全文获取类型
收费全文 | 41804篇 |
免费 | 3698篇 |
国内免费 | 205篇 |
专业分类
45707篇 |
出版年
2018年 | 494篇 |
2017年 | 480篇 |
2016年 | 622篇 |
2015年 | 700篇 |
2014年 | 903篇 |
2013年 | 1073篇 |
2012年 | 1222篇 |
2011年 | 1257篇 |
2010年 | 851篇 |
2009年 | 862篇 |
2008年 | 1174篇 |
2007年 | 1165篇 |
2006年 | 1177篇 |
2005年 | 1002篇 |
2004年 | 962篇 |
2003年 | 910篇 |
2002年 | 886篇 |
2001年 | 2774篇 |
2000年 | 2454篇 |
1999年 | 1799篇 |
1998年 | 612篇 |
1997年 | 525篇 |
1996年 | 470篇 |
1995年 | 400篇 |
1994年 | 386篇 |
1992年 | 1279篇 |
1991年 | 1158篇 |
1990年 | 1095篇 |
1989年 | 1043篇 |
1988年 | 928篇 |
1987年 | 922篇 |
1986年 | 802篇 |
1985年 | 793篇 |
1984年 | 563篇 |
1983年 | 513篇 |
1982年 | 366篇 |
1979年 | 630篇 |
1978年 | 451篇 |
1977年 | 414篇 |
1976年 | 368篇 |
1975年 | 494篇 |
1974年 | 575篇 |
1973年 | 553篇 |
1972年 | 571篇 |
1971年 | 548篇 |
1970年 | 520篇 |
1969年 | 526篇 |
1968年 | 426篇 |
1967年 | 373篇 |
1966年 | 409篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Judith Z. Drexler Isa Woo Christopher C. Fuller Glynnis Nakai 《Restoration Ecology》2019,27(5):1117-1127
Few comparisons exist between vertical accretion (VA) and carbon accumulation rates (CARs) in restored versus historic (i.e. reference) marshes. Here, we compare these processes in a formerly diked, sparsely vegetated, restored salt marsh (Six Gill Slough, SG), whose surface is subsided relative to the tidal frame, to an adjacent, relatively pristine, historic salt marsh (Animal Slough, AS). Six sediment cores were collected at both AS and SG approximately 6 years after restoration. Cores were analyzed for bulk density (BD), % loss of ignition, % organic carbon, and 210Pb. We found that sharp changes in BD in surface layers of SG cores were highly reliable markers for the onset of restoration. The mean VA since restoration at SG (0.79 [SD = 0.29] cm/year) was approximately twice that of AS (0.41 [SD = 0.16] cm/year). In comparison, the VA at AS over 50 years was 0.30 (SD = 0.09) cm/year. VA consisted almost entirely of inorganic sediment at SG whereas at AS it was approximately 55%. Mean CARs at SG were somewhat greater than at AS, but the difference was not significant due to high variability (SG: 81–210 g C m?2 year?1; AS: 115–168 g C m?2 year?1). The mean CAR at AS over the past 50 years was 118 (SD = 23) g C m?2 year?1. This study demonstrates that a sparsely vegetated, restored salt marsh can quickly begin to accumulate carbon and that historic and restored marshes can have similar CARs despite highly divergent formation processes. 相似文献
992.
L. R. Wang X. N. Yang Y. S. Gao X. Y. Zhang W. Hu Z. Zhou Y. L. Meng 《Plant biology (Stuttgart, Germany)》2019,21(5):911-919
- The dormancy of seeds of upland cotton can be broken during dry after‐ripening, but the mechanism of its dormancy release remains unclear.
- Freshly harvested cotton seeds were subjected to after‐ripening for 180 days. Cotton seeds from different days of after‐ripening (DAR) were sampled for dynamic physiological determination and germination tests. The intact seeds and isolated embryos were germinated to assess effects of the seed coat on embryo germination. Content of H2O2 and phytohormones and activities of antioxidant enzymes and glucose‐6‐phosphate dehydrogenase were measured during after‐ripening and germination.
- Germination of intact seeds increased from 7% upon harvest to 96% at 30 DAR, while embryo germination improved from an initial rate of 82% to 100% after 14 DAR. Based on T50 (time when 50% of seeds germinate) and germination index, the intact seed and isolated embryo needed 30 and 21 DAR, respectively, to acquire relatively stable germination. The content of H2O2 increased during after‐ripening and continued to increase within the first few hours of imbibition, along with a decrease in abscisic acid (ABA) content. A noticeable increase was observed in gibberellic acid content during germination when ABA content decreased to a lower level. Coat removal treatment accelerated embryo absorption of water, which further improved the accumulation of H2O2 and changed peroxidase content during germination.
- For cotton seed, the alleviation of coat‐imposed dormancy required 30 days of after‐ripening, accompanied by rapid dormancy release (within 21 DAR) in naked embryos. H2O2 acted as a core link between the response to environmental changes and induction of other physiological changes for breaking seed dormancy.
993.
994.
995.
996.
