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   841篇
  免费   110篇
  2020年   10篇
  2019年   11篇
  2018年   8篇
  2017年   16篇
  2016年   9篇
  2015年   25篇
  2014年   21篇
  2013年   25篇
  2012年   22篇
  2011年   24篇
  2010年   23篇
  2009年   18篇
  2008年   25篇
  2007年   31篇
  2006年   26篇
  2005年   31篇
  2004年   23篇
  2003年   19篇
  2002年   19篇
  2001年   20篇
  2000年   18篇
  1999年   23篇
  1998年   10篇
  1996年   10篇
  1995年   10篇
  1992年   12篇
  1991年   23篇
  1990年   10篇
  1988年   14篇
  1987年   12篇
  1986年   16篇
  1985年   11篇
  1984年   23篇
  1983年   12篇
  1982年   15篇
  1980年   9篇
  1979年   21篇
  1977年   9篇
  1976年   17篇
  1975年   16篇
  1974年   17篇
  1973年   15篇
  1972年   14篇
  1971年   17篇
  1970年   19篇
  1969年   13篇
  1968年   14篇
  1967年   12篇
  1966年   12篇
  1965年   9篇
排序方式: 共有951条查询结果,搜索用时 15 毫秒
891.
One of the most difficult problems faced by climatologists is how to translate global climate model (GCM) output into regional- and local-scale information that health and environmental effects researchers can use. It will be decades before GCMs will be able to resolve scales small enough for most effects research, so climatologists have developed climate downscaling methods to bridge the gap between the global and local scales. There are two main streams of climate downscaling research. First, high-resolution, limited-area climate models can be embedded in the coarse-scale GCMs, producing much finer resolution climate data. Second, empirical downscaling techniques develop transfer functions linking the large-scale atmospheric circulation generated by the GCMs to surface data. Examples of both types of downscaling, aimed at improving projections of future climate in the Susquehanna River Basin (the Mid-Atlantic Region of the United States), are presented. A third case is also described in which an even higher-resolution nested atmospheric model is being developed and linked to a hydrologic model system, with the ultimate goal of simulating the environmental response to climate forcing at all time and space scales.  相似文献   
892.
  1. Increasing rates of invasions in ecosystems worldwide necessitate experiments to determine the role of biotic interactions in the success and impact of multiple alien species. Here, we examined competitive and facilitative interactions among various combinations of three widespread and often co-occurring invaders: the zebra mussel Dreissena polymorpha, and the macrophytes Elodea canadensis and Elodea nuttallii.
  2. Using a mesocosm-based, factorial experimental design, we assessed the effect of interspecific competition on macrophyte growth rates in the absence and presence at varying biomass of D. polymorpha.
  3. Growth rates (wet g/day) of E. canadensis and E. nuttallii were similar when grown in isolation. When grown together, in the absence of D. polymorpha, E. canadensis growth was not significantly reduced in the presence of E. nuttallii and vice versa. In the presence of D. polymorpha (26.0 ± 1 mm), monocultural growth of E. canadensis was largely unaffected, while E. nuttallii growth was strongly enhanced. Low (2.64 g) and medium (3.96 g) mussel biomass led to negative interspecific effects between E. canadensis and E. nuttallii; at high (5.28 g) mussel biomass, the effect of interspecific competition was negated.
  4. Overall, D. polymorpha alleviated competitive interactions between the two invasive macrophytes when all three species co-occurred, and substantially enhanced growth of E. nuttallii with increasing mussel biomass, thereby suggesting a possible influence on the relative dominance of these macrophytes in the field.
  5. Our study demonstrates how facilitations can cause shifts in dominance among closely related invaders. The consequences of such facilitations for the structure and function of communities remain to be explored generally.
  相似文献   
893.
Evidence is presented for a transmembranous NADH-dehydrogenase in human erythrocyte plasma membrane. We suggest that this enzyme is responsible for the ferricyanide reduction by intact cells. This NADH-dehydrogenase is distinctly different from the NADH-cytochromeb 5 reductase on the cytoplasmic side of the membrane. Pretreatment of erythrocytes with the nonpenetrating inhibitor diazobenzene sulfonate (DABS) results in a 35% loss of NADH-ferricyanide reductase activity in the isolated plasma membrane. Since NADH and ferricyanide are both impermeable, the transmembrane enzyme can only be assayed in open membrane sheets with both surfaces exposed, and not in closed vesicles. The transmembrane dehydrogenase has affinity constants of 90 µM for NADH and 125 µM for ferricyanide. It is inhibited byp-chloromercuribenzoate, bathophenanthroline sulfonate, and chlorpromazine.  相似文献   
894.
895.
896.

Background  

Germline mutations in the tumor suppressor PTEN predispose human beings to breast cancer, and genetic and epigenetic alterations of PTEN are also detected in sporadic human breast cancer. Germline Pten mutations in mice lead to the development of a variety of tumors, but mammary carcinomas are infrequently found, especially in mice under the age of six months.  相似文献   
897.
898.
Papaver bracteatum, native to Iran and southern Russia, has been grown successfully in many countries. Research in the northwest United States has confirmed the potential for its commercial production as a source of the alkaloid thebaine. Potential for the chemical conversion of thebaine into codeine, one of man’s most widely used alkaloidal medicinal agents, is reviewed. Economic and social advantages of growing this species over opium poppy (Papaver somniferum) are discussed. The value of the seed oil for cooking and industrial use is considered.  相似文献   
899.
900.
Affinity chromatography purification of small amounts of penicillinase using cephalosporin C covalently linked to Sepharose 4B has been used in examining the turnover of cell-bound penicillinase by B. licheniformis 749/C. Under conditions in which most of the nascent penicillinase is retained by the cells, turnover could be demonstrated (15% of the cell-bound enzyme in 2.5 hours), although most of the secreted enzyme is derived from newly-formed molecules.  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