Tracheary elements in ferns: New techniques, observations, and concepts

Longisections of xylem were studied with scanning electron microscopy (SEM) for roots of Angiopteris, upright axes of Psilotum, and rhizomes of eight species of leptosporangiate ferns of diverse habits and varied ecological preferences. In contrast to earlier studies using macerations, razor-blade sections of fixed material from living plants were prepared. All materials studied showed porose or reticulate pit membranes present on presumptive end walls of tracheids. Contrasting non-porose pits were observed on lateral walls of some tracheids. Tracheid to parenchyma pit pairs may have porose pit membranes on the tracheid side and nonporose pit membranes on the parenchyma side; thus degree of porosity in a section can represent the degree to which one primary wall or the other is pared away. Reticulate pit membranes on tracheary element end walls are evidently widespread in ferns. Such cells should not be considered vessel elements, although the reticulate pit membranes suggest a degree of transition toward the membrane-free perforations of typical vessels. True vessels (pit membranes absent in perforations) do occur in roots in a limited number of fern genera. The preparation methods of the present study produced results freer from artifacts than did macerations, and interpretations must be altered accordingly. Reports of lateral, multiple, and interrupted perforation plates in ferns are probably the result of loss of pit membranes due to the oxidative action of maceration and should be rejected. Likewise, "pit dimorphism" (alternately wide and narrow pits) and "striate" (corrugated) pit membranes in ferns represent artifacts. True vessel elements in ferns probably always have secondary wall architecture of end walls different from that of lateral walls.