Osmunda cinnamomea forma frondosa at Mountain Lake, Virginia

Abstract

Pronounced dimorphism of sterile and fertile pinnae is a conspicuous morphological attribute of all species of Osmunda. Sporangia are borne on the margins of highly reduced segments that lack chlorophyll at maturity. The green spores initially give these fertile pinnae a greenish cast, but after release of the spores, they become wholly cinnamon-brown or gray-brown and soon wither. Of the three Osmunda species occurring in eastern North America, two bear fertile pinnae on otherwise undifferentiated sterile fronds: in Royal Fern, 0. regalis L., these fertile pinnae occur at the leaf tip, while in the Interrupted Fern, 0. claytoniana L., two to six fertile pinnae occur in the central portion of the frond. In the Cinnamon Fern, 0. cinnamomea L., the dimorphism is more complete, as sporophylls consist entirely of fertile pinnae and become completely achlorophyllous. Upon our arrival at Mountain Lake Biological Station in the summer of 1984, we were surprised to discover a large number of 0. cinnamomea plants bearing partially fertile fronds. These ranged continuously from those which were almost normal, i.e., with only a small portion of sterile blade tissue (usually at the tip) to those which were almost totally sterile, bearing only a few fertile segments, or even simply a few sporangia on sterile segment margins. The distribution of fertile pinnae on the fronds was highly irregular and unpredictable. The most frequent pattern was that with the lower portion of the frond fertile and intergrading into an upper sterile portion (Fig. 1A, B), but numerous fronds were found with a reverse pattern (Fig. 1C), or with sterile pinnae both above and below fertile ones (Fig. 1D). Numerous specimens were taken and have been deposited in the University of Richmond Herbarium. By midsummer, normal fertile fronds of 0. cinnamomea wither and fall to the ground or lie draped over sterile leaves. However, the sterile portions of the intermediate leaves remained turgid throughout the summer, making them especially conspicuous (a few of the nearly completely fertile fronds withered). This allowed us to take note of their distribution during our travels in the Mountain Lake area (Giles and Montgomery counties, Virginia) in the summers of 1984 and 1985. It became apparent that the intermediate fronds were very localized; we found them only in the immediate vicinity of the Biological Station and on Bald Knob and Bean Field Mountain near Mountain Lake (about 2 km southwest of the Biological Station). In these areas were patches where the in-