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   15026篇
  免费   1372篇
  国内免费   17篇
  2022年   85篇
  2021年   205篇
  2020年   103篇
  2019年   152篇
  2018年   204篇
  2017年   143篇
  2016年   293篇
  2015年   476篇
  2014年   547篇
  2013年   727篇
  2012年   937篇
  2011年   970篇
  2010年   635篇
  2009年   625篇
  2008年   831篇
  2007年   845篇
  2006年   826篇
  2005年   826篇
  2004年   856篇
  2003年   872篇
  2002年   895篇
  2001年   198篇
  2000年   114篇
  1999年   177篇
  1998年   233篇
  1997年   175篇
  1996年   131篇
  1995年   133篇
  1994年   138篇
  1993年   166篇
  1992年   123篇
  1991年   126篇
  1990年   121篇
  1989年   91篇
  1988年   105篇
  1987年   107篇
  1986年   95篇
  1985年   136篇
  1984年   114篇
  1983年   115篇
  1982年   138篇
  1981年   145篇
  1980年   117篇
  1979年   77篇
  1978年   96篇
  1977年   94篇
  1976年   85篇
  1975年   67篇
  1974年   105篇
  1973年   86篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Alan E. Davis 《CMAJ》1990,143(11):1162-1164
  相似文献   
62.
63.
Summary Ventral thoracic neurosecretory cells (VTNCs) of the blowflies, Calliphora erythrocephala and C. vomitoria, innervating thoracic neuropil and the dorsal neural sheath of the thoracico-abdominal ganglion have been shown to be immunoreactive to a variety of mammalian peptide antisera. In the neural sheath the VTNC terminals form an extensive neurohaemal network that is especially dense over the abdominal ganglia. The same areas are invaded by separate, ut overlapping serotonin-immunoreactive (5-HT-IR) projections derived from neuronal cell bodies in the suboesophageal ganglion. Immunocytochemical studies with different antisera, applied to adjacent sections at the lightmicroscopic level, combined with extensive cross-absorption tests, suggest that the perikarya of the VTNCs contain co-localized peptides related to gastrin/cholecystokinin (CCK), bovine pancreatic polypeptide (PP), Met- and Leuenkephalin and Met-enk-Arg6-Phe7 (Met-enk-RF). Electron-microscopic immunogold-labeling shows that some of the terminals in the dorsal sheath react with several of the individual peptide antisera, whilst others with similar cytology are non-immunoreactive. In the same region, separate terminals with different cytological characteristics contain 5-HT-IR. Both 5-HT-IR and peptidergic terminals are localized outside the cellular perineurium beneath the acellular permeable sheath adjacent to the haemocoel. Hence, we propose that various bioactive substances may be released from thoracic neurosecretory neurons into the circulating haemolymph to act on peripheral targets. The same neurons may also interact by synaptic or modulatory action in the CNS in different neuropil regions of the thoracic ganglion.  相似文献   
64.
Summary The distribution of FMRFamide-like immunoreactive neurons in the nervous system of the slug Limax maximus was studied using immunohistochemical methods. Approximately one thousand FMRFamide-like immunoreactive cell bodies were found in the central nervous system. Ranging between 15 m and 200 m in diameter, they were found in all 11 ganglia of the central nervous system. FMRFamide-like immunoreactive cell bodies were also found at peripheral locations on buccal nerve roots. FMRFamide-like immunoreactive nerve fibres were present in peripheral nerve roots and were distributed extensively throughout the neuropil and cell body regions of the central ganglia. They were also present in the connective tissue of the perineurium, forming an extensive network of varicose fibres. The large number, extensive distribution and great range in size of FMRFamide-like immunoreactive cell bodies and the wide distribution of immunoreactive fibres suggest that FMRFamide-like peptides might serve several different functions in the nervous system of the slug.  相似文献   
65.
Summary The human homologue of the fission yeast Schizosaccharomyces pombe cell cycle control gene cdc2 has been assigned to chromosome 10. DNA hybridization reveals that this gene is highly conserved in vertebrates. The human CDC2 gene probe detects a simple two-allele polymorphism in Taq1-digested DNA.  相似文献   
66.
