首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3388篇
  免费   236篇
  国内免费   2篇
  3626篇
  2023年   15篇
  2022年   46篇
  2021年   71篇
  2020年   33篇
  2019年   34篇
  2018年   78篇
  2017年   61篇
  2016年   95篇
  2015年   152篇
  2014年   146篇
  2013年   217篇
  2012年   238篇
  2011年   258篇
  2010年   164篇
  2009年   143篇
  2008年   191篇
  2007年   142篇
  2006年   154篇
  2005年   160篇
  2004年   156篇
  2003年   153篇
  2002年   114篇
  2001年   48篇
  2000年   48篇
  1999年   48篇
  1998年   46篇
  1997年   29篇
  1996年   32篇
  1995年   33篇
  1994年   21篇
  1993年   27篇
  1992年   41篇
  1991年   31篇
  1990年   30篇
  1989年   28篇
  1988年   24篇
  1987年   33篇
  1986年   25篇
  1985年   22篇
  1984年   20篇
  1983年   24篇
  1982年   24篇
  1981年   14篇
  1980年   16篇
  1979年   19篇
  1978年   18篇
  1976年   18篇
  1975年   13篇
  1974年   9篇
  1973年   9篇
排序方式: 共有3626条查询结果,搜索用时 0 毫秒
91.
Tracking changes in biodiversity through time requires an understanding of the relationship between modern diversity and how this diversity is preserved in the fossil record. Fossil pollen is one way in which past vegetation diversity can be reconstructed. However, there is limited understanding of modern pollen-vegetation diversity relationships from biodiverse tropical ecosystems. Here, pollen (palynological) richness and diversity (Hill N1) are compared with vegetation richness and diversity from forest and savannah ecosystems in the New World and Old World tropics (Neotropics and Palaeotropics). Modern pollen data were obtained from artificial pollen traps deployed in 1-ha vegetation study plots from which vegetation inventories had been completed in Bolivia and Ghana. Pollen counts were obtained from 15 to 22 traps per plot, and aggregated pollen sums for each plot were >?2,500. The palynological richness/diversity values from the Neotropics were moist evergreen forest?=?86/6.8, semi-deciduous dry forest?=?111/21.9, wooded savannah?=?138/31.5, and from the Palaeotropics wet evergreen forest?=?144/28.3, semi-deciduous moist forest?=?104/4.4, forest-savannah transition?=?121/14.1; the corresponding vegetation richness/diversity was 100/36.7, 80/38.7 and 71/39.4 (Neotropics), and 101/54.8, 87/45.5 and 71/34.5 (Palaeotropics). No consistent relationship was found between palynological richness/diversity, and plot vegetation richness/diversity, due to the differential influence of other factors such as landscape diversity, pollination strategy, and pollen source area. Palynological richness exceeded vegetation richness, while pollen diversity was lower than vegetation diversity. The relatively high global diversity of tropical vegetation was found to be reflected in the pollen rain.  相似文献   
92.
