全文获取类型
收费全文 | 1863篇 |
免费 | 176篇 |
出版年
2023年 | 12篇 |
2021年 | 20篇 |
2019年 | 33篇 |
2018年 | 28篇 |
2017年 | 28篇 |
2016年 | 51篇 |
2015年 | 68篇 |
2014年 | 70篇 |
2013年 | 96篇 |
2012年 | 91篇 |
2011年 | 104篇 |
2010年 | 62篇 |
2009年 | 51篇 |
2008年 | 82篇 |
2007年 | 94篇 |
2006年 | 69篇 |
2005年 | 81篇 |
2004年 | 88篇 |
2003年 | 94篇 |
2002年 | 70篇 |
2001年 | 37篇 |
2000年 | 50篇 |
1999年 | 33篇 |
1998年 | 23篇 |
1997年 | 25篇 |
1996年 | 27篇 |
1995年 | 17篇 |
1994年 | 19篇 |
1993年 | 20篇 |
1992年 | 15篇 |
1991年 | 28篇 |
1990年 | 16篇 |
1989年 | 24篇 |
1988年 | 20篇 |
1987年 | 21篇 |
1986年 | 18篇 |
1985年 | 22篇 |
1983年 | 15篇 |
1982年 | 18篇 |
1981年 | 15篇 |
1980年 | 13篇 |
1979年 | 16篇 |
1978年 | 20篇 |
1977年 | 12篇 |
1976年 | 17篇 |
1975年 | 25篇 |
1974年 | 16篇 |
1973年 | 16篇 |
1972年 | 13篇 |
1971年 | 18篇 |
排序方式: 共有2039条查询结果,搜索用时 593 毫秒
1.
Martyn Rix 《Curtis's Botanical Magazine》2010,27(3):290-295
Tropaeolum speciosum Poepp. & Endl. is described and illustrated. Its habitat in the wild, and its pollination are discussed, and instructions for its successful cultivation are given. 相似文献
2.
Geoffrey R. Banks David G. Barker 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1985,826(4):180-185
Yeast DNA ligase is radioactively labelled in vitro by incubating a crude cell extract with [α-32P]ATP. The product of this reaction is the stable covalent ligase-AMP adduct, which can be characterized by its reactivity with either pyrophosphate or nicked DNA and visualized by gel electrophoresis and autoradiography. The Saccharomyces cerevisiae DNA ligase was identified as an 89 kDa polypeptide by exploiting the fact that transformants with multiple copies of the plasmid-encoded DNA ligase (CDC9) gene overproduce the enzyme by two orders of magnitude. A similar strategy has been used to identify the Schizosaccharomyces pombe DNA ligase as an 87 kDa polypeptide. Both values agree well with the coding capacities of the respective cloned gene sequences. When the S. cerevisiae ligase is greatly overproduced with respect to wild-type levels, a second polypeptide of 78.5 kDa is also labelled and has the same properties as the 89 kDa adduct. We suggest that this polypeptide is generated by proteolysis. 相似文献
3.
Obesity-inducing lesions of the central nervous system alter leptin uptake by the blood-brain barrier. 总被引:1,自引:0,他引:1
Leptin regulates body adiposity by decreasing feeding and increasing thermogenesis. Obese humans and some obese rodents are resistant to peripherally administered leptin, suggesting a defect in the transport of leptin across the blood-brain barrier (BBB). Defective transport of exogenous leptin occurs in some models of obesity, but in other models transport is normal. This shows that factors other than obesity are associated with impairment of leptin transport across the BBB. In order to further investigate these factors, we determined leptin transport in rats made obese by lesioning of the ventromedial hypothalamus (VMH), paraventricular nucleus (PVN), or posterodorsal amygdala (PDA). These regions all contain leptin receptors and lesions there induce obesity and hyperleptinemia and alter the levels of many feeding hormones which might participate in leptin transporter regulation. We measured the uptake of radioactively labeled leptin by the BBB by multiple-time regression analysis which divides uptake into a reversible phase (Vi, e.g., receptor/transporter binding to the brain endothelial cell) and an irreversible phase (Ki, complete transport across the BBB). Leptin uptake was not affected in rats with VMH lesions. No significant change occurred in the entry rate (Ki) for any group, although Ki declined by over 35% in rats with PVN lesions. Decreased uptake was observed in rats with PVN lesions and with PDA lesions. This was primarily due to a reduced Vi (about 21% for the PDA). This decreased uptake is most likely explained by decreased binding of leptin to the brain endothelial cell, which could be because of decreased binding by either receptors or transporters. This suggests that some of the feeding hormones controlled by the PVN and PDA may participate in regulating leptin uptake by the BBB. 相似文献
4.
