全文获取类型
收费全文 | 5271篇 |
免费 | 389篇 |
国内免费 | 1篇 |
专业分类
5661篇 |
出版年
2023年 | 14篇 |
2022年 | 40篇 |
2021年 | 79篇 |
2020年 | 42篇 |
2019年 | 46篇 |
2018年 | 52篇 |
2017年 | 66篇 |
2016年 | 111篇 |
2015年 | 188篇 |
2014年 | 201篇 |
2013年 | 252篇 |
2012年 | 354篇 |
2011年 | 357篇 |
2010年 | 229篇 |
2009年 | 195篇 |
2008年 | 302篇 |
2007年 | 277篇 |
2006年 | 262篇 |
2005年 | 299篇 |
2004年 | 252篇 |
2003年 | 280篇 |
2002年 | 265篇 |
2001年 | 76篇 |
2000年 | 55篇 |
1999年 | 85篇 |
1998年 | 87篇 |
1997年 | 67篇 |
1996年 | 51篇 |
1995年 | 48篇 |
1994年 | 54篇 |
1993年 | 57篇 |
1992年 | 75篇 |
1991年 | 49篇 |
1990年 | 47篇 |
1989年 | 38篇 |
1988年 | 51篇 |
1987年 | 44篇 |
1986年 | 27篇 |
1985年 | 46篇 |
1984年 | 50篇 |
1983年 | 27篇 |
1982年 | 36篇 |
1981年 | 46篇 |
1980年 | 36篇 |
1979年 | 26篇 |
1978年 | 31篇 |
1977年 | 28篇 |
1976年 | 20篇 |
1974年 | 24篇 |
1961年 | 11篇 |
排序方式: 共有5661条查询结果,搜索用时 15 毫秒
91.
Siljanen HM Saari A Krause S Lensu A Abell GC Bodrossy L Bodelier PL Martikainen PJ 《FEMS microbiology ecology》2011,75(3):430-445
In lake ecosystems a major proportion of methane (CH(4) ) emissions originate from the littoral zone, which can have a great spatial variability in hydrology, soil quality and vegetation. Hitherto, spatial heterogeneity and the effects it has on functioning and diversity of methanotrophs in littoral wetlands have been poorly understood. A diagnostic microarray based on the particulate methane monooxygenase gene coupled with geostatistics was used to analyse spatial patterns of methanotrophs in the littoral wetland of a eutrophic boreal lake (Lake Kev?t?n, Eastern Finland). The wetland had a hydrology gradient with a mean water table varying from -8 to -25 cm. The wettest area, comprising the highest CH(4) oxidation, had the highest abundance and species richness of methanotrophs. A high water table favoured the occurrence of type Ib methanotrophs, whereas types Ia and II were found under all moisture conditions. Thus the spatial heterogeneity in functioning and diversity of methanotrophs in littoral wetlands is highly dependent on the water table, which in turn varies spatially in relation to the geomorphology of the wetland. We suggest that changes in water levels resulting from regulation of lakes and/or global change will affect the abundance, activity and diversity of methanotrophs, and consequently CH(4) emissions from such systems. 相似文献
92.
Hollingsworth KG Blamire AM Keavney BD Macgowan GA 《American journal of physiology. Heart and circulatory physiology》2012,302(4):H885-H892
This study determined, for the first time, whether the effects of normal aging on systolic and diastolic left ventricular function in subjects without cardiovascular disease are related to underlying energetic defects. Cardiac magnetic resonance imaging with tissue tagging and (31)P spectroscopy was used to determine global structure, function, myocardial strains, and the phosphocreatine-to-ATP ratio (PCr/ATP) in 49 healthy subjects aged 20-69 yr. The three major abnormalities that developed with increasing age were the early filling percentage (EFP, the left ventricular volume increase from end systole to mid-diastole divided by stroke volume × 100), which decreased with age, indicating impaired early diastolic filling (r = -0.72, P < 0.0001), the torsion-to-shortening ratio (TSR, measure of subepicardial torsion exerting mechanical advantage over subendocardial shortening), which increased with age indicating relative subendocardial dysfunction (r = 0.44, P < 0.02), and the PCr/ATP (decreased with increasing age, r = -0.52, P < 0.003). EFP and TSR were strongly correlated (r = -0.63, P < 0.0001), although they were not related to PCr/ATP [EFP vs. PCr/ATP: r = 0.34, not significant (NS) and TSR vs. PCr/ATP: r = -0.3, P = NS]. In normal aging, changes in EFP and TSR likely share the same pathophysiology, although it is unlikely that energetics have a major role in the functional effects of aging. 相似文献
93.
94.
95.
96.
