全文获取类型
收费全文 | 131617篇 |
免费 | 4404篇 |
国内免费 | 821篇 |
出版年
2022年 | 349篇 |
2021年 | 670篇 |
2020年 | 486篇 |
2019年 | 567篇 |
2018年 | 12365篇 |
2017年 | 11164篇 |
2016年 | 8389篇 |
2015年 | 2301篇 |
2014年 | 2149篇 |
2013年 | 2606篇 |
2012年 | 6999篇 |
2011年 | 15435篇 |
2010年 | 13640篇 |
2009年 | 9721篇 |
2008年 | 11826篇 |
2007年 | 13383篇 |
2006年 | 2364篇 |
2005年 | 2384篇 |
2004年 | 2832篇 |
2003年 | 2679篇 |
2002年 | 2410篇 |
2001年 | 901篇 |
2000年 | 761篇 |
1999年 | 643篇 |
1998年 | 439篇 |
1997年 | 373篇 |
1996年 | 348篇 |
1995年 | 350篇 |
1994年 | 314篇 |
1993年 | 334篇 |
1992年 | 434篇 |
1991年 | 392篇 |
1990年 | 357篇 |
1989年 | 370篇 |
1988年 | 381篇 |
1987年 | 347篇 |
1986年 | 268篇 |
1985年 | 298篇 |
1984年 | 310篇 |
1983年 | 286篇 |
1982年 | 259篇 |
1981年 | 231篇 |
1980年 | 210篇 |
1979年 | 215篇 |
1978年 | 181篇 |
1977年 | 183篇 |
1976年 | 182篇 |
1975年 | 203篇 |
1972年 | 386篇 |
1971年 | 398篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
861.
Abstract. This study reports on small-scale changes in the distribution of plant species in a 2.5 m2 plot of grazed, species-rich Veronica spicata - Avenula pratensis grassland on shallow, dry, nutrient-poor soil in the Great Alvar area (Stora Alvaret) of southern Öland, southeastern Sweden. Multivari-ate analysis of 0.001 m2 and 0.25 m2 quadrats within the plot showed that there is little floristic variation without any trend in the plot. Average species richness varied little throughout the study period from 1986 to 1991 with 1986 averages of 7.0 on 0.001m2, 16.3 on 0.01 m2, and 26.1 on 0.25 m2. On 0.001 m2the highest species number found was 12, on 0.01 m2, 27. However, cumulative species richness, i.e. species number in the first year plus new species appearing in later years (averaged over 40 quadrats) increased over the same period, on 0.001 m2 from 7.0 in 1986 to 14.9 in 1991, and on 0.01 m2 from 16.3 to 24.1. Cumulative frequency, i.e. number of quadrats out of 40 where a species occurred in the first year or/and in later years, increased as well; the number of cumulatively highly frequent (> 80%) species increased from 5 in 1986 to 18 in 1991. Species mobility on the scale of the small quadrats used implies both appearance and disappearance from these quadrats. Using six examples, species mobility is shown to vary individualistically, both in rate and degree. We postulate that in homogeneous, grazed, nutrient- and water-deficient environments many species can reach virtually all microsites within the plot, which we express through the idea of the ‘carousel model’. We also question the usefulness of the niche concept and re-interpret it by stating that all species of this plant community have the same habitat niche, while most of them are short-lived and have the same regeneration niche. The essential variation amongst the species is their individual ability to establish or re-establish by making use of favourable conditions appearing in microsites in an unknown, complex spatio-temporal pattern. 相似文献
862.
