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   100520篇
  免费   5730篇
  国内免费   25篇
  106275篇
  2021年   895篇
  2018年   1317篇
  2017年   1295篇
  2016年   1692篇
  2015年   1766篇
  2014年   2145篇
  2013年   3015篇
  2012年   3314篇
  2011年   3402篇
  2010年   2532篇
  2009年   2148篇
  2008年   3102篇
  2007年   3054篇
  2006年   2882篇
  2005年   2702篇
  2004年   2690篇
  2003年   2597篇
  2002年   2400篇
  2001年   4416篇
  2000年   4167篇
  1999年   3241篇
  1998年   1059篇
  1997年   1041篇
  1996年   928篇
  1995年   848篇
  1993年   835篇
  1992年   2449篇
  1991年   2464篇
  1990年   2459篇
  1989年   2218篇
  1988年   2070篇
  1987年   1982篇
  1986年   1829篇
  1985年   1826篇
  1984年   1502篇
  1983年   1305篇
  1982年   937篇
  1981年   880篇
  1979年   1437篇
  1978年   1135篇
  1977年   1018篇
  1976年   978篇
  1975年   1162篇
  1974年   1259篇
  1973年   1338篇
  1972年   1184篇
  1971年   1045篇
  1970年   926篇
  1969年   955篇
  1968年   840篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
The relative importance of the factors driving change in the population dynamics of nematodes in the soil is almost completely unknown. Top-down control by micro-arthropod predators may have a significant impact on nematode population dynamics. We report experiments showing that mites and Collembola were capable of reducing nematode numbers in the laboratory and were feeding on a targeted nematode species in the field. A PCR-based approach was developed for the detection of predation on three species of slug- and insect-pathogenic nematodes: Phasmarhabditis hermaphrodita, Heterorhabditis megidis and Steinernema feltiae. The collembolan Folsomia candida and the mesostigmatid mite Stratiolaelaps miles were employed as model predators to calibrate post-ingestion prey DNA detection times. Fragments of cytochrome oxidase I (COI) mtDNA were sequenced and species-specific primers were designed, amplifying 154-, 154- and 203-bp fragments for each of the nematode species. Detection times for nematode DNA within the guts of Collembola were longer than in mites, with half-lives (50% of samples testing positive) of 08.75 h and 05.03 h, respectively. F. candida significantly reduced numbers of the nematode H. megidis, with rates of predation of approximately 0.4 nematode infective juveniles per collembolan per hour over 10 h. Four taxa of field-caught micro-arthropod that had been exposed to the nematode P. hermaphrodita for a period of 12 h were analysed and significant numbers of three taxa tested positive. This is the first application of PCR techniques for the study of nematophagy and the first time these techniques have been used to measure predation on nematodes in the field.  相似文献   
62.
63.
64.
Dose-dependent effect of synthetic heptapeptides Semax (Met-Glu-His-Phe-Pro-Gly-Pro) and Selank (Thr-Lys-Pro-Arg-Pro-Gly-Pro) on the enkephalin-degrading enzymes of human serum was demonstrated. The inhibitory effects of Semax (IC5010 M) and Selank (IC5020 M) are more pronounced than that of puromycin (IC5010 mM), bacitracin, and some other inhibitors of peptidases. Beside the heptapeptides, their pentapeptide fragments also possessed an inhibitory effect; tri-, tetra- and hexapeptide fragments did not display such an effect. As the above enzymes take part in degradation of not only enkephalins but also other regulatory peptides, it can be assumed that one of the mechanisms of biological activity of Semax and Selank is related to this inhibitory activity of theirs.  相似文献   
65.
66.
Ritanserin and inmecarb hydrochloride, antagonists of serotonin, act cytostatically and teratogenically on early embryos ofTritonia diomedea, a nudibranch mollusk. On the basis of a pharmacological analysis and the type of developmental abnormalities observed, this action appears to be due to disturbances in the functional activity of endogenous serotonin and is associated with damage to the cytoskeleton. The effects of ritanserin and inmecarb are prevented or attenuated by lipophilic serotonin analogs (serotoninamides of polyenoic fatty acids), as well as by polypeptides isolated from neurons Pd5 and Pd6 of the pedal ganglia of the adultTritonia. In late embryos (stage of veligers), serotonin and to a lesser extent its lipophilic analogs strongly increase embryonic motility. This effect of serotonin is potentiated by some neuropeptides and inhibited by others. These results provide evidence for functional interaction between serotonin and neuropeptides in the control processes of embryogenesis.  相似文献   
67.
68.
The frequency of heterozygote carriers of risk zone alleles of the FMR1 gene (40–47 CGG repeats) was significantly higher in the group of patients with ovarian dysfunctions compared to control group I. The frequency of these alleles shows an increasing tendency in patients poorly responding to superovulation induction in IVF cycles. The average number of oocytes and follicles obtained from the stimulation of superovulation was significantly decreased in FMR1 gene heterozygous risk zone allele carriers as compared to patients with normal alleles of the FMR1 gene. The general average dosage of exogenous gonadotrophin necessary for superovulation induction was significantly higher in heterozygote carriers of FMR1 gene risk zone alleles than in patients with normal genotype. As well, the FMR1 gene risk zone alleles can be one of the hereditary susceptibility factors of impaired natural and stimulated ovulation.  相似文献   
69.
70.
A new method is proposed for reconstructing physical characteristics of nonideal systems by solving of an inverse problem in which the motion of interacting particles is described by a set of Langevin equations. The method allows one to simultaneously determine the interaction potential between dust grains, the friction coefficients, and the external confining potential. The method was verified by solving the problem numerically in a wide range of parameters typical of laboratory dusty plasmas.  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