Origins and nature of vessels in monocotyledons: 8. Orchidaceae

Xylem of the orchids studied provided unusually favorable material to demonstrate how conductive tissue evolves in monocotyledons. In the end walls of tracheary elements of many Orchidaceae, remnants of pit membranes were observed with scanning electron microscopy and minimally destructive methods. The full range from tracheids to vessel elements, featuring many intermediate stages, was illustrated with SEM in hand sections of fixed roots, stems, and inflorescence axes of 13 species from four subfamilies. Pit membranes in end walls of tracheary elements are porose to reticulate in roots of all species, but nonporose in stems of Cypripedioideae and Vanilloideae and porose to reticulate in stems of Orchidoideae and Epidendroideae. The distribution pattern of pit membranes and pit membrane remnants in end walls of tracheary elements of orchids parallels the findings of others. The position of Cypripedioideae and Vanilloideae as outgroups to Orchidoideae and Epidendroideae, claimed by earlier authors, is supported by clades based on molecular studies and by our studies. Little hydrolysis of pit membranes in tracheary element end walls was observed in pseudobulbs or inflorescence axes of epidendroids. The pervasiveness of network-like pit membranes of various extents and patterns in end walls of tracheary elements in Orchidaceae calls into question the traditional definitions of tracheids and vessel elements, not merely in orchids, but in angiosperms at large. These two concepts, based on light microscope studies, are blurred in light of ultrastructural studies. More importantly, the intermediate expressions of pit membranes in tracheary element end walls of Orchidaceae and some other families of angiosperms are important as indicators of steps in evolution of conduction with respect to organs (more rapid flow in roots than in succulent storage structures) and habitat (less obstruction to flow correlated with a shift from terrestrial to epiphytic).     

Vessel elements present in the secondary xylem of Trochodendron and Tetracentron (Trochodendraceae)

For almost 150 years, the two monotypic genera Trochodendron and Tetracentron (Trochodendraceae) have been considered to share an unusual and primitive feature in angiosperms - the lack of vessels in their wood. Therefore, they have been classified in a basal position in the angiosperms. Our observations by light microscopy, low-vacuum environmental scanning electron microscopy (ESEM) and high-vacuum scanning electron microscopy (SEM) both in fresh and FAA-fixed materials consistently showed the presence of tracheary elements differentiated into two types in both genera. In Trochodendron, the tracheary elements can be divided into perforate vessel elements and imperforate fiber-tracheids and tracheids. The vessel elements show end and lateral walls. The pits on the end walls are elongate- broadened and do not have membranes or only a few remnants of them forming the perforation plates. The fiber-tracheids show crossfield pit pairs and sharp ends, and the tracheids show bordered pits. In Tetracentron, the tracheary elements comprise vessel elements and fibers. The vessel elements are similar to those of Trochodendron, whereas the fibers have no crossfield pit pairs but, rather, elliptical pits and sharp ends. Thus, both Trochodendron and Tetracentron are vessel bearing rather than vesselless, although their vessel elements are primitive.     

PIT MEMBRANE REMNANTS IN PERFORATION PLATES OF PRIMITIVE DICOTYLEDONS AND THEIR SIGNIFICANCE

Perforations of vessel elements characteristically retain remnants of pit membranes (primary walls) in woods of species of more than 30 families of dicotyledons. Scanning electron microscopy is necessary to demonstrate presence and type of membrane remnant. Species with these remnants in perforations given in earlier literature as well as those newly reported here are listed. Perforation membrane remnants may take the form of flakes, strands, or webs, and particular types may characterize particular families (e.g., strands or bands in Illiciaceae). Some families have abundant perforation membrane remnants (e.g., Chloranthaceae, Illiciaceae). Where membranes are nearly intact, they are porose and closely resemble the porose pit membranes on end walls of Tetracentron tracheids. In Tetracentron, however, tracheary elements are monomorphic, so vessel origin cannot yet be said to have occurred. Membrane remnants in perforations are regarded as a relictual primitive feature that should be added to the list of primitive character states claimed for vessel elements in angiosperms; alternative hypotheses are considered and discussed, and evidence from DNA phylogenies is needed. In vessel-bearing dicotyledons with membrane remnants in perforations, many perforations are relatively clear, but an appreciable proportion of perforation plates do have membrane remnants.     

