Modélisation par éléments finis de la croissance du tube pollinique.

RÉSUMÉ Dans le domaine de la biologie végétale, la reproduction sexuelle des plantes par pollinisation est un sujet d’intérêt présentant plusieurs enjeux humains et économiques. Le tube pollinique est une cellule végétale indispensable à la pollinisation des fleurs. La croissance de la cellule végétale se fait par une déformation de la paroi l’entourant sous l’effet de la pression hydrostatique interne, la turgescence. Simultanément, de nouveaux matériaux doivent être livrés vers la paroi pour empêcher qu’elle n’éclate à cause de l'amincissement causé par l'étirement. Chez le tube pollinique, l'expansion de surface est localisée à l’apex de la cellule et le résultat du processus est une protubérance de forme cylindrique. Ce mode de croissance combiné au système biologique particulier du tube pollinique en fait un modèle privilégié pour les études mécaniques des cellules végétales en croissance. De nombreux modèles portant sur la croissance apicale ont été construits afin d’étudier les paramètres et processus impliqués. Ces modèles présentent différentes manières d’aborder le problème que ce soit des approches géométriques, mathématiques ou mécaniques mais dans chacun des constantes demeurent comme l’axisymétrie du tube. La méthode des éléments finis permet de représenter des formes complexes ou de changer facilement les modèles mathématiques utilisés pour les calculs. Ce potentiel est particulièrement adapté pour l’étude de structures biologiques amenées à changer de forme ou de propriétés mécaniques. Cette approche de modélisation par éléments finis a été privilégiée dans ce projet. Le premier objectif de ce projet de maitrise visait à développer un modèle par éléments finis représentant la croissance du tube pollinique permettant de changer facilement les paramètres mécaniques, géométriques et de chargement du tube pollinique. Le modèle a été construit en se basant sur l’observation expérimentale, soit un long cylindre avec un rayon de 6 μm, terminé par un apex de la forme d’un sphéroïde prolate. La paroi, d’une épaisseur moyenne fixée à 50 nm, a été modélisée par des éléments de type SHELL 181 via un maillage structuré. Ces éléments possèdent un comportement de type «coque» adéquat pour représenter la paroi et pourraient être modifiés dans le futur pour intégrer la viscoplasticité. Le tube a cependant été soumis à un comportement linéaire élastique dans un premier temps. Le coefficient de Poisson utilisé était alors fixé à 0.3. Les principaux paramètres du modèle incluaient entre autres les modules d’Young de la paroi cellulaire ainsi que leur gradient le long du tube, dans le but de tenir compte----------ABSTRACT In the field of plant biology, sexual reproduction through pollination is a topic of interest since it affects human life and economic issues. The pollen tube is a plant cell that is indispensable for the pollinisation of flowers. Plant cell growth occurs by a deformation of the surrounding wall under the effect of an internal hydrostatic pressure, the turgor pressure. At the same time, new material has to be delivered to the wall in order to prevent bursting caused by the thinning of the wall that in turn is a result of the turgor induced stretching. In the pollen tube, surface growth is located at the apex of the cell and it leads to the formation of a cylindrical protuberance. This type of growth, combined to the particular biological system of the pollen tube, provides an excellent model for mechanical studies of growing plant cells. Numerous models for apical growth have been used to study the parameters and processes involved. These models present different methods to approach the problem whether it is geometrical, mathematical or mechanical. However, most of these models depend on the axisymmetry of the tubular structure. The finite element method allows one to represent complex shapes or to easily change the mathematical models used for calculation. This potential is particularly suited for the study of biological structures that are able to change shape or to manipulate their mechanical properties. The finite element modeling approach has been adopted in this project. The first objective of this project was to develop a finite element model representing the growth of the pollen tube that would allow to easily change mechanical parameters, geometry and loading. The model was constructed based on experimental observation. It consisted in a long cylinder with a radius of 6 μm, terminated by an apex shaped as a prolate spheroid. The wall, with an average thickness fixed at 50 nm was modeled by elements of type SHELL 181 via a structured mesh. These elements behave as a thin "shell" and are thus an appropriate choice to represent the relatively thin wall of the pollen tube. Although they offer the possibility to incorporate viscoplastic material properties, we have chosen a linear elastic behavior for this first version of the model. The value for Poisson’s coefficient was fixed at 0.3. The main parameters of the model included the Young’s modulus of the cell wall and its gradient along the tube, in order to take into account the changing biochemical composition of the cell wall between the apex and the cylindrical region. To cover a large number of possible cases, different values fo

A mindset of competition versus cooperation moderates the impact of social comparison on self-evaluation

