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.     

SEM studies on vessels in ferns. 17. Psilotaceae

Perforation plates are reported in aerial and subaerial axes of Psilotum nudum and in aerial axes of Tmesipteris obliqua. In Psilotum, both perforations lacking pit membranes and perforations with pit membrane remnants were observed. Perforation plates in Psilotum may consist wholly of one type or the other. In Tmespteris, perforations have threadlike pit membranes or consist of porose pit membranes. Wide perforations alternating with narrow pits, a conformation observed in various ferns, were observed in Psilotum (subaerial axes). In Psilotum, perforations are more common in metaxylem than in protoxylem; perforations in protoxylem consist of primary wall areas containing small circular porosities or relatively large circular to oval perforations. There are no modifications in the secondary wall framework of protoxylem or metaxylem in Psilotum or Tmesipteris that would permit one to distinguish presence of perforations or perforation plates with light microscopy, and scanning electron microscopy (SEM) is required for demonstration of porose walls or perforations. The tracheary elements of the Psilotaceae studied have no features not also observed in other ferns with SEM.     

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.     

Pit membrane remnants in perforation plates of Hydrangeales with comments on pit membrane remnant occurrence, physiological significance and phylogenetic distribution in dicotyledons

Perforation plates from ten species of seven genera of Hydrangeales sensu Thorne were studied using scanning electron microscopy (SEM). The presence of pit membranes in perforations ranges from abundant, as in Carpenteria and Hydrangea, to minimal, as in Deutzia, Escallonia and Philadelphus. Abnormally great pit membrane presence may result from the presence of secondary compounds that inhibit lysis, as in Quintinia serrata; such interference with the natural lysis process may or may not be evident from coarseness and irregularity of pit membrane surface and of threads composing the pit membrane remnants. The presence of pit membrane remnants in perforation plates is hypothesized to be a symplesiomorphy, found in a fraction of dicotyledons with scalariform perforation plates (but still in an appreciable number of species). Pit membrane remnant presence may represent incomplete lysis of primary wall material (cellulose microfibrils) in species that occupy highly mesic habitats, where such impedance in the conductive stream does not have an appreciable negative selective value. This physiological interpretation of pit membrane remnants in perforations is enhanced by the phylogenetic distribution as well as the strongly mesic ecological preferences of species that exemplify this phenomenon in dicotyledons at large. Families with pit membrane presence in perforations are scattered throughout phylogenetic trees, but they occur most often in basal branches of major clades (superorders) or as basal branches of orders within the major clades. Further study will doubtless reveal other families and genera in which this phenomenon occurs, although it is readily detected only with SEM. Phylogenetic stages in the disappearance of pit membrane remnants from perforation plates are described, ranging from intact pit membranes except for presence of pores of various sizes, to presence of membrane remnants only at lateral ends of perforations and in one or two perforations (arguably pits) at the transition between a perforation plate and subadjacent lateral wall pitting. Developmental study of the mechanism and timing of lysis of pit membranes in perforations, and assessment of the role of the conductive stream in their removal, are needed to enhance present understanding of vessel evolution. © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society, 2004, 146 , 41–51.     

Vestigial pit membrane remnants in perforation plates and helical thickenings in vessels of Ericaceae

Perforation plates from 15 species of 10 genera with scalarifom perforation plates, representing three subfamilies of woody Ericaceae (Rhododendroideae, Arbutoideae, Vaccinioideae) were studied with scanning electron microscopy (SEM). In most of them, pit membrane remnants were present, but these remnants were less extensive than in the ericalean families Clethraceae, Cyrillaceae, and Sarraceniaceae. Pit membrane remnants in perforations of vessels of Ericaceae are characteristically found at lateral ends of the perforations and in perforations (which may alternatively be called pits) transitional to lateral wall pitting. Pit membrane remnants were most extensive in Enkianthus. Phylogenetic and physiological factors for vestigial membrane presence in the perforations are discussed. Helical thickenings on vessel walls as seen with SEM are figured and described for Leucothoe and Pieris, and their significance is assessed.     

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.     

Origins and nature of vessels in monocotyledons. 2. Juncaginaceae and Scheuchzeriaceae

SEM studies of roots and rhizomes of Triglochin (one species) and Maundia (monotypic) of Juncaginaceae and the sole species of Scheuchzeriaceae, Scheuchzeria palustris, reveals that vessels are present not only in roots, as previously reported, but also in rhizomes. The perforations contain pit membranes with pores of various sizes. Striate pit membranes, like those previously seen in Acorus, occur on pit remnants in peforations and on pit membranes of lateral walls in all genera studied. Grooves interconnecting pit apertures are illustrated for root tracheary elements of Triglochin; this is believed to be a first report of this feature for monocotyledons. The tracheary elements of Juncaginaceae and Scheuchzeriaceae are similar in their thick walls and narrow slitlike pits, lending support to the close relationship between the two families often claimed.     

