全文获取类型
收费全文 | 138703篇 |
免费 | 4268篇 |
国内免费 | 887篇 |
专业分类
143858篇 |
出版年
2023年 | 239篇 |
2021年 | 399篇 |
2020年 | 337篇 |
2019年 | 335篇 |
2018年 | 12589篇 |
2017年 | 11419篇 |
2016年 | 8327篇 |
2015年 | 1539篇 |
2014年 | 1449篇 |
2013年 | 2343篇 |
2012年 | 7456篇 |
2011年 | 16234篇 |
2010年 | 13620篇 |
2009年 | 9327篇 |
2008年 | 12570篇 |
2007年 | 14128篇 |
2006年 | 3175篇 |
2005年 | 3102篇 |
2004年 | 3439篇 |
2003年 | 3331篇 |
2002年 | 3069篇 |
2001年 | 1749篇 |
2000年 | 1902篇 |
1999年 | 961篇 |
1998年 | 500篇 |
1997年 | 387篇 |
1996年 | 456篇 |
1995年 | 392篇 |
1994年 | 420篇 |
1993年 | 393篇 |
1992年 | 397篇 |
1991年 | 343篇 |
1990年 | 288篇 |
1989年 | 274篇 |
1988年 | 263篇 |
1987年 | 266篇 |
1985年 | 300篇 |
1984年 | 378篇 |
1983年 | 349篇 |
1982年 | 311篇 |
1981年 | 298篇 |
1980年 | 263篇 |
1979年 | 259篇 |
1978年 | 264篇 |
1977年 | 251篇 |
1976年 | 245篇 |
1975年 | 288篇 |
1974年 | 211篇 |
1972年 | 375篇 |
1971年 | 414篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
32.
33.
Roberta L. Millstein 《Journal of the history of biology》2008,41(2):339-367
Biologists and philosophers have been extremely pessimistic about the possibility of demonstrating random drift in nature,
particularly when it comes to distinguishing random drift from natural selection. However, examination of a historical case
– Maxime Lamotte’s study of natural populations of the land snail, Cepaea nemoralis in the 1950s – shows that while some pessimism is warranted, it has been overstated. Indeed, by describing a unique signature
for drift and showing that this signature obtained in the populations under study, Lamotte was able to make a good case for
a significant role for␣drift. It may be difficult to disentangle the causes of drift and selection acting in a population,
but it is not (always) impossible. 相似文献
34.
D. Padilla F. Acosta J. A. García F. Real Jose R. Vivas 《Archives of microbiology》2009,191(3):191-198
Hafnia alvei, a Gram negative bacillus related to the Enterobacteriaceae family, is considered an opportunistic pathogen of several animal species and humans. In this communication, we describe
fimbrial-like structures from different strains of H. alvei that cannot be easily ascribed to any of the previously reported fimbrial types in this species (type I or type III). Polymerase
chain reaction (PCR) and immunofluorescence assays were carried out to study fimbriae and flagella in H. alvei strains isolated from different sources. No correlation between the results obtained by PCR and those obtained by phenotypic
methods were found, and the antibodies used gave cross or different recognition patterns of the surface structures present
in these strains. We report as well that strain and growth temperature influence fimbriation and expression of flagella in
human and animal isolates of H. alvei. This study also indicates that the absence of fimbriae have a significant positive influence on the initial adhesion of
H. alvei to human epithelial cells. 相似文献
35.
36.
Comparative analysis of the cattle and human genomes: detection of ZOO-FISH and gene mapping-based chromosomal homologies 总被引:6,自引:0,他引:6
Comparative chromosome painting with individual human chromosome-specific libraries (CSLs) on cattle metaphase chromosomes
delineated 46 homologous chromosomal segments between the two species. Continuous arrangement of these segments on individual
cattle chromosomes demonstrates a nearly complete coverage of the bovine karyotype and shows physical boundaries of bovine
chromosomal segments homologous to individual human chromosomes. Alignment of the available comparative gene mapping data
with the homologous segments strongly supports the detected gross homologies between the karyotypes of the two species. In
addition to cattle, four human CSLs were hybridized to sheep metaphase chromosomes also, to further verify the known karyotype
homology within the Bovidae. Besides its application to karyotype evolution research, the comparative knowledge provides for
rapid expansion of the much needed Type I locus-based bovine gene map.
