Asymmetrical development of biovulate cones resulting in uniovulate cones in Ephedra rhytidosperma (Ephedraceae)

Female cone morphology in Ephedra, including the number of initiated ovules and mature seeds per cone, provides important taxonomic characters used in sectional or species delimitation within Ephedra. Recent molecular phylogenies have indicated, however, that seed number per cone has changed repeatedly during the evolution of the genus. This study reports on the development of the female cone of E. rhytidosperma, based on a large sample of dissected cones studied under SEM. All cones were initially biovulate, and in the majority of cases, both female reproductive units (FRUs) developed a micropylar tube and formed mature seeds. In a few cases, the FRU pair developed asymmetrically in a cone, with one of them eventually aborting. There was no evidence of fusion of the FRU pair. Phylogenetically, E. rhytidosperma is in a clade with E. equisetina, which has uniovulate cones, and E. gerardiana and E. minuta, which have biovulate cones that also become unispermous via abortion. The biovulate condition may thus be ancestral in this clade.     

Induction of Near-vessellessness in Ephedra campylopoda C. A. Mey.

Near-vessellessness was induced in the secondary xylem of Ephedracampylopoda C. A. Mey. by mechanical bark blocking or by wounding.Both treatments resulted in regions of near-vesselless xylem.Xylem formed after the mechanical bark blocking also had regionsin which the orientation of the axial components was changedfrom axial to lateral. Since either mechanical arrest of phloemand cambial transport or wounding of the cambium almost stoppeddifferentiation into vessels, and instead induced differentiationinto tracheids, it seems that the developmental signal for tracheiddifferentiation is not the same as that for vessels. The possibleregulation of near-vessellessness in Ephedra is discussed.Copyright1994, 1999 Academic Press Differentiation, Ephedra campylopoda, near-vessellessness, wood formation, xylem     

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.     

Variable wood formation and adaptation to the alpine environment of Ephedra pachyclada (Gnetales: Ephedraceae) in the Mustang district, western Nepal

BACKGROUND AND AIMS: Plants of Ephedra normally have vessels, but are known to become nearly vessel-less in some alpine localities. Previous studies implied that wood formation in Ephedra differs fundamentally from that in dicotyledons in which vessel-bearing and vessel-less taxa are systematically distinct. Using E. pachyclada in the Mustang district of Nepal, growing in an altitudinal range of over 2000 m, variation in wood formation and adaptation to alpine environment was studied in this normally vessel-bearing species. METHODS: Variation in wood anatomy and wood formation was observed with conventional optical microscopy. The lengths of three kinds of tracheary elements were measured and statistically analysed against habitat altitude and plant size of the individuals studied. KEY RESULTS: In E. pachyclada three kinds of tracheary elements, vessel elements, tracheids and fibre-tracheids, were nearly equal in length within individuals showing no elongation after differentiation from cambial initials. Tracheary element lengths among individuals had a negative correlation with altitude and a positive correlation with plant size. Multivariate analyses showed that altitude has a stronger correlation with tracheary element lengths than plant height or stem diameter. Moreover, several individuals from high elevations completely lacked vessels, and vessel formation fluctuated even in individuals from lower elevations. CONCLUSIONS: Wood anatomical trends in E. pachyclada are considered as an adaptation to extremely dry conditions in high mountains. Fluctuation in vessel formation in individuals from low elevations indicated that vessels differentiate only when their lateral expansion is allowed. These results showed that E. pachyclada has a different system of wood formation from dicotyledons and supported the opinion that the wood structure of Gnetales is fundamentally different from that of angiosperms.     

From near extinction to diversification by means of a shift in pollination mechanism in the gymnosperm relict Ephedra (Ephedraceae,Gnetales)

Pollination in gymnosperms is usually accomplished by means of wind, but some groups are insect‐pollinated. We show that wind and insect pollination occur in the morphologically uniform genus Ephedra (Gnetales). Based on field experiments over several years, we demonstrate distinct differences between two Ephedra species that grow in sympatry in Greece in pollen dispersal and clump formation, insect visitations and embryo formation when insects are denied access to cones. Ephedra distachya, nested in the core clade of Ephedra, is anemophilous, which is probably the prevailing state in Ephedra. Ephedra foeminea, sister to the remaining species of the genus, is entomophilous and pollinated by a range of diurnal and nocturnal insects. The generalist entomophilous system of E. foeminea, with distinct but infrequent insect visitations, is in many respects similar to that reported for Gnetum and Welwitschia and appears ancestral in Gnetales. The Ephedra lineage is well documented already from the Early Cretaceous, but the diversity declined dramatically during the Late Cretaceous, possibly to near extinction around the Cretaceous–Palaeogene boundary. The clade imbalance between insect‐ and wind‐pollinated lineages is larger than expected by chance and the shift in pollination mode may explain why Ephedra escaped extinction and began to diversify again.     

