Néoformation de jeunes plantes d'Elaeis guineensis à partir de cals primaires obtenus sur fragments foliaires cultivés in vitro

Une méthode rapide de multiplication végétative du palmier à huile a été obtenue par la voie d'une embryogenèse somatique sur cals primaires. Ces cals sont obtenus à partir de tissus foliaires, dans un délai de 2 à 3 mois après la mise en culture. Les embryoïdes apparaissent sur cals isolés environ 2 mois après leur isolement et leur repiquage en conditions "organogènes

La multiplication végétative in vitro du palmier à huile par embryogenèse somatique

Le procédé de multiplication végétative in vitro du palmier à huile, proposé par Rabéchault et Martin [1976] a été amélioré. Soixante et un clones sous forme de cals primaires ont été obtenus à partir de jeunes feuilles d'arbres adultes ou de plants de pépinière. Les cals primaires évoluent, après une période qui a été considérablement abrégée, en cultures à croissance rapide (CCR). Les CCR, dans des conditions appropriées, différencient des embryoïdes somatiques. Le traitement des embryoïdes permet leur développement en plantules viables. Les palmiers issus des premiers embryoïdes obtenus en 1976 ont fleuri en juillet 198

Influence of wiring cost on the large-scale architecture of human cortical connectivity

In the past two decades some fundamental properties of cortical connectivity have been discovered: small-world structure, pronounced hierarchical and modular organisation, and strong core and rich-club structures. A common assumption when interpreting results of this kind is that the observed structural properties are present to enable the brain's function. However, the brain is also embedded into the limited space of the skull and its wiring has associated developmental and metabolic costs. These basic physical and economic aspects place separate, often conflicting, constraints on the brain's connectivity, which must be characterized in order to understand the true relationship between brain structure and function. To address this challenge, here we ask which, and to what extent, aspects of the structural organisation of the brain are conserved if we preserve specific spatial and topological properties of the brain but otherwise randomise its connectivity. We perform a comparative analysis of a connectivity map of the cortical connectome both on high- and low-resolutions utilising three different types of surrogate networks: spatially unconstrained (‘random’), connection length preserving (‘spatial’), and connection length optimised (‘reduced’) surrogates. We find that unconstrained randomisation markedly diminishes all investigated architectural properties of cortical connectivity. By contrast, spatial and reduced surrogates largely preserve most properties and, interestingly, often more so in the reduced surrogates. Specifically, our results suggest that the cortical network is less tightly integrated than its spatial constraints would allow, but more strongly segregated than its spatial constraints would necessitate. We additionally find that hierarchical organisation and rich-club structure of the cortical connectivity are largely preserved in spatial and reduced surrogates and hence may be partially attributable to cortical wiring constraints. In contrast, the high modularity and strong s-core of the high-resolution cortical network are significantly stronger than in the surrogates, underlining their potential functional relevance in the brain

Low-rank matrix decompositions for ab initio nuclear structure

The extension of ab initio quantum many-body theory to higher accuracy and larger systems is intrinsically limited by the handling of large data objects in form of wave-function expansions and/or many-body operators. In this work we present matrix factorization techniques as a systematically improvable and robust tool to significantly reduce the computational cost in many-body applications at the price of introducing a moderate decomposition error. We demonstrate the power of this approach for the nuclear two-body systems, for many-body perturbation theory calculations of symmetric nuclear matter, and for non-perturbative in-medium similarity renormalization group simulations of finite nuclei. Establishing low-rank expansions of chiral nuclear interactions offers possibilities to reformulate many-body methods in ways that take advantage of tensor factorization strategies

Least-square approach for singular value decompositions of scattering problems

It was recently observed that chiral two-body interactions can be efficientlyrepresented using matrix factorization techniques such as the singular valuedecomposition. However, the exploitation of these low-rank structures in a few-or many-body framework is nontrivial and requires reformulations thatexplicitly utilize the decomposition format. In this work, we present a generalleast-square approach that is applicable to different few- and many-bodyframeworks and allows for an efficient reduction to a low number of singularvalues in the least-square iteration. We verify the feasibility of theleast-square approach by solving the Lippmann-Schwinger equation in factorizedform. The resulting low-rank approximations of the $T$ matrix are found tofully capture scattering observables. Potential applications of theleast-square approach to other frameworks with the goal of employing tensorfactorization techniques are discussed.<br

Low-Rank Decompositions of Three-Nucleon Forces via Randomized Projections

Ab initio calculations for nuclei and nuclear matter are limited by the computational requirements of processing large data objects. In this work, we develop low-rank singular value decompositions for chiral three-nucleon interactions, which dominate these limitations. In order to handle the large dimensions in representing three-body operators, we use randomized decomposition techniques. We study in detail the sensitivity of different three-nucleon topologies to low-rank matrix factorizations. The developed low-rank three-nucleon interactions are benchmarked in Faddeev calculations of the triton and ab initio calculations of medium-mass nuclei. Exploiting low-rank properties of nuclear interactions will be particularly important for the extension of ab initio studies to heavier and deformed systems, where storage requirements will exceed the computational capacities of the most advanced high-performance-computing facilities.Comment: 7 pages, 4 figure

Oxidative Coupling as a Biomimetic Approach to the Synthesis of Scytonemin

The first total synthesis of the dimeric alkaloid pigment scytonemin is described. The key transformations In Its synthesis from 3-indole acetic acid are a Heck carbocyclization and a Suzuki-Miyaura cross-coupling, orchestrated In a stereospecific tandem fashion, followed by a biosynthetically inspired oxidative dimerization. The tandem sequence generates a tetracyclic (E)-3-(arylidene)-3,4-dihydrocyclopenta[b]indol-2(1H)-one that is subsequently dimerized into the unique homodimeric core structure of scytonemin