全文获取类型
收费全文 | 2976篇 |
免费 | 237篇 |
出版年
2023年 | 16篇 |
2022年 | 9篇 |
2021年 | 57篇 |
2020年 | 34篇 |
2019年 | 40篇 |
2018年 | 60篇 |
2017年 | 49篇 |
2016年 | 73篇 |
2015年 | 175篇 |
2014年 | 173篇 |
2013年 | 220篇 |
2012年 | 276篇 |
2011年 | 253篇 |
2010年 | 151篇 |
2009年 | 125篇 |
2008年 | 201篇 |
2007年 | 181篇 |
2006年 | 176篇 |
2005年 | 153篇 |
2004年 | 140篇 |
2003年 | 112篇 |
2002年 | 115篇 |
2001年 | 33篇 |
2000年 | 30篇 |
1999年 | 33篇 |
1998年 | 43篇 |
1997年 | 21篇 |
1996年 | 20篇 |
1995年 | 19篇 |
1994年 | 28篇 |
1993年 | 20篇 |
1992年 | 22篇 |
1991年 | 16篇 |
1990年 | 11篇 |
1989年 | 22篇 |
1988年 | 8篇 |
1987年 | 17篇 |
1986年 | 8篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1976年 | 3篇 |
1964年 | 3篇 |
1940年 | 2篇 |
1912年 | 2篇 |
排序方式: 共有3213条查询结果,搜索用时 31 毫秒
941.
942.
Non-indigenous species (NIS) are important components of global change, and in order to manage such species it is important
to understand which factors affect their success. Interactions with enemies in the new range have been shown to be important
for the outcome of introductions, but thus far most studies on NIS–enemy interactions have considered only specialist herbivores
in terrestrial systems. Here we present the results from the first biogeographic study that compares herbivore resistance
between populations in the native and new region of a non-indigenous seaweed. We show that low consumption of the non-indigenous
seaweed by a generalist herbivore is caused by higher chemical defence levels and herbivore resistance in the new range—and
not by the failure of the herbivore to recognise the non-indigenous seaweed as a suitable host. Since most seaweed–herbivore
interactions are dominated by generalist herbivores, this pattern could be common in marine communities. Our results also
reveal that traits used to predict the invasive potential of species, such as their resistance to enemies, can change during
the invasion process, but not always in the way predicted by dominant theories. 相似文献
943.
Henrik Svedäng Carl André Patrik Jonsson Mikael Elfman Karin E. Limburg 《Environmental Biology of Fishes》2010,89(3-4):383-397
The question whether temperate marine fishes typically consist of self-sustaining populations or “open” populations still remains unresolved. At the heart of this population connectivity problem lays the nature of the stock separation mechanisms. Fish populations could be segregated either by environmental forcing, accompanied with opportunistic recruitment of juveniles to spawning areas, or by philopatric behaviours (i.e., inclination of an individual to return to or remain in its natal area). Here we report three, partly independent, studies on Atlantic Cod (Gadus morhua) stock separation in the Kattegat and Öresund (eastern North Sea): characterisation of spawning aggregations with genetic markers, tagging experiments, and analysis of chemical constituents in otolith cores of recaptured fish that could be linked to a specific spawning site. While the genetic investigation showed no population segregation, the observed migratory patterns indicated three separate spawning sites at close distances. The natal dependence on the choice of spawning site was tested by measuring the contents of various trace elements in the otolith core of recaptured tagged fish. Quantification of the trace elements: Ba, Br, Co, Cr, Cu, Fe, Mn, Mo, Ni, Sr, Ti, and Zn expressed as ratios to Ca were obtained using scanning micro PIXE. These results indicated that natal origin could be differentiated between spawning sites, supporting the hypothesis that natal homing is an important stock separating mechanism even over short distances (<100 km). 相似文献
944.
