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<jats:p>The embryos of pelagic corals break up in waves yet survive to form smaller colonies.</jats:p>

<jats:title>Earth's Sluggish Past</jats:title> <jats:p> Based on their affinity for iron or silicon, some elements partition into different compartments of Earth's interior. In the early stages of the formation of the solar system, when Earth was rapidly accreting and differentiating to form a core and mantle, elements such as tungsten and hafnium went along for the ride. By studying the signatures of these elements and their isotopes in ancient volcanic rocks from the mantle, <jats:bold> Touboul <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1065" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1216351">1065</jats:related-article> , published online 16 February; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6072" page="1051" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1219126">Bennett</jats:related-article> </jats:bold> ) reveal that there was a long-lived <jats:sup>182</jats:sup> W reservoir that formed deep within Earth during the first 30 million years of the solar system. Because the reservoir persisted for over 1.7 billion years, it appears the mantle was poorly mixed and indeed it was so sluggish that it did not even homogenize during the massive collision that formed the Moon. </jats:p>

<jats:title>Critically Cold Atoms</jats:title> <jats:p> Unlike classical phase transitions, such as the freezing of water into ice, which is driven by lowering the temperature of the system, quantum phase transitions occur at absolute zero and are driven by other parameters, including magnetic field or pressure. In the vicinity of a quantum phase transition, a critical region forms where physical observables obey scaling laws as a consequence of the self-similarity of the system. Quantum phase transitions and quantum criticality are usually observed in solid state, but <jats:bold> Zhang <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1070" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1217990">1070</jats:related-article> , published online 16 February) used an optical lattice filled with a cold gas of atoms to simulate a quantum phase transition—from an insulator to a superflnuid in two dimensions. They observed the characteristic scaling of the equation of state, a finding that will facilitate the building of a platform in a tunable system for further investigations of quantum criticality. </jats:p>

<jats:title>Setting Atoms Free</jats:title> <jats:p> In a plasma, gaseous atoms and molecules accumulate so much energy that they fall apart into electrons and all manner of bigger charged fragments. Free electrons can also wreak a degree of havoc within a liquid environment as they generate local reactive fragments, which then couple with the surrounding solvent to form a slew of products. <jats:bold> Alexander <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1072" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1215956">1072</jats:related-article> ) explored what happens if reactive electrons emerged at the surface of such a liquid. To generate these conditions, they bombarded a shallow film of deuterated glycerol with sodium atoms that ionized on impact and discovered, in contrast with bulk liquid, that solvent-derived deuterium atoms escaped from the surface into the gas phase before they could react any further. </jats:p>

<jats:title>Exclusive Roaming</jats:title> <jats:p> How do polyatomic molecules fall apart? The basic model, supported by centuries of chemical theory and experiment, invokes a series of internal rearrangements that lead to a fleeting high-energy transition-state geometry from which lower-energy products emerge. Over the past decade, several molecules have been shown to manifest a competing dissociation mechanism such that alongside trajectories that pass through the transition state, there are energetically accessible pathways that roam around it. <jats:bold> Grubb <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1075" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1216911">1075</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6072" page="1054" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1218767">Jordan and Kable</jats:related-article> </jats:bold> ) showed that the light-induced reaction of NO <jats:sub>3</jats:sub> to form NO and O <jats:sub>2</jats:sub> proceeded exclusively by roaming. Although there are distinct pathways to the products in two different electronic states, neither one passes through a conventional transition state. </jats:p>

<jats:title>Cold Dust</jats:title> <jats:p> Atmospheric dust affects air quality, air and ocean chemistry, ocean biology, and climate, so understanding its origins is important to many fields. Hot, dry, desert regions at low latitudes are well-understood sources, but the role of higher-latitude regions in dust production has not been considered. <jats:bold> Prospero <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1078" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1217447">1078</jats:related-article> ) present a 6-year record of measurements made on an island south of Iceland, which revealed frequent episodes of dust-production associated with glacial outwash plains and outburst floods. Much of this dust is transported southward and deposited in the North Atlantic, making it a potentially important supply of iron to drive ocean production in that region. </jats:p>

