary elements in fascicles in ferns: facets of tips of elements cont act numerous facets of adjacent elements; all such contacts are potential s ites for conduction by means of perforations. This packing differs from tha t in primary xylem of dicotyledons and monocotyledons. Porosities in pit me mbranes represent a way of interconnecting vessel elements within a rhizome or root. In addition, these porosities can interconnect rhizome Vessel ele ments with those of roots, a feature of importance because roots are advent itious in ferns as opposed to those of vascular plants with taproots. Fully -formed or incipient (small-to-medium sized porosities in pit membranes) pe rforation plates are widespread in ferns. These are believed to represent ( 1) ease of lysis of pit membranes via pectinase and cellulase; (2) numerous potential sites for perforation plate formation because of fasciculate pac king of tracheary elements; (3) evolution of ferns over a long period of ti me, so that lysis pathways have had time to form; (4) lack of disadvantage in perforation plate presence, regardless of whether habitat moisture fluct uates markedly or little, because ferns likely have maintaining integrity o f water columns that override the embolism-confining advantage of tracheids . Although all ferns share some common features, the diversity in xylem ana tomy discovered thus far in ferns suggests that much remains to be learned.