Distribution of gymnosperms in West Pakistan

Summary In West Pakistan 17 species of Gymnosperms are found wild. Out of these 13 species belong to the conifers and 4 species are of Ephedra. In the Plains only Ephedra foliata Borss. is found. In the outer ranges of Kashmir, Murree Hills, Hazara, Swat and Dir which are under the dominant influence of summer monsoon rains only Conifers consisting of Pinus roxburgii Sarg. Pinus wallichiana A.B.Jacks., Picea smithiana Boiss., Taxus baccata L., Abies pindrow Spach., Abies spectabis Royle are found and are indigenous to the Western Himalayas with the exception of Taxus baccata L. These species show relationship with those found in the Mediterranean and temperate Central European regions. There is not much resemblance between these and the Conifers of adjoining Eastern Himalayas. The Conifers and the Ephedra species found in Quetta, Waziristan, Kurram, Chitral, Gilgit, Indus Kohistan and Upper parts of Dir, Swat, Kaghan and Kishan Ganga which are under the dominant influence of winter rains are inhibited by elements of Pak-Turanio floristic region. The species belonging to this category are Cedrus deodara Loud., Pinus gerardiana Wall., Juniperus macropoda Boiss., Juniperus communis L., Ephedra gerardiana Wall. and Ephedra intermedia Sabr. et C.A.May. Plants of this region show clear affinities with the flora of the Mediterranean region. In the northern parts like Chitral and Gilgit central Asian elements are represented by Ephedra przewalskii Stapf and Juniperus turkestanica Kom.

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Zusammenfassung In West Pakistan gibt es 17 Arten von Gymnospermen, von denen 13 Coniferen und 4 Ephedra sind. Im Flachland gibt es nur Ephedra foliata Boiss. Am äusseren Rande von Kashmir, Murree Hills, Hazara, Swat und Dir, wo der Einfluss der Sommermonsun Regen gross ist findet man Pinus roxburgii Sarg, Pinus wallichiana A.B.Jacks., Picea smithiana Boiss., Taxus baccata L. Abies pindrow Spach. und Abies spectabis Royle. Dies sind einheimische Arten aus West Himalaya mit Ausnahme von Taxus baccata L. Sie haben Verwandtschaft mit den Arten aus der Mediterran und gemässigten Zone Mitteleuropas, aber überhaupt keine mit dem benachbarten östlichen Himalaya Gebiet. Die Coniferen und Ephedra-Arten aus Quetta, Waziristan, Kurram, Chitral, Gilgit, Indus Kohistan, und dem oberen Teil aus Dir, Swat, Kaghan und Kishan Ganga stehen unter dem Einfluss der Winterregen, und bilden das Pak-Turanio floristische Gebiet. Es sind die Arten Cedrus deodara Loud., Pinus gerardiana Wall., Juniperus macropoda Boiss., Juniperus communis L., Ephedra gerardiana Wall., Ephedra intermedia Sabr. et C.A.May. Die Vegetation dieses Gebietes zeigt eindeutige Verwandtschaft mit der Vegetation des Mediterran-Gebietes. In dem nördlichen Teil, wie Chitral und Gilgit sind Ephedra przewalskii Stapf. und Juniperus turkestanica Kom. zentral-asiatische Elemente.