Gauri Dixit Rachael Baker Carly M. Sacks Matthew P. Torres Henrik G. Dohlman 《The Journal of biological chemistry》2014,289(21):15052-15063
Heterotrimeric G proteins are well known to transmit signals from cell surface receptors to intracellular effector proteins. There is growing appreciation that G proteins are also present at endomembrane compartments, where they can potentially interact with a distinct set of signaling proteins. Here, we examine the cellular trafficking function of the G protein α subunit in yeast, Gpa1. Gpa1 contains a unique 109-amino acid insert within the α-helical domain that undergoes a variety of posttranslational modifications. Among these is monoubiquitination, catalyzed by the NEDD4 family ubiquitin ligase Rsp5. Using a newly optimized method for G protein purification together with biophysical measures of structure and function, we show that the ubiquitination domain does not influence enzyme activity. By screening a panel of 39 gene deletion mutants, each lacking a different ubiquitin binding domain protein, we identify seven that are necessary to deliver Gpa1 to the vacuole compartment including four proteins (Ede1, Bul1, Ddi1, and Rup1) previously not known to be involved in this process. Finally, we show that proper endocytosis of the G protein is needed for sustained cellular morphogenesis and mating in response to pheromone stimulation. We conclude that a cascade of ubiquitin-binding proteins serves to deliver the G protein to its final destination within the cell. In this instance and in contrast to the previously characterized visual system, endocytosis from the plasma membrane is needed for proper signal transduction rather than for signal desensitization. 相似文献
945.
Uwe Karsten Gisela Papsdorf Annedore Pauly Borivoj Vojtesek Roland Moll E. Birgitte Lane Henrik Clausen Peter Stosiek Michael Kasper 《Differentiation; research in biological diversity》1993,54(3):55-66
Abstract. Normal (non-transformed) human mammary epithelial cell lines derived from reduction mammoplasties were analyzed by immunocytochemistry with more than 80 monoclonal antibodies (mAbs) and other specific reagents to tissue-specific and developmentally regulated antigens at different passage levels. A subpopulation of poorly differentiated, proliferating epithelial cells, corresponding to the 'selected' cell type of late passages, is shown to be characterized by a new marker, the histo-blood group antigen H type 2, probably carried on a membrane-bound glycolipid. These cells also express a number of other onco-developmental carbohydrate antigens [Ley , Lex , sialosyl-Lea , precursor of Thomsen Friedenreich antigen (Tn ), but not Thomsen-Friedenreich antigen and sialosyl-Tn ). Their cytokeratin (CK) phenotype, as assessed by reactivity with monospecific mAbs and two-dimensional gel electrophoresis, is CK 5, 6, 14 and 17, with CK 19 being consistently absent, and varying minor amounts of CK 7, 8 and 18, as well as 15 and 16. The reactivity of these cells with a panel of 11 mAbs specific for CK 18 varies considerably even after cloning, indicating heterogeneity of epitope expression or accessibility. Our data strongly suggest that the H type 2+ cells develop from the basal cell layer of the mammary gland. 相似文献
946.
Thea Kristiansen Jens-Christian Svenning Csar Grndez Jukka Salo Henrik Balslev 《Acta Oecologica》2009,35(4):554-562
The mechanisms that cause variation in commonness (abundances and range sizes) of species remain debated in ecology, and a repeatedly observed pattern is the positive relation between local abundances and larger scale range sizes. We used the Amazonian palm species (Arecaceae) to investigate the dependence between and potential determinants of commonness across three (local, landscape, continental) spatial scales. Commonness at the smaller scales (local abundance, landscape frequency) was estimated using data from 57 transects (5 × 500 m) in primary, non-inundated (terra firme) rainforest in a western Amazonian landscape, while commonness at the largest scale (continental range size) was estimated from digitized distribution maps. Landscape frequency was positively related to both local abundance and continental range size, which, however, were not related to each other. Landscape frequency was positively related to topographic niche breadth. Stem height correlated with continental range size and was the only species life-history trait related to any commonness measure. Distance from the study area to a species' range centre did not influence any of the commonness measures. The factors determining commonness in the Amazonian palm flora appear to be scale-dependent, with the unrelated local scale abundance and continental range size probably being controlled by different driving factors. Interestingly, commonness at the intermediate, landscape scale seems linked to both the smaller and the larger scale. Our results point towards topographic niche breadth at the smaller scales and stem height, possibly reflecting species' dispersal potential, at the continental scale as important determinants of commonness. 相似文献
947.
