STRUCTURE OF TRACHEIDS IN THREE SPECIES OF LYCOPODIUM

The structure of tracheids in Lycopodium lucidulum, L. clavatum, and L. tristachyum was studied with the light microscope. Protoxylem development is at least sometimes and possibly always mesarch in indeterminate axes of all three species. Centrifugally formed protoxylem elements are reticulate and discontinuities in the secondary walls of these elements are sometimes conspicuously bordered. Wall thickenings of first formed protoxylem elements consist mainly of indirectly connected rings. Late centripetally formed protoxylem elements and transitional elements have a reticulate secondary wall pattern. The narrowest metaxylem elements have circular bordered pits while in wider metaxylem elements pits are bordered and may vary from circular to scalariform. Pitting is uniseriate to triseriate in tracheids of all three species, and intermittent tetraseriate pitting was occasionally observed in L. lucidulum. Crassulae occur in tracheids of the three species, and in L. clavatum an additional framework, probably representing thickened compound middle lamella, is also present. Pits often appear helically arranged, and in all three species pits are connected by thin areas in the secondary wall. Macrofibrils approximately 0.5 μ wide were observed in tracheids of the three species. In L. clavatum the arrangement of macro-fibrils was predominantly bidirectional.     

Mechanism for formation of the secondary wall thickening in tracheary elements: Microtubules and microfibrils of tracheary elements of Pisum sativum L. and Commelina communis L. and the effects of amiprophosmethyl

Arrangements of microfibrils (MFs) and microtubules (MTs) were examined in tracheary elements (TEs) of Pisum sativum L. and Commelina communis L. by production of replicas of cryo-sections, and by immunofluorescence microscopy, respectively. The secondary wall thickenings of TEs of Pisum and Commelina roots have pitted and latticed patterns, respectively. Most MFs in the pitted thickening of Pisum TEs retain a parallel alignment as they pass around the periphery of pits. However, some groups of MFs grow into the pits but then terminate at the edge of the thickening, indicating that cellulose-synthase complexes are inactivated in the plasma membrane under the pit. Microtubules of TEs of both Pisum and Commelina are localized under the secondary thickening and few MTs are detected in the areas between wall thickenings. In the presence of the MT-disrupting agent, amiprophosmethyl, cellulose and hemicellulose, which is specific to secondary thickening, are deposited in deformed patterns in TEs of Pisum roots, Pisum epicotyls and Commelina roots. This indicates that the localized deposition of hemicellulose as well as cellulose involves MTs. The deformed, but heterogeneous pattern of secondary thickening is still visible, indicating that MTs are involved in determining and maintaining the regular patterns of the secondary thickening but not the spatial heterogeneous pattern of the wall deposition. A working hypothesis for the formation of the secondary thickening is proposed.Abbreviations APM amiprophosmethyl - DMSO dimethyl sulfoxide - F-WGA fluorescein-conjugated wheat-germ agglutinin - M F microfibril - MT microtubule - PEG polyethyleneglycol - TE tracheary element I thank Ms. Aiko Hirata (Institute of Applied Microbiology, University of Tokyo, Japan) for help in taking stereomicrographs. This work was supported in part by a Grant-in-Aid from the Ministry of Education, Science and Culture of Japan.     

PATTERNS OF ENDOTHECIAL WALL THICKENINGS IN ARACEAE: SUBFAMILIES POTHOIDEAE AND MONSTEROIDEAE

A survey of the patterns of endothecial wall thickenings in 106 representative species from 20 genera in the Pothoideae and Monsteroideae was made using cleared anthers, sections and macerations. The wide variety of wall thickenings that is present is based on an annular-helical pattern. Variations in thickenings are related to differences in cell shape, cell orientation, intergradation between helical and annular patterns, pitch of helices, presence of branched thickenings, and various types of discontinuities in thickenings. Notable exceptions to the annular-helical pattern include Culcasia, which lacks a differentiated endothecial layer with thickenings, and Acorus, which has a peculiar stellate pattern that is unique in the family. No single pattern consistently characterizes either subfamily, although continuous helices are common in the Monsteroideae, and rare in the endothecium of Pothoideae (except Anadendrum). Monsteroideae frequently exhibit a series of slanted separate thickenings on anticlinal walls, which is absent from Pothoideae except in Heteropsis. The slanted pattern is considered a variation on a rectangular helix, involving discontinuities of thickenings on the periclinal walls. Some monsteroid genera show considerably more interspecific variation (Rhaphidophora) than others (Monstera). Endothecial thickenings constitute an anatomical character that is useful in the systematic study of Araceae; present results support other anatomical studies in identifying Culcasia and Acorus as highly divergent genera in the Pothoideae.     

