首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   8016篇
  免费   799篇
  国内免费   3篇
  8818篇
  2023年   46篇
  2022年   75篇
  2021年   130篇
  2020年   81篇
  2019年   120篇
  2018年   134篇
  2017年   114篇
  2016年   216篇
  2015年   309篇
  2014年   384篇
  2013年   419篇
  2012年   557篇
  2011年   551篇
  2010年   335篇
  2009年   329篇
  2008年   403篇
  2007年   388篇
  2006年   351篇
  2005年   388篇
  2004年   359篇
  2003年   364篇
  2002年   336篇
  2001年   111篇
  2000年   132篇
  1999年   109篇
  1998年   118篇
  1997年   79篇
  1996年   92篇
  1995年   74篇
  1994年   74篇
  1993年   66篇
  1992年   98篇
  1991年   78篇
  1990年   81篇
  1989年   64篇
  1988年   79篇
  1987年   68篇
  1986年   49篇
  1985年   60篇
  1984年   68篇
  1983年   59篇
  1982年   53篇
  1981年   66篇
  1980年   41篇
  1979年   57篇
  1978年   48篇
  1977年   49篇
  1976年   41篇
  1974年   54篇
  1973年   36篇
排序方式: 共有8818条查询结果,搜索用时 14 毫秒
91.
92.
Skeletons of chimpanzees with recorded life stories allow assessment of the potential relationships among hard tissue features and expressed behaviors. We analyze bone size, weight, and mineralization to assess osteological characters for identification of laterality of expressed behaviors involving the upper body. Results show that associations are not yet clearly defined.  相似文献   
93.
Wounding hybrid poplar (Populus trichocarpa × P. deltoides) trees results in the expression of novel wound-inducible (win) mRNAs thought to encode proteins involved in defense against pests and pathogens. Members of thewin6 gene family encode acidic multi-domain chitinases, with combined structure and charge characteristics that differ from previously described chitinases.Win6 expression has been shown to occur in pooled unwounded leaves of a wounded (on multiple leaves) poplar plant. Here we demonstrate that wounding a single leaf induceswin6 expression locally, in the wounded leaf, and remotely, in specific unwounded leaves with strong vascular connections to the wounded leaf. We also demonstrate that awin6 promoter--glucuronidase (GUS) gene fusion (win6-GUS) responds to wounding locally and remotely in transgenic tobacco. These data indicate that the poplarwin6 promoter has regulatory elements that are responsive to wound signals in the heterologous host. In addition,win6-GUS is developmentally activated in unwounded young leaves and floral tissues of transgenic tobacco. Similar developmental expression patterns are found to occur forwin6 in poplar trees, demonstrating that a herbaceous plant can serve as a host for woody tree transgene analysis and can accurately predict expression patterns in tree tissues (e.g. flowers) that would be difficult to study in free-living trees.  相似文献   
94.
Monoclonal antibody 4E9, which was raised against a partially purified detergent extract of rat caudal epididymal sperm, recognizes the tail of sperm from the cauda, but not from caput epididymidis, as well as epithelial cells in a restricted region of the distal caput/corpus epididymidis and proteins in epididymal fluid from corpus and cauda epididymidis. The antigen is apparently a glycoprotein, since it is retained on a Ricinus communis agglutinin l lectin column. Epididymal fluid antigens have apparent MrS of 38–26 kD, whereas the memrane-associated form of the molecule has an Mr of 26 kD. Immunocytochemical data and Western immunoblot data suggest that the membrane antigen is derived from the fluid antigen, which, in turn, is secrteted by the epididymal epithelium. Characterization of the membrane antigen indicates that it is tightly associated with the sperm surface, behaving as though it is an integral membrane protein. The antigen persists on ejaculated sperm. © 1994 Wiley-Liss, Inc.  相似文献   
95.
The epithelial lining of the gastrointestinal (GI) tract is in a state of continuous cell renewal, and the proliferating and differentiating/differentiated cell populations are spatially clearly demarcated. Members of the epidermal growth factor (EGF) family of peptides, the trefoil peptides and enteroglucagon appear to be the most important enterotrophic molecules for both normal cell renewal and healing after cell damage. Transforming growth factor-a (TGF-a) appears to be the primary physiological ligand for the EGF receptor (EGFR), promoting normal cell renewal, and TGF-a/EGFR are part of an autocrine loop in many intestinal cancers. In response to damage, a differentiating cell lineage arises from adjacent epithelium secreting EGF, TGF-a and trefoil peptides; this may be viewed as part of a ‘repair kit’ in damaged endodermally-derived tissue.  相似文献   
96.
