Monocot Xylem Revisited: New Information, New Paradigms

摘要: Five sources of data force extensive revision ideas about the nature and evolution monocot xylem: scanning electron microscopy (SEM) studies thick sections; availability molecular phylogenies covering a relatively large number families genera; information on ecology habitat; concerning habit; observations from xylem physiology. These five new data, absent Cheadle, plus added light microscopy, lead to fresh understanding how has evolved in monocots. Tracheary elements hitherto recorded as vessel with scalariform end walls prove instances, retain pit membranes (often porous or reticulate) walls. There is not an inexorable progression "primitive" "specialized" monocots; apparent accelerations reversions are also possible. The latter include such changes result production narrower elements; less metaxylem, which probably heterochronic (an extreme form juvenilism). intermediate between tracheids must be recognized for what they are, forced into mutually exclusive categories. Original various types related here habit groups Asteliaceae, Boryaceae, Cyclanthaceae, Orchidaceae, Pandanaceae, Taccaceae, Typhaceae, dracaenoid Asparagaceae, Zingiberales. Data palm shows nearly unique syndrome features that can explained aid physiology ecology. Vessellessness stems leaves characterizes species; physiological ecological significance these highlighted. An non-palm arborescent monocots combine all-tracheid stem addition bundles vegetative modifications attempted. effect disjunction xylems adventitious roots stems, providing physiologically demonstrated valve ("rectifier") discussed. "Ecological iteration" occurred some lineages, so early-departing branches cases may have more because entry xeric habitats, whereas nearby crown groups, retained xylem, represent long occupation mesic habitats. Cheadle's use "negations" phyletic pathways no longer accepted. Symplesiomorphic mesomorphic patterns do characterize many earlier-departing whole, however. idea non-monocot angiosperms attained vessels independently improbable trees angiosperms. Vessels seem adaptation major swings moisture compared taproots. commonness plans clades feature requires further clarification but primarily have. Secondary vessellessness something very close it hypothesized Campynemataceae, Philesiaceae, Orchidaceae. Eleven salient shifts our conceptual views proposed conclude paper. Monocot collection historical information, rigorously parsimonious system contemporary habits