MODIFIED REMNANTS OF CENTRIPETAL XYLEM IN THE LEAF TRACE OF CEPHALOTAXUS

A study of the leaf traces at the nodes in various species and varietal forms of Taxus, Torreya, Amentotaxus and Cephalotaxus reveals, only in Cephalotaxus, an unusual type of parenchymatous tissue associated with the xylem of the leaf trace. The cells of this tissue occur in one to three layers, have abundant cytoplasm and conspicuous nuclei. The thin walls of these cells are devoid of pits and show spiral or spiral-reticulate thickenings. These thickenings, although readily taking the counterstain, reveal the presence of lignin as determined by the phloroglucin test and by fluorescence microscopy. This tissue is always internal—that is, adaxial—to the protoxylem of the trace. From the node it accompanies the trace for a short distance into the leaf base, where it gives way centrally to the typical leaf parenchyma and laterally to the transfusion tissue on the flanks of the bundle. In the basipetal direction it is in contact with the inner face of the annular protoxylem of the stele, eventually disappearing in the typical pith parenchyma. This tissue occasionally reveals instances of well-lignified tracheid-like centripetal elements. On the basis of the characteristics of this tissue, it is suggested that its origin lies in former centripetal xylem. The significance of this tissue to the evolution of the stele and the systematic position of Cephalotaxus is discussed.     

Arrangement of cortical microtubules during formation of bordered pit in the tracheids ofTaxus

Summary The arrangement of cortical microtubules during the development of the secondary wall and bordered pits in the tracheids ofTaxus was examined by immunofluorescence and electron microscopy. The cambial region of radial longitudinal sections of developing young shoots (2–3 years old) contains cells at various stages of differentiation from cambial cells to tracheids. At the early stage of formation of bordered pits, circular bands of microtubules were seen to be associated with the inner edge of the border of the developing pit. In other regions than the pit secondary wall of uniform thickness was laid down, and obliquely oriented cortical microtubules ran parallel to one another. These cortical microtubules also covered the surface of the border of the developing pit on the side facing the center of the cell. As the border of the pit developed, a circular band of MTs remained associated with the inner edge of border, suggesting that the MTs were involved in the formation of the rim of the bordered pit, extending the initial border thickening, which consisted of concentrically oriented cellulose microfibrils. After completion of the formation of the bordered pit, helical thickenings became apparent. The obliquely oriented microtubules were organized in bands parallel to one another, being superimposed on the helical thickenings. The involvement of MTs in the formation of bordered pits and helical thickening is discussed.     

A phi Layer in Roots of Ceratonia siliqua L

The central cylinder of the primary root of the carob tree (Ceratonia siliqua) is encircled by a layer of cells with wall thickenings, known as a phi (φ) cell layer. The development of the φ layer and the chemical composition of the cell wall thickenings have been studied in roots of C. siliqua. The results reveal the presence of condensed tannins in the mature phi thickenings and that the development of the φ layer is asynchronous: at 0–1 cm from the root tip φ thickenings appear before endodermis differentiation at the sites opposite phloem, at 1–4 cm new φ thickenings are developed at the sites opposite xylem, at 4–7 cm the φ layer consists of two layers of cells and it completely encloses the central cylinder.     

Formation of the pits with taste buds at the lingual root in the striped dolphin,Stenella coeruleoalba

The tongue of the striped dolphin, Stenella coeruleoalba, shows a V-shaped row of pits on its posterior dorsum. Their development is described on the basis of macroscopic and light microscopic observations on fetal, young, and adult stages. Four to eight pits occur, most often five in the adult. Anlagen of the pits first protrude as round epithelial thickenings which later increase in diameter and become thin. The circular primordia then sink, and grooves oriented both circularly and radially develop in the walls of the shallow pits thus formed. Pits and grooves deepen with development so that older pits become lined with conical projections. As pits grow further, they become elongated anterolaterally, retaining slit-like openings. Each pit in the adult is 2–8 mm long and about 1 mm wide. The pits are not derived from lingual gland ducts but develop independently. Taste buds resembling those of other mammalian tongues can be found in young dolphins but are few in number and limited to the thin epithelium of the pit projections and to that of the side wall of the pits. They first appear in the late prenatal period but degenerate in the adult. A rich nerve supply is observable in the lamina propria below taste buds in the calf. The pits and their projections in the dolphin correspond to the vallate papillae of other mammals, but whether each projection or a whole pit corresponds to a single vallate papilla is undecided.     

