SEM Studies on Vessels in Ferns. 1. Woodsia obtusa

Abstract

ABsT1cT.—Circular to oval pores are present in primary walls (pit membranes) of end walls of tracheary elements in roots and rhizomes of Woodsici obtusa (Spreng.) Torr, (Dryopteridaceae). The pores are at least as abundant in rhizomes as in roots. We regard tracheary elements with end walls bearing these pores as vessel elements (such pores are absent on lateral walls). Pores occur in thin areas of the primary wall of pit membranes; thicker portions of pit membranes, both on lateral and end walls, occur in the form of striations oriented parallel to the long axis of the cell. These striations, recently demonstrated by means of scanning electron microscopy (SEM) for pri mary xylem of Nymphaeales and the monocotyledon Acorus, are likely widespread in vascular plants. Pore formation in W obtusa is only moderate compared to that in Nymphaeales and Acei-us, and may correlate with the fact that W obtusa occupies habitats relatively more mesic than those in which other species of Woodsia grow. The presence of vessel elements in ferns was Claimed over a century ago by Russow (1872), who cited only two species, Pteridium aquilinum (L.) Kuhn and Nephrodium fihix-mas Rich. (= Dryopteris fihix-mas (L.) Schott), as having true perforations in the end walls. White (1962), who reviewed the history of claimed vessel presence in ferns, demonstrated that opinions varied widely: De Bary (1884) agreed with Russow (1872), whereas Gwynne-Vaughan (1908) thought that vessels could be found in Cyatheaceae, Gleicheniaceae, Hymen ophyllaceae, Marattiaceae, Ophioglossaceae, Polypodiaceae, and Schizaeaceae. Bancroft (1911), using a variety of techniques, thought that vessels exist in ferns only in Pteridium aquilinum, a conclusion shared by Duerden (1940). White (1961, 1962) presented clear evidence for the presence of vessels in roots of Marsilea. In addition, White (1962, 1963) suspected vessel presence in two other species of ferns, Woodsia ilvensis (L.) R. Br. and Astrolepis sin uata (Lag. ex Sw.) D.M. Benham & Windham (= Notholaena sin uata (Lag. ex Sw.) Kaulf.), although he did not demonstrate perforations. White’s (1962, 1963) reason for suspecting vessels in Astrolepis sin uata and Woodsia ilvensis was marked differentiation (as indicated by distribution of pits and therefore sec ondary wall) between the lateral walls and the end walls of tracheary elements. In addition, moderate differences between pits of lateral walls and pits (or perforations) of end walls of tracheary elements were claimed by White (1962, p. 75) for 10 other species of Woodsia, and one hybrid Woodsia. The species we have elected to study here, W. obtusa (Spreng.) Torr., was cited by White (1962) as having “slight” differentiation of end walls as compared to lateral