Cacti (Cactaceae) represent a family of highly specialized angiosperm plants with a native range of distribution restricted to the American continents. Columnar cacti of the sub-family Cactoideae evolved in adaptation to their arid or semi-arid habitats characteristics that distinguish them from most other dicot plants, e.g. the stem succulence with a strongly vascularized storage parenchyma and the presence of the spine wearing areoles. Although cacti have been in cultivation since the discovery of America, some studies even suggest the agricultural use in pre-colombian times, and many scientific investigations were carried out on the functional morphology and anatomy with regard to biomechanical adaptations of the found structures, no research focused on the branch-stem attachment. The most conspicuous features of such a ramification are the pronounced constrictions at the branch-stem junctions that are also present in the lignified vascular structures within the succulent cortex. Based on Finite Element Analyses of ramification models it could be demonstrated that these indentations in the region of high flexural and torsional stresses are not regions of structural weakness, e.g. allowing vegetative propagation. On the contrary, they can be regarded as anatomical adaptations to increase the stability by fine-tuning the stress state and stress directions in the junction along prevalent fiber directions. The development of the woody support structure within the succulent cortex of the parental shoot can be traced back to the leaf and bud traces of the dormant axillary buds. Surprisingly, these initials also develop into another woody structure supporting the flowers of the cacti. As these two support structures differ significantly in their macroscopic and microscopic anatomy and as they develop from the same initial state as leaf/bud traces, another objective of this work was to analyze the secondary growth of the two structures with traditional botanic investigation methods. The results of these investigations reveal a wood dimorphism consisting of an early parenchymatous phase followed later by fibrous wood in both kind of support structure. In vegetative branches, the woody support structures have the typical ringlike arrangement as found in the stele of the parental shoot, whereas the flower support structures have a reticular arrangement of interconnected woody strands. This fundamentally different anatomy of the support structures results from the formation of an interfascicular cambium between the leaf/bud traces when a vegetative branch forms or its absence in the case of a flower. After shedding light on the functional morphology and anatomy of the cactus ramification and their development the question arises if the found load adaptation strategies may serve to improve technical fiber composite structures analogue to the design recommendation developed from the biomechanical analyses of tree ramifications. Such a biomimetic transfer from the cactus ramification as biological role model to a technical implementation and the adaptation of the fine-tuned geometric shape and arrangement of lignified strengthening tissues might contribute to the development of alternative concepts for branched fiber-reinforced composite structures within a limited design space.

Cette étude se situe dans le cadre des recherches menées par le CEAEA sur la production massive d'hydrogène, sans émission de gaz à effet de serre, via un cycle thermo-chimique de décomposition de l'eau couplé à une source de chaleur à haute température d'origine nucléaire. Dans le cas particulier du cycle dit« Iode-Soufre», on doit extraire H2 à partir d'un mélange H2/HI/H20 très corrosif, opération pour laquelle des procédés membranaires ont été proposés. L'objectif de ce travail est le développement de membranes zéolithiques de type MFI susceptibles d'être utilisées dans ce contexte. Nous présentons les différents matériaux utilisés, la méthodologie de synthèse de couches minces de Silicalite-1 et de ZSM-5 synthétisée sans structurant organique, les techniques de caractérisation des membranes. Une étude cinétique nous a permis d'optimiser et de contrôler les conditions d'obtention de ces couches minces déposées sur des substrats tubulaires en Ti02 et plans en Al2O3-α. De nombreuses expériences de perméation ont été réalisées, pour des gaz simples (H2, He, Ar, N 2, C02, SF6) et des mélanges gazeux (H2/H20/Ar) et (H2/H20/HI/Ar). Les effets de la température, de la pression amont, de l'épaisseur et de la longueur de la couche mince ainsi que du gaz vecteur ont été étudiés en détail. Il apparaît que la présence de molécules d'H20 dans le système joue un rôle prépondérant sur la perméation des autres molécules. / In the general context of massive and "carbon free" hydrogen production studies, the aim of this work was the development of zeolitic MFI membranes for hydrogen extraction and separation. The methodology of synthesis, the membranes characterization techniques as well as the permeation experimental setup are presented. Optimization and control of the elaboration of Ti02 supported Silicalite-1 and template free ZSM-5 membranes have been reached. Details of the full kinetic study that we performed are given. Numerous permeation experiments, involving pure gas (H2, He, Ar, N2, C02, SF6) and mixtures (H2/H20/Ar) and (H2/H 20/HI/Ar) have been carried on. The effects of temperature, feed pressure, thickness and length of the membranes, as well as the role of the sweeping gas have been emphasized. In the case of gas mixtures, the presence of H20 molecules appears to be a predominant factor.