<jats:title>Tree Refugia</jats:title> <jats:p> Ideas of how and when boreal plants spread to the formerly glaciated parts of the world following the retreat of the glaciers 9000 years ago are long debated. Models of the postglacial spread of boreal plants argue for dispersal from southern refugia; however, <jats:bold> Parducci <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1083" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1216043">1083</jats:related-article> ) have shown that both spruce and pine were present in small ice-free regions of Scandinavia much earlier than thought. DNA haplotyping confirmed that a remnant mitochondrial type of spruce, once unique to Scandinavia, now lives alongside the more common spruce originating from Eastern Europe. Evidence from lake cores collected from central and northern Norway indicated the survival of conifers as early as 22,000 years before the present, when apart from ice-free pockets, most of Scandinavia was covered by ice. </jats:p>

<jats:title>Random and Directed</jats:title> <jats:p> Natural selection drives populations to adapt to new environments; the raw material or “founder effects” provided by the first colonizing individuals can thus have a formative influence on the population's future. <jats:bold> Kolbe <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1086" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1209566">1086</jats:related-article> , published online 2 February; see the cover) tested the relative contributions of selection and founder effects in Bahamian lizards. Founders were taken from an island covered in forest: These lizards had long hindlimbs for sprinting across the broad expanses of tree trunks. Long-limbed lizards were introduced to seven smaller islands covered in scrub that, before hurricane Frances in 2004 swept them away, had been populated by lizards with short hindlimbs better suited for navigating a twiggy habitat. After several generations, all the new lizard populations had adapted to their new habitats by evolving shorter hindlimbs but they also retained other morphological and genetic signatures from their founding ancestors. Thus, evolution occurs by a combination of arbitrary events, as well as those shaped by selection. </jats:p>

<jats:title>The Constant Pollinator</jats:title> <jats:p> In ecology, reinforcement is the process by which species prevent hybridization and maintain species boundaries, but the underlying genetic mechanisms are unclear. <jats:bold>Hopkins and Rausher</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1090" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1215198">1090</jats:related-article> , published online 2 February) examined reinforcement between two species of a wild flowering plant called <jats:italic>Phlox</jats:italic> that show incomplete hybrid sterility. Down-regulation of a flavonoid gene produces red flowers and operates in concert with a color intensity locus to adjust flower color and tone. A distinct geography of flower color has emerged in which it appears that dark coloration causes less hybridization between the species because the butterfly pollinators tend to favor light-blue flower color variants. If pollinators visit flowers with similar phenotypes more frequently than flowers with dissimilar phenotypes, this will decrease gene flow between the unlike flowers. </jats:p>

<jats:title>Shape-Shifting Signals</jats:title> <jats:p> Although orthogonal signaling systems seem to direct various developmental processes, few tissues remain in the same shape as they are at initiation to that of the final form. <jats:italic>Arabidopsis</jats:italic> leaves are free of the cell migrations that complicate animal development, and thus allowed <jats:bold> Kuchen <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1092" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1214678">1092</jats:related-article> ) to track and model the trajectory of leaf growth under a variety of perturbations. Varying the values of parameters in their model produced outputs of different leaf shapes ranging from obcordate, ovate, and oval to elliptic, and offered predictions for genes that regulate the developmental process. The meristem at the growing tip of plants is home to stem cells and is the source of newly differentiating shoots and leaves. New leaves make their first appearance as bulges at the side of the dome-shaped meristem. Although these developmental events are under hormonal control, they also seem to be constrained by the physical properties of the meristem. <jats:bold> Kierzkowski <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1096" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1213100">1096</jats:related-article> ) tested physical effects acting on the shoot apical meristem of growing tomato shoots that alter turgor pressure. Again, mathematical modeling combined with observations of plant tissue helped to define the different zones in the meristem that respond to diverse mechanical stimuli. </jats:p>