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On the Limitations of Manipulation Checks: An Obstacle Toward Cumulative Science

Manipulation checks do not allow ruling out or accepting alternative explanations of causal effects (Sigall & Mills, 1998). In order to gauge the influence of this argument on current research practices, we surveyed the views of researchers on manipulation checks. Results confirmed that a manipulation check still stands as a totem of experimental rigor. Except in rare circumstances, such as when pilot testing, manipulation checks do not provide information relevant to construct validity. While it seems cost free to include seemingly informative manipulation checks, we claim it is actually costly because it wrongly enhances subjective confidence in the validity of research findings. We conclude that manipulation checks may hinder efforts to adopt a cumulative culture and practice of hypothesis testing

Fitting motivational content and process:A systematic investigation of fit between value-framing and self-regulation

Objective: Values are often phrased as ideals that people seek to approach, but they can also be conceptualized as counter-ideals that people seek to avoid. We aimed to test whether individuals endorse more strongly values that are framed in line with their predominant self-regulatory motivation, using individual difference scales in promotion/prevention (Higgins, 1997) and in behavioral approach/inhibition (Carver & White, 1994). To address this systematically, we developed approach- and avoidance-framed versions of the Portrait Value Questionnaire-RR (PVQ-RR; Schwartz et al., 2012). Method: Participants completed approach- and avoidance-framed PVQ-RR versions in two studies measuring regulatory focus or motivational orientation (together 414 U.S. adults, 48% female, ages 18-69) and one study manipulating motivational orientation (39 UK high school students, 79% female, ages 16-19). Results: Value framing consistently interacted with both self-regulation variables. However, a fit between self-regulation and value framing resulted in greater value endorsement only for promotion-focused and approach-oriented (not prevention-focused and avoidance-oriented) participants. This may be because values are more naturally understood as ideal states that people seek to approach. Conclusions: Our findings provide first insights into the psychological process of person-value framing fit affecting value endorsement. We discuss implications for cross-cultural value research and research on value-congruent behavior

An Osmotic Model of the Growing Pollen Tube

Pollen tube growth is central to the sexual reproduction of plants and is a longstanding model for cellular tip growth. For rapid tip growth, cell wall deposition and hardening must balance the rate of osmotic water uptake, and this involves the control of turgor pressure. Pressure contributes directly to both the driving force for water entry and tip expansion causing thinning of wall material. Understanding tip growth requires an analysis of the coordination of these processes and their regulation. Here we develop a quantitative physiological model which includes water entry by osmosis, the incorporation of cell wall material and the spreading of that material as a film at the tip. Parameters of the model have been determined from the literature and from measurements, by light, confocal and electron microscopy, together with results from experiments made on dye entry and plasmolysis in Lilium longiflorum. The model yields values of variables such as osmotic and turgor pressure, growth rates and wall thickness. The model and its predictive capacity were tested by comparing programmed simulations with experimental observations following perturbations of the growth medium. The model explains the role of turgor pressure and its observed constancy during oscillations; the stability of wall thickness under different conditions, without which the cell would burst; and some surprising properties such as the need for restricting osmotic permeability to a constant area near the tip, which was experimentally confirmed. To achieve both constancy of pressure and wall thickness under the range of conditions observed in steady-state growth the model reveals the need for a sensor that detects the driving potential for water entry and controls the deposition rate of wall material at the tip

Abundance and scarcity: classical theories of money, bank balance sheets and business models, and the British restriction of 1797‐1818.

The thesis looks through the lens of bank balance sheet accounting to investigate the structural change in the British banking system between 1780 and 1832, and how classical quantity theorists of money attempted to respond to the ensuing financialisation of the wartime economy with its growing reliance on credit funded with paper-based instruments (the ‘Vansittart system’ of war finance). The thesis combines contributions to three separate fields to construct a holistic historical example of the challenges faced by monetary economists when ‘modelling’ financial innovation, credit growth, ‘fringe’ banking, and agent incentives – at a time of radical experimentation: the suspension of the 80-year-old gold standard (“the Restriction”). First, critical text analysis of the history of economics argues that the 1809-10 debate between Ricardo and Bosanquet at the peak of the credit boom, bifurcated classical theory into two timeless competing policy paradigms advocating the ‘Scarcity’ or ‘Abundance’ of money relative to exchange transactions. The competing hypotheses regarding the role of money and credit are identified and the rest of the thesis examines the archival evidence for each. Second, the core of the thesis contributes to the historical literature on banking in relation to money by reconstructing a taxonomy of bank business models, their relationships with the London inter-bank settlement system, and their responses to the Restriction - drawing on some 17,000 mostly new data points collected from the financial records of London and Country banks. The final section contributes to the economic history of money by constructing aggregated views of total bank liabilities from the firm-level data, scaled to recently available British GDP estimates. These are examined to establish (with hindsight) the relative merits and lacuna of the competing theoretical hypotheses postulated by political economists. It was the period of deleveraging after 1810 that revealed the lacuna of both paradigms

