Mutants of Arabidopsis thaliana deficient in gibberellin synthesis (gal-3 and gal-6), and a gibberellin-insensitive mutant (gai) were compared to the wild-type (WT) Landsberg erecta line for flowering time and leaf number when grown in either short days (SD) or continuous light (CL). The ga 1-3 mutant, which is severely defective in ent-kaurene synthesis because it lacks most of the GAI gene, never flowered in SD unless treated with exogenous gibberellin. After a prolonged period of vegetative growth, this mutant eventually underwent senescence without having produced flower buds. The gai mutant and the "leaky' gal-6 mutant did flower in SD, but took somewhat longer than WT. All the mutants flowered readily in CL, although the gal-3 mutant showed some delay. Unlike WT and gal-3, the gai mutant failed to respond to gibberellin treatment by accelerating flowering in SD. A cold treatment promoted flowering in the WT and gai, but failed to induce flowering in gal-3. From these results, it appears that gibberellin normally plays a role in initiating flowering of Arabidopsis.

The root hairs of plants are tubular projections of root epidermal cells and are suitable for investigating the control of cellular morphogenesis. In wild-type Arabidopsis thaliana (L.) Heynh, growing root hairs were found to exhibit cellular expansion limited to the apical end of the cell, a polarized distribution of organelles in the cytoplasm, and vesicles of several types located near the growing tip. The rhd3 mutant produces short and wavy root hairs with an average volume less than one-third of the wild-type hairs, indicating abnormal cell expansion. The mutant hairs display a striking reduction in vacuole size and a corresponding increase in the relative proportion of cytoplasm throughout hair development. Beadlabeling experiments and ultrastructural analyses indicate that the wavy-hair phenotype of the mutant is caused by asymmetric tip growth, possibly due to abnormally distributed vesicles in cortical areas flanking the hair tips. It is suggested that a major effect of the rhd3 mutation is to inhibit vacuole enlargement which normally accompanies root hair cell expansion.

It is well-established that brain structures and cognitive functions change across the lifespan. A longstanding hypothesis called age differentiation additionally posits that the relations between cognitive functions also change with age. To date however, evidence for age-related differentiation is mixed, and no study has examined differentiation of the relationship between brain and cognition. Here we use multi-group Structural Equation Modeling to study differences within and between brain and cognition across the adult lifespan (18-88 years) in a large (N>646), population-derived sample of healthy adults from the Cambridge Centre for Ageing and Neuroscience (www.cam-can.org). After factor analyses of grey-matter volume (from T1-and T2-weighted MRI) and white-matter organisation (fractional anisotropy from Diffusion-weighted MRI), we found evidence for differentiation for grey and white matter, such that the covariance between brain factors decreased with age. However, we found no evidence for age differentiation between fluid intelligence, language and memory, suggesting a relatively stable covariance pattern between cognitive factors. This pattern is compatible with adaptive reorganization of cognitive functions in the face of neural decline, and/or with the emergence of specific subpopulations in old age. Finally, we observed a specific pattern of age differentiation between brain and cognitive factors, such that a white matter factor, which loaded most strongly on the hippocampal cingulum, became less correlated with memory performance in later life. Key words: Ageing, Differentiation, Grey Matter, White Matter, Structural Equation Modeling Significance statementThe theory of age differentiation posits age-related changes in the relationships between brain structures and between cognitive domains, either weakening (differentiation) or strengthening (de-differentiation), but evidence for this hypothesis is mixed. Using multigroup models in a large cross-sectional adult lifespan sample we found age-related reductions in the covariance among both brain measures (neural differentiation), but no significant changes in the covariance between cognitive factors of fluid intelligence, language and memory. We also observed evidence of uncoupling (differentiation) between white matter factor and memory in older age. Together our findings support age-related differentiation as a complex, multifaceted pattern that differs for brain and cognition, and discuss several mechanisms that might explain the changing relationship between brain and cognition across the lifespan.

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We describe conventional and high-resolution transmission electron microscopy ͑HRTEM͒ characterization of the microstructure of sputtered NiFe/Cu giant magnetoresistance spin valves ͑Cu/FeMn/NiFe/Cu/NiFe͒ sandwiched between thick Nb contact layers. Six spin valves, sputtered at different temperatures, three with thin ͑3 nm͒ and three with thick ͑24 and 30 nm͒ NiFe layers, were studied. All of the spin-valve layers were smooth and continuous, consisting of columnar grains generally 20-90 nm wide. In most cases, the grains had grown epitaxially from the bottom contact, through the entire multilayer, to the top contact layer. The columnar grains grew on the closest-packed planes ͑i.e., ͕110͖ planes for bcc Nb and ͕111͖ planes for fcc Cu, FeMn, and NiFe spin-valve components͒. This epitaxial growth yields an apparent Kurdjumov-Sachs ͕111͖ fcc ʈ ͕110͖ bcc ; ͗110͘ fcc ʈ ͗111͘ bcc orientation relationship. However, HRTEM imaging supported by fast Fourier transform analysis reveals that in some of the columnar grains the Cu, FeMn, and NiFe layers take up a nonequilibrium bcc structure. In these cases, the bcc Cu, FeMn, and NiFe layers grow on ͕110͖ planes and are epitaxial with the Nb contacts for the individual grain columns. While bcc Cu has been observed elsewhere, the length scale of the nonequilibrium bcc phases reported here is an order of magnitude greater than previously observed.

Summary Ultramicrotomy is an effective method of sample preparation for TEM of metal multilayer thin‐films. Large, thin, cross‐sectional areas can be obtained for TEM observations from a small amount of thin‐film which has been mechanically removed from its substrate. An example of a TEM cross‐section made from an AuFe0.03/a‐silicon multilayer used in the study of metallic spin‐glass properties illustrates the usefulness of ultramicrotomy in preparing samples that are largely free of the deleterious effects associated with ion‐milling such as irradiation damage, differential thinning and variable thickness.

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