国产对囊蕨亚科(蹄盖蕨科)植物的管状分子

利用扫描电镜观察了国产蹄盖蕨科(Athyriaceae)对囊蕨亚科(Deparioideae)10种植物及双盖蕨属(Diplazium Sw.)3种植物根状茎的管状分子。结果显示, 这些管状分子端壁和侧壁的形态及结构分别相同且侧壁具有穿孔板(多穿孔板)。根据穿孔板的形态特征, 将该亚科的管状分子分为5种类型: (1)梯状穿孔板, 无穿孔的二型性现象; (2)梯状穿孔板, 有穿孔的二型性现象; (3)网状穿孔板; (4)梯状-网状混合的穿孔板; (5)大孔状穿孔板。按照纹孔膜残留的程度又可分为3种: 部分区域有完整的纹孔膜、残留呈网状或线状以及很少或无纹孔膜残留。结合前人的研究资料, 发现蕨类植物的管状分子与被子植物的导管分子在形态和输导机理上存在明显差异, 管胞和导管分子不能仅仅根据纹孔膜的存在与否来确定, 而应根据穿孔板存在于端壁还是侧壁进行判断, 即穿孔板仅存在于端壁的管状分子为导管分子; 端壁和侧壁形态及结构分别相同, 有或无穿孔板的管状分子为管胞。由此可以推测蕨类植物和裸子植物中输导水分和矿物质的管状分子主要为管胞。单叶双盖蕨属(Triblemma(J. Sm.) Ching)与双盖蕨属管状分子的特征并不相似, 显示了将单叶双盖蕨属从双盖蕨属独立出来归入对囊蕨亚科的合理性。根据管状分子的特征, 推测假蹄盖蕨属(Athyriopsis Ching)和蛾眉蕨属(Lunathyrium Koidz.)可能是比较进化的属, 而介蕨属 (Dryoathyrium Ching)相对比较原始, 单叶双盖蕨属的系统位置应介于假蹄盖蕨属与介蕨属之间。     

国产对囊蕨亚科（蹄盖蕨科）植物的管状分子

利用扫描电镜观察了国产蹄盖蕨科（Athyriaceae）对囊蕨亚科（Deparioideae）10种植物及双盖蕨属（Diplazium Sw．）3种植物根状茎的管状分子。结果显示,这些管状分子端壁和侧壁的形态及结构分别相同且侧壁具有穿孔板（多穿孔板）。根据穿孔板的形态特征,将该亚科的管状分子分为5种类型：（1）梯状穿孔板,无穿孔的二型性现象：（2）梯状穿孔板,有穿孔的二型性现象：（3）网状穿孔板：（4）梯状-网状混合的穿孔板：（5）大孔状穿孔板。按照纹孔膜残留的程度又可分为3种：部分区域有完整的纹孔膜、残留呈网状或线状以及很少或无纹孔膜残留。结合前人的研究资料,发现蕨类植物的管状分子与被子植物的导管分子在形态和输导机理上存在明显差异,管胞和导管分子不能仅仅根据纹孔膜的存在与否来确定,而应根据穿孔板存在于端壁还是侧壁进行判断,即穿孔板仅存在于端壁的管状分子为导管分子：端壁和侧壁形态及结构分别相同,有或无穿孔板的管状分子为管胞。由此可以推测蕨类植物和裸子植物中输导水分和矿物质的管状分子主要为管胞。单叶双盖蕨属（Triblemma（J．Sm．）Ching）与双盖蕨属管状分子的特征并不相似,显示了将单叶双盖蕨属从双盖蕨属独立出来归人对囊蕨亚科的合理性。根据管状分子的特征,推测假蹄盖蕨属（Athyriopsis Ching）和蛾眉蕨属（Lunathyrium Koidz．）可能是比较进化的属,而介蕨属（Dryoathyrium Ching）相对比较原始,单叶双盖蕨属的系统位置应介于假蹄盖蕨属与介蕨属之间。     

