A system of one-shot induction of flowering in Arabidopsis thaliana, ecotype Columbia, is described. Plants from vernalized seeds are grown for 2 months in 8 h short days at an irradiance of 48 mumol m-2 sec-1 (fluorescent light only). At that age they can be induced to flower by exposure to either a single long day or a single displaced short day. Non-induced plants stay vegetative for at least a further month. Synchrony of induction among the individuals of the population exposed to one long day is of the same order as in the best classical model plants, that is, the fastest individuals are only 6 h ahead of the slowest ones. A further advantage of this system is the large size of plants at the time of induction, allowing easy analysis of changes in leaves, leaf exudate and shoot meristem. The design of such a synchronous system will allow the timings of gene activations and deactivations to be established in the different plant parts, before flowers are initiated.
The structure of the £=9 and £=11 tilt grain boundaries about [011] axis is studied by means of complementary electron microscopy techniques. Using α'fringes it is shown that the £=1l boundary translations amount to a large expansion and that the £ =9 is little dilated while the translation along [411] is not accessible. The diffraction technique yields the missing result : there is no translation along [411] : the boundary periodicity is not in both cases that of the coincidence site lattice. A supplementary symmetry occurs in the £ =9 boundary which leads to a shortest periodicity. The observations are consistent with the model of five- and seven-fold rings describing the £ =9 boundary. A model, without dangling bond, which consists in a combination of the £ =9 and £ =3 (coherent twin) patterns, is proposed for the £ =ll boundary.
L'affinement de la structure de U₂₀C₃Si₁₆ est exposé ici. L'accent a été mis sur les méthodes expérimentales mises en œuvre pour surmonter les difficultés dues à l'absorption importante du monocristal examiné.
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