     

THE AERODYNAMICS OF POLLEN CAPTURE IN TWO SYMPATRIC EPHEDRA SPECIES

Wind-tunnel analyses of the behavior of airborne pollen around ovules of two Ephedra species (E. trifurca and E. nevadensis) indicate that at certain airflow speeds (0.5 m/sec and 1.0 m/sec) each species is capable of biasing pollination in favor of conspecific pollen. A computer procedure was designed to evaluate the physical basis for this aerodynamic discrimination. This procedure indicates that differences in size and density confer significantly different inertial properties to the two pollen species. Operating within the specific aerodynamic environments generated around ovules from each species, these differences are sufficient to account for the biases observed in the probability of pollination. Within natural populations, there exists significant variation in pollen size (and possibly in density). Accordingly, it is possible that, under certain ambient wind conditions, ovules from each species can select subsets of the entire airborne population of Ephedra pollen.     

Influence of climatic factors upon tree rings of Larix decidua and L. decidua × L. kaempferi from Pulawy,Poland

Summary Dendroclimatological techniques are used to assess the impact of climatic factors on tree-ring width of Larix decidua and L. decidua × L. kaempferi (= L. x eurolepis) growing in two experimental plots established in 1914 in south-west Poland. One plot included F₁ progeny grown from seeds of an artificial crossing between European and Japanese larch. The other plot included progeny from maternal trees of European larch. Total ring width, earlywood width and latewood widths were dated, standardized and related to monthly climatic data using response function and stepwise multiple regression analyses. Wide rings in larch are associated with high precipitation in May–July, cool conditions in July–September of the preceding year, and cool dry conditions in August. Ring widths in L. x eurolepis are more dependent upon precipitation than ring widths in L. decidua. Latewood widths in L. x eurolepis are more dependent on high temperatures in June and July than latewood in L. decidua as well as total width and earlywood measurements. Variations in latewood were relatively independent of variations in earlywood and total wood. The variability of ring widths in these larches was greater than the variability reported for larches in many alpine sites and for other conifer species in some regions of North America.     

A high frequency of allopolyploid speciation in the gymnospermous genus Ephedra and its possible association with some biological and ecological features

The origin and evolution of polyploids have been studied extensively in angiosperms and ferns but very rarely in gymnosperms. With the exception of three species of conifers, all natural polyploid species of gymnosperms belong to Ephedra, in which more than half of the species show polyploid cytotypes. Here, we investigated the origin and evolution of polyploids of Ephedra distributed in the Qinghai–Tibetan Plateau (QTP) and neighbouring areas. Flow cytometry (FCM) was used to measure the ploidy levels of the sampled species that are represented by multiple individuals from different populations, and then, two single‐copy nuclear genes (LFY and DDB2) and two chloroplast DNA fragments were used to unravel the possible origins and maternal donors of the polyploids. The results indicate that the studied polyploid species are allopolyploids, and suggest that allotetraploidy is a dominant mode of speciation in Ephedra. The high percentage of polyploids in the genus could be related to some of its biological attributes such as vegetative propagation, a relatively high rate of unreduced gamete formation, and a small genome size relative to most other gymnosperms. Significant ecological divergences between allotetraploids and their putative progenitors were detected by PCAs and anova and Tukey's tests, with the exception of E. saxatilis. The overlap of geographical distributions and ecological niches of some diploid species could have provided opportunities for interspecific hybridization and allopolyploid speciation.     

Evolution of seed dispersal in North American <Emphasis Type="Italic">Ephedra</Emphasis>

Related plants often produce seeds that are dispersed in very different ways, raising questions of how and why plants undergo adaptive shifts in key aspects of their reproductive ecology. Here we analyze the evolution of seed dispersal syndromes in an ancient group of plants. Ephedra (Gymnospermae; Gnetales; Ephedraceae) is a genus containing ≈50 species in semiarid ecosystems worldwide and with three distinct types of cones. We collected mature cones and seeds of ten species of Ephedra in southwestern United States and measured nine morphological traits for each species. Principal component analysis and other data characterized three types of Ephedra cones and seeds. Species with dry, winged cone bracts are dispersed by wind (i.e., E. torreyana and E. trifurca), those with succulent, brightly-colored cone bracts are dispersed by frugivorous birds (i.e., E. antisyphilitica), and those with small, dry cone bracts and large seeds are dispersed by seed-caching rodents (e.g., E. viridis and E. californica). Two species (E. funerea and E. nevadensis) have cone and seed morphologies intermediate between two seed dispersal syndromes. Seed and cones traits were mapped onto two recent phylogenies to help reveal the evolutionary history of seed dispersal syndromes. Bird dispersal is thought to be the ancestral form of seed dispersal in ephedras as it is common in the Old World where Ephedra originated, but the three North American species dispersed by birds are not monophyletic. The two wind dispersed species in North America also do not cluster together, suggesting separate origins. Seed dispersal by seed-caching rodents is common in North America and appears to have evolved several times, but this syndrome is absent form other continents. The evolutionary history of Ephedra in North America suggests that the means of seed dispersal has been malleable. Evolutionary shifts were likely linked to changes in ecological conditions.     