The anatomy of lotus fibers found in petioles of Nelumbo nucifera

Lotus fibers are the isolated helical secondary cell wall thickenings from tracheary elements of lotus (Nelumbo nucifera Gaertn) petioles. In this study the anatomical characteristics of lotus petioles and microstructures of tracheary elements were studied using light microscopy (LM) and scanning electron microscopy (SEM). The results show that vascular bundles of lotus petioles are scattered throughout ground tissue. Their tracheary elements are of various sizes and there are several patterns of secondary wall thickening present. However, only secondary thickening in a ribbon-like helical pattern can be drawn out from the petiole to form lotus fibers for subsequent utilization. Study of the microstructure of the tracheary elements reveals that there are two pit structures present in the end walls in addition to pits with intact pit membranes: those with porose or web-like remnants pit membrane and those that lack pit membranes. This is an indication of the transitional stage between tracheids and vessel elements. This study provides supportive evidence that lotus fibers are found in both helically thickened tracheids and helically thickened primitive vessels.     

SEM studies on vessels in ferns. 11. Ophioglossum

With scanning electron microscopy (SEM), the nature of metaxylem vessel elements and tracheids was examined in Ophioglossum crotalophomides, 0. pendulum subsp. falcatum , and 0. vulgatum roots and rhizomes. Vessels were identified in all species. End walls of vessel elements, which bear perforations, are like lateral wall pitting of those elements in the secondary wall framework and differ only in absence of pit membranes or presence of pit membrane remnants. Some of the perforations contain pit membrane remnants that have large pores, small porosities, or are threadlike or weblike in structure. Dimorphic perforations were found in some vessel elements of rhizomes of 0. pendulum subsp. falcatum. Tracheids are very likely present in addition to vessels in all three species. The secondary wall framework of both tracheids and vessels is basically scalariform, although deviations in pattern are present. Vessel elements of Ophiglossum are entirely comparable to those of leptosporangiate ferns.     

PRESENCE OF VESSELS IN WOOD OF SARCANDRA (CHLORANTHACEAE); COMMENTS ON VESSEL ORIGINS IN ANGIOSPERMS

Sarcandra is the only genus of Chloranthaceae hitherto thought to be vesselless. Study of liquid-preserved material of S. glabra revealed that in root secondary xylem some tracheary elements are wider in diameter and have markedly scalariform end walls combined with circular pits on lateral walls. Examination of these wider tracheary elements with scanning electron microscope (SEM) demonstrated various degrees of pit membrane absence in the end walls. Commonly a few threadlike fibrils traverse the pits (perforations); these as well as intact nature of pit membranes in pits at ends of some perforation plates are evidence that lack of pit membranes does not result from damage during processing. Some perforations lack any remnants of pit membranes. Although perforation plates and therefore vessels are present in Sarcandra roots, no perforations were observed in tracheary elements of stems or lignotubers. Further, stem tracheids do not have the prominently scalariform end walls that the vessel elements in roots do. Presence of vessels in Sarcandra removes at least one (probably several) hypothetical events of vessel origin that must be postulated to account for known patterns of vessel distribution in angiosperms, assuming that they are primitively vesselless. Seven (perhaps fewer) vessel origin events in angiosperms could account for these patterns; two of those events (Nelumbo and monocotyledons) are different from the others in nature. Widely accepted data on trends of vessel specialization in woody dicotyledons yield an unappreciated implication: vessel specialization has happened in a highly polyphyletic manner in dicotyledons, and therefore multiple vessel origins represent a logical extension backward in time. If a group of vesselless dictyoledons ancestral to other angiosperms existed, they can be hypothesized to have had a relatively homogeneous floral plan now that Sarcandra-like plants no longer need be imagined within that group. Sarcandra and other Chloranthaceae show that the borderline between vessel absence and presence is less sharp than generally appreciated.     