ENDOTHECIUM IN IRIDACEAE AND ITS SYSTEMATIC IMPLICATIONS

An examination of the endothecial thickenings in 44 species of Iridaceae, selected from the four subfamilies and all major tribes, provides useful information about generic and tribal relationships in the family. U-shaped thickenings occur in Nivenioideae and Iridoideae—Sisyrinchieae, the latter the least specialized tribe of its subfamily. The occurrence of helical thickenings in all members examined of Iridiodeae tribes Irideae, Mariceae, and Tigridieae (a putatively monophyletic group) and Ixioideae is consistent with the recognition of these two lines as distinct taxa based on anatomical, morphological, phytochemical, and in the case of Ixioideae, palynological criteria. Baseplate thickenings are restricted to Patersonia. However, the shrubby Cape genera—Nivenia, Klattia, and Witsenia—have U-shaped thickenings which show a tendency for the bars on the inner periclinal cell walls to anastomose, suggesting a trend towards the baseplate condition in Patersonia. This accords with a suggested relationship between these genera, based on anatomical and flavonoid similarities. The pattern of variation in endothecial thickenings in Iridaceae is consistent with the phylogeny proposed by Goldblatt (1990). The distribution of thickening types within the family does not make it possible to polarize this character, but the most parsimonious interpretation assumes that U-shapes are basic. However, in at least some other monocotyledonous families the pattern suggests that U-shaped thickenings are derived from helices.     

A SYSTEMATIC STUDY OF ENDOTHECIAL THICKENINGS IN THE ORCHIDACEAE

Endothecial cell thickenings were examined in anther macerations of representative species from 210 genera of the Orchidaceae. Nearly all species examined possessed the characteristic thickened walls which, in several tested species, gave a positive reaction to phloroglucinol, indicating the presence of lignin. Four basic thickening types were identified; distribution of the types was found to be largely in agreement with previously recognized suprageneric groups. Type I thickenings are tightly packed channels of loops or helices and were found in the “neottioid” genera, the Apostasioideae, and putatively basal genera in the remaining subfamilies. Because of its occurrence in the Apostasioideae, which is believed to be the most basal subfamily, Type I is hypothesized to be the plesiomorphic thickening type for the remainder of the Orchidaceae. Type II thickenings appear as scattered loops and may be a synapomorphy for the Orchidoideae, as they were found in all genera sampled from the subfamily except Disperis. Type III thickenings are circular in appearance and were found in the Cypripedioideae and in some members of the Spiranthoideae and Epidendroideae. Type IV thickenings show little regular arrangement, appearing to be scattered bars, and were observed primarily in the Epidendroideae and also in some Spiranthoideae. Three subtypes were recognized in Type III and Type IV. Some genera, such as Triphora, Goodyera, and Elythranthera, had thickenings that appeared intermediate between the recognized types. In general, terrestrial genera were found to have regularly arranged, well-developed thickenings, while many epiphytic groups showed congested, irregular, thinner thickenings.     

CLADISTIC ANALYSIS OF PATTERNS OF ENDOTHECIAL THICKENINGS IN THE POALES/RESTIONALES

The three-dimensional structure of the endothecial thickenings in the anthers was investigated in 87 species from 70 genera, chosen to provide representative coverage of the families Cyperaceae, Restionaceae, Anarthriaceae, Ecdeiocoleaceae, Centrolepidaceae, Joinvilleaceae, Flagellariaceae, Poaceae, Xyridaceae, and Eriocaulaceae. There is complex variation in the patterns of endothecial thickening: the Eriocaulaceae, Flagellariaceae, and most Poaceae have thickenings with a complete baseplate; the Cyperaceae and most of the Restionaceae are characterized by helical thickenings; some genera in the Bambusoideae have annular thickenings; and U-shaped thickenings occur in the Xyridaceae and Eriocaulaceae and in some Poaceae and Restionaceae. Joinvillea and Ecdeiocolea have unique thickening types. Endothecial characters were subjected to cladistic analysis. Including endothecial characters in an existing cladogram of the group indicates that there is no single, well-corroborated cladogram available for the Poales/Restionales.     