The aim of this review of the landmark HMG-CoA reductase inhibitors (statins) studies is to enable the clinician to draw practical lessons from these trials. The Scandinavian Simvastatin Survival Study (4S) established the importance of treating the hypercholesterolemic patient with established cardiovascular heart disease. The West of Scotland Coronary Prevention Study (WOSCOPS) showed the benefit of treating healthy hypercholesterolemic men who were nevertheless at high risk of developing cardiovascular heart disease in the future. The Cholesterol and Recurrent Events (CARE) study, a secondary prevention trial, proved the benefit of treating patients with myocardial ischemia and cholesterol levels within normal limits. This conclusion was confirmed by the Long-term Intervention With Pravastatin in Ischemic Disease (LIPID) study, another secondary prevention study that enrolled patients with a wide range of cholesterol levels (4-7 mmol/dL), into which the large majority of patients would belong. The importance of treating patients with established ischemic heart disease (IHD), and those at high risk of developing cardiovascular heart disease, regardless of cholesterol level, was being realized. The Air Force/Texas Coronary Artery Prevention Study (AFCAPS/TexCAPS) then showed that treatment can reduce adverse cardiovascular events even in the primary prevention of patients with normal cholesterol levels. The Myocardial Ischemia Reduction With Aggressive Cholesterol Lowering (MIRACL) trial showed that hypocholesterolemic therapy is useful in the setting of an acute coronary syndrome, while the Atorvastatin Versus Revascularisation Treatment (AVERT) study showed that aggressive statin therapy is as good as angioplasty in reducing ischemic cardiac events in patients with stable angina pectoris. Finally, the Heart Protection Study (HPS) randomized more than 20,000 patients, and the value of statins in reducing adverse cardiovascular events in the high-risk patient, including the elderly, women, and even in those with low cholesterol levels, is beyond doubt. The emphasis is now on the risk level for developing cardiovascular events, and treatment should target the high-risk group and not be dependent on the actual cholesterol level of the patient. It is interesting to compare the large amount of data on the value and safety of the statins with the much more limited and less convincing data on antioxidant vitamins.  相似文献   
93.
An emerging model of auxin transport regulation   总被引:15,自引:0,他引:15       下载免费PDF全文
Muday GK  Murphy AS 《The Plant cell》2002,14(2):293-299
  相似文献   
94.
95.
alpha(2)-Macroglobulin (alpha(2)M) is a broad-spectrum protease-binding protein abundant in plasma from vertebrates and several invertebrate phyla. This protein was purified from cell-free hemolymph of the white shrimp, Penaeus vannamei, using Blue-Sepharose and Phenyl-Sepharose chromatography. The shrimp alpha(2)M is a 380 kDa protein, a homodimer of two apparently identical subunits of approximately 180 kDa linked by disulphide bridges. The amino acid sequence of the N-terminus is similar to the Limulus alpha(2)M counterpart. The shrimp alpha(2)M has a wide inhibition spectrum against different proteinase types including trypsin, leucine amino peptidase, chymotrypsin, elastase and papain. The secondary structure of shrimp alpha(2)M is mainly beta-sheet (36%), with a characteristic minimum elipticity at 217 nm. Evidence for a thiolester-mediated inhibition mechanism of proteases by alpha(2)M was provided by inactivation with methylamine.  相似文献   
96.
Cultured human skin fibroblasts take up α- -iduronidase by receptor-mediated pinocytosis. Certain lysosomotropic amines such as chloroquine, ammonia and procaine inhibit this process, without affecting the fluid endocytosis of dextran. In contrast to the competitive inhibition by mannose 6-phosphate, the inhibition by amines is non-competitive and is therefore presumed not to affect binding of the enzyme to receptors. The dose response curves are very steep, and equations that best fit the data use a power of inhibitor concentration (i2 for procaine, i4 for chloroquine), indicating interaction of several amine molecules at the inhibitory site(s). The inhibition is reversed by removal of the amine from the medium and does not result from accelerated efflux of endocytosed enzyme. We suggest that the amines interfere with delivery of receptor-bound enzyme to lysosomes.  相似文献   
97.