Xylem of early angiosperms: Novel microstructure in stem tracheids of Barclaya (Nymphaeaceae)

Pit membranes of stem tracheids of all recognized species of Barclaya, an Indomalaysian genus of Nymphaeaceae, were studied with scanning electron microscopy (SEM). Pit membranes of the tracheids are composed of two thick layers, both constructed of fibrils much larger than those of tracheary elements of angiosperms other than Nymphaeaceae. The outer (distal) layer, which comprises the continuous primary wall around the tracheids, is spongiform, perforated by porosities of relatively uniform size, and confined to or most prominent on end walls of stem tracheids. The second layer consists of thick widely spaced fibrils that are oriented axially and are laid down proximally (facing the cell lumen) to the first (outer) layer, although continuous with it. These axial fibrils are attached at their ends to the pit cavities. This peculiar microstructure is not known outside Nymphaeaceae except in Brasenia and Cabomba (Cabombaceae, Nymphaeales), and has not been previously described for Barclaya. The longitudinally oriented threads and strands in perforation plates of secondary xylem of wood and stems of a variety of primitive woody angiosperms (e.g., Illicium) are not homologous to the pit membrane structure observed in stem tracheids of Barclaya, which, like other Nymphaeaceae, has only primary xylem and no perforation plates. The tracheid microstructure reported here is different from pit structures observed in any other group of vascular plants, living or fossil. The tracheid stem microstructures of Barclaya and other Nymphaeaceae appear to be a synapomorphy of Nymphaeaceae and Cabombaceae, and need further study with respect to ultrastructure and function.     

Origin and nature of vessels in monocotyledons. 5. Araceae subfamily Colocasioideae

Tracheary elements from macerations of roots and stems of one species each of five genera of Araceae subfamily Colocasioideae were studied by means of SEM (scanning electron microscopy). All of the genera have vessel elements not merely in roots, as previously reported for the family as a whole, but also in stems. The vessel elements of stems in all genera other than Syngonium are less specialized than those of roots; stem vessel elements are tracheid-like and have porose pit membrane remnants in perforations. The perforations with pit membrane remnants demonstrate probable early stages in evolution of vessels from tracheids in primary xylem of monocotyledons. The vessel elements with such incipient perforation plates lack differentiation in secondary wall thickenings between perforation plate and lateral wall, and such vessel elements cannot be identified with any reliability by means of light microscopy. The discrepancy in specialization between root and stem vessel elements in genera other than Syngonium is ascribed to probable high conductive rates in roots where soil moisture fluctuates markedly, in contrast with the storage nature of stems, in which selective value for rapid conduction is less. Syngonium stem vessels are considered adapted for rapid conduction because the stems in that genus are scandent. Correlation between vessel element morphology and ecology and habit are supported. Although large porosities in vessel elements facilitate conduction, smaller porosities may merely represent rudimentary pit membrane lysis.     

国产球盖蕨科植物管状分子的比较研究

利用扫描电镜观察了国产球盖蕨科10种植物,鳞毛蕨科6种植物的管状分子,结果显示:它们的管状分子端壁和侧壁的形态及结构分别相同,且侧壁具有穿孔板。它们具有4种类型的管状分子:(1)梯状穿孔板,无穿孔板的二型性现象;(2)梯状穿孔板,具有二型性现象;(3)梯状-网状混合穿孔板;(4)大孔状穿孔板。穿孔板仅存在于端壁的管状分子为导管分子,而端壁和侧壁形态、结构相似,有或无穿孔板的管状分子为管胞,蕨类植物中的管状分子主要为管胞,这与传统观点不同。管状分子的形态特征表明:球盖蕨科是鳞毛蕨群的成员,但不是原始成员,可能属于其中较为进化的类群,与鳞毛蕨科有许多共同特征,但仍存在较大差异,所以将其作为独立的科是合理的,推测球盖蕨科中的鱼鳞蕨属是比较进化的属,柄盖蕨属相对原始,红腺蕨属的系统位置应介于二者之间。     

