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   14496篇
  免费   42篇
  国内免费   54篇
系统科学   41篇
丛书文集   66篇
教育与普及   34篇
理论与方法论   26篇
现状及发展   5935篇
研究方法   707篇
综合类   7589篇
自然研究   194篇
  2013年   77篇
  2012年   225篇
  2011年   411篇
  2010年   80篇
  2008年   232篇
  2007年   296篇
  2006年   275篇
  2005年   292篇
  2004年   308篇
  2003年   259篇
  2002年   277篇
  2001年   566篇
  2000年   543篇
  1999年   383篇
  1994年   288篇
  1992年   343篇
  1991年   262篇
  1990年   298篇
  1989年   244篇
  1988年   242篇
  1987年   262篇
  1986年   244篇
  1985年   338篇
  1984年   272篇
  1983年   227篇
  1982年   185篇
  1981年   188篇
  1980年   207篇
  1979年   482篇
  1978年   383篇
  1977年   318篇
  1976年   292篇
  1975年   301篇
  1974年   337篇
  1973年   288篇
  1972年   315篇
  1971年   385篇
  1970年   482篇
  1969年   355篇
  1968年   361篇
  1967年   309篇
  1966年   348篇
  1965年   243篇
  1959年   113篇
  1958年   186篇
  1957年   124篇
  1956年   93篇
  1955年   84篇
  1954年   105篇
  1948年   83篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Summary In this paper I discuss the development of mathematical analysis during the second and third decades of the nineteenth century; and in particular I assert that the well-known correspondence of new ideas to be found in the writings of Bolzano and Cauchy is not a coincidence, but that Cauchy had read one particular paper of Bolzano and drew on its results without acknowledgement. The reasons for this conjecture involve not only the texts in question but also the state of development of mathematical analysis itself, Cauchy both as personality and as mathematician, and the rivalries which were prevalent in Paris at that time.  相似文献   
92.
J L Wilkens  M I Mote 《Experientia》1970,26(3):275-276
  相似文献   
93.
Summary Normal human spermatozoa were demonstrated by dot immunoblot analysis and immunohistochemistry to possess transglutaminase (TGase). The immunological identification of spermatozoal TGase is consistent with reports by others of its biochemical identification and suggested role in sperm motility,and provides, in view of the immunoregulatory properties of seminal plasma TGase, presumptive identification of a means whereby spermatozoa, under normal physiological conditions, may possibly be protected from immunological attack within the female reproductive tract.  相似文献   
94.
Summary The ability of liposomes composed of different kinds of phospholipid materials to adhere to the surface of the cornea was studied in the rabbit. The liposomes were labelled with tracer amounts of an I125-labelled phosphatidylethanolamine derivative and were instilled in 10 l drops onto the cornea. The retention of radioactivity was monitored. The results show that liposomes containing positively charged phospholipids are better retained than an albumin control. Thus, it may be possible to develop a drug delivery, liposome system which would permit long-term sustained release of ophthalmic drugs onto the cornea.  相似文献   
95.
Summary Histamine content in the rat hypothalamus was lower at 4°C and higher at 31°C compared to that at 21°C. Pretreatment with -fluoromethylhistidine, a suicide inhibitor of histidine decarboxylase, attenuated both the increased level of hypothalamic histamine and rat adaptive behavior at 31°C. Increase of histamine content in the hypothalamus appears to be an important factor contributing to rat adaptive behavior to high environmental temperature.  相似文献   
96.
MDMethodinSolvingMultipleCriteriaDecisionProblemsArisingfromInvestmentSystemsDUWenzhong;CIJianhua(EconomicInformationCenterof...  相似文献   
97.
Phytoalexins, defensive compounds produced by plants against microbial infections, were purified fromSophora exigua (Leguminosae) and their growth inhibitory effects on oral cariogenic bacteria were determined in vitro. Among three isolated compounds, 5,7,2,4-tetrahydroxy-8-lavandulylflavanone completely inhibited the growth of oral bacteria including primary cariogenic mutans streptococci, other oral streptococci, actinomycetes, and lactobacilli, at concentrations of 1.56 to 6.25 g/ml.  相似文献   
98.
The Indian antHarpegnathos saltator may be unique among insects in using its jumping capacity not only as an escape mechanism but also as a normal means of locomotion, and for catching its prey in flight. High-speed cinematography used to analyse the various phases of the jump suggests thatHarpegnathos employs a novel jumping mechanism to mediate these behaviours: namely the synchronous activation of its middle and hindlegs. Electrophysiological recordings from muscles or nerves in pairs of middle and hindlegs show remarkably synchronous activity during fictive jumping, supporting the synchronous activation hypothesis.Harpegnathos is not the only ant to jump, and a cladistic analysis suggests that jumping behaviour evolved independently three times during ant evolutionary history.  相似文献   
99.
There is considerable evidence suggesting that the switch from to and chain production after birth is due, in part, to silencing of the genes by stage-specific factors which bind to their promoters and to the competition from the adult ( and ) genes for a common enhancer element located in the locus control region. As a consequence one can expect that the increased Hb F production in adults with hereditary persistence of fetal hemoglobin or -thalassemia is directed mainly by -globin genes in cis to the deletion(s) responsible for these conditions. Here we review data on heterozygotes with -, -, or -thalassemia, who also had anAT mutation, in cis or in trans, which was used as a marker of gene expression. The results show that a deletion affecting adult genes favors the expression of genes in cis, while the deletion of a single gene does not affect the expression of the gene in cis but leads to a faster switch postnatally.  相似文献   
100.
Highly active metabolites have been detected in the hemolymph of the lepidopteranSpodoptera exigua infected with the mycopathogen,Beauveria bassiana. A combination of phenyl sepharose and CM ion exchange chromatography was utilized to extract the active metabolites from infected hemolymph samples. The active in vivo metabolites, having a molecular mass greater than 10 KDa, were thermolabile and were inactivated by proteinase K. These metabolites were characterized by their ability to disrupt metamorphosis, killing treated larvae at the wandering or pupal stage. Additionally, injection ofS. exigua larvae with active samples caused a reduction in the number of filopodial-producing hemocytes. The biological activities and biochemical properties suggest that novel compounds are produced duringB. bassiana mycosis.  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