Fruit and Seed Structures in Menodora (Oleaceae): a Comparison with Jasminum

The fruit development in Menodora agrees largely with that in Jasminum, except for the final stages. The seed coat resembles that of Jasminum in the presence of ribbon-shaped wall thickenings in the mesotestal layers, a character not known from any other Oleaceae. In several characters there is more similarity between Menodora and Jasminum sect. Alternifolia, or between Menodora and J. nudiflorum, than there is among the species of Jasminum. This supports the idea that Jasminum might be paraphyletic, if Menodora is retained as a separate genus.     

Effects of environmental changes on sugars,tannins, and organized growth in cell suspension cultures of white spruce

Summary Callus from hypocotyls of white spruce (Picea glauca [Moench] Voss) was grown on agar under defined conditions with high levels of calcium nitrate. Transfer of callus to liquid suspension cultures and maintenance of suspensions either under a regime of constant temperature and light or under alternating conditions similar to those of a late spring day, affected the content of free sugars, tannins, and aldehydes. Under the alternating conditions the levels of these substances increased greatly compared to those under the constant environment. By contrast, vascularization of cell clumps, which was comparable to the differentiation of hypocotyls in seedlings, was obtained only under constant conditions. Cells at the centre of the clumps developed secondary wall thickenings and bordered pits, and were surrounded by cambial-like initials.     

Duality of Secretory Inclusions in Neurones — Ultrastructure of the Corresponding Sites of Release in Invertebrate Nervous Systems

Central nerve terminals have been examined for ultrastructural signs of release of neurochemical mediators in the annelids Nereis diversicolor, Harmothoe imbricata and Lumbricus terrestris. Two categories of presumptive secretory inclusions are readily distinguished. Exocytosis of ‘storage granules’ is widespread in the neuropile, and involves probable peptidergic terminals as well as more conventional terminals. Plasma membranes at such sites of release are apparently unmodified. In contrast, ‘synaptic vesicles’ are aggregated adjacent to membrane thickenings and specialized clefts, and signs of their fusion with the presynaptic membranes have been observed rarely. The presence of coated pits surmounting omega profiles involving storage granules may indicate that membrane is retrieved in the form of microvesicles from the site of exocytosis. Coated pits associated with synapses have only been observed in areas of membrane adjacent to presumed sites of vesicle exocytosis. The incidence of dual sites of release, often relating to individual terminals, may be indicative of the segregated storage and independent secretion of distinct active principles. Materials released by granule exocytosis may have the role of neuromodulators.     

Systematic and phylogenetic value of wood anatomy in Heteromorpheae (Apiaceae,Apioideae)

The wood anatomy of all four woody genera of the tribe Heteromorpheae (Apiaceae, subfamily Apioideae) has been described and compared, based on 40 wood samples (representing nine species of Anginon, one species of Glia, three species of Heteromorpha and two species of Polemannia). The four genera were found to be relatively similar in their wood anatomy. Helical thickenings on the vessel walls occur in all species investigated and appear to represent an ancestral character state and a symplesiomorphy for the tribes Bupleurieae and Heteromorpheae. Each of four genera has a diagnostically different combination of character states relating to the diameter of vessels, size of intervessel pits, length of fibres, presence and arrangement of banded axial parenchyma, size of rays and ray cells, and presence of septate fibres and crystals in the ray cells. The occurrence of marginal axial parenchyma in Anginon and Glia may be an additional synapomorphy for these taxa. Variation in the wood anatomy of 31 samples from nine species of Anginon is not correlated with habitat (Fynbos or Succulent Karoo Biomes), but instead appears to reflect adaptations to seasonal aridity found in both ecosystems. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158 , 569–583.     

A cytoskeletal basis for wood formation in angiosperm trees: the involvement of cortical microtubules

Rearrangements of cortical microtubules (CMTs) during the differentiation of axial secondary xylem elements within taproots and shoots of Aesculus hippocastanum L. (horse-chestnut) are described. A correlative approach was employed using indirect immunofluorescence microscopy of α-tubulin in 6- to 10-μm sections and transmission electron microscopy of ultrathin sections. All cell types – fibres, vessel elements and axial parenchyma – derive from fusiform cambial cells which contain randomly oriented CMTs. At the early stages of development, fibres and axial parenchyma cells possess helically arranged CMTs, which increase in number as secondary wall thickening proceeds and simple pits develop. In contrast, incipient vessel elements are distinguished by the marking out of sites of bordered pits; these sites first appear as microtubule-free regions within the reticulum of randomly oriented CMTs that characterises their precursor fusiform cambial cells. Subsequently, the ring of CMTs which develops at the periphery of the microtubule-free region decreases in diameter as the over-arching pit border is formed. Like bordered pits, large-diameter, non-bordered pits (contact pits) which develop between vessel elements and adjacent contact ray cells originate as microtubule-free regions and are also associated with development of a ring of CMTs at the periphery. In the case of contact pits, however, there is no reduction in the diameter of the CMT ring during pit development. Tertiary cell wall thickenings are also a feature of vessel elements and appear to form at sites where bands of laterally associated, transversely oriented CMTs, separated from each other by microtubule-free zones, are found. Later, these bands of CMTs become narrower, and separate into pairs of microtubule bundles located on each side of the developing wall thickening. Development of perforations between vessel elements is also associated with the presence of a ring of CMTs at their periphery. Received: 13 July 1998 / Accepted: 30 November 1998     