Quirijn R. J. G. Tummers Charlotte E. S. Hoogstins Alexander A. W. Peters Cor D. de Kroon J. Baptist M. Z. Trimbos Cornelis J. H. van de Velde John V. Frangioni Alexander L. Vahrmeijer Katja N. Gaarenstroom 《PloS one》2015,10(6)
Objective
In ovarian cancer, two of the most important prognostic factors for survival are completeness of staging and completeness of cytoreductive surgery. Therefore, intra-operative visualization of tumor lesions is of great importance. Preclinical data already demonstrated tumor visualization in a mouse-model using near-infrared (NIR) fluorescence imaging and indocyanine green (ICG) as a result of enhanced permeability and retention (EPR). The aim of this study was to determine feasibility of intraoperative ovarian cancer metastases imaging using NIR fluorescence imaging and ICG in a clinical setting.Methods
Ten patients suspected of ovarian cancer scheduled for staging or cytoreductive surgery were included. Patients received 20 mg ICG intravenously after opening the abdominal cavity. The mini-FLARE NIR fluorescence imaging system was used to detect NIR fluorescent lesions.Results
6 out of 10 patients had malignant disease of the ovary or fallopian tube, of which 2 had metastatic disease outside the pelvis. Eight metastatic lesions were detected in these 2 patients, which were all NIR fluorescent. However, 13 non-malignant lesions were also NIR fluorescent, resulting in a false-positive rate of 62%. There was no significant difference in tumor-to-background ratio between malignant and benign lesions (2.0 vs 2.0; P=0.99).Conclusions
This is the first clinical trial demonstrating intraoperative detection of ovarian cancer metastases using NIR fluorescence imaging and ICG. Despite detection of all malignant lesions, a high false-positive rate was observed. Therefore, NIR fluorescence imaging using ICG based on the EPR effect is not satisfactory for the detection of ovarian cancer metastases. The need for tumor-specific intraoperative agents remains.Trial Registration
ISRCTN Registry ISRCTN16945066 相似文献997.
[35S] trithiomolybdate was administered intravenously to a group of four steer at two dose rates, 1 and 26 mg Mo per animal. Radioactivity appeared rapidly in the liver and was distributed in all the subcellular fractions examined. Examination by Sephadex G-100 gel-filtration of the cytosol fraction showed that distinct 35S-binding protein peaks were present. The protein-bound radioactivity was displaceable and was identified as [35S]thiomolybdates. No radioactivity eluted with metallothionein, but 35S was associated with the high molecular weight copper fraction, eluted in the void volume of the column, which increased transiently after the administration of the higher dose. It was suggested that the presence of protein-bound thiomolybdates in the liver gave rise to new ligands, which altered the equilibrium of copper between the different metal-binding proteins. This might be similar to the alteration in the copper-binding of albumin produced by the presence of thiomolybdates. 相似文献
998.
Z H Zhao B Watschinger C D Brown M M Beyer E A Friedman 《Hormones et métabolisme》1987,19(11):534-537
The variations of susceptibility to alloxan induced Diabetes in a total of seventeen rabbits was described. Our study was designed to explore dosage schedules which might improve rabbit responsiveness to and survival after alloxan treatment. A wide range of response to intravenously administered alloxan was observed. Permanent diabetes (blood glucose 350 mg/dl) was found in three rabbits after a single injection (60 mg/kg in one, 100 mg/kg in two). This effect has persisted for eight months. By contrast, two other rabbits injected with a single dose of alloxan (60 mg/kg) developed only transient hyperglycemia. Similarly, four other rabbits either did not respond or had an incomplete response after receiving a total dose of 120 mg/kg. These data suggest that there is extreme variability in individual rabbits susceptibility to the diabetogenic affects of alloxan. 相似文献
999.
The conformations of the H+-ATPase complex and F1-ATPase in low concentrations of methanol, ethanol, n-propanol, iso-propanol and t-butanol were studied by circular dichroism. For F1-ATPase, all but methanol first increased and then decreased the circular dichroism magnitude of helical bands as the alcohol concentration was increased. With ethanol, n-propanol, iso-propanol and t-butanol, the alpha-helix content reached a maximum at about 5% alcohol and began to decrease at 10%. The content of beta-sheet showed the opposite effect, reaching a minimum at 5% and increasing slightly at higher concentrations. None of the alcohols studied had a significant effect on the conformation of the H+-ATPase complex. This difference implies that the alcohols had a greater effect on free F1-ATPase than on the membrane-bound F1-ATPase. The hydrophobic protein F0 and the membrane lipids in the H+-ATPase complex may stabilize and protect F1 from the effects of the alcohols. 相似文献
1000.
Although [3H]imipramine is a selective radioligand for the 5-hydroxytryptamine (5-HT) transporter in human platelets, its affinity for binding to the 5-HT transporter complex at 0 degrees C (0.6 nM) is significantly higher than its potency for inhibition of [3H]5-HT uptake at the physiological temperature of 37 degrees C (Ki = 29 nM). As this apparent discrepancy could be related to the assay temperature, we studied the thermodynamics of drug interaction with the 5-HT transporter at assay temperatures between 0 degrees C and 37 degrees C, using as radioligands [3H]imipramine (0 degrees C and 20 degrees C) and [3H]paroxetine (20 degrees C and 37 degrees C), a newly available probe for the 5-HT transporter. At 20 degrees C, Ki values of 14 tricyclic and nontricyclic drugs for inhibition of [3H]imipramine and [3H]paroxetine binding to human platelet membranes were highly significantly correlated (r = 0.98, p less than 0.001), validating the use of these two radioligands to study the 5-HT transporter over a temperature range larger than was previously possible with [3H]imipramine alone. The affinity of imipramine for the 5-HT transporter is progressively enhanced with decreasing incubation temperature, thus favoring the selectivity of [3H]imipramine for the 5-HT transporter at 0 degrees C. At 37 degrees C, the Ki of imipramine for inhibition of [3H]paroxetine binding is 32 nM, and equals its Ki value for inhibition of 5-HT uptake into human platelets. With the exception of chlorimipramine, other tricyclic 5-HT uptake inhibitors showed a temperature sensitivity in their interaction with the 5-HT transporter similar to that of imipramine.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献