In an attempt to identify invertebrate homologs of Thy-1 antigen, the optic and central nervous tissue of squid was solubilized in deoxycholate and fractionated by lentil lectin affinity chromatography and gel filtration to yield small abundant glycoproteins. Material with biochemical similarities to Thy-1 was found and shown to consist of two glycoproteins that were ultimately purified using monoclonal antibody affinity columns. Both glycoproteins were sequenced to yield sequences of 84 residues for Sgp-1 and 92 residues for Sgp-2. The sequences were analyzed for similarities to Thy-1 and other Ig-related sequences, and Sgp-1 showed some similarities that were > 3 standard deviation units away from mean random scores when tested with the ALIGN program. However, the sequence patterns were not typical of Ig-related domains and the relationship of Sgp-1 to the Ig superfamily remains problematical. Sgp-2 showed no relationship to the Ig superfamily, but similarities to Ly-6 antigen sequences were noted that are in accord with an evolutionary relationship. The similarities included ten Cys residues in each sequence of which eight were matched in the best alignment given by the ALIGN program. Chemical evidence was obtained for glycophospholipid tails at the COOH-termini of Sgp-1 and Sgp-2 as is the case for Thy-1 and Ly-6 antigens.Abbreviations used in this paper GPL glycophospholipid - mAb monoclonal antibody - SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis  相似文献   
67.
Measurement of light within thin plant tissues with fiber optic microprobes   总被引:1,自引:0,他引:1  
Vogelmann, T. C., Knapp, A. K., McClean, T. M. and Smith, W. K. 1988. Measurement of light within thin plant tissues with fiber optic microprobes. - Physiol. Plant. 72: 623–630.
The measurement of light with fiber optic microprobes has been extended to thin (200–300 μm) plant tissue samples. To test the method, light measurements were made in thin aqueous films and paradermal sections from 10-day-old etiolated Cucurbita pepo L. cv. Fordhook cotyledons. The measurements obtained were highly reproducible. Paradermal sections of spongy mesophyll that were irradiated with collimated light scattered light more effectively than the palisade layer of intact cotyledons. These results demonstrate that different plant tissues have different light scattering characteristics. The successful extension of the fiber optic microprobe technique to thin systems makes it possible to examine the optical properties of different cell layers within leaves and other plant organs.  相似文献   
68.
Three extracellular proteases produced by Legionella pneumophila during growth in liquid medium were examined for their effects on human alpha-1-antitrypsin (alpha-1-AT). One of these proteases, tissue-destructive protease (TDP) destroyed completely the trypsin-inhibitory capacity of alpha-1-AT at protease: inhibitor molar ratios down to 0.002:1. After inactivation by TDP, the Mr of alpha-1-AT was reduced by 5000 in SDS-PAGE. This suggested that inactivation entailed only limited cleavage.  相似文献   
69.
Immunogold labelling has been used to determine the cellular distribution of glycine decarboxylase in leaves of C3, C3–C4 intermediate and C4 species in the genera Moricandia, Panicum, Flaveria and Mollugo. In the C3 species Moricandia foleyi and Panicum laxum, glycine decarboxylase was present in the mitochondria of both mesophyll and bundle-sheath cells. However, in all the C3–C4 intermediate (M. arvensis var. garamatum, M. nitens, M. sinaica, M. spinosa, M. suffruticosa, P. milioides, Flaveria floridana, F. linearis, Mollugo verticillata) and C4 (P. prionitis, F. trinervia) species studied glycine decarboxylase was present in the mitochondria of only the bundle-sheath cells. The bundle-sheath cells of all the C3–C4 intermediate species have on their centripetal faces numerous mitochondria which are larger in profile area than those in mesophyll cells and are in close association with chloroplasts and peroxisomes. Confinement of glycine decarboxylase to the bundle-sheath cells is likely to improve the potential for recapture of photorespired CO2 via the Calvin cycle and could account for the low rate of photorespiration in all C3–C4 intermediate species.Abbreviation and symbol kDa kilodaltons - CO2 compensation point  相似文献   
70.
Glyphosate tolerant flax plants from Agrobacterium mediated gene transfer   总被引:5,自引:0,他引:5  
Agrobacterium tumefaciens carrying a disarmed Ti-plasmid vector containing a chimeric NPT-II gene and a glyphosate resistance plant-derived 5-enolpyruvylshikimate-3-phosphate synthase gene was used to transform flax hypocotyl tissues. Transformed shoots could be regenerated from the inoculated tissue and were proven to be transgenic by the combination of leaf callus assays, nopaline assays and progeny tests. Co-segregation was observed in the progeny for kanamycin and glyphosate resistance.  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