Characteristics of the aeropollen dynamics at several localities in Spain

In the Iberian Peninsula and Balearic Islands there are several bioclimatic regions which result in many different and fragmentary landscapes. Results obtained from monitoring 18 localities in Spain allow an outline of the aeropollen dynamics of some of these environments. The major pollen types are: I. trees: Pinus, Qvercus, Olea, Cupressaceae, 2. shrubs: Corylus, Pislacia and 3. herbs: Poaccae, Urticaceae, Chenopodiaceae-Amaranthaceae, Plantago and Asteraceae. In some locations Alnus, Artemisia, Buxus, Betula, Castanea, Eucalyptus, Populus, Ulmus, and Ephedra also have to be considered. Quantitatively, pollen from trees predominates over that from herbs and shrubs, although at some localities this trend varies. The first pollen grains found in the weekly atmospheric analyses are from trees (winter). In spring tree pollen grains share the atmosphere with herb pollen. For this reason it is usual to find peak values of pollen content in the atmosphere during this part of the year, while winter and autumn are characterized by low pollen counts.     

Aerodynamics of Ephedra trifurca

Computer simulations are used to predict the behavior of pollen grains with different physical properties within the acceleration field created around the ovules of the gymnosperm Ephedra trifurca. A modelling procedure is given that (1) calculates the number of pollen grains captured by an ovule's pollination-droplet and (2) gives a correlation between pollination efficiency and the physical properties (= mass, size) of different types of pollen. Based on this procedure, the number of Ephedra pollen grains captured by micropyles can be less than the number captured from other species. However, the mass and size of Ephedra pollen grains appear to coincide with those predicted to yield a local maximum of pollination efficiency, i.e. slightly larger or smaller values of either mass or size would decrease the probability of capture. In addition, the properties of Ephedra pollen grains operate synergistically in the aerodynamic environment around ovules and are focused to collide with pollination-droplets. By analogy, the properties of Ephedra pollen coincide with those predicted for a localized adaptive peak. The physical properties of pollen grain types other than E. trifurca that can maximize pollen capture are not generally represented in the aerobiology of Ephedra during the pollination season. Therefore, the phenology of pollen release, community taxonomic-composition, and the physics of particle capture play collectively important roles in the reproductive success of Ephedra trifurca.     

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.     

The climate sensitivity of Norway spruce [Picea abies (L.) Karst.] in the southeastern European Alps

To investigate the potential of Norway spruce (Picea abies L. Karst) as a palaeoclimate archive in the southeastern European Alps, tree ring chronologies were developed from trees growing at two sites in Slovenia which differed in their ecological and climatological characteristics. Ring width, maximum latewood density, annual height increment and latewood cellulose carbon isotope composition were determined at both sites and the resulting time-series compared with and verified against instrumental climate data for their common period (AD 1960–AD 2002). Results indicate that ring width sensitivity to summer temperature is very site-dependent, with opposing responses at alpine and lowland sites. Maximum density responds to September temperatures, indicating lignification after cell division has ceased. Stable carbon isotopes have most potential, responding strongly to summer temperature in both alpine and lowland stands. Height increment appears relatively insensitive to climate, and is likely to be dominated by local stand dynamics.     

Pollen germination in Welwitschia mirabilis Hook. f.: differences between the polyplicate pollen producing genera of the Gnetales

Pollen grains of the seed plant genera Ephedra L. and Welwitschia Hook. f. (Gnetales) are of similar size, shape, and have a polyplicate exine with alternating thicker and thinner regions. Ephedra pollen is considered inaperturate and the exine is shed during germination, leaving the male gametophyte naked. The shed exine curls up and forms a characteristic structure with transverse striations. Such upcurled exines have been found in situ in Early Cretaceous seeds with affinities to Ephedra. The purpose of this study was to document the germination of Welwitschia pollen and investigate whether they also discard their exine during this process.