Does maternal condition or predation risk influence small mammal population dynamics? 总被引:2,自引:0,他引:2
There is strong debate over whether the intrinsic traits of individuals or the extrinsic environment exert the greater influence on small mammal population dynamics. We test the roles of maternal effects (an intrinsic factor) and predation risk (an extrinsic factor) in the population dynamics of wild strain house mice using a 2-factor enclosure experiment. Pre-release supplemental feeding with a high-fat diet created female treatment founders that were 6–10% heavier than controls, a condition that we predicted would be passed on as a maternal effect. Predation risk was enhanced using regular application of predator (red fox Vulpes vulpes ) scats. Founder populations of six females and six males released into eight, 15×15 m enclosures showed near exponential population growth over 17 weeks (maximum 3 generations). But there were no responses to either treatment in terms of survival, inherited body weights, fecundity or population size. We suggest that elevated maternal condition may have only minor and transient intergenerational effects with little long-term consequence. We also suggest that the general significance of predator scats as a cue to predation risk to alter prey behaviour may have been overestimated. Hence our results question the role of either factor in causing long-term responses that influence condition to affect population processes. 相似文献
5.
Household‐level correlates of children's physical activity levels in and across 12 countries
下载免费PDF全文
![点击此处可从《Obesity (Silver Spring, Md.)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Deirdre M. Harrington Fiona Gillison Stephanie T. Broyles Jean‐Philippe Chaput Mikael Fogelholm Gang Hu Rebecca Kuriyan Anura Kurpad Allana G. LeBlanc Carol Maher Jose Maia Victor Matsudo Timothy Olds Vincent Onywera Olga L. Sarmiento Martyn Standage Mark S. Tremblay Catrine Tudor‐Locke Pei Zhao Peter T. Katzmarzyk for the ISCOLE Research Group 《Obesity (Silver Spring, Md.)》2016,24(10):2150-2157
6.
7.
Pair-formation behaviour among the Eiders began after the annual moult, in late September. There was a small peak of display in October and November and a large one in the following April and May. As a result, the female population became paired in two phases, up to 50% in the autumn and the remainder in the following spring. The seasonal cycle of the abundance of Leydig cells was bimodal, with a peak in October and a large one in April-May. That the peaks represent increased androgen production was supported by the cycles of Leydig cell enzyme activity and lipid content and of penis weight. The monthly means of Leydig cell abundance and of the rate of display were statistically correlated. Experiments with sexually quiescent males in eclipse plumage demonstrated that testosterone was capable of inducing pair-formation behaviour. Relatively advanced stages of spermatogenesis were found in autumn, and were maintained through the winter, complete development occurring in the spring. Seasonal changes in the production of pituitary gonadotropin followed a bimodal pattern with peaks in autumn and in the spring. Field evidence is presented which suggests that Follicle Stimulating Hormone and Luteinizing Hormone were produced at different phases of a circadian rhythm of photosensitivity. 相似文献
8.
9.
Martyn Kelly 《Ecological Indicators》2011,11(5):1492-1494
Many diatom-based methods have been proposed in recent years. Besse-Lototskaya et al. (2011) compare some of those developed for assessing inorganic nutrients. However, they fail to address the two questions of greatest interest to those who wish to use such metrics to inform decision-making. These questions are: what is the role of diatom-based trophic metrics in environmental management in Europe in the twenty-first century and do these indices represent causal relationships with underlying pressure gradients? The present economic climate means that Member States of the European Union will look more critically at all methods for assessing freshwater quality and developers need to ensure that their methods are “fit-for-purpose”. 相似文献
10.