全文获取类型
收费全文 | 13731篇 |
免费 | 1221篇 |
国内免费 | 880篇 |
出版年
2024年 | 26篇 |
2023年 | 249篇 |
2022年 | 207篇 |
2021年 | 402篇 |
2020年 | 467篇 |
2019年 | 726篇 |
2018年 | 590篇 |
2017年 | 603篇 |
2016年 | 571篇 |
2015年 | 528篇 |
2014年 | 643篇 |
2013年 | 1349篇 |
2012年 | 413篇 |
2011年 | 581篇 |
2010年 | 523篇 |
2009年 | 735篇 |
2008年 | 796篇 |
2007年 | 656篇 |
2006年 | 704篇 |
2005年 | 548篇 |
2004年 | 546篇 |
2003年 | 478篇 |
2002年 | 416篇 |
2001年 | 305篇 |
2000年 | 304篇 |
1999年 | 274篇 |
1998年 | 261篇 |
1997年 | 234篇 |
1996年 | 215篇 |
1995年 | 193篇 |
1994年 | 169篇 |
1993年 | 128篇 |
1992年 | 139篇 |
1991年 | 117篇 |
1990年 | 122篇 |
1989年 | 79篇 |
1988年 | 63篇 |
1987年 | 69篇 |
1986年 | 59篇 |
1985年 | 71篇 |
1984年 | 74篇 |
1983年 | 19篇 |
1982年 | 44篇 |
1981年 | 39篇 |
1980年 | 32篇 |
1979年 | 13篇 |
1978年 | 20篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1973年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 280 毫秒
11.
Abstract The conformations of the chains constituting the hydrophilic component of alkyl monolayers and bilayers are investigated by performing molecular dynamics atomistic simulations on these systems at different temperatures. Several monitoring techniques are used to reveal the chain conformations, including atom pair radial distribution functions, evolutions of the torsional angles over thousands of timesteps, frequency distributions of the torsionl angles and ‘snapshot’ plots of the atomic configurations. These methods consistently testify to the stability of the trans (fully extended) character of the strain-free alkyl chains up to room temperature. The chains retain much of this conformation even when the layers are compressed by the application of pressure, to which the chains respond by ‘folding’ at the ends attaching them to the substrate planes while maintaining directions which are mainly normal to these planes. A non-zero gap between the layers is also maintained. A pressure of about 50 kbar abruptly causes all motion in the chains to cease, resulting in a highly ordered lattice structure. 相似文献
12.
13.
14.
15.
Climatic forcing and larval dispersal capabilities shape the replenishment of fishes and their habitat‐forming biota on a tropical coral reef 下载免费PDF全文
Shaun K. Wilson Martial Depcyznski Rebecca Fisher Thomas H. Holmes Mae M. Noble Ben T. Radford Michael Rule George Shedrawi Paul Tinkler Christopher J. Fulton 《Ecology and evolution》2018,8(3):1918-1928
Fluctuations in marine populations often relate to the supply of recruits by oceanic currents. Variation in these currents is typically driven by large‐scale changes in climate, in particular ENSO (El Nino Southern Oscillation). The dependence on large‐scale climatic changes may, however, be modified by early life history traits of marine taxa. Based on eight years of annual surveys, along 150 km of coastline, we examined how ENSO influenced abundance of juvenile fish, coral spat, and canopy‐forming macroalgae. We then investigated what traits make populations of some fish families more reliant on the ENSO relationship than others. Abundance of juvenile fish and coral recruits was generally positively correlated with the Southern Oscillation Index (SOI), higher densities recorded during La Niña years, when the ENSO‐influenced Leeuwin Current is stronger and sea surface temperature higher. The relationship is typically positive and stronger among fish families with shorter pelagic larval durations and stronger swimming abilities. The relationship is also stronger at sites on the coral back reef, although the strongest of all relationships were among the lethrinids (r = .9), siganids (r = .9), and mullids (r = .8), which recruit to macroalgal meadows in the lagoon. ENSO effects on habitat seem to moderate SOI–juvenile abundance relationship. Macroalgal canopies are higher during La Niña years, providing more favorable habitat for juvenile fish and strengthening the SOI effect on juvenile abundance. Conversely, loss of coral following a La Niña‐related heat wave may have compromised postsettlement survival of coral dependent species, weakening the influence of SOI on their abundance. This assessment of ENSO effects on tropical fish and habitat‐forming biota and how it is mediated by functional ecology improves our ability to predict and manage changes in the replenishment of marine populations. 相似文献
16.
