全文获取类型
收费全文 | 3062篇 |
免费 | 276篇 |
国内免费 | 2篇 |
专业分类
3340篇 |
出版年
2023年 | 22篇 |
2022年 | 43篇 |
2021年 | 84篇 |
2020年 | 62篇 |
2019年 | 54篇 |
2018年 | 93篇 |
2017年 | 68篇 |
2016年 | 144篇 |
2015年 | 172篇 |
2014年 | 178篇 |
2013年 | 234篇 |
2012年 | 260篇 |
2011年 | 247篇 |
2010年 | 150篇 |
2009年 | 118篇 |
2008年 | 187篇 |
2007年 | 172篇 |
2006年 | 138篇 |
2005年 | 133篇 |
2004年 | 117篇 |
2003年 | 113篇 |
2002年 | 87篇 |
2001年 | 25篇 |
2000年 | 21篇 |
1999年 | 31篇 |
1998年 | 21篇 |
1997年 | 18篇 |
1996年 | 20篇 |
1995年 | 21篇 |
1994年 | 11篇 |
1993年 | 20篇 |
1992年 | 19篇 |
1991年 | 12篇 |
1990年 | 21篇 |
1989年 | 19篇 |
1988年 | 17篇 |
1987年 | 13篇 |
1986年 | 18篇 |
1985年 | 13篇 |
1984年 | 15篇 |
1983年 | 10篇 |
1982年 | 14篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 15篇 |
1976年 | 9篇 |
1974年 | 5篇 |
1973年 | 6篇 |
1970年 | 5篇 |
1962年 | 5篇 |
排序方式: 共有3340条查询结果,搜索用时 0 毫秒
111.
Brian Keane Stephan Pelikan Greg P. Toth M. Kate Smith Steven H. Rogstad 《American journal of botany》1999,86(9):1226-1238
Genetic diversity at variable-number-tandem-repeat (VNTR) loci was examined in the common cattail, Typha latifolia (Typhaceae), using three synthetic DNA probes composed of tandemly repeated “core” sequences (GACA, GATA, and GCAC). The principal objectives of this investigation were to determine whether: (1) the previously reported almost complete lack of polymorphism at allozyme loci in this species was indicative of a reduced amount of genetic diversity at VNTR loci as well; (2) VNTR markers were informative about possible clonal propagation; and (3) significant differences in genetic structure of sampling sites were associated with differences in environmental levels of pollutants at those sites. Previously, widespread sampling across the eastern United States, surveying across ten allozyme loci, has detected only two genotypes, involving a difference at a single locus, among 104 populations. In this study, the amount of genetic diversity detected at VNTR loci: (1) among ramets (N = 40; 40 genotypes detected) collected at ∼8-km intervals along a 320-km transect; (2) among ramets (N = 220; 117 genotypes detected) from five study sites separated by 50–3000 m; and (3) even among ramets within each study site [N = 44 per site; from 13 to 34 genotypes detected per site (270 m2)] exceeds that previously found in those more geographically widespread allozyme surveys. Among the 260 ramets analyzed here, the mean number of bands scored per individual was 48.61 (SD = 2.80). Mean genetic similarity among ramets collected along the 320-km transect was 0.91, which was within the range of mean genetic similarity within the five study sites (range: 0.89–0.95). Among the five study sites, 61% of the samples analyzed appeared to be clonal ramets, with up to 12 clones detected for 44 ramets sampled within a site. Clones grew intermingled and ranged up to 39 m in extent. Permutation tests of genetic similarity revealed significant genetic differentiation between each of the five study sites. Consistent with the previous allozyme studies, T. latifolia was characterized by extremely low genetic variation relative to levels of polymorphism detected at VNTR loci in other plant species. Estimated heterozygosity among ramets along the 320-km transect ranged from 0.11 to 0.13, while that within the five study sites ranged from 0.05 to 0.12. Estimates of Fst (0.32–0.41) also indicated considerable genetic subdivision among these stands. Significantly higher genetic diversity was detected at the two study sites that chemistry and toxicity data indicate to be the most severely impacted by pollutants. Although this correlation does not establish cause and effect, the results of this study indicate that the analysis of genetic diversity at VNTR loci may be a useful tool for monitoring anthropogenic-induced changes in the genetic structure of natural populations of plants. 相似文献
112.
Kate?ina Ku?elová Barbora Brodská Ota Fuchs Marie Dobrovolná Petr Soukup Petr Cetkovsky 《PloS one》2015,10(5)
Nucleophosmin 1 (NPM1) mutations are frequently found in patients with acute myeloid leukemia (AML) and the newly generated sequences were suggested to induce immune response contributing to the relatively favorable outcome of patients in this AML subset. We hypothesized that if an efficient immune response against mutated nucleophosmin can be induced in vivo, the individuals expressing HLA alleles suitable for presenting NPM-derived peptides should be less prone to developing AML associated with NPM1 mutation. We thus compared HLA class I frequencies in a cohort of patients with mutated NPM1 (63 patients, NPMc+), a cohort of patients with wild-type NPM1 (94 patients, NPMwt) and in normal individuals (large datasets available from Allele Frequency Net Database). Several HLA allelic groups were found to be depleted in NPMc+ patients, but not in NPMwt compared to the normal distribution. The decrease was statistically significant for HLA B*07, B*18, and B*40. Furthermore, statistically significant advantage in the overall survival was found for patients with mutated NPM1 expressing at least one of the depleted allelic groups. The majority of the depleted alleles were predicted to bind potent NPM-derived immunopeptides and, importantly, these peptides were often located in the unmutated part of the protein. Our analysis suggests that individuals expressing specific HLA allelic groups are disposed to develop an efficient anti-AML immune response thanks to aberrant cytoplasmic localization of the mutated NPM protein. 相似文献
113.