Morphology and taxonomy of Chondria tenuissima and Chondria dasyphylla (Rhodomelaceae,Rhodophyta) from European waters

Recent collections of fertile Chondria tenuissima, the type species of the genus, and Chondria dasyphylla (Woodward) C. Ag. from European waters have clarified details of their morphology and reproduction. This has allowed more detailed comparisons with southern Australian material hitherto placed under these species and has shown that neither occurs on these coasts. Records of these species from other countries may thus be open to doubt.

Ruthenium Red-positive cell wall thickenings are present in some populations of both species. In C. tenuissima the thickenings are thin and lenticular or band-like, occurring on both the upper ends and inner and radial walls of pericentral and subcortical cells. In C. dasyphylla the thickenings occur as band-like caps on upper ends of these cells. Older pericentral cells may also develop additional, separate thickenings on the inner and radial walls, and in cells near the base of the plant these become lobed.

The production of an auxiliary cell after fertilization of the procarp has been observed in both species. However its purported absence in material of C. tenuissima examined by Phillips (1896, p. 19) is not discounted as this situation has been observed in a number of southern Australian species of Chondria in which the division of the supporting cell is delayed until after the diploid nucleus has been transferred from the carpogonium. This variation appears to be more common in the Ceramiales than previously realized, however it does not appear to be sufficient to invalidate Kylin's differentiation of the order.     

Eucalyptus stomata with occluded anterior chambers

Summary This paper describes variations of a mode of stomatal development already described in a species (E. orbifolia) ofEucalyptus L'Herit. (Carr andCarr, Protoplasma 96, 127, 1978) in which the outer part of the stomatal pore (ostiole) is formed by the creation of a break in the thin layer of cuticle lying over the stomatal chamber. In a number of species with a thick cuticle (e.g., E. cooperana) the process of breakthrough is different: additions to the guard cell upper thickenings extend from them as ridges, pressing the leaf cuticle outwards. Breakthrough of the cuticle occurs above the tips of these extensions. The anterior chamber is lined throughout by the extensions, which become heavily cutinized. This mode of stomatal development is typical of many other species of eucalypts, including those dealt with in this paper.In addition, inE. halophila thickenings develop on the end walls of the anterior chamber above the unusual upturned poles of the guard cells. Cutinized thickenings, pseudo-outer stomatal ledges are also formed on the upper guard cell walls. All these wall thickenings occlude the anterior chamber, leaving only a narrow passage in the form of a letter H.Similar occlusions are found inE. balladoniensis but here the thickenings are developed into the chamber from its lateral walls. InE. gracilis, and the related speciesE. celastroides andE. calycogona, less regular occluding thickenings develop principally from the lateral walls of the chamber. In addition, large pseudo-outer stomatal ledges may be formed.These phenomena are discussed in terms of the mechanism underlying the formation of the occluding thickenings and the possibility of their adaptive significance.     

Pit membranes in tracheary elements of Rosaceae and related families: new records of tori and pseudotori

The micromorphology of pits in tracheary elements was examined in 35 species representing 29 genera of Rosaceae and related families to evaluate the assumption that angiosperm pits are largely invariant. In most Rosaceae, pit membranes between fibers and tracheids frequently appear to have amorphous thickenings with an irregular distribution. Although these structures are torus-like under the light microscope, observations by electron microscopy illustrate that they represent "pseudotori" or plasmodesmata-associated thickenings. These thickenings frequently extend from the periphery of the pit membrane and form a cap-like, hollow structure. Pseudotori are occasionally found in few Elaeagnaceae and Rhamnaceae and appear to be related to species with fiber-tracheids and/or tracheids. True tori are strongly associated with round to oval pit apertures and are consistently present in narrow tracheary elements of Cercocarpus (Rosaceae), Planera (Ulmaceae), and ring-porous species of Ulmus and Zelkova (Ulmaceae). Vestured pits with homogenous pit membranes are reported for Hemiptelea (Ulmaceae). The homoplastic nature of pit membrane characteristics may be related to functional adaptations in terms of safety and efficiency of water transport or may reflect different developmental processes of xylem elements. These observations illustrate that there is more variation in angiosperm pits than previously thought.     