Both nitric oxide and prostaglandins are potent paracrine mediators of intercellular communication. An endotoxin-lipopolysaccharide (LPS) inducible form of nitric oxide synthase (mac-NOS) has recently been cloned from murine macrophages. An inducible prostaglandin synthase (TIS1O/PGS-2), cloned from 3T3 cells, is also induced in LPS-activated macrophage. Because of the wide range of ligands that induce primary response genes in 3T3 cells, the ease of studying chimeric promoter constructs in 3T3 cells, and the importance of both nitric oxide and prostaglandins as paracrine mediators, we examined expression of mac-NOS in 3T3 cells. Tetradecanoyl phorbol-13 acetate (TPA), forskolin, platelet-derived growth factor, fibroblast growth factor, and serum all induce mac-NOS expression in Swiss 3T3 cells. Thus the mac-NOS gene can respond to a far wider range of inducers than previously suspected. mac-NOS is a primary response gene; cycloheximide does not block induction. TPA-induced mac-NOS and TIS10/PGS-2 mRNA accumulation patterns are similar. LPS is a potent inducer of mac-NOS in Swiss 3T3 cells but cannot induce TIS10/PGS-2. In contrast, v-src expression induces TIS10/PGS-2 message, but not iNOS message in a BALB/c 3T3 cell line containing a temperature-sensitive v-src gene. Dexamethasone (DEX) prevents induction of TIS10/PGS-2, but not most other primary response genes. DEX also blocks mac-NOS induction in Swiss 3T3 cells. The inducible TIS10/PGS-2 and mac-NOS genes, responsible for the production of two distinct paracrine agents, appear to share many regulatory features in 3T3 cells. © 1993 Wiley-Liss, Inc.  相似文献   
97.
The 10-mer primer OPM-01 (5'-GTT GGT GGC T-3') was used to generate random amplification of polymorphic DNA (RAPD) profiles by polymerase chain reaction for 91 strains of Listeria monocytogenes from raw milk, food, and veterinary, medical, and food-environmental sources. The profiles obtained contained 1 to 10 bands within the molecular size range of 0.5 to 5.0 kbp. Reproducibility was enhanced by annealing at low stringency and introducing a 1-min ramp time between annealing and extension temperatures. Thirty-three RAPD profiles were observed, with specific profiles being observed for strains from each source. RAPD profiles allowed discrimination within serogroups, although five RAPD profiles which were not confined to one serotype were found. Within food strains, one RAPD profile was more common than others, suggesting this to be a common type among strains from this source.  相似文献   
98.
The molecular dynamics algorithm (MD), which simulates intramolecular motions on the subnanosecond timescale, has been modified to allow the investigation of slow conformational transitions that do not necessarily occur spontaneously in MD simulations. The method is designated CONTRA MD (CONformational TRAnsitions by Molecular Dynamics with minimum biasing). The method requires the prior definition of a single conformational variable that is required to vary monotonically from an initial conformation to a final target conformation. The simulation is broken up into a series of short free MD segments, and we determine, after each segment of MD, whether or not the system has evolved toward the final conformation. Those segments that do not move the system in that direction are deleted. Those that do move it toward the final conformation are patched together sequentially to generate a single representative trajectory along the transition pathway. The CONTRA MD method is demonstrated first by application to the simultaneous C2′-endo to C3′-endo repucker and anti to syn N-glycosidic torsion transitions in 2′-deoxyadenosine and then to the large-scale bending in phenylalanine transfer RNA. © 1993 John Wiley & Sons, Inc.  相似文献   
99.
Wheat is a major crop worldwide, mainly cultivated for human consumption and animal feed. Grain quality is paramount in determining its value and downstream use. While we know that climate change threatens global crop yields, a better understanding of impacts on wheat end-use quality is also critical. Combining quantitative genetics with climate model outputs, we investigated UK-wide trends in genotypic adaptation for wheat quality traits. In our approach, we augmented genomic prediction models with environmental characterisation of field trials to predict trait values and climate effects in historical field trial data between 2001 and 2020. Addition of environmental covariates, such as temperature and rainfall, successfully enabled prediction of genotype by environment interactions (G × E), and increased prediction accuracy of most traits for new genotypes in new year cross validation. We then extended predictions from these models to much larger numbers of simulated environments using climate scenarios projected under Representative Concentration Pathways 8.5 for 2050–2069. We found geographically varying climate change impacts on wheat quality due to contrasting associations between specific weather covariables and quality traits across the UK. Notably, negative impacts on quality traits were predicted in the East of the UK due to increased summer temperatures while the climate in the North and South-west may become more favourable with increased summer temperatures. Furthermore, by projecting 167,040 simulated future genotype–environment combinations, we found only limited potential for breeding to exploit predictable G × E to mitigate year-to-year environmental variability for most traits except Hagberg falling number. This suggests low adaptability of current UK wheat germplasm across future UK climates. More generally, approaches demonstrated here will be critical to enable adaptation of global crops to near-term climate change.  相似文献   
100.
Soils contain biotic and abiotic legacies of previous conditions that may influence plant community biomass and associated aboveground biodiversity. However, little is known about the relative strengths and interactions of the various belowground legacies on aboveground plant–insect interactions. We used an outdoor mesocosm experiment to investigate the belowground legacy effects of range-expanding versus native plants, extreme drought and their interactions on plants, aphids and pollinators. We show that plant biomass was influenced more strongly by the previous plant community than by the previous summer drought. Plant communities consisted of four congeneric pairs of natives and range expanders, and their responses were not unanimous. Legacy effects affected the abundance of aphids more strongly than pollinators. We conclude that legacies can be contained as soil ‘memories’ that influence aboveground plant community interactions in the next growing season. These soil-borne ‘memories’ can be altered by climate warming-induced plant range shifts and extreme drought.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号