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Leaf traits in succulent and non-succulent plants show both similar constraints and divergent strategies in water storage and resources use
No full textAxial variation of xylem conduits in giant cacti
Full text linkGiant columnar cacti store massive amounts of water in their parenchymous storage tissues in order
to persist under conditions of extreme aridity. Nevertheless, the relationship between stem water
storage capacity and the maximum efficiency to deliver water from the roots to stem storage tissues
via xylem vessels remains largely unknown. Indeed, the relationship between the axial water flow in
xylem and the lateral flow through the storage tissue may affect the xylem structure and, therefore,
the plant water conduction strategies. Since the axial structure of vascular conduits has been
demonstrated to be universal (i.e. in a broad spectrum of plant species xylem conduits widen
basipetally at the same rate), we wanted to determine if both the vessel size and wall thickness in
giant cactae follow the same general rule in spite of the buffer action of water storage tissue. To
address these hypotheses, we are investigating anatomical variation in xylem structural traits and
storage volume in the stems of giant cacti species belonging to different phylogenetic lineages that
are native to both the Northern and Southern hemisphere (e.g.Pachycereus weberi, Echinopsis
terschekii, Carnegiea gigantea). We collected cross-sections from 6 to 13 samples along the stem
of each plant. We found that vessel lumina increased basipetally following a widening rate similar to
what has been documented by the theoretical model (WBE model) and from existing surveys on a
wide range of tree species. The conduits double wall thickness (t) and its span (s) ratio decrease
basipetally and interplay to reduce the risk of cell collapse. We concluded that the xylem architecture
of columnar cacti in this study was not influenced by the buffering action of the surrounding storage
tissue, and that axial water transport efficiency is maintained for the length of the path as in many
other plant species
Tracheid and pit anatomy vary in tandem in a tall Sequoiadendron giganteum tree
No full textAcross land plants there is a general pattern of xylem conduit diameters widening towards the stem base thus reducing the accumulation of hydraulic resistance as plants grow taller. In conifers, xylem conduits consist of cells with closed end-walls and water must flow through bordered pits imbedded in the side walls. As a consequence both cell size, which determines the numbers of walls that the conductive stream of water must cross, as well as the characteristics of the pits themselves, crucially affect total hydraulic resistance. Because both conduit size and pit features influence hydraulic resistance in tandem, we hypothesized that features of both should vary predictably with one another. To test this prediction we sampled a single tall (94.8 m) Sequoiadendron giganteum tree (giant sequoia), collecting wood samples from the most recent annual ring progressively downwards from the tree top to the base. We measured tracheid diameter and length, number of pits per tracheid, and the areas of pit apertures, tori, and margos. Tracheid diameter widened from treetop to base following a power law with an exponent (tracheid diameterstem length slope) of approximately 0.20. A similar scaling exponent was found between tracheid length and distance from tree top. Additionally, pit aperture, torus, and margo areas all increased (again with a power of ∼0.20) with distance from tree top, paralleling the observed variation in tracheid diameter and length. Pit density scaled isometrically with tracheid length. Within individual tracheids, total permeable area of pits, measured as the sum of the margo areas, scaled isometrically with lumen area. Given that pores of the margo membrane are believed to increase in parallel with membrane area, from a strictly anatomical perspective, our results support the interpretation that pit resistance remains a relatively constant fraction of total resistance along the hydraulic pathway. © 2016 International Association of Wood Anatomists
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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