全文获取类型
收费全文 | 6985篇 |
免费 | 549篇 |
国内免费 | 5篇 |
专业分类
7539篇 |
出版年
2023年 | 44篇 |
2022年 | 47篇 |
2021年 | 106篇 |
2020年 | 79篇 |
2019年 | 99篇 |
2018年 | 213篇 |
2017年 | 207篇 |
2016年 | 264篇 |
2015年 | 408篇 |
2014年 | 325篇 |
2013年 | 460篇 |
2012年 | 586篇 |
2011年 | 565篇 |
2010年 | 321篇 |
2009年 | 228篇 |
2008年 | 369篇 |
2007年 | 329篇 |
2006年 | 333篇 |
2005年 | 303篇 |
2004年 | 294篇 |
2003年 | 251篇 |
2002年 | 216篇 |
2001年 | 193篇 |
2000年 | 195篇 |
1999年 | 141篇 |
1998年 | 60篇 |
1997年 | 28篇 |
1996年 | 38篇 |
1995年 | 46篇 |
1994年 | 24篇 |
1993年 | 29篇 |
1992年 | 86篇 |
1991年 | 52篇 |
1990年 | 57篇 |
1989年 | 53篇 |
1988年 | 46篇 |
1987年 | 61篇 |
1986年 | 45篇 |
1985年 | 45篇 |
1984年 | 46篇 |
1983年 | 18篇 |
1982年 | 21篇 |
1979年 | 16篇 |
1977年 | 14篇 |
1975年 | 19篇 |
1974年 | 29篇 |
1973年 | 17篇 |
1971年 | 10篇 |
1970年 | 11篇 |
1968年 | 15篇 |
排序方式: 共有7539条查询结果,搜索用时 15 毫秒
141.
Functional composition of plant communities determines the spatial and temporal stability of soil microbial properties in a long‐term plant diversity experiment 下载免费PDF全文
Stable provisioning of ecosystem functions and services is crucial for human well‐being in a changing world. Two essential ecological components driving vital ecosystem functions in terrestrial ecosystems are plant diversity and soil microorganisms. In this study, we tracked soil microbial basal respiration and biomass over a time period of 12 years in a grassland biodiversity experiment (the Jena Experiment) and examined the role of plant diversity and plant functional group composition for the spatial and temporal stability of soil microbial properties (basal respiration and biomass) in bulk‐soil. Spatial and temporal stability were calculated as the inverse coefficient of variation (CV?1) of soil microbial respiration and biomass measured from soil samples taken over space and time, respectively. We found that 1) plant species richness consistently increased soil microbial properties after a time lag of four years since the establishment of the experimental plots, 2) plant species richness had minor effects on the spatial stability of soil microbial properties, whereas 3) the functional composition of plant communities significantly affected spatial stability of soil microbial properties, with legumes and tall herbs reducing both the spatial stability of microbial respiration and biomass, while grasses increased the latter, and 4) the effect of plant diversity on temporal stability of soil microbial properties turned from being negative to neutral, suggesting that the recovery of soil microbial communities from former arable land‐use takes more than a decade. Our results highlight the importance of plant functional group composition for the spatial and temporal stability of soil microbial properties, and hence for microbially‐driven ecosystem processes, such as decomposition and element cycling, in temperate semi‐natural grassland. 相似文献
142.
Mira E Lacalle RA González MA Gómez-Moutón C Abad JL Bernad A Martínez-A C Mañes S 《EMBO reports》2001,2(2):151-156
143.
Rodolfo Valdés Andrés Tamayo Marcos González Sigifredo Padilla Déborah Geada William Ferro Lorely Milá Leonardo Gómez Rosario Alemán Alberto Leyva Cristina García Otto Mendoza Tatiana Alvarez Lamay Dorta Yanet Villega Dobián Cecilia Hasel Aragón Tatiana González Maylín La O Jorge López 《Biotechnology and Bioprocess Engineering》2012,17(1):145-159
Monoclonal antibody (mAb) production methods (ascites, in vitro technologies, transgenic animals, and dicot or monocot transgenic plants; moss, algae) have been improved since they were first developed in 1975. In this study, we illustrate a summary of a study case in which mice, a hollow fiber system, and tobacco transgenic plants were assessed for the production of mAb for vaccine manufacturing and vaccine production. 相似文献
144.
145.