Bordered pits in xylem of vesselless angiosperms and their possible misinterpretation as perforation plates

Vesselless wood represents a rare phenomenon within the angiosperms, characterizing Amborellaceae, Trochodendraceae and Winteraceae. Anatomical observations of bordered pits and their pit membranes based on light, scanning and transmission electron microscopy (SEM and TEM) are required to understand functional questions surrounding vesselless angiosperms and the potential occurrence of cryptic vessels. Interconduit pit membranes in 11 vesselless species showed a similar ultrastructure as mesophytic vessel‐bearing angiosperms, with a mean thickness of 245 nm (± 53, SD; n = six species). Shrunken, damaged and aspirated pit membranes, which were 52% thinner than pit membranes in fresh samples (n = four species), occurred in all dried‐and‐rehydrated samples, and in fresh latewood of Tetracentron sinense and Trochodendron aralioides. SEM demonstrated that shrunken pit membranes showed artificially enlarged, > 100 nm wide pores. Moreover, perfusion experiments with stem segments of Drimys winteri showed that 20 and 50 nm colloidal gold particles only passed through 2 cm long dried‐and‐rehydrated segments, but not through similar sized fresh ones. These results indicate that pit membrane shrinkage is irreversible and associated with a considerable increase in pore size. Moreover, our findings suggest that pit membrane damage, which may occur in planta, could explain earlier records of vessels in vesselless angiosperms.     

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.     

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.     

Pit membrane remnants in perforation plates and other vessel details of Cornales

Perforation plates and other vessel details as studied with scanning electron microscopy (SEM) have been reported for four species of Cornaceae (s.l.): similar features are shown by the four, suggesting that a more extensive sampling of the family might reveal similar phenomena. Perforation plates contain pit membrane remnants in the form of threads or, less commonly, laminar portions perforated by pores. When least well-represented, the pit membrane remnants are restricted to lateral ends of perforations and to the perforations transitional to lateral wall pitting. Perforations are all clearly bordered. Helical thickenings that do not form a continuous gyre are reported for the vessel walls ofAucuba. The presence of pit membrane remnants in vessel elements of Cornaceae correlates with the mesic habitats occupied by species in this family. The presence and type of pit membrane remnants reported by us in the three genera is very similar, although pit membrane remnants are doubtless a symplesiomorphy and thus not an indicator of relationships. The presence of pit membrane remnants in the three genera, however, does attest to the primitiveness of wood and other features of Cornaceae s.l.     

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. 2. Pteridium

Xylem from roots and rhizomes of two infraspecific taxa of Pteridium aquilinum was studied by means of scanning electron microscopy (SEM). All tracheary elements proved to be vessels. End wall perforation plates were all scalariform, lacked pit membrane remnants in at least the central part of the perforation plate, and varied with respect to width of bars, from wide to tenuous, and with respect to presence of pit membrane remnants. In addition, porose pit membranes on walls that are likely all lateral vessel-to-vessel walls must be considered to be perforations also, although different from those on end walls. Lateral wall perforation plates, hypothesized by one worker on the basis of tylosis presence but denied by another on the basis of light microscopy, were confirmed by demonstration of pores with SEM. In addition, lateral walls of Pteridium vessels bear some grooves interconnecting pit apertures; this feature is newly figured by SEM for ferns. Lateral wall pitting that is not porose may either have striate thickenings of the primary wall or be smooth. Vessel presence and degree of specialization in Pteridium vessels may bear a relationship to the wide ecological tolerances of the genus.     

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.     

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).     

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.     

From the sap's perspective: The nature of vessel surfaces in angiosperm xylem

Premise of the Study

Xylem sap in angiosperms moves under negative pressure in conduits and cell wall pores that are nanometers to micrometers in diameter, so sap is always very close to surfaces. Surfaces matter for water transport because hydrophobic ones favor nucleation of bubbles, and surface chemistry can have strong effects on flow. Vessel walls contain cellulose, hemicellulose, lignin, pectins, proteins, and possibly lipids, but what is the nature of the inner, lumen‐facing surface that is in contact with sap?

Methods

Vessel lumen surfaces of five angiosperms from different lineages were examined via transmission electron microscopy and confocal and fluorescence microscopy, using fluorophores and autofluorescence to detect cell wall components. Elemental composition was studied by energy‐dispersive X‐ray spectroscopy, and treatments with phospholipase C (PLC) were used to test for phospholipids.

Key Results

Vessel surfaces consisted mainly of lignin, with strong cellulose signals confined to pit membranes. Proteins were found mainly in inter‐vessel pits and pectins only on outer rims of pit membranes and in vessel‐parenchyma pits. Continuous layers of lipids were detected on most vessel surfaces and on most pit membranes and were shown by PLC treatment to consist at least partly of phospholipids.

Conclusions

Vessel surfaces appear to be wettable because lignin is not strongly hydrophobic and a coating with amphiphilic lipids would render any surface hydrophilic. New questions arise about these lipids and their possible origins from living xylem cells, especially about their effects on surface tension, surface bubble nucleation, and pit membrane function.     

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.     

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.     

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.