Tracheary Elements in Ferns: New Techniques, Observations, and Concepts

Abstract

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. Progress in study of the nature of tracheary elements in ferns has been gradual, limited by techniques and technology. With light microscopy and macerations, vessels were early reported in Pteridium aquilinum (L.) Kuhn (Russow, 1873; Bliss, 1939) and Nephrodium filix-mas (L.) Schott (Russow, 1873). Gwynne-Vaughan's (1908) reports that vessels occurred in a number of other ferns have been discounted (Bancroft, 1911). Light microscope studies of macerations revealed distinctive perforation plates in roots of Marsilea (White, 1961; Loyal and Singh, 1978). White (1962) reported possible vessels in Astrolepis, Phlebodium, Polystichum ,a ndWoodsia. The report of possible vessels in these four genera by White (1962) was based on the presence of tracheary element end walls that were perceptibly different from lateral walls, an appearance which in turn is based on differences between end walls and lateral walls in secondary wall architecture. Because we had access to a scanning electron microscope (SEM), we endeavored to confirm White's (1962) reports. In our studies of xylem of Woodsia obtusa (Spr.) Torr. (Carlquist et al., 1997) and Phlebodium and Polystichum (Schneider and Carlquist, 1997), we obtained similar results. In the case of Woodsia obtusa, we used SEM study of sections prepared by paraffin sectioning techniques. Excessive fracturing of tracheid walls by