Martina Ferraguti Sergio Magallanes Jéssica Jiménez-Peñuela Josué Martínez-de la Puente Luz Garcia-Longoria Jordi Figuerola Jaime Muriel Tamer Albayrak Staffan Bensch Camille Bonneaud Rohan H. Clarke Gábor Á. Czirják Dimitar Dimitrov Kathya Espinoza John G. Ewen Farah Ishtiaq Wendy Flores-Saavedra László Zsolt Garamszegi Olof Hellgren Dita Horakova Kathryn P. Huyvaert Henrik Jensen Asta Križanauskienė Marcos R. Lima Charlene Lujan-Vega Eyðfinn Magnussen Lynn B. Martin Kevin D. Matson Anders Pape Møller Pavel Munclinger Vaidas Palinauskas Péter L. Pap Javier Pérez-Tris Swen C. Renner Robert Ricklefs Sergio Scebba Ravinder N. M. Sehgal Manuel Soler Eszter Szöllősi Gediminas Valkiūnas Helena Westerdahl Pavel Zehtindjiev Alfonso Marzal 《Global Ecology and Biogeography》2023,32(5):809-823
Aim
The increasing spread of vector-borne diseases has resulted in severe health concerns for humans, domestic animals and wildlife, with changes in land use and the introduction of invasive species being among the main possible causes for this increase. We explored several ecological drivers potentially affecting the local prevalence and richness of avian malaria parasite lineages in native and introduced house sparrows (Passer domesticus) populations.Location
Global.Time period
2002–2019.Major taxa studied
Avian Plasmodium parasites in house sparrows.Methods
We analysed data from 2,220 samples from 69 localities across all continents, except Antarctica. The influence of environment (urbanization index and human density), geography (altitude, latitude, hemisphere) and time (bird breeding season and years since introduction) were analysed using generalized additive mixed models (GAMMs) and random forests.Results
Overall, 670 sparrows (30.2%) were infected with 22 Plasmodium lineages. In native populations, parasite prevalence was positively related to urbanization index, with the highest prevalence values in areas with intermediate urbanization levels. Likewise, in introduced populations, prevalence was positively associated with urbanization index; however, higher infection occurred in areas with either extreme high or low levels of urbanization. In introduced populations, the number of parasite lineages increased with altitude and with the years elapsed since the establishment of sparrows in a new locality. Here, after a decline in the number of parasite lineages in the first 30 years, an increase from 40 years onwards was detected.Main conclusions
Urbanization was related to parasite prevalence in both native and introduced bird populations. In invaded areas, altitude and time since bird introduction were related to the number of Plasmodium lineages found to be infecting sparrows. 相似文献863.
Katherine Miclau William S. Hambright Johnny Huard Martin J. Stoddart Chelsea S. Bahney 《Aging cell》2023,22(1):e13759
Mesenchymal-derived stromal or progenitor cells, commonly called “MSCs,” have attracted significant clinical interest for their remarkable abilities to promote tissue regeneration and reduce inflammation. Recent studies have shown that MSCs' therapeutic effects, originally attributed to the cells' direct differentiation capacity into the tissue of interest, are largely driven by the biomolecules the cells secrete, including cytokines, chemokines, growth factors, and extracellular vesicles containing miRNA. This secretome coordinates upregulation of endogenous repair and immunomodulation in the local microenvironment through crosstalk of MSCs with host tissue cells. Therapeutic applications for MSCs and their secretome-derived products often involve in vitro monolayer expansion. However, consecutive passaging of MSCs significantly alters their therapeutic potential, inducing a broad shift from a pro-regenerative to a pro-inflammatory phenotype. A consistent by-product of in vitro expansion of MSCs is the onset of replicative senescence, a state of cell arrest characterized by an increased release of proinflammatory cytokines and growth factors. However, little is known about changes in the secretome profile at different stages of in vitro expansion. Some culture conditions and bioprocessing techniques have shown promise in more effectively retaining the pro-regenerative and anti-inflammatory MSC phenotype throughout expansion. Understanding how in vitro expansion conditions influence the nature and function of MSCs, and their associated secretome, may provide key insights into the underlying mechanisms driving these alterations. Elucidating the dynamic and diverse changes in the MSC secretome at each stage of in vitro expansion is a critical next step in the development of standardized, safe, and effective MSC-based therapies. 相似文献
864.
Elizabeth J. Fear Frida H. Torkelsen Elisa Zamboni Kuan-Ju Chen Martin Scott Glenn Jeffery Heidi Baseler Aneurin J. Kennerley 《Aging cell》2023,22(11):e14005
Mitochondrial function declines with age, and many pathological processes in neurodegenerative diseases stem from this dysfunction when mitochondria fail to produce the necessary energy required. Photobiomodulation (PBM), long-wavelength light therapy, has been shown to rescue mitochondrial function in animal models and improve human health, but clinical uptake is limited due to uncertainty around efficacy and the mechanisms responsible. Using 31P magnetisation transfer magnetic resonance spectroscopy (MT-MRS) we quantify, for the first time, the effects of 670 nm PBM treatment on healthy ageing human brains. We find a significant increase in the rate of ATP synthase flux in the brain after PBM in a cohort of older adults. Our study provides initial evidence of PBM therapeutic efficacy for improving mitochondrial function and restoring ATP flux with age, but recognises that wider studies are now required to confirm any resultant cognitive benefits. 相似文献
865.