Hermès dans les Dionysiaques de Nonnos de Panopolis

The careful study of the passages relating to Hermes (interventions in the narration, mentions made by other characters) in the Dionysiacs by Nonnos of Panopolis shows a very intricate god's figure, composed of borrowings from various traditions, old or new, well known or unusual. Though that complex figure does not allow to perceive the philosophical or religious poet's believes, it appears as a manifestation of the poikilia which defines his poetics.L'étude attentive des passages consacrés à Hermès (interventions dans le récit, mentions par d'autres personnages) dans les Dionysiaques de Nonnos de Panopolis fait apparaître une image du dieu très composite, faite d'emprunts à des traditions diverses, anciennes ou récentes, courantes ou rares. Si cette image complexe ne permet guère de saisir les convictions philosophiques ou religieuses du poète, elle apparaît en revanche comme une manifestation de la poikilia qui définit son art poétique.Fayant Marie-Christine. Hermès dans les Dionysiaques de Nonnos de Panopolis. In: Revue des Études Grecques, tome 111, Janvier-juin 1998. pp. 145-159

Ampélos, Carpos et Hylas. Nonnos face à Théocrite et Apollonios de Rhodes

L’influence, souvent soulignée, de la poésie hellénistique sur les Dionysiaques de Nonnos de Panopolis revêt des formes diverses. Dans l’épisode d’Ampélos, lorsqu’il évoque la disparition du jeune Satyre aimé de Dionysos, dont la métamorphose posthume donne naissance à la vigne (chants 10-12), Nonnos, par un jeu subtil d’intertextualité avec le chant I des Argonautiques d’Apollonios de Rhodes et l’Idylle XIII de Théocrite, invite à rapprocher le drame d’Ampélos et la légende d’Hylas, éromène d’Héraclès dont la disparition met fin à la participation du héros à l’expédition des Argonautes. L’article analyse les procédés mis en œuvre par Nonnos pour faire référence à ses prédécesseurs hellénistiques et les significations possibles, tant littéraires que mythologiques, de ces renvois à la légende d’Hylas.The influence, often remarked upon, of hellenistic poetry on Nonnus’ Dionysiaca, assumes different forms. In the Ampelus episode, when Nonnus evokes the disappearance of the young satire, loved by Dionysus, whose posthumous metamorphosis gives birth to the vine (Books 10-12), the poet, through subtle intertextual play with Book I of Apollonius of Rhodes’ Argonautica and Theocritus’ Idyll XIII, invites his reader to compare Ampelus’ drama with the legend of Hylas, Heracles’ eromenos, whose disappearance puts an end to the participation of that hero in the Argonauts’ expedition. The article analyses the processes Nonnus uses to reference his hellenistic predecessors, and also the possible meanings, both literary and mythological, of these reflections upon the Hylas legend.L’influsso, spesso sottolineato, della poesia ellenistica sulle Dionisiache di Nonno di Panopoli si riveste di forme differenti. Nell’episodio di Ampelo, quando egli evoca la scomparsa le giovane satiro amato da Dioniso la cui metamorfosi postuma dà vita alla vigna (canti 10-12), Nonno, in un sottile gioco di intertestualità con il primo canto delle Argonautiche di Apollonio Rodio e l’Idillio XIII di Teocrito, invita a confrontare il dramma di Ampelo e la leggenda di Ila, eromenos di Eracle la cui scomparsa mette fine alla partecipazione dell’eroe alla spedizione degli Argonauti. L’articolo analizza i procedimenti messi in atto da Nonno per far riferimento ai suoi predecessori ellenistici e i possibili significati, tanto letterari quanto mitologici, di queste allusioni alla leggenda di Ila

La création lexicale dans les Hymnes orphiques

Une des caractéristiques formelles des Hymnes orphiques, qui a d’ailleurs contribué à leur discrédit, c’est d’être constitués de séries d’épithètes qui tentent de cerner la personnalité de la divinité invoquée. Même si cette caractéristique est moins systématique qu’on ne le prétend parfois, certains hymnes comportant aussi des parties narratives, il n’en demeure pas moins que chaque divinité reçoit de nombreuses épithètes, traditionnelles ou nouvelles. Paradoxalement, peu d’études ont été co..