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   69036篇
  免费   283篇
  国内免费   576篇
系统科学   1270篇
丛书文集   855篇
教育与普及   319篇
理论与方法论   554篇
现状及发展   35990篇
研究方法   1422篇
综合类   27295篇
自然研究   2190篇
  2013年   827篇
  2012年   769篇
  2011年   2874篇
  2009年   667篇
  2008年   1026篇
  2007年   1116篇
  2006年   1228篇
  2005年   1424篇
  2004年   2501篇
  2003年   2132篇
  2002年   1834篇
  2001年   1894篇
  2000年   1350篇
  1999年   1219篇
  1998年   655篇
  1997年   784篇
  1994年   691篇
  1993年   698篇
  1992年   1154篇
  1991年   991篇
  1990年   1070篇
  1989年   886篇
  1988年   867篇
  1987年   861篇
  1986年   906篇
  1985年   1094篇
  1984年   949篇
  1983年   822篇
  1982年   889篇
  1981年   911篇
  1980年   1055篇
  1979年   1807篇
  1978年   1641篇
  1977年   1624篇
  1976年   1294篇
  1975年   1335篇
  1974年   1451篇
  1973年   1544篇
  1972年   1610篇
  1971年   1789篇
  1970年   2076篇
  1969年   1723篇
  1968年   1655篇
  1967年   1595篇
  1966年   1350篇
  1965年   1059篇
  1959年   622篇
  1958年   826篇
  1957年   709篇
  1956年   606篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
241.
Conclusion Current enological research does not aim at developing technological wines whose organoleptic characteristics are basically determined by the methods used in the transformation of grape wine, but rather to refine methods which will highlight the original qualities of different cultivars and viticultural conditions.  相似文献   
242.
The median procedure for n-trees   总被引:2,自引:2,他引:0  
Let (X,d) be a metric space The functionM:X k 2 x defined by is the minimum } is called themedian procedure and has been found useful in various applications involving the notion of consensus Here we present axioms that characterizeM whenX is a certain class of trees (hierarchical classifications), andd is the symmetric difference metricWe would like to thank the referees and Editor for helpful comments  相似文献   
243.
Summary During molt-induced atrophy of the pectoralis muscle of the Giant Canada Goose (Branta canadensis maxima), the zinc content of the muscle was significantly reduced (p0.0139), though the concentration of zinc per unit weight of muscle appeared higher (p0.0232). Zinc lost from the muscle during molt could be utilized for growth of the new flight feathers.Acknowledgments. Funds for this study were obtained from an operating grant awarded to J. C. G. by the Natural Sciences and Engineering Research Council of Canada.  相似文献   
244.
Summary Porcine or human follicular fluid inhibited the spontaneous maturation of isolated hamster oocytes in vitro during the first 1.5 h of culture. Moreover, the presence of 50% follicular fluid combined with 100 M dbcAMP cooperatively reduced the incidence of germinal vesicle breakdown. The addition of FSH also inhibited the resumption of meiosis, and the presence of LH did not overcome the inhibitory effects of follicular fluid and tended to impede isolated hamster oocyte maturation in vitro.  相似文献   
245.
246.
Knoll AH  Hayes JM  Kaufman AJ  Swett K  Lambert IB 《Nature》1986,321(6073):832-838
Analyses of stratigraphically continuous suites of samples from Upper Proterozoic sedimentary successions of East Greenland, Spitsbergen and Nordaustlandet (Svalbard) provide an approximation to the secular variation in carbon isotope ratios during a geologically and biologically important period of change from around 900 million years ago to the beginning of the Cambrian period. Late Riphean carbonates and organic material show a stratigraphically useful pattern of enrichment in 13C relative to Phanerozoic or earlier Proterozoic samples. Isotopic compositions of isolated samples from other localities are consistent with a worldwide extended interval of enhanced organic burial and consequent net survival of oxidized material, probably O2, just before the initial radiation of metazoans.  相似文献   
247.
Knoll AH  Golubic S  Green J  Swett K 《Nature》1986,321(6073):856-857
Diverse microorganisms ranging from cyanobacteria to eukaryotic algae and fungi live endolithically within ooids, hardgrounds and invertebrate shells on the present-day sea floor. These organisms are involved in the mechanical destruction of carbonates, and are useful ecological indicators of water depth and pollution. The Phanerozoic history of microbial endoliths has been elucidated through the study of microborings (the trace fossils of endolithic microorganisms) and rare cellularly preserved individuals, but nothing was known of the possible Precambrian evolution of comparable microorganisms until Campbell documented the occurrence of microborings in late Proterozoic ooids from central East Greenland. We now report the discovery of large populations of organically preserved endolithic microorganisms in silicified pisolites from 700-800-Myr-old Limestone-Dolomite Series of East Greenland. This fossil assemblage is significant for three reasons: (1) It confirms the prediction that oolites, pisolites and hardgrounds--the substrates for pre-Phanerozoic endoliths--provide a hitherto poorly explored but rewarding set of environments into which the search for early microfossils must be broadened; (2) the assemblage is diverse, containing about 12 taxa of morphologically distinct and previously unknown endolithic cyanobacteria, plus associated epilithic and interstitial populations; and (3) at least six of the fossil populations are indistinguishable in morphology, pattern of development, reproductive biology and inferred ecology from distinctive cyanobacterial species that bore ooids today in the Bahama Banks.  相似文献   
248.
Summary Two new butenolides, seiridin andiso-seiridin, were isolated from culture filtrates ofSeiridium cardinale, the pathogen of cypress canker, a destructive disease ofCupressus and relatedConiferae These metabolites were characterized as 3-methyl-4-(2-hydroxyheptyl)-2(5H)-furanone and its 4-(3-hydroxyheptyl) isomer, respectively. Chlorotic, and necrotic symptoms were produced on leaves of either host or non-host test plants by absorption of 0.3 mg/ml solutions of either compound. These also showed antibacterial activity.This study was supported in part by the National Research Council, Rome (special ad hoc project Chimica fine e secondaria) and in part by the Ministry of Education, Rome.  相似文献   
249.
Summary The suitability of urethane anesthesia for physiopharmacological investigations is reviewed. Total dose administered and route of administration are recognized as factors having a great influence on both resting parameters and biological responses to drugs. A peculiar characteristic of urethane is represented by its ability to induce a surgical plane of anesthesia without affecting neurotransmission in various subcortical areas and the peripheral nervous system. This makes urethane a suitable general anesthetic for studying neural function in both central and peripheral nervous systems and accounts for the preservation of a number of reflex responses in urethane-anesthetized animals.  相似文献   
250.
Summary A random distribution of the Y-chromosome at somatic metaphase was found in 50 patients with Ph' positive chronic myelogenous leukemia (CML). Thus, it is concluded that the positive of the Y-chromosome at somatic metaphase does not appear to influence the loss from bone marrow cells.  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