Amphisteginid foraminifera are larger symbiont-bearing foraminifera that are abundant in tropical and subtropical reef and shelf regions of the world’s oceans. There is now unequivocal evidence that climate change has led to an expansion of tropical belts. As temperatures rise, surface isotherms are shifting poleward and habitat ranges are moving towards higher latitudes. Fueled by substantial warming, the Mediterranean has been identified as one of the most severely affected areas. This study documents for the first time that amphisteginid foraminifera have now crossed the Strait of Otranto and invaded the eastern Adriatic coast along southern Albania. Sampling on the opposite side along the southern part of the Italian boot shows, however, that amphisteginid foraminifera have not yet colonized the western coast of the Adriatic. The asymmetric invasion of the Adriatic displays spatial heterogeneity, where the progressive colonization follows the major surface currents with a northerly flow along the eastern coast and a southerly return along the western coastline. Previous sampling along Ionian Sea sample sites provides a baseline chronology allowing computations of range expansion rates. The observed recent rate of range expansion in Amphistegina is computed between 4.0 and 10 km/year?1 and provides strong support for current species model projections. Based on the new data, the range boundary shift is projected to lead to a total northward range expansion of 5.2° latitude in the year 2100. The latitudinal range extension computed for the Mediterranean is thus almost twice as large as those computed for the southern hemisphere. The ongoing range extension into new areas of the Mediterranean Sea is shown to trigger changes in community structures with potential consequences for ecosystem functioning. Numerical abundances and impacts of invasive amphisteginids are greatest at shallow depths <20 m. Evidence is now accumulating that the invasion of amphisteginid foraminifera results in (1) a loss of benthic foraminiferal biodiversity, (2) alterations of foraminiferal community structures and (3) shifts in abundances of functional groups of foraminifera.  相似文献   
98.
99.
The present study examines the diagnostic challenges of identifying ante-mortem illicit substance use in human postmortem cases. Substance use, assessed by clinical case history reviews, structured next-of-kin interviews, by general toxicology of blood, urine and/or brain, and by scalp hair testing, identified 33 cocaine, 29 cannabis, 10 phencyclidine and nine opioid cases. Case history identified 42% cocaine, 76% cannabis, 10% phencyclidine and 33% opioid cases. Next-of-kin interviews identified almost twice as many cocaine and cannabis cases as Medical Examiner (ME) case histories, and were crucial in establishing a detailed lifetime substance use history. Toxicology identified 91% cocaine, 68% cannabis, 80% phencyclidine and 100% opioid cases, with hair testing increasing detection for all drug classes. A cocaine or cannabis use history was corroborated by general toxicology with 50% and 32% sensitivity, respectively, and with 82% and 64% sensitivity by hair testing. Hair testing corroborated a positive general toxicology for cocaine and cannabis with 91% and 100% sensitivity, respectively. Case history corroborated hair toxicology with 38% sensitivity for cocaine and 79% sensitivity for cannabis, suggesting that both case history and general toxicology underestimated cocaine use. Identifying ante-mortem substance use in human postmortem cases are key considerations in case diagnosis and for characterization of disorder-specific changes in neurobiology. The sensitivity and specificity of substance use assessments increased when ME case history was supplemented with structured next-of-kin interviews to establish a detailed lifetime substance use history, while comprehensive toxicology, and hair testing in particular, increased detection of recent illicit substance use.  相似文献   
100.
Lee LY  Ong SL  Ng HY  Hu JY  Koh YN 《Bioresource technology》2008,99(14):6614-6620
Simultaneous ammonium-nitrogen (NH(4)(+)-N) and copper removal, and copper recovery in synthetic wastewater using nitrifying biofilm from an ultra-compact biofilm reactor (UCBR) was demonstrated in batch studies, which consisted of three phases: Phase 1 for NH(4)(+)-N and copper removals, Phase 2 for copper recovery, and Phase 3 for NH(4)(+)-N removal. The results showed that more than 96.3% of copper was removed within 60min, while 60.1% of the adsorbed copper was recovered through rinsing the biofilms with 0.1mM of ethylenediaminetetraacetic acid (EDTA). The nitrifying biofilm was able to adsorb 0.245mg of copper/g of biofilms. After recovery treatment, 29.4% of copper remained bound within the nitrifying biofilms. No significant inhibitory effects towards NH(4)(+)-N removal in the presence of 0.92mg copper/L was noted in Phase 1 compared with the control test. However, lower initial pH condition in the recovery process and the accumulation of copper on the biofilm led to 50% inhibition on NH(4)(+)-N removal efficiency in the subsequent phase.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号