The diverse, aquatic Hydrocharitaceae have defied stable classification for nearly two centuries. Anatomical and morphological convergence characterize many aquatic plants and undoubtedly have hindered the ability of researchers to ascertain accurately those features representing reliable phylogenetic markers within Hydrocharitaceae. Most prior classifications of the family have emphasized few characters to define major taxonomic subdivisions (i.e., they were highly artificial). Previous studies using molecular data have shown that DNA sequences provide novel indications of phylogeny not indicated previously by morphologically based classifications; however, they have not yet recommended alterations to the classification for the family. We conducted a more comprehensive phylogenetic study of Hydrocharitaceae to better elucidate evolutionary relationships among the genera that in turn could be used to provide insight for improvements in classification. We analyzed different data sets (55 morphological characters; chloroplast rbcL, matK, trnK intron sequences; nuclear ribosomal ITS region sequences) singly and in various combinations using maximum parsimony and maximum likelihood methods of phylogenetic reconstruction. Phylogenetic analysis of combined data yielded a fully resolved tree depicting four well-supported, major clades within Hydrocharitaceae. We use these results to propose a phylogenetic classification of Hydrocharitaceae recognizing four subfamilies that correspond to these clades: Anacharidoideae, Hydrilloideae, Hydrocharitoideae, and Stratioideae. Phylogenetic analysis also indicated the pattern of derivation with respect to submersed lifeforms, hydrophilous pollination, and marine habitation in the family. Character reconstructions indicated that several features, (e.g., ovule type; occurrence of detaching male flowers), once thought to provide strong phylogenetic markers in Hydrocharitaceae, actually are highly homoplasious and have acutely mislead past attempts at classification of the family.
Four variants of Kranz anatomy occur in the Cyperaceae. Three of these anatomical types (fimbristyloid, chlorocyperoid, and eleocharoid) are unique among taxa with C(4) photosynthesis in that the photosynthetic carbon reduction tissue (PCR, functional equivalent of bundle sheath) is located within the vascular strand and is separated from the primary carbon assimilation tissue (PCA, positional equivalent of mesophyll) by the mestome sheath layer. In the fourth anatomical type, rhynchosporoid, PCR tissue is located in the position of the mestome sheath. In this study, we compared two aspects of development of PCR and PCA tissues in representatives of the C(3) and C(4) types: (1) ontogenetic derivation and (2) cellular differentiation. Analysis of the planes of cell division associated with procambial strand formation indicated that PCR tissue is always derived from the procambium, while PCA tissue is derived from the ground meristem. These cell lineages remain distinct after the initial organization of vascular strands. Analysis of cell differentiation using accumulation of cell-type-specific photosynthetic enzymes as markers of differentiation indicated that, with one exception, a low level of non-cell-specific enzyme accumulation preceded abundant and cell-specific accumulation of photosynthetic enzymes at the distal end of the leaf elongation zone. Enzyme accumulation coincided spatially (and temporally) with structural aspects of cell differentiation. Previous cladistic analyses have indicated that these anatomical types represent separate evolutionary origins of the C(4) pathway, and the differences in developmental pathways observed here reflect these independent origins from C(3) ancestors.
We identified the zones of leaf extension, cell division, cell elongation, and cell differentiation in developing leaves of a sedge species, Cyperus eragrostis Lam. (Cyperaceae). The zone of leaf extension was located by measuring the separation between pinhole markers and by observing the staining pattern of Calcofluor White after pulse-labelling growing leaves. These observations were supported by determining growth rates of control and punctured leaves and by scanning electron and light microscopy of developing leaves. The location of the zone of cell division was assessed by enumerating mitotic figures, and the zone of cell elongation was established by measuring lengths of epidermal cells in cleared leaves. These studies indicated that the zone of leaf elongation is within the basal 10–15 mm of a leaf and that cell divisions are restricted to the basal 0.2–1 mm. Radial enlargement of internal tissues begins in the basal half of the elongation zone and cells are fully differentiated within a short distance above it. Expanding leaves can be divided into three zones: zone 1, a basal meristematic zone where cell division and some cell elongation occur; zone 2, a zone above the base where cells are elongating but cell division has ceased; and zone 3, a zone where elongation is complete and cells have reached their final length. This pattern of leaf development is similar to, but more condensed than, feat found in the related monocotyledonous family, the Poaceae. Keywords: Cyperus eragrostis, leaf development, leaf extension zone, Cyperaceae, cell enlargement.
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