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排序方式: 共有188条查询结果,搜索用时 296 毫秒
121.
122.
A new tricyclic agent with an allenyl side chain experimentally shows antidepressant activity similar to amitriptyline and imipramine but also exerts marked CNS depression. Such dual activity should be of clinical interest for treatment of mixed anxiety and depression. 相似文献
123.
Social values and research in human embryology 总被引:1,自引:0,他引:1
124.
WF Laurance DC Useche J Rendeiro M Kalka CJ Bradshaw SP Sloan SG Laurance M Campbell K Abernethy P Alvarez V Arroyo-Rodriguez P Ashton J Benítez-Malvido A Blom KS Bobo CH Cannon M Cao R Carroll C Chapman R Coates M Cords F Danielsen B De Dijn E Dinerstein MA Donnelly D Edwards F Edwards N Farwig P Fashing PM Forget M Foster G Gale D Harris R Harrison J Hart S Karpanty WJ Kress J Krishnaswamy W Logsdon J Lovett W Magnusson F Maisels AR Marshall D McClearn D Mudappa MR Nielsen R Pearson N Pitman 《Nature》2012,489(7415):290-294
The rapid disruption of tropical forests probably imperils global biodiversity more than any other contemporary phenomenon. With deforestation advancing quickly, protected areas are increasingly becoming final refuges for threatened species and natural ecosystem processes. However, many protected areas in the tropics are themselves vulnerable to human encroachment and other environmental stresses. As pressures mount, it is vital to know whether existing reserves can sustain their biodiversity. A critical constraint in addressing this question has been that data describing a broad array of biodiversity groups have been unavailable for a sufficiently large and representative sample of reserves. Here we present a uniquely comprehensive data set on changes over the past 20 to 30 years in 31 functional groups of species and 21 potential drivers of environmental change, for 60 protected areas stratified across the world’s major tropical regions. Our analysis reveals great variation in reserve ‘health’: about half of all reserves have been effective or performed passably, but the rest are experiencing an erosion of biodiversity that is often alarmingly widespread taxonomically and functionally. Habitat disruption, hunting and forest-product exploitation were the strongest predictors of declining reserve health. Crucially, environmental changes immediately outside reserves seemed nearly as important as those inside in determining their ecological fate, with changes inside reserves strongly mirroring those occurring around them. These findings suggest that tropical protected areas are often intimately linked ecologically to their surrounding habitats, and that a failure to stem broad-scale loss and degradation of such habitats could sharply increase the likelihood of serious biodiversity declines. 相似文献
125.
Gregori G Ravasio A Murphy CD Schaar K Baird A Bell AR Benuzzi-Mounaix A Bingham R Constantin C Drake RP Edwards M Everson ET Gregory CD Kuramitsu Y Lau W Mithen J Niemann C Park HS Remington BA Reville B Robinson AP Ryutov DD Sakawa Y Yang S Woolsey NC Koenig M Miniati F 《Nature》2012,481(7382):480-483
The standard model for the origin of galactic magnetic fields is through the amplification of seed fields via dynamo or turbulent processes to the level consistent with present observations. Although other mechanisms may also operate, currents from misaligned pressure and temperature gradients (the Biermann battery process) inevitably accompany the formation of galaxies in the absence of a primordial field. Driven by geometrical asymmetries in shocks associated with the collapse of protogalactic structures, the Biermann battery is believed to generate tiny seed fields to a level of about 10(-21)?gauss (refs 7, 8). With the advent of high-power laser systems in the past two decades, a new area of research has opened in which, using simple scaling relations, astrophysical environments can effectively be reproduced in the laboratory. Here we report the results of an experiment that produced seed magnetic fields by the Biermann battery effect. We show that these results can be scaled to the intergalactic medium, where turbulence, acting on timescales of around 700 million years, can amplify the seed fields sufficiently to affect galaxy evolution. 相似文献
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127.
Zusammenfassung Die morphologische Beobachtung von getrennten cytoplasmatischen Phasen in geschlossenen Membranen führt zur Annahme von Membranpotentialen innerhalb der Zelle. Schreibt man den Membranen des endoplasmatischen Reticulums die Eigenschaften der Plasmamembran zu, so leiten sie die Erregung ins Zellinnere und dienen als Potentialspeicher. 相似文献
128.
DINGLili LIANGHanwen ZHUYixin MOWeiheng WANGQiang RENHongqiang WANGXiaorong M.Edwards D.Glindemann 《科学通报(英文版)》2005,50(12):1274-1276
Phosphine (PH3), a highly toxic and reductive gas, has been explored in biogases and it proves also to be ubiquitous even in remote atmospheric air at a concentration in the order of (pg-ng) /m^3 [3]. For more than one hundred years, sources and mechanisms of biological phosphine formation in natural and engineered environments have been investigated and discussed. Anaerobic environments were reported to be the main source of phosphine emission. 相似文献
129.
The specificity of histones in nucleated erythrocytes 总被引:3,自引:0,他引:3
130.
Fertilization and cleavage in vitro of preovulator human oocytes 总被引:5,自引:0,他引:5