SEM Studies on Vessels in Ferns. 13. Nephrolepis

Abstract

Vessels are present in roots, rhizomes, and stolons of N. exaltata; vessel elements are probably present, but little different from tracheids, in the tubers of N. cordifolia. This contrast correlates with putatively slower conductive rates in an organ that stores water. The vessels have perforation plates both on end walls and lateral walls. Both types of perforation plates are similar to lateral walls except for absence of pit membranes. Perforation plates comprise a large area of vessel surfaces; this characteristic has been observed in other ferns from habitats in which marked fluctuation of water availability occurs. As shown in other papers of this series as well as the present paper, adaptation to ecological conditions is more important than phylogenetic position in explaining the presence and degree of specialization of vessel elements in ferns. Nephrolepis exaltata (L.) Schott is a fern that has been described as epi- phytic or epipetric (Lellinger, 1985); our specimen was collected in crevices of recent lava flows in the Hawaiian Islands. The species extends from central Florida into tropical Central and South America and tropical portions of the Pacific and the Old World (Hillebrand, 1888; Lellinger, 1985; Tryon and Tryon, 1982). Thus, N. exaltata is a tropical and subtropical species that occupies exposed and pioneering habitats within its range, although it is also found in shady and moist localities, as in the Hawaiian Islands (Neal, 1965). The pio- neering characteristics of N. exaltata and its ability to withstand full sun and periodic drought are features of interest with respect to morphology of trache- ary elements. In at least some ferns of dry localities, vessel elements with more specialized perforation plates occur, as in Pteridium (Carlquist and Schneider, 1997a), Astrolepis (Carlquist and Schneider, 1997b), Woodsia scopulina D.C. Eaton (Schneider and Carlquist, 1998a), and Woodsia ilvensis (L.) R. Br. (Carl- quist and Schneider, 1998a), whereas in ferns of moist localities, vessels may be present, but perforation plates are like lateral wall pitting except for absence of pit membranes (e.g., Osmundaceae and Schizaeaceae; Carlquist and Schnei- der, 1998b). The habit of Nephrolepis offers distinctive organs that invite study with respect to potential diversity in morphology of tracheary elements. In addition to presence of relatively thick rhizomes, Nephrolepis exaltata plants bear rel- atively slender stolons. Tubers are formed on the slender stolons of N. cordi- folia (L.) C. Presl. Because flow rates might be expected to be slower in the tubers because they function in storage of water and probably other substances, one might not expect perforation plates adapted to promoting rapid conduc- tion rates in the tubers. Our studies have concentrated on ferns from habitats that show pronounced fluctuation of temperature and water availability, such as Polystichum from areas that freeze in winter and Phlebodium, a tropical epiphyte (Schneider