Martin S. Williamson Ian Denholm Caroline A. Bell Alan L. Devonshire 《Molecular genetics and genomics : MGG》1993,240(1):17-22
The voltage-sensitive sodium channel is generally regarded as the primary target site of dichlorodiphenyl-trichloro-ethane (DDT) and pyrethroid insecticides, and has been implicated in the widely reported mechanism of nerve insensitivity to these compounds. This phenomenon is expressed as knockdown resistance (kdr) and has been best characterised in the housefly where several putative alleles, including the more potent super-kdr factor, have been identified. We report the isolation of cDNA clones containing part of a housefly sodium channel gene, designated Msc, which show close homology to the para sodium channel of Drosophila (99% amino acid identity within the region of overlap). Using Southern blots of insect DNA, restriction fragment length polymorphisms (RFLPs) at the Msc locus were identified in susceptible, kdr and super-kdr housefly strains. These RFLPs showed tight linkage to resistance in controlled crosses involving these strains, thus providing clear genetic evidence that kdr, and hence pyrethroid mode of action, is closely associated with the voltage-sensitive sodium channel. 相似文献
866.
Martin SJ 《Trends in cell biology》1993,3(5):141-144
Apoptosis, a controlled form of cell death, appears to be regulated in several ways. Early studies indicated that de novo protein synthesis was required for apoptosis of thymocytes, but more recent studies have found that other cells can undergo apoptosis when protein synthesis is blocked or that inhibition of protein or RNA synthesis can induce apoptosis. Whether these findings reflect distinct forms of apoptosis or variations on a single pathway is not yet known. In this article the case for a single pathway to apoptosis, accessible at multiple points, is discussed. 相似文献
867.
868.
In vitro, many unfolded polypeptides are able to fold to the native state spontaneously, indicating that the amino acid sequence of a protein contains all the information necessary to specify its three-dimensional conformation. It had been assumed that protein folding in vivo also generally occurs in a spontaneous process. This view has changed only recently due to the discovery of a number of proteins, now commonly called 'molecular chaperones', which are essential for cellular protein folding and occur ubiquitously in eubacteria, archaebacteria and in eukaryotic cells. 相似文献
869.
Summary The spatial distribution and species composition of high-Antarctic ichthyonekton was investigated during the EPOS 3 cruise by RV Polarstern in the eastern Weddell Sea during January–February 1989. A multiple rectangular midwater trawl was used to collect samples from the surface to near the sea floor at 11 stations along a 245 kra transect off Halley Bay. Early larval stages of 18 species, representing about 24% of the known Weddell Sea ichthyofauna, were present in the water column. The Antarctic silver-fish, Pleuragramma antarcticum, over-whelmingly dominated the catches comprising 84.5% of the 5022 specimens caught. The abundance of this species markedly increased towards the offshore end of the transect with the highest numbers occurring near the shelf-break front associated with the westerly current of the southern limb of the Weddell Gyre. The increased abundance of P. antarcticum in continental slope waters was attributed to deflection of the East Weddell Coastal Current beyond the shelf/slope break by fringing ice shelves. Most larval and juvenile fish were found in the seasonally warmed upper 0–70 m layer of the Antarctic Surface Water where conditions occurred that appeared to be favourable to both feeding and growth. Cluster analysis indicated that inner-, central-and outer-shelf assemblages were represented and that the species composition was most effectively described by reference to water mass and depth. 相似文献
870.
Molecular phylogenies in angiosperm evolution 总被引:8,自引:0,他引:8
Martin W; Lydiate D; Brinkmann H; Forkmann G; Saedler H; Cerff R 《Molecular biology and evolution》1993,10(1):140-162
We have cloned and sequenced cDNAs for the glyceraldehyde-3-phosphate
dehydrogenase of glycolysis, gapC, from a bryophyte, a gymnosperm, and
three angiosperms. Phylogenetic analyses are presented for these data in
the context of other gapC sequences and in parallel with published
nucleotide sequences for the chloroplast encoded gene for the large subunit
of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcL). Relative-rate
tests were performed for these genes in order to assess variation in
substitution rate for coding regions, along individual plant lineages
studied. The results of both gene analyses suggest that the deepest
dichotomy within the angiosperms separates not magnoliids from remaining
angiosperms, but monocotyledons from dicotyledons, in sharp contrast to
prediction from the Euanthial theory for angiosperm evolution. Furthermore,
these chloroplast and nuclear sequence data taken together suggest that the
separation of monocotyledonous and dicotyledonous lineages took place in
late Carboniferous times [approximately 300 Myr before the present (Mybp)].
This date would exceed but be compatible with the late-Triassic
(approximately 220 Mybp) occurrence of fossil reproductive structures of
the primitive angiosperm Sanmiguelia lewisii.
相似文献