This paper comprises the scientific justification for the Farm Constructed Wetland (FCW) Design Manual for Northern Ireland and Scotland. Moreover, this document addresses an international audience interested in applying wetland systems in the wider agricultural context. Farm constructed wetlands combine farm wastewater (predominantly farmyard runoff) treatment with landscape and biodiversity enhancements, and are a specific application and class of integrated constructed wetlands (ICW), which have wider applications in the treatment of other wastewater types such as domestic sewage. The aim of this review paper is to propose guidelines highlighting the rationale for FCW, including key water quality management and regulatory issues, important physical and biochemical wetland treatment processes, assessment techniques for characterizing potential FCW sites and discharge options to water bodies. The paper discusses universal design, construction, planting, maintenance and operation issues relevant specifically for FCW in a temperate climate, but highlights also catchment-specific requirements to protect the environment. 相似文献
114.
Gene E. Likens Donald C. Buso Brian K. Dresser Emily S. Bernhardt Robert O. Hall Jr Kate H. Macneale Scott W. Bailey 《Restoration Ecology》2004,12(3):419-428
Ground and pelletized Wollastonite (Wo; CaSiO3) was added to a 50‐m reach of an anthropogenically acidified stream within the Hubbard Brook Experimental Forest, New Hampshire, to evaluate its buffering and restoration potential. The Wo was highly effective in raising the pH, acid‐neutralizing capacity (ANC), dissolved inorganic carbon (DIC), and Ca2+ concentrations of the stream water, but during the short duration of the experiment had no discernable effect on the stream biota. After initial, spike‐like fluctuations in pH and concentrations of ANC, DIC, and Ca2+, the relatively slow dissolution rates of the Wo dampened extreme concentrations and contributed to relatively long‐lasting (4 months) amelioration of streamwater acidity. Changes in concentrations of Ca2+, dissolved Si, ANC, and DIC were inversely related to streamflow. After several high, stream‐discharge events, concentrations quickly and consistently returned to pre‐event conditions. 相似文献
115.
Kate Swift Natalie Moltschaniwskyj 《Journal of experimental marine biology and ecology》2005,315(2):177-186
The cephalopod digestive gland plays an important role in the efficient assimilation of nutrients and therefore the fast growth of the animal. The histological and enzymatic structure of Euprymna tasmanica was studied and used in this experiment to determine the dynamics of the gland in response to feeding. The major roles of the digestive gland were secretion of digestive enzymes in spherical inclusions (boules) and excretion of metabolic wastes in brown body vacuoles. High levels of trypsin, chymotrypsin and α-amylase, low levels of α-glucosidase and negligible carboxypeptidase activity were produced by the gland. There was no evidence of secretion of digestive enzymes in other organs of the digestive tract. Within 60 min of a feeding event, the gland produced increasing numbers of boules to replace those lost from the stomach during the feeding event. Initially, small boules were seen in the digestive cells, they increased in size until they are released into the lumen of the gland where they are transported to the stomach. There was no evidence of an increase in activity of digestive enzymes following a feeding event, despite structural changes in the gland. However, there was large variation among individuals in the level of digestive enzyme activity. A negative correlation between boule and brown body vacuole density suggested that the large variation in enzyme activity may be due to the digestive gland alternating between enzyme production and excretion. 相似文献
116.
Taylor LM James A Schuller CE Brce J Lock RB Mackenzie KL 《The Journal of biological chemistry》2004,279(42):43634-43645
117.
Verónica C. Neves Kate Griffiths Fiona R. Savory Robert W. Furness Barbara K. Mable 《European Journal of Wildlife Research》2010,56(1):95-100
The European starling (Sturnus vulgaris) has recently been found to eat eggs of the endangered roseate tern (Sterna dougallii) in the Azores. Azorean starlings are considered an endemic subspecies (S. vulgaris granti), so we investigated how much genetic divergence has accumulated between the Azores and other European populations in order
to assess whether lethal control measures might be possible, as previous experiments have found that taste aversion is not
likely to be successful. For this purpose, we sequenced a region of the protein-coding mitochondrial gene ND2 for samples
from six different populations. Of the 1,026 base pairs sequenced, 19 (1.7%) were variable and formed 15 different haplotypes.