There has been much recent research interest in the existence of a major axis of life‐history variation along a fast–slow continuum within almost all major taxonomic groups. Eco‐evolutionary models of density‐dependent selection provide a general explanation for such observations of interspecific variation in the "pace of life." One issue, however, is that some large‐bodied long‐lived “slow” species (e.g., trees and large fish) often show an explosive “fast” type of reproduction with many small offspring, and species with “fast” adult life stages can have comparatively “slow” offspring life stages (e.g., mayflies). We attempt to explain such life‐history evolution using the same eco‐evolutionary modeling approach but with two life stages, separating adult reproductive strategies from offspring survival strategies. When the population dynamics in the two life stages are closely linked and affect each other, density‐dependent selection occurs in parallel on both reproduction and survival, producing the usual one‐dimensional fast–slow continuum (e.g., houseflies to blue whales). However, strong density dependence at either the adult reproduction or offspring survival life stage creates quasi‐independent population dynamics, allowing fast‐type reproduction alongside slow‐type survival (e.g., trees and large fish), or the perhaps rarer slow‐type reproduction alongside fast‐type survival (e.g., mayflies—short‐lived adults producing few long‐lived offspring). Therefore, most types of species life histories in nature can potentially be explained via the eco‐evolutionary consequences of density‐dependent selection given the possible separation of demographic effects at different life stages. 相似文献
17.
18.
This research presents the results of constructing and parameterizing an individual-based model of spatiotemporal dynamics of mixed forest stands. The model facilitates computerized experiments with forest stands having different combinations of species and age structures. These forest stands grow on temperate areas where light is the main system-forming factor that shapes and develops forest ecosystems. The model TEMFORM (TEMperate FORests Model) is developed with few equations and parameters, most of which can be estimated using standard forest inventory data. Parameterization of the model used the growth tables of a set of basic forest-forming species in Far East Russia. Simulation results of the development of the natural single- and mixed-species stands and the effects of different types of disturbances on the stand dynamics and compositions are presented. 相似文献
19.
20.
YONG-JIANG ZHANG FREDERICK C. MEINZER GUANG-YOU HAO FABIAN G. SCHOLZ SANDRA J. BUCCI FREDERICO S. C. TAKAHASHI RANDOL VILLALOBOS-VEGA JUAN P. GIRALDO KUN-FANG CAO WILLIAM A. HOFFMANN & GUILLERMO GOLDSTEIN 《Plant, cell & environment》2009,32(10):1456-1466
Size-related changes in hydraulic architecture, carbon allocation and gas exchange of Sclerolobium paniculatum (Leguminosae), a dominant tree species in Neotropical savannas of central Brazil (Cerrado), were investigated to assess their potential role in the dieback of tall individuals. Trees greater than ∼6-m-tall exhibited more branch damage, larger numbers of dead individuals, higher wood density, greater leaf mass per area, lower leaf area to sapwood area ratio (LA/SA), lower stomatal conductance and lower net CO2 assimilation than small trees. Stem-specific hydraulic conductivity decreased, while leaf-specific hydraulic conductivity remained nearly constant, with increasing tree size because of lower LA/SA in larger trees. Leaves were substantially more vulnerable to embolism than stems. Large trees had lower maximum leaf hydraulic conductance ( K leaf ) than small trees and all tree sizes exhibited lower K leaf at midday than at dawn. These size-related adjustments in hydraulic architecture and carbon allocation apparently incurred a large physiological cost: large trees received a lower return in carbon gain from their investment in stem and leaf biomass compared with small trees. Additionally, large trees may experience more severe water deficits in dry years due to lower capacity for buffering the effects of hydraulic path-length and soil water deficits. 相似文献