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   2592篇
  免费   237篇
  国内免费   485篇
  2024年   10篇
  2023年   34篇
  2022年   62篇
  2021年   66篇
  2020年   65篇
  2019年   75篇
  2018年   58篇
  2017年   53篇
  2016年   56篇
  2015年   86篇
  2014年   95篇
  2013年   155篇
  2012年   152篇
  2011年   157篇
  2010年   119篇
  2009年   150篇
  2008年   172篇
  2007年   152篇
  2006年   156篇
  2005年   140篇
  2004年   130篇
  2003年   119篇
  2002年   119篇
  2001年   119篇
  2000年   97篇
  1999年   82篇
  1998年   35篇
  1997年   22篇
  1996年   21篇
  1995年   24篇
  1994年   19篇
  1993年   25篇
  1992年   42篇
  1991年   52篇
  1990年   45篇
  1989年   49篇
  1988年   40篇
  1987年   34篇
  1986年   20篇
  1985年   32篇
  1984年   23篇
  1983年   16篇
  1981年   12篇
  1980年   11篇
  1979年   13篇
  1978年   11篇
  1977年   12篇
  1976年   11篇
  1974年   10篇
  1969年   9篇
排序方式: 共有3314条查询结果,搜索用时 15 毫秒
1.
Three types of cloned cDNA sequences for rat low molecular weight prekininogens were isolated and determined by molecular cloning and sequence analysis. The deduced amino acid sequences indicated that one, termed K-prekininogen, represents the counterpart of the known low molecular weight prekininogen present in other mammals, while the other two, called T-prekininogens, contain a novel T-kinin sequence which was recently identified from rat plasma. Although T- and K-prekininogens are highly homologous with each other, both of the T-prekininogens contain methionine, instead of arginine or lysine, as an amino acid preceding T-kinin and exhibit two consecutive amino acid deletions in the preceding region of T-kinin as compared with K-prekininogen. The former finding accounts for the previous observation of strong resistance of T-kininogens to cleavage with trypsin or kallikreins, while the latter finding has been explained by the structural analysis of genomic clones in which T-kinin-coding exon is contracted at its intron junction. A partial nucleotide sequence reported recently for the rat major acute phase protein (alpha 1-MAP) mRNA was found to be extremely related to the corresponding portion of the rat T-prekininogen mRNA. Furthermore, consistent with the previous report of the structural identity of major acute phase protein and alpha 1-cysteine proteinase inhibitor, kininogen closely resembles not only the former but also the latter in the amino acid compositions. The interrelationship among the triad of these proteins has been discussed.  相似文献   
2.
The capillaries of the cardiac muscle were investigated in the goat by means of light microscopy, transmission electron microscopy and scanning electron microscopy, and the following results were obtained. The capillaries of the working cardiac muscles were numerous, arranged mainly parallel to the long axis of muscle cells and formed dense elongated networks. On the contrary, those of the terminal Purkinje fibers were relatively few in number, oriented in various directions and formed loose and circularly meshed networks surrounding the fibers. Such findings were discussed in correlation with the physiology and functional morphology of various types of the cardiac muscle cells.  相似文献   
3.
The recombinants of the mandibular molar bud epithelia with cranial ectomesenchymal cell groups from several different sources--mandibular molar area, tongue anlagen, and lateral nasal process--were cultured. Dental laminalike buds were developed in each of the recombinants (incidence of development 38-86%). In the heterotrophic recombinants, heterotypic differentiation of mandibular epithelium was also induced. However, the foreign ectomesenchymal cells were not induced heterotypically by the epithelial genetic factor, but the mesenchymal genetic factor is maintained. It is suggested that mandibular molar bud epithelia have potency to proliferate into mesenchyme under non-organ-specific influences of ectomesenchymal cells and that presumptive mandibular mucosal epithelia have multipotency for differentiation sensitive to inductive influences by the heterotypic cranial ectomesenchymal cells but that the mandibular molar bud epithelia have no heterotypic inductive activity for the differentiation of cranial ectomesenchymal cells.  相似文献   
4.
A new species, Galearis huanglongensis Q.W.Meng & Y.B.Luo, is described and illustrated. It is similar to Galearis cyclochila (Franch. & Sav.) Soó and Galearis diantha (Schltr.) P.F.Hunt, but differs in having a short spur, two elliptical lateral stigma lobes and distinctly separated bursicles. This new species is known only from the type locality, the Huanglong Valley, Songpan County, western Sichuan, China, growing amongst mosses under alpine shrubs at an elevation of about 3000 m. Based on two years of observations of its population size, the species was categorized as critically endangered CR (B1a, B2a) according to the World Conservation Union (IUCN) Red List Categories and Criteria, Version 3.1. The micromorphology of pollinia and seeds was observed by scanning electron microscopy and compared with that of G. cyclochila and G. diantha. The results supported G. huanglongensis Q.W.Meng & Y.B.Luo as a new species. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158 , 689–695.  相似文献   
5.
