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   2702篇
  免费   222篇
  2023年   27篇
  2022年   66篇
  2021年   118篇
  2020年   54篇
  2019年   75篇
  2018年   81篇
  2017年   69篇
  2016年   104篇
  2015年   143篇
  2014年   165篇
  2013年   204篇
  2012年   251篇
  2011年   234篇
  2010年   129篇
  2009年   118篇
  2008年   137篇
  2007年   133篇
  2006年   155篇
  2005年   112篇
  2004年   100篇
  2003年   71篇
  2002年   60篇
  2001年   23篇
  2000年   19篇
  1999年   18篇
  1998年   20篇
  1997年   18篇
  1996年   18篇
  1995年   16篇
  1994年   14篇
  1993年   10篇
  1992年   6篇
  1991年   10篇
  1990年   6篇
  1989年   10篇
  1988年   5篇
  1986年   10篇
  1982年   7篇
  1981年   8篇
  1980年   9篇
  1979年   8篇
  1978年   7篇
  1977年   3篇
  1976年   3篇
  1975年   4篇
  1974年   4篇
  1973年   6篇
  1971年   9篇
  1968年   3篇
  1963年   3篇
排序方式: 共有2924条查询结果,搜索用时 15 毫秒
1.
International Journal of Primatology - The selection of sleeping sites and sleeping trees in nonhuman primates is related to social and ecological factors. We investigate the role of body...  相似文献   
2.
3.
Multiple sclerosis, the most common cause of neurological disability in young population after trauma, represents a significant public health burden. Current challenges associated with management of multiple sclerosis (MS) patients stem from the lack of biomarkers that might enable stratification of the different clinical forms of MS and thus prompt treatment for those patients with progressive MS, for whom there is currently no therapy available. In the present work we analyzed a set of thirty different plasma cytokines, chemokines and growth factors present in circulation of 129 MS patients with different clinical forms (relapsing remitting, secondary progressive and primary progressive MS) and 53 healthy controls, across two independent cohorts. The set of plasma analytes was quantified with Luminex xMAP technology and their predictive power regarding clinical outcome was evaluated both individually using ROC curves and in combination using logistic regression analysis. Our results from two independent cohorts of MS patients demonstrate that the divergent clinical and histology-based MS forms are associated with distinct profiles of circulating plasma protein biomarkers, with distinct signatures being composed of chemokines and growth/angiogenic factors. With this work, we propose that an evaluation of a set of 4 circulating biomarkers (HGF, Eotaxin/CCL11, EGF and MIP-1β/CCL4) in MS patients might serve as an effective tool in the diagnosis and more personalized therapeutic targeting of MS patients.  相似文献   
4.
5.
6.
Early humans were obligately social, living in nested kin groups or close associations of related individuals. Theoretical and empirical research has demonstrated that group life is characterized by both costs (e.g. increased likelihood of disease transmission) and benefits (e.g. enhanced predator defense). This paper addresses the evolution of exploitation in humans (e.g. slavery, infanticide) as a response to within‐group competition for limiting resources (e.g. food, mates), a potential cost of living in groups. Exploitation is defined as one individual's use of another for selfish ends, in particular, the acquisition and/or use of another's resources for the optimization of inclusive fitness. It is argued that exploitation is most likely to occur in relationships characterized by asymmetries such as dependence, intimacy, and/or differential access to resources. A simple mathematical treatment assesses exploitation as a facultative response to local competition among relatives, providing insights into the conditions favorable and adverse to exploitation of conspecifics. Possible applications of the formulations are discussed, including the conditions under which intraspecific exploitation may be beneficial to both actor and recipient(s). Constraints on the evolution of exploitation in humans are identified, and suggestions are made for testing hypotheses related to the differential costs and benefits of exploitation to conspecifics. Future studies may promote the mitigation of exploitation's deleterious effects in Homo sapiens, a body of research which may apply, as well, to other social mammals.  相似文献   
7.
Use of tissue culture to bypass wheat hybrid necrosis   总被引:1,自引:0,他引:1  
Summary Hybrid necrosis in wheat is a barrier to gene transfer in wheat breeding practice. It is based on two complementary genes, Ne1 and Ne2. Recovery mutants (Re1, Re2 and Re3) which can grow to maturity were recovered from immature embryo cultures of necrotic hybrids between T. aestivum and T. durum. Cytological observation demonstrated that Re1 had 34 chromosomes instead of 35. This indicated that one of the chromosomes carrying the Ne genes was lost. Genetic study suggested that for Re1, the lost chromosome was chromosome 5B of the durum parental line. Re mutants are male sterile but can be maintained through a young ear culture method. Re mutants could be successfully pollinated by either parental line and the BC1 progeny is partially fertile. Re mutants were repeatedly induced in about 1% of the regenerated plants from immature embryo culture. This technique provides a practical way to bypass hybrid necrosis.  相似文献   
8.
Summary A hybrid of Saccharomyces cerevisiae with the ability to utilize starch and to produce the killer toxin was constructed by the protoplast fusion technique. The hybrid was obtained in two steps. In the first, a wild killer strain was fused with a laboratory strain (S. cerevisiae STA2). A fusion product which carried the killer factor and the ability to grow on starch was selected. In the second step, this hybrid was fused with a baker's yeast.  相似文献   
9.
Metabolism of a subtropical Brazilian lagoon   总被引:1,自引:1,他引:0  
Total community, planktonic and benthic metabolisms were measured by using the carbon dioxide production and consumption, the diurnal curve' method and the in situ bottle incubation technique over an annual cycle in two sublagoons of the Saquarema Lagoon, Brazil. Metabolic rates of the phytoplankton-based lagoon were characterized by considerable daytime and daily variability in production and respiration, by a seasonal shift between net autotrophy and heterotrophy and by an annual balance of production (P = 105 ± 65 mmoles/m2/dayn = 25) and respiration (R = 102 ± 50 mmoles/m2/dayn = 25). Total community metabolism was similar throughout the lagoon, but phytoplankton assimilation rates and benthic respiration showed spatial differences. Bottle incubations compared to total community free water respiration suggested that the pelagic community was 2–5 times more active than the benthos  相似文献   
10.
Summary Wild strains of Saccharomyces cerevisiae were isolated from dairy products, bakery goods, fresh fruit and vegetables, and tested for killer activity. Four isolates out of 238 strains possessed killer activity. The best of these was converted to the petite form and hybridized with an industrial strain of Saccharomyces cerevisiae by protoplast fusion. Thirty-eight out of 104 isolates had killer activity, and some of these had good dough-raising activity as well.  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