全文获取类型
收费全文 | 251022篇 |
免费 | 23118篇 |
国内免费 | 254篇 |
出版年
2018年 | 2807篇 |
2017年 | 2757篇 |
2016年 | 3700篇 |
2015年 | 4147篇 |
2014年 | 5090篇 |
2013年 | 7079篇 |
2012年 | 7899篇 |
2011年 | 8534篇 |
2010年 | 5739篇 |
2009年 | 5102篇 |
2008年 | 7264篇 |
2007年 | 7433篇 |
2006年 | 7001篇 |
2005年 | 6684篇 |
2004年 | 6549篇 |
2003年 | 6304篇 |
2002年 | 6155篇 |
2001年 | 12037篇 |
2000年 | 11980篇 |
1999年 | 9163篇 |
1998年 | 2681篇 |
1997年 | 2756篇 |
1996年 | 2697篇 |
1995年 | 2481篇 |
1994年 | 2427篇 |
1993年 | 2318篇 |
1992年 | 7184篇 |
1991年 | 6988篇 |
1990年 | 7058篇 |
1989年 | 6845篇 |
1988年 | 6361篇 |
1987年 | 6015篇 |
1986年 | 5359篇 |
1985年 | 5667篇 |
1984年 | 4466篇 |
1983年 | 3870篇 |
1982年 | 2677篇 |
1981年 | 2497篇 |
1980年 | 2307篇 |
1979年 | 4114篇 |
1978年 | 3137篇 |
1977年 | 2890篇 |
1976年 | 2830篇 |
1975年 | 3270篇 |
1974年 | 3505篇 |
1973年 | 3528篇 |
1972年 | 3059篇 |
1971年 | 2845篇 |
1970年 | 2534篇 |
1969年 | 2301篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The level of urine selenium in healthy adult population, 230 persons, was examined. Persons were selected regarding sex, ages, and smoking habits. No differences versus these observations have been found. For a total, group values are 16.96 Se nmol/creatinine mmol, SD=5.44. It is possible from a single-void specimen to express daily excretion of selenium. 相似文献
992.
Plants often suffer reductions in fecundity due to insect herbivory. Whether this loss of seeds has population-level consequences
is much debated and often unknown. For many plants, particularly those with long-lived seedbanks, it is frequently asserted
that herbivores have minimal impacts on plant abundance because safe-site availability rather than absolute seed number determines
the magnitude of future plant recruitment and hence population abundance. However, empirical tests of this assertion are generally
lacking and the interplay between herbivory, spatio-temporal variability in seed- or safe-site-limited recruitment, and seedbank
dynamics is likely to be complex. Here we use a stochastic simulation model to explore how changes in the spatial and temporal
frequency of seed-limited recruitment, the strength of density-dependent seedling survival, and longevity of seeds in the
soil influence the population response to herbivory. Model output reveals several surprising results. First, given a seedbank,
herbivores can have substantial effects on mean population abundance even if recruitment is primarily safe-site-limited in
either time or space. Second, increasing seedbank longevity increases the population effects of herbivory, because annual
reductions in seed input due to herbivory are accumulated in the seedbank. Third, population impacts of herbivory are robust
even in the face of moderately strong density-dependent seedling mortality. These results imply that the conditions under
which herbivores influence plant population dynamics may be more widespread than heretofore expected. Experiments are now
needed to test these predictions.
Received: 3 November 1999 / Accepted: 15 February 2000 相似文献
993.
Both short-term and continuous red radiation stimulated while far red radiation inhibited growth and ethylene production in
Chlorella pyrenoidosa. Kinetin and linuron also affected culture density and ethylene production, depending on their concentration.
Phytochrome might participated in the regulation of growth and ethylene production.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
994.
V V Iaglov N I Sheina M A Fesenko 《Biulleten' eksperimental'no? biologii i meditsiny》1990,109(5):508-510
Morphofunctional indices of pyracryl (phosphate poly-2-pyridylethylmethacrilate) effecting reproduction system of Wistar rats have been studied. It has been shown that monthly administration of pyracryl in doses 1 mg/kg and 5 mg/kg lead to infiltration of mast cells and their pronounced degranulation and impairment of blood and lymph microcirculation. As far as gonadotropic effect is concerned, its parametres depend on the administration of pyracryl. Pyracryl in doses of 1 mg/kg stimulates folliculogenesis in ovaries, a dose of 5 mg/kg may cause expressed atresia of growing follicules and changes of oestrous cycle. It is suggested that biologically active substances of mast cells in female reproduction system play an important role in realization of gonadotropic effects of pyracryl. 相似文献
995.