The pollen grains of Welwitschia are monoaperturate with a distinct, distal sulcus. During germination, the sulcus splits open and the gametophyte expands to a spherical form that extends out of the exine. The pollen tube starts to grow one or two hours later and as in Ephedra, it is displaced towards one side. The exine is not shed but remains as a “cap” that partly covers the male gametophyte. Thus, in this respect the germination process is distinctly different from that in Ephedra and this study demonstrates that discharging the exine during pollen germination is unique to Ephedra, among the polyplicate pollen producing genera in the Gnetales.     

Tetracentron Wood from the Miocene of Noto Peninsula,Central Japan,with a short revision of homoxylic fossil woods

A vesselless fossil wood was discovered in the Miocene Yanagida Formation in the Noto Peninsula, central Japan. This fossil has distinct growth rings with gradual transition from the early- to the latewood ; tracheids, which are called 'usual traeheids' here, constitute the ground mass of the wood and have typical scalariform bordered pits on radial walls in the earlywood and circular sparse pits on those in the latewood ; rays are 1\2-4 cells wide and heterogeneous with low to high uniseriate wings; axial parenchyma strands are scattered in the latewood. This wood has a peculiar feature; sporadic radial files of broad tracheids whose tangential walls have crowded alternate bordered pits. The radial walls have crowded half-bordered pits to ray cells, but no pits to the usual tracheids. Among all of the extant and extinct angiosperms and gymnosperms, these unusual tracheids occur only in Tetracentron. From these features, we refer the fossil to the extant genus Tetracentron, and name it T. japonoxylum. A revision of homoxylic woods is made for comparision with the present fossil. Tetracentron japonoxylum is the only fossil wood of Tetracentron.     

Hardly a relict: freezing and the evolution of vesselless wood in Winteraceae

The Winteraceae are traditionally regarded as the least-specialized descendents of the first flowering plants, based largely on their lack of xylem vessels. Since vessels have been viewed as a key innovation for angiosperm diversification, Winteraceae have been portrayed as declining relicts, limited to wet forest habitats where their tracheid-based wood does not impose a significant hydraulic constraints. In contrast, phylogenetic analyses place Winteraceae among angiosperm clades with vessels, indicating that their vesselless wood is derived rather than primitive, whereas extension of the Winteraceae fossil record into the Early Cretaceous suggests a more complex ecological history than has been deduced from their current distribution. However, the selective regime and ecological events underlying the possible loss of vessels in Winteraceae have remained enigmatic. Here we examine the hypothesis that vessels were lost as an adaptation to freezing-prone environments in Winteraceae by measuring the responses of xylem water transport to freezing for a diverse group of Winteraceae taxa as compared to Canella winterana (Canellaceae, a close relative with vessels) and sympatric conifer taxa. We found that mean percent loss of xylem water transport capacity following freeze-thaw varied from 0% to 6% for Winteraceae species from freezing-prone temperate climates and approximately 20% in those taxa from tropical (nonfreezing) climates. Similarly, conifers exhibit almost no decrease in xylem hydraulic conductivity following freezing. In contrast, water transport in Canella stems is nearly 85% blocked after freeze-thaw. Although vessel-bearing wood of Canella possesses considerably greaterhydraulic capacity than Winteraceae, nearly 20% of xylem hydraulic conductance remains, a value that is comparable to the hydraulic capacity of vesselless Winteraceae xylem, if the proportion of hydraulic flow through vessels (modeled as ideal capillaries) is removed. Thus, the evolutionary removal of vessels may not necessarily require a deleterious shift to an ineffective vascular system. By integrating Winteraceae's phylogenetic relationships and fossil history with physiological and ecological observations, we suggest that, as ancestors of modern Winteraceae passed through temperate conditions present in Southern Gondwana during the Early Cretaceous, they were exposed to selective pressures against vessel-possession and returned to a vascular system relying on tracheids. These results suggest that the vesselless condition is advantageous in freezing-prone areas, which is supported by the strong bias in the ecological abundance of Winteraceae to wet temperate and tropical alpine habitats, rather than a retained feature from the first vesselless angiosperms. We believe that vesselless wood plays an important role in the ecological abundance of Winteraceae in Southern Hemisphere temperate environments by enabling the retention of leaves and photosynthesis in the face of frequent freeze-thaw events.     