Vessels in ferns: structural, ecological, and evolutionary significance

We have studied macerated xylem of ferns, supplemented by sections, by means of scanning electron microscopy (SEM) in a series of 20 papers, the results of which are summarized and interpreted here. Studies were based mostly on macerations, but also on some sections; these methods should be supplemented by other methods to confirm or modify the findings presented. Guidelines are cited for our interpretations of features of pit membranes. Fern xylem offers many distinctive features: (1) presence of numerous vessels and various numbers of tracheids in most species; (2) presence of vessels in both roots and rhizomes in virtually all species; (3) presence of specialized end walls in vessels of only a few species; (4) multiple end-wall perforation plates in numerous species; (5) lateral-wall perforation plates in numerous species; (6) porose pit membranes associated with perforation plates in all species; and (7) pit dimorphism, yielding wide membrane-free perforations alternating with extremely narrow pits. Multiple end wall perforation plates and lateral wall perforation plates are associated with the packing of tracheary elements in fascicles in ferns: facets of tips of elements contact numerous facets of adjacent elements; all such contacts are potential sites for conduction by means of perforations. This packing differs from that in primary xylem of dicotyledons and monocotyledons. Porosities in pit membranes represent a way of interconnecting vessel elements within a rhizome or root. In addition, these porosities can interconnect rhizome vessel elements with those of roots, a feature of importance because roots are adventitious in ferns as opposed to those of vascular plants with taproots. Fully-formed or incipient (small-to-medium sized porosities in pit membranes) perforation plates are widespread in ferns. These are believed to represent (1) ease of lysis of pit membranes via pectinase and cellulase; (2) numerous potential sites for perforation plate formation because of fasciculate packing of tracheary elements; (3) evolution of ferns over a long period of time, so that lysis pathways have had time to form; (4) lack of disadvantage in perforation plate presence, regardless of whether habitat moisture fluctuates markedly or little, because ferns likely have maintaining integrity of water columns that override the embolism-confining advantage of tracheids. Although all ferns share some common features, the diversity in xylem anatomy discovered thus far in ferns suggests that much remains to be learned.     

Monocot Xylem Revisited: New Information,New Paradigms

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

Xylem of early angiosperms: Nuphar (Nymphaeaceae) has novel tracheid microstructure1

SEM studies of xylem of stems of Nuphar reveal a novel feature, not previously reported for any angiosperm. Pit membranes of tracheid end walls are composed of coarse fibrils, densest on the distal (outside surface, facing the pit of an adjacent cell) surface of the pit membrane of a tracheid, thinner, and disposed at various levels on the lumen side of a pit membrane. The fibrils tend to be randomly oriented on the distal face of the pit membrane; the innermost fibrils facing the lumen take the form of longitudinally oriented strands. Where most abundantly present, the fibrils tend to be disposed in a spongiform, three-dimensional pattern. Pores that interconnect tracheids are present within the fibrillar meshwork. Pit membranes on lateral walls of stem tracheids bear variously diminished versions of this pattern. Pits of root tracheids are unlike those of stems in that the lumen side of pit membranes bears a reticulum revealed on the outer surface of the tracheid after most of the thickness of a pit membrane is shaved away by the sectioning process. No fibrillar texturing is visible on the root tracheid pits when they are viewed from the inside of a tracheid. Tracheid end walls of roots do contain pores of various sizes in pit membranes. These root and stem patterns were seen in six species representing the two sections of Nuphar, plus one intersectional hybrid, as well as in one collection of Nymphaea, included for purposes of comparison. Differences between root and stem tracheids with respect to microstructure are consistent in all species studied. Microstructural patterns reported here for stem tracheid pits of Nymphaeaceae are not like those of Chloranthaceae, Illiciaceae, or other basal angiosperms. They are not referable to any of the patterns reported for early vascular plants. The adaptational nature of the pit membrane structure in these tracheids is not apparent; microstructure of pit membranes in basal angiosperms is more diverse than thought prior to study with SEM.     