948.
949.
Henrik Lundegårdii 《Planta》1926,2(2-3):152-240
Ohne ZusammenfassungMit 44 Textabbildungen 相似文献
950.
Laurentiu Benga Peter M Benten Eva Engelhardt Karl Khrer Barbara Hueber Werner Nicklas Henrik Christensen Martin Sager 《Comparative medicine》2020,70(6):487
The internal transcribed spacer (ITS) regions of Rodentibacter pneumotropicus, R. heylii, R. rarus, R. ratti, and R. heidelbergensis and of a Rodentibacter-related β-hemolytic Pasteurellaceae taxon isolated from laboratory rodents were studied for their feasibility to discriminate among these species. The 6 species analyzed showed species-specific ITS patterns that were shared by the type strains and clinical isolates and that allowed their identification. Nevertheless, differentiating between the ITS band patterns of R. pneumotropicus and R. ratti is visually challenging. In all species tested, sequence analysis of the ITS fragments revealed a larger ITSile+ala, which contained the genes for tRNAIle(GAU) and tRNAAla(UGC), and a smaller ITSglu with the tRNAGlu(UUC) gene. The ITS sequences varied among the 6 species evaluated, displaying identity levels ranging from 62% to 86% for ITSile+ala and 68% to 90% for ITSglu. Overall, ITS amplification proved to be a reliable method to differentiate among these important Pasteurellaceae species of laboratory rodents. Moreover, the ITS sequence variations recorded here might facilitate the design of probes for specific identification of these species. The ability to diagnose these organisms to the species level could increase our understanding of their clinical significance.The members of the Pasteurellaceae isolated from rodents are among the most prevalent bacterial agents from experimental animal facilities.18 Recently, [Pasteurella] pneumotropica and its closely related rodent Pasteurellaceae were reclassified into 8 distinct species and 2 genomospecies within the new genus Rodentibacter.1 The uncertain taxonomic position of [P.] pneumotropica complex has hindered understanding of the epidemiology, pathogenesis, diagnosis, and control of infections caused by these microorganisms.8 Currently R. pneumotropicus and R. heylii are considered to have tropism mainly toward mice, whereas R. ratti and R. heidelbergensis are rather rat-specific.8,17 Despite their relatedness, isolates of Rodentibacter spp. of rat origin infected only a few mice by contact and experimentally, whereas mouse isolates infected all rats, thus showing that the rat isolates are more species specific than the mice isolates.15 We recently documented that R. heylii naturally infects both rats and mice.7 In addition to the known Rodentibacter species, a Rodentibacter-related β-hemolytic taxon is apparently often present in laboratory mice.6,12 The members of the former [P.] pneumotropica complex (now Rodentibacter spp.) are generally regarded as classic opportunistic pathogens of rodents. However, little is known about the pathogenicity of the individual bacterial species.8The new taxonomic separation of the previous [P.] pneumotropica complex into Rodentibacter species supports better diagnostic assays among this complex group of bacteria. Nevertheless, simple molecular techniques to differentiate these organisms are currently available only for R. pneumotropicus and R. heylii; 16S rRNA gene sequencing is the only alternative for the remaining Rodentibacter species.The internal transcribed spacer (ITS) region has been used as a target for PCR-based identification and typing of many closely related bacteria, including Pasteurellaceae.13 We previously used this method to differentiate among R. pneumotropicus, R. heylii, and R. rarus, which were known as [P.] pneumotropica biotypes Jawetz and Heyl and Bisgaard Taxon 17 at that time.4In this current investigation, we extended the analysis of the 16S–23S ITS of the rRNA operons to R. ratti, R. heidelbergensis, and the Rodentibacter-related β-hemolytic taxon and compared them with the ITS of R. pneumotropicus, R. heylii, and R. rarus, as a basis for identification and differentiation within this group of closely related bacterial species. The length and sequence polymorphisms of the ITS allowed differentiation among the 6 species. The ability to diagnose Rodentibacter taxa at the species level could improve our understanding of their clinical significance. 相似文献