Common and Confusing Bipinnate-Dimidiate Adiantums of Tropical America

Abstract

and are commonly misidentified in herbaria. These species, A. cajennense, fructuosum, fuliginosum, pulverulentum, serratodentatum, terminatum, and tetraphyllum, can reliably be distinguished from one another on the basis of costa indument and abaxial lamina indument (observed at 50 x) and often by frond and pinna morphology and rhizome morphology. However, pinna morphology is not always distinctive and is difficult to characterize adequately, and rhizome morphology is often not observable because collectors do not excavate and press the tenacious rhizomes. (In this regard, species with longcreeping rhizomes tend to have stipes that are straight at the base, whereas those with short-creeping rhizomes tend to have stipes that curve at the base, and so an educated guess about rhizome morphology sometimes can be made in the absence of the rhizome itself.) Many of the species of Adiantum hybridize quite readily, which can lead to difficulty in making identifications. Kramer (1978, p. 95), for instance, suggested eight possible hybrids in Surinam, four of them involving one or two of the species treated here. If one has a clear concept of the species, the intermediateness of hybrids in pinna and pinnule shape is often evident, and at least one parent often can be deduced. According to Dr. Brigitte Zimmer (pers. comm.), many of the specimens that have passed as variants, such as pinnate or bipinnate forms of certain Adiantum species, are actually hybrids. Dr. Robbin Moran (pers. comm.) has begun to find a similar situation in the unrelated genus Lindsaea (Dennstaedtiaceae). It is interesting that both genera occupy similar, edaphically dry habitats, have evolved sufficiently similar habits to be mistaken for each other by non-pteridologists, and may grow in genus communities (Wagner & Wagner, 1983). The following key will serve to distinguish these critical species of Adiantum. Although rhizome morphology is a useful character, I have avoided using it as a single character in the keys because of its frequent absence from herbarium sheets. 1. Abaxial surface of the laminae bearing simple, jointed hairs. Indument of the costae reddish brown, a mixture of simple, jointed hairs and linear scales up to 2(3) cells wide proximally and hair-like distally. Rhizomes short-creeping, knotted; stipes crowded. 2. Indusia brown (rarely blackish) at maturity, rarely any of them elongate,