Inhibition of phenylalanine ammonia-lyase activity causes the depression of lignin accumulation of secondary wall thickening in isolatedZinnia mesophyll cells

Summary Isolated mesophyll cells ofZinnia elegans L. cv. Canary Bird differentiate into tracheary elements in differentiation (D) medium. These elements develop lignified secondary wall thickenings. The influence of 2-aminoindan-2-phosphonic acid (AIP), an inhibitor of phenylalanine ammonia-lyase (PAL), on lignification ofZinnia tracheary elements was examined. The mesophyll cells were cultured in D and AIP media. The latter medium, in which 100 M AIP was added to the D medium, inhibited PAL activity, though the differentiation proceeded. Morphological differences of secondary wall thickenings cultured in these two types of media were investigated under an UV microscope and a transmission electron microscope. The secondary wall thickenings at 96 h in the D medium showed strong UV absorption. The fibrillar structure of the thickenings observed clearly at 72 h was covered with electron opaque materials by 96 h. The secondary wall thickenings at 96 h in the AIP medium showed weak UV absorption. The thickenings at 96 h had a cracked appearance. Furthermore, the thickenings showed a little irregular or wavy arrangement of cellulose microfibrils and had many pores and spaces between microfibrils. From these results, the role of lignin accumulation in the formation of secondary wall thickenings was discussed.Abbreviations AIP 2-aminoindan-2-phosphonic acid - PAL phenylalanine ammonia-lyase     

Strukturelle Grundlagen des Parasitismus beiOrobanche III. Die Differenzierung des Xylemanschlusses beiO. crenata

Zusammenfassung Innerhalb des Wirtsxylems wurden Haustorialzellen des WurzelparasitenOrobanche lichtund elektronenoptisch untersucht. Diese Zellen durchlaufen eine ungewöhnliche Differenzierung bis sie wasserleitendes Xylemanschlu\element sind. Von Haustorialzellen mit stark verdickten FrontwÄnden entwickeln sie sich bei Eintritt in das Wirtsxylem zu einer typischen Transferzelle mit polar zum Holzelement des Wirts angelegtem Wandlabyrinth. Erst durch einen zweiten Differenzierungsschritt wird die Transferzelle zum Wasserleitelement, indem die typischen SekundÄrwandverdickungen des Xylems in der Zelle angelegt werden. Diese entstehen teilweise innerhalb des Wandlabyrinths und sind stets gegenüber denjenigen des Wirtselements angelegt. Zuletzt wird das Labyrinth — bis auf gelegentliche Reststrukturen — abgebaut, der Protoplast degeneriert, und es entsteht ein haustoriales Wasserleitelement, das über kommunizierende Tüpfel an das Wirtselement angeschlossen ist.

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Structural features of parasitism ofOrobanche III. The differentiation of xylem connexion ofO. crenata

Summary Haustorial cells of the root parasiteOrobanche within the xylem of the host tissue were investigated by light- and electronmicroscopy. Coming into contact with the tracheary elements of the host these cells show an unusual differentiation before turning into a water conducting xylem element. From haustorial cells with thickened front walls they develop into typical transfer cells, bearing wall ingrowth in those parts of the wall orientated towards the tracheary elements of the host. During further differentiation the transfer cell changes into a water conducting element by developing the typical secondary wall thickenings of xylem elements within the cell. Partly these wall thickenings are formed inside the labyrinth structures of the transfer cell, always situated opposite those of the tracheary element of the host. Simultaneously the labyrinth disintegrates—some small remnants of wall ingrowths may persist. The protoplast degenerates, and finally a haustorial water conducting element results. Host- und parasitic tracheary elements are connected by pits.

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Wir danken FrauChristl Glockmann für ihre stets verantwortungsvolle Mitarbeit. Den GÄrtnern des Botanischen Gartens in Kiel sei Dank für die oft schwierige Anzucht des Pflanzenmaterials. Die Untersuchungen wurden durch Mittel der Deutschen Forschungsgemeinschaft gefördert.