The Azores had high and significant genetic differentiation from all the other populations studied. Haplotype diversity was
high in the mainland populations studied, ranging from 0.767 to 0.900, but there was no variation among the Azores samples,
which were collected from a geographically broad region. Given the lack of genetic variability in the Azores birds and their
abundance throughout the archipelago, lethal control on a local basis and as part of an integrated control plan can be seen
as a reasonable measure to protect tern colonies. 相似文献
118.
Michelle J. Boyle Prasanna Jagannathan Lila A. Farrington Ijeoma Eccles-James Samuel Wamala Tara I McIntyre Hilary M. Vance Katherine Bowen Felistas Nankya Ann Auma Mayimuna Nalubega Esther Sikyomu Kate Naluwu John Rek Agaba Katureebe Victor Bigira James Kapisi Jordan Tappero Mary K Muhindo Bryan Greenhouse Emmanuel Arinaitwe Grant Dorsey Moses R. Kamya Margaret E. Feeney 《PLoS pathogens》2015,11(7)
FoxP3+ regulatory CD4 T cells (Tregs) help to maintain the delicate balance between pathogen-specific immunity and immune-mediated pathology. Prior studies suggest that Tregs are induced by P. falciparum both in vivo and in vitro; however, the factors influencing Treg homeostasis during acute and chronic infections, and their role in malaria immunopathogenesis, remain unclear. We assessed the frequency and phenotype of Tregs in well-characterized cohorts of children residing in a region of high malaria endemicity in Uganda. We found that both the frequency and absolute numbers of FoxP3+ Tregs in peripheral blood declined markedly with increasing prior malaria incidence. Longitudinal measurements confirmed that this decline occurred only among highly malaria-exposed children. The decline of Tregs from peripheral blood was accompanied by reduced in vitro induction of Tregs by parasite antigen and decreased expression of TNFR2 on Tregs among children who had intense prior exposure to malaria. While Treg frequencies were not associated with protection from malaria, there was a trend toward reduced risk of symptomatic malaria once infected with P. falciparum among children with lower Treg frequencies. These data demonstrate that chronic malaria exposure results in altered Treg homeostasis, which may impact the development of antimalarial immunity in naturally exposed populations. 相似文献
119.
Repeated inoculation as a strategy for the remediation of low concentrations of phenanthrene in soil
Phenanthrene, a polycyclic aromatic hydrocarbon, becomes increasingly unavailable to microorganisms for degradation as it ages in soil. Consequently, many bioaugmentation efforts to remediate polycyclic aromatic hydrocarbons in soil have failed. We studied theeffect of repeatedly inoculating a soil with a phenanthrene-degrading Arthrobacter sp. on the mineralization kinetics of low concentrations of phenanthrene. After the first inoculation, the initial mineralization rate of 50 ng/g phenanthrene declined in a biphasicexponential pattern. By three hundred hours after inoculation, there was no difference in mineralization rates between the inoculated and uninoculated treatments even though a large fraction of the phenanthrene had not yet been mineralized. A second and third inoculation significantly increased the mineralization rate, suggesting that, though themineralization rate declined, phenanthrene remained bioavailable. Restirring the soil, without inoculation, did not produce similar increases in mineralization rates, suggesting absence of contact between cells and phenanthrene on a larger spatial scale (>mm) is not the cause of the mineralization decline. Bacteria inoculated into soil 280 hours beforethe phenanthrene was added could not maintain phenanthrene degradation activity. We suggest sorption lowered bioavailability of phenanthrene below an induction threshold concentration for metabolic activity of phenanthrene-degrading bacteria. 相似文献
120.
Gill JJ Summer EJ Russell WK Cologna SM Carlile TM Fuller AC Kitsopoulos K Mebane LM Parkinson BN Sullivan D Carmody LA Gonzalez CF LiPuma JJ Young R 《Journal of bacteriology》2011,193(19):5300-5313
Within the Burkholderia cepacia complex, B. cenocepacia is the most common species associated with aggressive infections in the lungs of cystic fibrosis patients, causing disease that is often refractive to treatment by antibiotics. Phage therapy may be a potential alternative form of treatment for these infections. Here we describe the genome of the previously described therapeutic B. cenocepacia podophage BcepIL02 and its close relative, Bcep22. Phage Bcep22 was found to contain a circularly permuted genome of 63,882 bp containing 77 genes; BcepIL02 was found to be 62,714 bp and contains 76 predicted genes. Major virion-associated proteins were identified by proteomic analysis. We propose that these phages comprise the founding members of a novel podophage lineage, the Bcep22-like phages. Among the interesting features of these phages are a series of tandemly repeated putative tail fiber genes that are similar to each other and also to one or more such genes in the other phages. Both phages also contain an extremely large (ca. 4,600-amino-acid), virion-associated, multidomain protein that accounts for over 20% of the phages' coding capacity, is widely distributed among other bacterial and phage genomes, and may be involved in facilitating DNA entry in both phage and other mobile DNA elements. The phages, which were previously presumed to be virulent, show evidence of a temperate lifestyle but are apparently unable to form stable lysogens in their hosts. This ambiguity complicates determination of a phage lifestyle, a key consideration in the selection of therapeutic phages. 相似文献