J. Guy Dr. O. Bosler G. Dusticier G. Pelletier A. Calas 《Cell and tissue research》1987,250(3):657-662
Summary The morphological substrate of putative serotonin (5-HT)/neuropeptide Y (NPY) interactions in thé suprachiasmatic nucleus (SCN) was investigated by combined radioautography and immunocytochemistry after intraventricular administration of (3H)5-HT in the rat. In the ventral portion of the SCN, the distribution of (3H)5-HT uptake sites overlapped closely the NPY-immunoreactive terminals. Previous investigations have shown that the dense 5-HT and NPY innervations of the SCN originate in different structures, i.e., the midbrain raphe nuclei and the ventral lateral geniculate nucleus, respectively. Accordingly, in the present study, destruction of 5-HT afferents by 5,7-dihydroxytryptamine was not found to induce any modification in NPY staining and, in ultrastructural immuno-radioautographic preparations, two distinct pools of axonal varicosities could be identified. Both 5-HT and NPY terminals established morphologically defined synaptic junctions, sometimes on the same neuronal target. Some cases of direct axo-axonic appositions between the two types of terminals were also encountered. These data constitute additional criteria for characterizing the cytological basis of the multiple transmitter interactions presumably involved in the function of the SCN as a central regulator of circadian biological rhythms. 相似文献
97.
Pascal Poindron Pierre Orgeur Pierre Le Neindre Guy Kann Ildiko Raksanyi 《Hormones and behavior》1980,14(2):173-177
The fading of postpartum maternal interest for the neonate (sensitive period) in ewes separated from their young at lambing is delayed when parturition is induced with 20 mg of estradiol benzoate (EB). An experiment was carried out to investigate the role of prolactin in this phenomenon. The sensitive period was studied in three groups of parturient ewes. In all groups lambs were removed at birth and reintroduced to their mothers 24 hr later. Maternal acceptance was tested at this time. In group 1 (dexamethasone D), ewes were induced to lamb with dexamethasone (15 mg im). In group 2 (EB), ewes were treated with 20 mg of estradiol benzoate (im). In group 3 (EB + CB 154) ewes received 20 mg of EB as in group 2 and 1 mg of CB 154 (sc) every 12 hr to prevent the enhanced secretion of prolactin which normally occurs after EB injection. The concentration of prolactin was highest in group 2 (EB), lowest in group 3 (EB + CB), and intermediate in group 1 (D) (p ? 0.001 between groups). By contrast, the proportion of ewes showing maternal behavior was similar in groups 2 and 3 ( and ), both of which differed from group 1 (). It is concluded that the lengthening of the sensitive period for establishing maternal behavior in sheep following EB induced parturition is not related with high levels of prolactin in the peripheral circulation. 相似文献
98.
Moises Joao Zotti Anderson Dionel Grutzmacher Isac Heres Lopes Guy Smagghe 《Insect Science》2013,20(6):743-752
The comprehensive knowledge that the delayed systemic and reproduction side effects can be even more deleterious than acute toxicity, has caused a shift in focus toward sublethal effects assessment on physiology and behavior of beneficial insects. In this study, we assessed the risks posed by some insecticides with different mode of action through lethal and delayed systemic sublethal effects on the pupation, adult emergence, and reproduction of the chrysopid Chrysoperla externa (Hagen, 1861; Neuroptera: Chrysopidae), an important predator in pest biological control. The maximum field recommended dose (MFRD) and twice (2×MFRD) for chlorantraniliprole, tebufenozide, and pyriproxyfen were harmless to C. externa. In contrast, all the tested chitin synthesis inhibitors (CSIs) were highly detrimental to the predator, despite of their lack of acute lethal toxicity. Therefore, the safety assumed by using IGRs toward beneficial insects is not valid for chrysopids. Dose–response data showed that although all CSIs have a similar mechanism of action, the relative extent of toxicity may differ (novaluron > lufenuron > teflubenzuron). For CSIs, the delayed systemic effects became obvious at adult emergence, where the predicted no observable effect dose (NOED) was 1/2 048 of the MFRD for novaluron (0.085 ng/insect), and 1/256 of the MFRD for both lufenuron (0.25 ng/insect) and teflubenzuron (0.6 ng/insect). Finally, this work emphasized the significance of performing toxicity risk assessments with an adequate posttreatment period to avoid underestimating the toxicities of insecticides, as the acute lethal toxicity assays may not provide accurate information regarding the long‐range effects of hazardous compounds. 相似文献
99.
Guy G.S. Dutton Keith L. Mackie Angela V. Savage Mary D. Stephenson 《Carbohydrate research》1978,66(1):125-131
Klebsiella K23 capsular polysaccharide has been investigated by the techniques of hydrolysis, methylation, Smith degradation-periodate oxidation, and base-catalysed degradation, either on the original or the carboxyl-reduced polysaccharide. The structure was found to consist of a tetrasaccharide repeating-unit, as shown below. The anomeric configurations of the sugar residues were determined by 1H-and 13C-n.m.r. spectroscopy on the original and degraded polysaccharides. 相似文献
100.
Mitochondria are remarkably dynamic organelles that migrate, divide and fuse. Cycles of mitochondrial fission and fusion ensure metabolite and mitochondrial DNA mixing and dictate organelle shape, number and bioenergetic functionality. There is mounting evidence that mitochondrial dysfunction is an early and causal event in neurodegeneration. Mutations in the mitochondrial fusion GTPases mitofusin 2 and optic atrophy 1, neurotoxins and oxidative stress all disrupt the cable-like morphology of functional mitochondria. This results in impaired bioenergetics and mitochondrial migration, and can trigger neurodegeneration. These findings suggest potential new treatment avenues for neurodegenerative diseases. 相似文献