Caryophyllales: a key group for understanding wood anatomy character states and their evolution

Definitions of character states in woods are softer than generally assumed, and more complex for workers to interpret. Only by a constant effort to transcend the limitations of glossaries can a more than partial understanding of wood anatomy and its evolution be achieved. The need for such an effort is most evident in a major group with sufficient wood diversity to demonstrate numerous problems in wood anatomical features. Caryophyllales s.l., with approximately 12 000 species, are such a group. Paradoxically, Caryophyllales offer many more interpretive problems than other ‘typically woody’ eudicot clades of comparable size: a wider range of wood structural patterns is represented in the order. An account of character expression diversity is presented for major wood characters of Caryophyllales. These characters include successive cambia (more extensively represented in Caryophyllales than elsewhere in angiosperms); vessel element perforation plates (non‐bordered and bordered, with and without constrictions); lateral wall pitting of vessels (notably pseudoscalariform patterns); vesturing and sculpturing on vessel walls; grouping of vessels; nature of tracheids and fibre‐tracheids, storying in libriform fibres, types of axial parenchyma, ray anatomy and shifts in ray ontogeny; juvenilism in rays; raylessness; occurrence of idioblasts; occurrence of a new cell type (ancistrocladan cells); correlations of raylessness with scattered bundle occurrence and other anatomical discoveries newly described and/or understood through the use of scanning electron microscopy and light microscopy. This study goes beyond summarizing or reportage and attempts interpretations in terms of shifts in degrees of juvenilism, diversification in habit, ecological occupancy strategies (with special attention to succulence) and phylogenetic change. Phylogenetic change in wood anatomy is held to be best interpreted when accompanied by an understanding of wood ontogeny, species ecology, species habit and taxonomic context. Wood anatomy of Caryophyllales demonstrates problems inherent in binary character definitions, mapping of morphological characters onto DNA‐based trees and attempts to analyse wood structure without taking into account ecological and habital features. The difficulties of bridging wood anatomy with physiology and ecology are briefly reviewed. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 164 , 342–393.     

Perforated pit membranes in imperforate tracheary elements of some angiosperms

BACKGROUND AND AIMS: The structure of pit membranes in angiosperms has not been fully examined and our understanding about the structure is incomplete. Therefore, this study aims to illustrate the micromorphology of pit membranes in fibres and tracheids of woody species from various families. METHODS: Specimens from ten species from ten genera and eight families were prepared using two techniques and examined by field-emission scanning electron microscopy. KEY RESULTS: Interfibre pit membranes with an average diameter of <4 microm were frequently perforated or appeared to be very porous. In contrast, pit membranes in imperforate tracheary elements with distinctly bordered pits and an average diameter of >or=4 microm were homogeneous and densely packed with microfibrils. These differences were observed consistently not only among species but also within a single species in which different types of imperforate tracheary elements were present. CONCLUSIONS: This study demonstrates that the structure of interfibre pit membranes differs among cell types and the differences are closely associated with the specialization of the fibre cells. It is suggested that perforated pit membranes between specialized fibres contribute to the dehydration of the fibre cells at or soon after maturation.     

Distinctive tracheid microstructure in stems of Victoria and Euryale (Nymphaeaceae)

Scanning electron microscopy (SEM) photographs of thick sections from liquid‐preserved stems of Victoria cruziana and Euryale ferox show accretions of coarse fibrils on pit membranes of tracheids. The first‐deposited fibrils are randomly orientated; on top of them (facing the tracheid lumina) are axially orientated coarse fibrils. The two systems are interconnected. Axially orientated fibrils were more extensively observed in Euryale than in Victoria and tips of fibrils in Euryale extend over the pit apertures onto secondary wall surfaces. Tracheid–parenchyma interfaces bear rudimentary coarse fibrils on the tracheid side. End walls of Victoria tracheids have highly porose pit membranes, thinner and less complex than those of the lateral intertracheid walls. The structures reported in Victoria and Euryale are consistent with those concurrently reported for stems of other Nymphaeaceae. Although also present in Cabombaceae, the coarse fibrils are otherwise not reported for stems of angiosperms and are not yet reported in roots of any species. Pit membrane remnants in perforation plates of various woody dicotyledons represent a nonhomologous phenomenon. The accretions of coarse fibrils in stem tracheids of Nymphaeaceae do not appear to enhance conduction, although they do contain porosities interconnecting tracheids. Removal of pit membrane remnants from perforation plates of primitive dicotyledon woods by hydrolysis does, on the contrary, suggest conduction enhancement. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 159 , 52–57.     