Received: 9 September 1995 / Accepted: 4 December 1995 相似文献
37.
Major knowledge gaps exist with respect to light-quality regimes in the coastal-zone Strandzha Quercus frainetto (Q.f.) forest region adjoining the southern Bulgarian Black Sea. This paper presents preliminary results that help narrow these
gaps. In conjunction with leaf area index (LAI) field campaigns we undertook measurements with an array of 7 broad-band (ca
40 nm) sensors covering the range 0.40–0.94 μm, plus 1 sensor for UVB (0.297 μm peak) and 1 for photosynthetically active
radiation (PAR). Measurements focused on inside-forest shade conditions at sites 0 to ca 15 km from the Black Sea and at altitudes
up to ca 120 m above sea level. Some of the sites were also studied using a high-resolution spectroradiometer. A sequential
measuring strategy was necessary. This involves potentially large uncertainties, here addressed through estimations of the
variability around the sinusoidal course of daylight. Light-quality regimes were found to be in general support of earlier
studies of deciduous forests. Our data from the broad-band sensors and from the spectroradiometer are mutually supportive.
They indicate a stronger red-shift below Q.f. canopies than below canopies in enclaves dominated by Fagus orientalis and Pinus sylvestris. Transmission in the range 0.50–0.55 μm increases beneath the three types of canopies, most pronounced in the Q.f. case. Analysis of relationships between the inside-forest to open-field irradiance ratio and LAI supports the use of Beer’s
Law. We found a fairly strong relationship between the red (0.66 μm) to far-red (0.73 μm) irradiance ratios (R/FR) and LAI
for the Q.f. forest. In quantitative terms, the result is new for this Q.f. region, and suggests further research to explore whether a two-sensor approach (0.66 and 0.73 μm) might offer possibilities
for further low-cost mapping of the spatio-temporal patterns of R/FR and LAI in Strandzha. Such mapping would assist in further
studies of the region’s forest biogeochemistry and vitality. 相似文献
38.
The microbial metabolism of organic matter (OM) in seagrass beds can create sulfidic conditions detrimental to seagrass growth;
iron (Fe) potentially has ameliorating effects through titration of the sulfides and the precipitation of iron-sulfide minerals
into the sediment. In this study, the biogeochemical effects of Fe availability and its interplay with sulfur and OM on sulfide
toxicity, phosphorous (P) availability, seagrass growth and community structure were tested. The availability of Fe and OM
was manipulated in a 2 × 2 factorial experiment arranged in a Latin square, with four replicates per treatment. The treatments
included the addition of Fe, the addition of OM, the addition of both Fe and OM as well as no addition. The experiment was
conducted in an oligotrophic, iron-deficient seagrass bed. Fe had an 84.5% retention efficiency in the sediments with the
concentration of Fe increasing in the seagrass leaves over the course of the experiment. Porewater chemistry was significantly
altered with a dramatic decrease in sulfide levels in Fe addition plots while sulfide levels increased in the OM addition
treatments. Phosphorus increased in seagrass leaves collected in the Fe addition plots. Decreased sulfide stress was evidenced
by heavier δ34S in leaves and rhizomes from plots to which Fe was added. The OM addition negatively affected seagrass growth but increased
P availability; the reduced sulfide stress in Fe added plots resulted in elevated productivity. Fe availability may be an
important determinant of the impact that OM has on seagrass vitality in carbonate sediments vegetated with seagrasses. 相似文献
39.