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   47466篇
  免费   143篇
  国内免费   224篇
系统科学   247篇
丛书文集   858篇
教育与普及   74篇
理论与方法论   175篇
现状及发展   21916篇
研究方法   1841篇
综合类   21947篇
自然研究   775篇
  2013年   377篇
  2012年   669篇
  2011年   1429篇
  2010年   298篇
  2008年   811篇
  2007年   948篇
  2006年   916篇
  2005年   901篇
  2004年   962篇
  2003年   820篇
  2002年   846篇
  2001年   1357篇
  2000年   1306篇
  1999年   927篇
  1992年   878篇
  1991年   642篇
  1990年   729篇
  1989年   729篇
  1988年   690篇
  1987年   760篇
  1986年   754篇
  1985年   955篇
  1984年   733篇
  1983年   592篇
  1982年   547篇
  1981年   576篇
  1980年   704篇
  1979年   1499篇
  1978年   1236篇
  1977年   1175篇
  1976年   962篇
  1975年   985篇
  1974年   1382篇
  1973年   1204篇
  1972年   1260篇
  1971年   1389篇
  1970年   1846篇
  1969年   1455篇
  1968年   1366篇
  1967年   1347篇
  1966年   1201篇
  1965年   872篇
  1964年   304篇
  1959年   471篇
  1958年   847篇
  1957年   597篇
  1956年   491篇
  1955年   452篇
  1954年   496篇
  1948年   318篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
281.
Summary A novel sesquiterpene lactone, 9--hydroxyparthenolide (1) was isolated from a chloroform extract ofAnvillea garcini that was collected in Iran. The structure of1 was assigned on the basis of high field NMR decoupling experiments and other spectroscopic evidence. This -methylene lactone has shown significant activity in both the 9KB cell culture and P388 mouse leukemia test systems.The authors acknowledge the support of contract No. N01-CM-97296 from the National Cancer Institute and the use of the Purdue University Biochemical Magnetic Resonance Laboratory (NIH grant No. RR01077). This is paper 16 in the series Potential Antitumor Agents.  相似文献   
282.
Summary A needle type of colloid osmometer has been devised with special attachments which made it useable with 3-l samples. It has been used in oncometry of fresh water teleosts to give the following results: 5.1±1.0 mm Hg for goldfish, 5.2±0.71 mm Hg for carp, and 14.3±0.75 mm Hg for eel.This study was supported in part by Research Grant 387036 from the Ministry of Education of Japan.invited to National Institute of Physiological Sciences, Okazaki, Japan.  相似文献   
283.
284.
Summary Food ingestion of male guinea-pigs was restricted to 10 g/day. Total carnitine content of liver of the underfed animals fell to 3.75 moles per liver (46.1% of control). Serum level of total carnitine was 35.0 nmoles/ml (64.4%) in the underfed animals while they developed high ketonemia, 384.2 nmoles/ml (555.2%). In restricted animals the carnitine levels also decreased in the muscles.  相似文献   
285.
Summary Autoradiographic, stereological and histological studies have been carried out to determine the origin of muscle fibre splitting which supposedly occurs during muscle hypertrophy. The results obtained clearly indicate that the supposedly split fibres are a transient response probably derived from satellite cells and are not derived from pre-existing fibres by true splitting. Similarly, increases in muscle fibre size are not achieved by recruitment of satellite structures as indicated by lack of myonuclear recruitment.Acknowledgment. This work was carried out with the aid of a grant from the Medical Research Council of Great Britain. The authors are grateful for the excellent technical assistance of Miss H. Caulton, M.J. Wild and M. Fenner.  相似文献   
286.
Summary Growth-promoting alpha-globulin (GPAG), a specific serum protein complex which induces mitotic activity in continually replicating metazoan cells in vitro, was shown in this study to support growth of astrocytes and mesenchymal cells as well as process formation of nerve cells isolated from cerebral hemispheres of chick embryos.  相似文献   
287.
Summary The conduction velocity of peristaltic movements of the canine ureter was measured under anaesthesia with a new type of diameter gauge using an image sensor. The peristaltic velocity was 34.1±6.2 mm/sec in 10 experiments. Noradrenaline at a low dosage of 1 g/kg i.v. reduced the resting diameter, increased the conduction velocity to 47–56 mm/sec, and approximately doubled the frequency of contraction. The application of acetylcholine also caused an increase in both frequency and conduction velocity (42–46 mm/sec). A plot of the conduction velocity against the mean period of peristaltic contraction was hyperbolic in shape.  相似文献   
288.
    
Summary -Galactosidase activity in intact cells of 21 species ofStreptomyces was measured using ONPG hydrolysis, without addition of a permeabilizing agent. Differences in the induction efficiency of ONPG-hydrolytic activity by lactose or galactose, which could have taxonomic implications, were observed among the species.  相似文献   
289.
Summary Adult spontaneously hypertensive rats (SHR) are less sensitive to painful stimuli than their normotensive controls, Wistar-Kyoto (WKY) rats. This difference can be eliminated by the specific opiate antagonist, naloxone, and by the -adrenergic blocking agent, propranol.Acknowledgment. The author wishes to thank Miss R. L. Holcomb for her editorial assistance.  相似文献   
290.
Summary Several high molecular weight polyacetylenes have been isolated from the spongePetrosia ficiformis found in dark caves. These compounds are related to, but different from, the polyacetylenes isolated from the same sponge living in its usual habitat.This work is a part of the Progetto finalizzato per l'Oceanografia e i Fondi Marini, CNR, Roma.The authors thank Mr G. Scognamiglio for the HPLC work, Mr C. Di Pinto for the NMR spectra and Mr G. Villani for the collection of the sponge.  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