Radial-growth and wood-anatomical changes in overaged Quercus pyrenaica coppice stands: functional responses in a new Mediterranean landscape

Recent land-use changes in intensively managed forests such as Mediterranean coppice stands might profoundly alter their structure and function. We assessed how the abandonment of traditional management practices in coppice stands, which consisted of short cutting-cycles (10–15 years), has caused overaging (stems are usually much older than when they were coppiced) and altered their wood anatomy and hydraulic architecture. We studied the recent changes of wood anatomy, radial growth, and hydraulic architecture in two stands of Quercus pyrenaica, a transitional Mediterranean oak with ring-porous wood forming coppice stands in W–NW Spain. We selected a xeric and a mesic site because of their contrasting climates and disturbance histories. The xeric site experienced an intense defoliation after the severe 1993–1994 summer drought. The mesic site was thinned in late 1994. We studied the temporal variability in width, vessel number and diameter, and predicted the hydraulic conductivities (K _h) of earlywood and latewood. In the mesic site, we estimated the vulnerability to xylem cavitation of earlywood vessels. Overaging caused a steep decline in latewood production at a cambial age of 14 years., which was close to the customary cutting cycle of Q. pyrenaica. The diameter distribution of vessels was bimodal, and latewood vessels only accounted for 4% of the K _h. Overaging, acting as a predisposing factor in the decline episode, was observed at the xeric site, where most trees did not produce latewood in 1993–1995. At the mesic site, thinned trees formed wider tree-rings, more latewood and multiseriate tree-rings than overaged trees. The growth enhancement remained 8 years after thinning. Most of the hydraulic conductivity in earlywood was lost in a narrow range of potentials, between −2.5 and −3.5 MPa. We have shown how hydraulic conductivity and radial growth are closely related in Q. pyrenaica and how aging modulates this relationship.     

The karyology of some Taraxacum species from alpine regions of Europe

The karyotypes of 18 samples assigned to 13 agamospecies in Taraxacum are described. The species are classified among six sections; Eiythrosperma Lindb. f., Vulgaria Dahlst., Alpestria v. Soest, Alpina Hagl., Fontana v. Soest and Cucullata v. Soest, all from alpine localities in Europe. No previous cytological work is known for agamospecies in the last four sections. Two small chromosome types and the occurrence of accessory chromosomes are described for the first time in Taraxacum. Sibling karyotypes were found to be invariable, but extensive karyotypic variation was found between samples, even intraspecifically. Karyotypes were compared by means of a “karyotype similarity index”. Samples with a high overall similarity tended to have smaller chromosomes. Eight species exhibited regular chromosome sets, the remainder being irregular. The nature of the origin of the latter is discussed in relation to the origin of agamospecies in the genus.     

A fossil‐calibrated relaxed clock for Ephedra indicates an Oligocene age for the divergence of Asian and New World clades and Miocene dispersal into South America

Abstract Ephedra comprises approximately 50 species, which are roughly equally distributed between the Old and New World deserts, but not in the intervening regions (amphitropical range). Great heterogeneity in the substitution rates of Gnetales (Ephedra, Gnetum, and Welwitschia) has made it difficult to infer the ages of the major divergence events in Ephedra, such as the timing of the Beringian disjunction in the genus and the entry into South America. Here, we use data from as many Gnetales species and genes as available from GenBank and from a recent study to investigate the timing of the major divergence events. Because of the tradeoff between the amount of missing data and taxon/gene sampling, we reduced the initial matrix of 265 accessions and 12 loci to 95 accessions and 10 loci, and further to 42 species (and 7736 aligned nucleotides) to achieve stationary distributions in the Bayesian molecular clock runs. Results from a relaxed clock with an uncorrelated rates model and fossil‐based calibration reveal that New World species are monophyletic and diverged from their mostly Asian sister clade some 30 mya, fitting with many other Beringian disjunctions. The split between the single North American and the single South American clade occurred approximately 25 mya, well before the closure of the Panamanian Isthmus. Overall, the biogeographic history of Ephedra appears dominated by long‐distance dispersal, but finer‐scale studies are needed to test this hypothesis.