Five popular but iron-inefficient cultivars were crossed with three efficient genotypes and both parents and F1s were evaluated for iron-efficiency in potted calcareous and noncalcareous soil. The iron-efficient genotypes were dark green or green in both noncalcareous and calcareous soils whereas inefficient types were light green to yellow in calcareous soil. The chlorophyll and active iron (Fe2+) concentration of leaves was less in iron-efficient genotypes compared to efficient types in calcareous soil and reduction of both the parameters from noncalcareous to calcareous soil was considerably high in iron-inefficient lines. There was significant correlation between visual scores, chlorophyll and active iron content. There were no differences among F1s for iron chlorosis and they were all iron-inefficient. The frequency of iron-inefficient plants was higher than the efficient plants in all F2 populations. But most of the productive plants came from iron-efficient segregants indicating strong association between iron-efficiency and productivity. Based on the results selection for iron-efficiency in early generations and extensive evaluation for productivity in advanced generations is suggested for developing varieties for cultivation in calcareous soils. 相似文献
996.
Biophysics - Abstract—The goal of the present study was to investigate the molecular mechanisms that underlie heart and skeletal muscle damage in male Wistar rats weighing 200–250 g in... 相似文献
997.
998.
Raul J. Cano Jessica Rivera-Perez Gary A. Toranzos Tasha M. Santiago-Rodriguez Yvonne M. Narganes-Storde Luis Chanlatte-Baik Erileen García-Roldán Lucy Bunkley-Williams Steven E. Massey 《PloS one》2014,9(9)
Coprolites are fossilized feces that can be used to provide information on the composition of the intestinal microbiota and, as we show, possibly on diet. We analyzed human coprolites from the Huecoid and Saladoid cultures from a settlement on Vieques Island, Puerto Rico. While more is known about the Saladoid culture, it is believed that both societies co-existed on this island approximately from 5 to 1170 AD. By extracting DNA from the coprolites, followed by metagenomic characterization, we show that both cultures can be distinguished from each other on the basis of their bacterial and fungal gut microbiomes. In addition, we show that parasite loads were heavy and also culturally distinct. Huecoid coprolites were characterized by maize and Basidiomycetes sequences, suggesting that these were important components of their diet. Saladoid coprolite samples harbored sequences associated with fish parasites, suggesting that raw fish was a substantial component of their diet. The present study shows that ancient DNA is not entirely degraded in humid, tropical environments, and that dietary and/or host genetic differences in ancient populations may be reflected in the composition of their gut microbiome. This further supports the hypothesis that the two ancient cultures studied were distinct, and that they retained distinct technological/cultural differences during an extended period of close proximity and peaceful co-existence. The two populations seemed to form the later-day Taínos, the Amerindians present at the point of Columbian contact. Importantly, our data suggest that paleomicrobiomics can be a powerful tool to assess cultural differences between ancient populations. 相似文献
999.
Aptamers constitute a potential class of therapeutic molecules typically selected from a large pool of oligonucleotides against a specific target. With a scope of developing unique shorter aptamers with very high biostability and affinity, locked nucleic acid (LNA) nucleotides have been investigated as a substrate for various polymerases. Various reports showed that some thermophilic B-family DNA polymerases, particularly KOD and Phusion DNA polymerases, accepted LNA-nucleoside 5′-triphosphates as substrates. In this study, we investigated the docking of LNA nucleotides in the active sites of RB69 and KOD DNA polymerases by molecular docking simulations. The study revealed that the incoming LNA-TTP is bound in the active site of the RB69 and KOD DNA polymerases in a manner similar to that seen in the case of dTTP, and with LNA structure, there is no other option than the locked C3′-endo conformation which in fact helps better orienting within the active site. 相似文献
1000.
S A Bitko L O Dynga A R Shikhman Z F Shmakova N I Briko 《Zhurnal mikrobiologii, epidemiologii, i immunobiologii》1990,(4):3-7
The amino acid composition of cell walls and surface proteins, isolated from virulent (M+) and avirulent (M-) streptococcal strains (group A, type 29) has been determined by the method of E. H. Beachey et al. The kinetics of the lysis and proteolysis of streptococcal cell walls with muramidase and protease obtained from Actinomyces levoris and streptolysin has been studied. The constants describing the progress rates of these processes has been determined; their values in case of both lysis and proteolysis are higher in virulent strains than in avirulent ones. 相似文献