When energetic particles irradiate matter, it becomes activated by nuclear reactions. Radioactivation induced cellular effects are not clearly understood, but it could be a part of bystander effects. This investigation is aimed at understanding the biological effects from radioactivation in solution induced by hadron radiation. Water or phosphate buffered saline was activated by being exposed to hadron radiation including protons, carbon- and iron-ions. 1 mL of radioactivated solution was transferred to flasks with Chinese hamster ovary (CHO) cells cultured in 5 mL of complete media. The induction of sister chromatid exchanges (SCE) was used to observe any increase in DNA damage responses. The energy spectrum and the half-lives of the radioactivation were analyzed by NaI scintillation detector in order to identify generated radionuclides. In the radioactivated solution, 511 keV gamma-rays were observed, and their half-lives were approximately 2 min, 10 min, and 20 min. They respectively correspond to the beta+ decay of 15O, 13N, and 11C. The SCE frequencies in CHO cells increased depending on the amount of radioactivation in the solution. These were suppressed with a 2-hour delayed solution transfer or pretreatment with dimethyl sulfoxide (DMSO). Our results suggest that the SCE induction by radioactivated solution was mediated by free radicals produced by the annihilated gamma-rays. Since the SCE induction and DMSO modulation are also reported in radiation-induced bystander effects, our results imply that radioactivation of the solution may have some contribution to the bystander effects from hadron radiation. Further investigations are required to assess if radioactivation effects would attribute an additional level of cancer risk of the hadron radiation therapy itself.  相似文献   
6.
易扬  胡昕利  史明昌  康宏樟  王彬  张辰  刘春江 《生态学报》2021,41(19):7796-7807
基于1999-2015年的MODIS NDVI时间序列遥感数据,应用趋势分析、变异系数、重标极差分析和偏相关分析等方法,分析了长江中游的植被时空变化特征及其与气象因子的关系。结果表明,长江中游地区NDVI均值总体上呈上升趋势(从0.72增加到0.80)。从空间分布来看,NDVI低值区域(0.1-0.5)占1.40%,高值区域(>0.7)占87.15%;NDVI空间格局呈"西高东低、北高南低"的分布特征,低值区域表现为以三省省会城市为中心向外辐射。Hurst指数显示,研究区大部分区域(60.54%)的NDVI变化趋势具有不确定性,持续性改善区域(34.78%)主要分布在西部山地区,持续性退化区域(3.26%)主要分布在人类活动频繁的较发达城市区域。在年际尺度上,研究区NDVI与各气象因子关系均不显著;月际尺度上,NDVI与降水、相对湿度和日照时数显著相关,降水和日照时数有明显的时滞性。区域内NDVI动态趋势以不确定性发展为主,城市群周边NDVI呈现持续退化的区域应该引起关注。  相似文献   
7.
8.
C6-Aldehydes emitted from intact tea leaves were analyzed quantitatively.Emission of the aldehydes increased temporarily in mid-May whenenzymatic activities involved in aldehyde formation from lipidsbegan to increase. Levels of C6-aldehydes in tea leaves alsoincreased temporarily. However, the accumulated C6-aldehydesdid not always correspond to emitted ones. (Received December 1, 1991; Accepted March 18, 1992)  相似文献   
9.
Juvenile three-spined stickleback Gasterosteus aculeatus in their nursery pond had a bimodal length distribution after October in their first year of life. A sea-run migration of all individuals belonging to the lower modal group was observed, with a peak in early November. These data suggest that partial migration based on the early status of individual juveniles occurs in the Pacific Ocean form of three-spined stickleback.  相似文献   
10.
This paper discusses results of a computer simulation for designing a new pulse duplicator system for mechanical heart valves. The design objective of the system is to obtain a compact, efficient pulse duplicator system capable of accurately measuring the volume flow rate across a valve. The volume flow rate is determined as the derivative of the volume displacement of an actuator piston which directly drives the fluid. The system does not need any circulatory loop, since the piston is controlled on-line to follow command signals representing an aortic impedance. The results of the computer simulation show: the designed PI-controllers of the actuator can precisely control valve motion to follow given command signals, the eigenvalues of the controllers have to be carefully chosen to prevent unstable behaviors of a valve in diastole, and the dimensions of the actuator is optimized by minimizing a cost function of the total efficiency of the system.  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