Systematic Botany(1986), 11(2): pp. 309-321 ? Copyright1986 by the American Society of Plant Taxonomists
The Early Evolutionary and Morphological Diversity of Isoetes, with Descriptions of Two New Neotropical Species R. JAMES HICKEY Botany Department, Miami University, Oxford, Ohio 45056 Two new Isoetes fromSouth America are described and contrastedwith morphologABSTRACT. ically similar and sympatricspecies. The firstof these species, I. baculata, representsa relictual morphotypewithin the Isoetaceae as indicated by the presence of completelylaminate leaves. The other species, I. eshbaughii, is unique in formingdesiccation resistantfoliar gemmae which are completelyenclosed by sclerifiedscales. Analyses on the distributionof alate leaves and scales, as well as peripheral fibrousbundles and sporangial pigmentation,show that thereare nested sets of species withinthe Isoetaceae which representmajorphylogeneticlineages. The patternof character nestingsuggestsan historicaltransitionfroma primitivelyaquatic habit to a seasonally amphibious one, with subsequent parallel reversionsback to aquatic zones. These habitatchanges have resulted in convergent morphologies which are in large part responsible for the previous confusion regarding the evolution of the genus and the dependence on megaspore morphology as the sole basis of infragenericclassification.A major advantage of the proposed phylogeny is that the demarcationof majorlineages now allows forthe establishmentof functionalingroupsand outgroups, and thus provides a frame of referencefromwhich additional phylogenetic hypotheses can be formulated.
The Isoetaceae are a small, cosmopolitan familyof aquatic, heterosporouspteridophytes which are related to the extantSelaginellaceae and Lycopodiaceae. The family is monotypic and comprises some 150 species (Tryon and Tryon 1982) which are classified primarilyon the basis of megaspore surface morphology (Pfeiffer1922; Fuchs-Eckert1982). The classification of such a large numberof species on the basis of variation in a single characteris intuitively suspect and several taxonomic studies indicate that this suspicion is warranted (Duthie 1929; Williams 1943). Beforethe establishment of an infragenericclassificationsystem 1922), based on megasporemorphology(Pfeiffer the significanceof megaspore charactersin Isoetestaxonomywas questioned by Eaton (1900). A number of workershave regarded Pfeiffer's classificationas unreliable and unnatural (Williams 1943; Berthetand Lecocq 1977). Although megaspore surfacemorphologyis unreliable in delineating naturalsubdivisions within Isoetes, it does provide a ready means forspecies sorting and is of great importancein species identification(Boom 1982; Croft1980; Hickey 1981; Kott and Britton1983; Stolze and Hickey 1983; Taylor et al. 1975). In recent years, a number of investigators (Reed 1965; Boom 1982; Hickey 1985), working within restrictedgeographical regions, have
adopted informalspecies alliances ratherthan use the currentclassificationsystem.Their reluctance to propose a new classificationis appropriatein light of the meager and often inconsistent data base currently available for Isoetes. This paper is divided into four major sections. The firstintroduces and describes two new species of Isoetes fromSouth America.Each description is accompanied by an account of that species' distinctive features and each species is compared with morphologicallysimilar or sympatrictaxa. The second summarizes the variation and systematic distribution of vegetativereproductionin the genus, with particular referenceto a novel form of gemmae found in one of the newly described species. The third section is concerned with the taxonomic and phylogeneticsignificanceof several unusual character states found in these new species. Correlations among these and other charactersand characterstatesfound in the genus are pointed out. In particular,I discuss the presence or absence of a subula, the development of peripheral fibrousbundles, the morphology and taxonomic distribution of scale leaves and phyllopodia, and the production of sporangial pigmentation.This suite of characters,when polarized and taken in concert,suggests a reinterpretationof the early evolution-
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