RESUMO
CeRhIn5 is a new heavy-electron material that crystallizes in a quasi-2D structure that can be viewed as alternating layers of CeIn3 and RhIn2 stacked sequentially along the tetragonal c axis. Application of hydrostatic pressure induces a first-order-like transition from an unconventional antiferromagnetic state to a superconducting state with T(c) = 2.1 K.
RESUMO
Muon spin rotation ( &mgr;SR) measurements of the temperature dependence of the &mgr;(+) Knight shift in single crystals of U0. 965Th0.035Be13 have been used to study the static spin susceptibility chi(s) below the transition temperatures T(c1) and T(c2). While an abrupt reduction of chi(s) with decreasing temperature is observed below T(c1), chi(s) does not change below T(c2) and remains at a value below the normal-state susceptibility chi(n). In the normal state we find an anomalous anisotropic temperature dependence of the transferred hyperfine coupling between the &mgr;(+) spin and the U 5f electrons.
RESUMO
Floral display (the size, number, and arrangement of open flowers) influences pollinator visitation to animal-pollinated plants and should be an important determinant of reproductive success. We examined variation in the size and number of open flowers in wild daffodils (Narcissus). Our analysis of published data on 45 taxa showed that flower number varied negatively with flower diameter among Narcissus species, which supports the widespread assumption that there is a trade-off between these traits. In contrast, field measurements indicated a positive relation between flower number and diameter within two populations of Narcissus dubius, and no relation was evident after we controlled for variation in bulb size. The discrepancy between inter- and intraspecific patterns may have occurred because variable resource levels obscure trade-offs when variation in flower size is low (e.g., within species). Size-related increases in floral tube length were half as great as corresponding increases in flower diameter, a result that is consistent with stronger stabilizing selection on tube length. Staggered flowering within N. dubius inflorescences limited the mean number of open flowers to <66% of total flower number, and slow expansion by later opening flowers resulted in significant differences in flower size throughout flowering. Although pollinators preferred large flowers, experimental reductions in flower diameter did not affect seed production. Our results illustrate how the relative importance of the factors influencing floral display can vary among levels of biological organization. Interspecific variation in flower size and number appeared to be constrained by allocation trade-offs, but intraspecific variation in both traits was more greatly influenced by plant resource status. Within plants, the size and number of open flowers reflected the relative age of individual flowers and floral longevity.
RESUMO
Colonization and isolation are critical events in the evolutionary dynamics of plant populations. In this paper I review how spatial population structure of genetic markers provides insights into the evolutionary significance of episodes of colonization and isolation in the Mediterranean flora. I use as themes to structure my review the following topics: spatial structure induced by historical associations among populations of widespread species; population differentiation in relation to the evolution of closely related species with disjunct distributions; the potential effect of founder events during colonization on character evolution; and the conservation implications of spatial population structure. My review illustrates that the Mediterranean flora is full of examples that provide key insights into such evolutionary and conservation issues.