The dwarf lanternshark (Etmopterus perryi) is the smallest known described shark, and practically no information has been available on this species since first described in the mid-1980s. Therefore, the aim of this work is to describe, for the first time, the population structure regarding dwarf lanternshark in the Colombian Caribbean Sea. During deep-water research surveys conducted along the Colombian Caribbean coast, 87 stations were sampled using the swept area method. A total of 153 dwarf lanternshark individuals were caught in depths ranging from 230 to 530 m. This information was used to describe the size structures, morphological measurements including length-at-weight relationship, length at maturity in females and the spatial distribution of mean length and biomass of the species. The lengths of individuals ranged from 78.02 to 289.00 mm total length (TL), which is a new record of maximum length for this species. The spatial distribution of mean lengths and biomass distributions show high abundances and high relative mean lengths in the northeast area off Santa Marta and the area northwest of Riohacha. The mean biomass density in the whole prospected area was 5.52 kg km−2. Length at 50% maturity in females was estimated in 203 mm TL (95% C.I. : 190–214 mm). Deep-water elasmobranch species, such as the dwarf lantern shark, are expected to show extremely low resilience to fishing exploitation, even when they are not targeted by commercial fishing. Therefore, the information reported in this study can serve as a baseline upon which management measurements can be proposed for the conservation of this shark species. 相似文献
146.
147.
148.
González-Rosa JM Martín V Peralta M Torres M Mercader N 《Development (Cambridge, England)》2011,138(9):1663-1674
The zebrafish heart has the capacity to regenerate after ventricular resection. Although this regeneration model has proved useful for the elucidation of certain regeneration mechanisms, it is based on the removal of heart tissue rather than its damage. Here, we characterize the cellular response and regenerative capacity of the zebrafish heart after cryoinjury, an alternative procedure that more closely models the pathophysiological process undergone by the human heart after myocardial infarction (MI). Localized damage was induced in 25% of the ventricle by cryocauterization (CC). During the first 24 hours post-injury, CC leads to cardiomyocyte death within the injured area and the near coronary vasculature. Cell death is followed by a rapid proliferative response in endocardium, epicardium and myocardium. During the first 3 weeks post-injury cell debris was cleared and the injured area replaced by a massive scar. The fibrotic tissue was subsequently degraded and replaced by cardiac tissue. Although animals survived CC, their hearts showed nonhomogeneous ventricular contraction and had a thickened ventricular wall, suggesting that regeneration is associated with processes resembling mammalian ventricular remodeling after acute MI. Our results provide the first evidence that, like mammalian hearts, teleost hearts undergo massive fibrosis after cardiac damage. Unlike mammals, however, the fish heart can progressively eliminate the scar and regenerate the lost myocardium, indicating that scar formation is compatible with myocardial regeneration and the existence of endogenous mechanisms of scar regression. This finding suggests that CC-induced damage in zebrafish could provide a valuable model for the study of the mechanisms of scar removal post-MI. 相似文献
149.
150.
Meirav Meiri Adrian M. Lister Thomas F. G. Higham John R. Stewart Lawrence G. Straus Henriette Obermaier Manuel R. González Morales Ana B. Marín‐Arroyo Ian Barnes 《Molecular ecology》2013,22(18):4711-4722
The Pleistocene was an epoch of extreme climatic and environmental changes. How individual species responded to the repeated cycles of warm and cold stages is a major topic of debate. For the European fauna and flora, an expansion–contraction model has been suggested, whereby temperate species were restricted to southern refugia during glacial times and expanded northwards during interglacials, including the present interglacial (Holocene). Here, we test this model on the red deer (Cervus elaphus) a large and highly mobile herbivore, using both modern and ancient mitochondrial DNA from the entire European range of the species over the last c. 40 000 years. Our results indicate that this species was sensitive to the effects of climate change. Prior to the Last Glacial Maximum (LGM) haplogroups restricted today to South‐East Europe and Western Asia reached as far west as the UK. During the LGM, red deer was mainly restricted to southern refugia, in Iberia, the Balkans and possibly in Italy and South‐Western Asia. At the end of the LGM, red deer expanded from the Iberian refugium, to Central and Northern Europe, including the UK, Belgium, Scandinavia, Germany, Poland and Belarus. Ancient DNA data cannot rule out refugial survival of red deer in North‐West Europe through the LGM. Had such deer survived, though, they were replaced by deer migrating from Iberia at the end of the glacial. The Balkans served as a separate LGM refugium and were probably connected to Western Asia with genetic exchange between the two areas. 相似文献