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   11302篇
  免费   49篇
  国内免费   160篇
系统科学   75篇
丛书文集   65篇
教育与普及   55篇
理论与方法论   28篇
现状及发展   3967篇
研究方法   526篇
综合类   6627篇
自然研究   168篇
  2014年   77篇
  2013年   86篇
  2012年   212篇
  2011年   390篇
  2010年   110篇
  2009年   111篇
  2008年   242篇
  2007年   278篇
  2006年   253篇
  2005年   269篇
  2004年   252篇
  2003年   246篇
  2002年   270篇
  2001年   485篇
  2000年   469篇
  1999年   335篇
  1998年   77篇
  1996年   79篇
  1994年   298篇
  1992年   258篇
  1991年   218篇
  1990年   240篇
  1989年   210篇
  1988年   194篇
  1987年   194篇
  1986年   206篇
  1985年   264篇
  1984年   202篇
  1983年   163篇
  1982年   135篇
  1981年   140篇
  1980年   144篇
  1979年   325篇
  1978年   263篇
  1977年   215篇
  1976年   191篇
  1975年   195篇
  1974年   206篇
  1973年   176篇
  1972年   206篇
  1971年   245篇
  1970年   303篇
  1969年   229篇
  1968年   226篇
  1967年   194篇
  1966年   223篇
  1965年   149篇
  1959年   75篇
  1958年   123篇
  1957年   81篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
河南信阳南湾水库牌坊鸟岛鸟类调查报告   总被引:1,自引:0,他引:1  
自1986年以来,我们多次对牌坊鸟岛的鸟类进行了调查。目前,现巳查明分布于该岛的鸟类有95种,隶属16目,30科。其中留鸟38种,夏候鸟33种,冬候鸟5种,旅鸟19种。该岛鸟类区系具有东洋种和古北种混杂分布的特点,以东洋种为主。  相似文献   
992.
993.
Summary Pyrrolizidine alkaloid sequestered by adult maleDanaus gilippus from plants is transferred in large measure to the female at mating, and by the female to the eggs. The eggs, presumably, are protected as a result. The male's courtship pheromone, danaidone, derived from the sequestered alkaloid, may function to advertise the male's alkaloid-donating capacity.Acknowledgments. Paper Nr. 89 of the series Defense Mechanisms of Arthropods. Study supported by NIH (Grants AI-02908 and AI-12020; Predoctoral Traineeship to DED), Hatch funds (191-7416), the Bache Fund (National Academy of Sciences, to DED), and the Theodore Roosevelt Memorial Fund (American Museum of Natural History, to DED). We thank M. Weingarner and J. Miller forD. gilippus collection, J. Boggan for technical assistance, and the staff of the Archbold Biological Station, Lake Placid, Florida, for assistance and hospitality. One of us (DED) is indebted to M. D. Achey, who by demonstrating nuptial cardenolide transfer in the monarch butterfly [Honors Thesis, Amherst College, Amherst, Massachusetts (1979)] raised the possibility that other plant metabolites might be similarly transferred in danaines.Deceased July, 1985. This paper is affectionately dedicated to his memory.  相似文献   
994.
995.
扫描隧道显微镜针尖的电化学腐蚀制备方法   总被引:8,自引:0,他引:8  
本文利用电化学腐蚀方法设计并搭建了一种扫描隧道显微镜(STM)针尖制备的直流腐蚀电路。电路中使用了9013三极管和LM311快速比较器,使电解池切断时间低于500ns。成功地制备了质量较高的钨针类。  相似文献   
996.
Association of a Ras-related protein with cytochrome b of human neutrophils   总被引:24,自引:0,他引:24  
Activation of the superoxide generating system in human neutrophils is thought to involve the interaction or assembly of cytochrome b with other cytosolic and membrane proteins. We have now co-isolated by conventional purification procedures a protein of relative molecular mass 22,000 with cytochrome b. This Ras-related protein is not a fragment of either of the subunits of cytochrome b, and its primary structure, as determined by the sequencing of its complementary DNA, is identical to that predicted from a recently cloned ras-related gene, rap1 (also termed Krev-1). Immunoaffinity purification on anti-cytochrome and anti-Ras immunoaffinity matrices indicates an association between cytochrome b and the Ras-related protein. The association of a Ras-related GTP-binding protein with cytochrome b of human neutrophils could indicate a role for such a protein in the transduction, regulation or structure of the superoxide generating system.  相似文献   
997.
Summary Both normetanephrine and metanephrine were found to be oxidized by both types of monoamine oxidase in mouse liver mitochondria. Both Km and Vmax values of type B MAO for both substrates were higher than those of type A MAO, which caused the shift of inhibition curves with clorgyline and deprenyl according to the increase in substrate concentration.  相似文献   
998.
T Franz  J L?hler  A Fusco  I Pragnell  P Nobis  R Padua  W Ostertag 《Nature》1985,315(6015):149-151
The study of retrovirus-induced leukaemias in mice is a powerful tool for the elucidation of the normal regulation of the haematopoietic system. The acute murine spleen focus-forming viruses (SFFV) can be classified according to the haematopoietic lineage on which they exert their effects in the adult mouse. Here we report a new SFFV isolate, the AF-1 virus, with the novel ability to transform cells of the mononuclear phagocyte lineage. The virus was isolated from sarcomas that were induced on passage of a cloned Friend helper virus (F-MuLV, 643/22F) in newborn BALB/c mice. We have cloned the transforming defective subunit of the AF-1 viral complex in NRK cells and isolated several subclones. Analysis of the proviral genome in two non-producer cell clones reveals that AF-1 virus contains Harvey v-ras-specific sequences (Fig. 1). Thus, AF-1 virus is closely related to Harvey murine sarcoma virus (Ha-MSV), and is, at present, the only tool by which permanent cell lines can be obtained from mononuclear phagocytes in the mouse.  相似文献   
999.
1000.
论述了转炉热喷补技术的发展与应用,在对宝钢转炉热喷补机进行充分调查的情况下,对喷补机喷射系统的结构及参数作了分析,提出了有关参数的确定与计算方法.  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