Anatomical features associated with water transport in imperforate tracheary elements of vessel-bearing angiosperms

Background and Aims

Imperforate tracheary elements (ITEs) in wood of vessel-bearing angiosperms may or may not transport water. Despite the significance of hydraulic transport for defining ITE types, the combination of cell structure with water transport visualization in planta has received little attention. This study provides a quantitative analysis of structural features associated with the conductive vs. non-conductive nature of ITEs.

Methods

Visualization of water transport was studied in 15 angiosperm species by dye injection and cryo-scanning electron microscopy. Structural features of ITEs were examined using light and electron microscopy.

Key Results

ITEs connected to each other by pit pairs with complete pit membranes contributed to water transport, while cells showing pit membranes with perforations up to 2 µm were hydraulically not functional. A close relationship was found between pit diameter and pit density, with both characters significantly higher in conductive than in non-conductive cells. In species with both conductive and non-conductive ITEs, a larger diameter was characteristic of the conductive cells. Water transport showed no apparent relationship with the length of ITEs and vessel grouping.

Conclusions

The structure and density of pits between ITEs represent the main anatomical characters determining water transport. The pit membrane structure of ITEs provides a reliable, but practically challenging, criterion to determine their conductive status. It is suggested that the term tracheids should strictly be used for conductive ITEs, while fibre-tracheids and libriform fibres are non-conductive.     

水青树特殊管胞的分布位置及其形态特征的研究

针对水青树（Tetracentron sinense）中一类特殊管胞进行较为全面的观察研究,判断细胞种类并分析维管组织输导机理及树木进化过程中的细胞演化规律。通过切片和解离技术,借助光学显微镜和电子显微镜对34年生水青树特殊管胞的分布位置和形态特征进行观察。结果表明：(1)特殊管胞在树木水平方向自内向外径向呈串排列,并贯穿年轮界限,多为一列,少数两列,且较为稀见。每个特殊管胞弦向左右两侧或单侧均与木射线细胞相连通。纵向上,特殊管胞单独或数个上下端接相连。(2)特殊管胞主要有以下3种类型：无端壁的纺锤形,有一个倾斜端壁,以及有两个倾斜端壁。特殊管胞的平均长度为286.44μm;横切面为四边形,平均弦向宽度为55.22μm,其平均壁厚为1.53μm。(3)特殊管胞两端封闭,无穿孔。(4)特殊管胞侧面壁上的纹孔数量较多且纹孔膜明显可见,具体表现为：弦面壁上布满特殊管胞之间的具缘纹孔,呈对列、互列偶见梯状排列;径面壁上存在与射线细胞间的具狭缘单纹孔,呈大圆形至椭圆形,每区域多为2～10个纹孔,呈1～4排横列;径面壁上与正常管胞间几无纹孔。水青树特殊管胞分布有一定规律,其长度远小于水青树正常管胞...     

Vestured pits and vestured vessel member walls in some Indian dicotyledonous woods

NAIR, M. N. B. AND MOHAN RAM, H. Y., 1989. Vestured pits and vestured vessel member walls in some Indian dicotyledonous woods. The woods of 144 taxa belonging to 38 families of angiosperms were examined for vestured pits and vestured vessel member walls using scanning electron microscopy. Vestured pits are present in 66 taxa (belonging to ten families) and vestured vessel member walls occur in only six taxa (belonging to three families). In Ehretiaceae and Euphorbiaceae vestures are present only in certain vessel members. In Wrighlia tinctoria , perforation plates containing vestures have been observed in addition to the presence of vestured pits. A classification of vestured pits based on their morphology and distribution is proposed by us. In all the types of vestured pits, vestures are present on the margin of the outer pit aperture or on the pit chamber wall. Occasionally, they are present in the pit canal, on the margin and in the vicinity of the inner pit aperture and rarely over the inner walls of the vessel members. The functions of vestured pits are not clear, although several suggestions are made. Whether or not these structures affect wood processing is not presently understood. It appears that vestured pits and vestured vessel member walls have diagnostic rather than phylogenetic value.