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   139028篇
  免费   3723篇
  国内免费   898篇
  143649篇
  2024年   52篇
  2023年   184篇
  2022年   578篇
  2021年   930篇
  2020年   585篇
  2019年   709篇
  2018年   12578篇
  2017年   11281篇
  2016年   8749篇
  2015年   2964篇
  2014年   3003篇
  2013年   3365篇
  2012年   8113篇
  2011年   16299篇
  2010年   14123篇
  2009年   10222篇
  2008年   12618篇
  2007年   14045篇
  2006年   2853篇
  2005年   2917篇
  2004年   3164篇
  2003年   2908篇
  2002年   2427篇
  2001年   1561篇
  2000年   1396篇
  1999年   1024篇
  1998年   410篇
  1997年   364篇
  1996年   260篇
  1995年   215篇
  1994年   218篇
  1993年   206篇
  1992年   349篇
  1991年   334篇
  1990年   273篇
  1989年   234篇
  1988年   191篇
  1987年   179篇
  1986年   132篇
  1985年   105篇
  1984年   86篇
  1983年   101篇
  1982年   65篇
  1981年   53篇
  1980年   55篇
  1979年   69篇
  1978年   51篇
  1974年   63篇
  1972年   278篇
  1971年   308篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
Hostile intercommunity relations, including attacking and killing extra-community infants of both sexes have occurred at most wild chimpanzee sites. We describe three recent cases of intercommunity attacks on infants committed by members of the Ngogo chimpanzee community in Kibale National Park, Uganda. Two of the attacks resulted in confirmed infanticides while a third attack probably resulted in the infant's death. In common with previous accounts of chimpanzee infanticides, the attacks described here occurred during boundary patrols outside the Ngogo community's usual range, adult and adolescent males were the main participants, one infant was cannibalized after being killed, and the victims’ mothers did not accompany the attacking party back to the Ngogo range. However, the patrol parties during each infanticide were larger than before and included females from the Ngogo community. Our observations indirectly support both the range expansion and imbalance of power hypotheses, which address why and under which conditions chimpanzee intercommunity encounters lead to aggression. These cases of intercommunity infanticide add to the growing database of the phenomenon in wild chimpanzees.  相似文献   
3.
4.
5.
6.
Secretion of levansucrase from Zymomonas mobilis in Escherichiacoli by glycine supplement was investigated. A significant amount of levansucrase (about 25% of total activity) was found in intact whole-cells. Cell fractionation experiments showed that levansucrase was found both in the periplasmic space and in the cytoplasmic fraction of E. coli. None or only trace amounts of levansucrase was detected in the extracellular culture broth at 24 h of cultivation and it accrued with the increasing concentration of glycine in the culture medium and duration of the culture period. Optimal glycine concentration for the maximum secretion of levansucrase was in the range of 0.8-1%, in which approximately 20-50% of levansucrase was released into the extracellular fraction at 24 h of cultivation, although glycine retarded the bacterial growth.  相似文献   
7.
IN addition to its well known antiviral activity, interferon has recently been shown to inhibit the multiplication of tumour and mammalian cells in cell culture1–6. We report here the inhibition by interferon of DNA synthesis induced in mouse spleen lymphocytes by the non-viral stimuli phytohaemagglutinin (PHA) and allogeneic lymphocytes. These findings are in accord with our contention that interferon affects cell function and, furthermore, they suggest that by acting on lymphocytes, interferon plays a role in the immunological response of the host.  相似文献   
8.
9.
CHLOROPLASTS are more permeable to amino-acids1,2 than are cell membranes3 and also discriminate between their optical isomers2. This has led to the suggestion that the passage of amino-acids through the limiting membranes of chloroplasts is facilitated by carriers2. Using the additional criteria of saturation and competition, we support this suggestion and show that at least two carriers are probably involved.  相似文献   
10.
The probability that protected areas will deliver their potential for maintaining or enhancing biodiversity is likely to be maximised if they are appropriately and effectively managed. As a result, governments and conservation agencies are devoting much attention to the management of protected areas. In the U.K., the demand for performance accountability has resulted in Public Service Agreements (PSA) that set out targets for government departments to deliver results in return for investments being made. One such target for England is to ensure that all nationally important wildlife sites are in favourable condition by 2010. Here, we tested the hypothesis, of potential strategic importance, that the ecological condition of these sites is predictable from relationships with a range of physical, environmental and demographic variables. We used binary logistic regression to investigate these relationships, using the results of English Nature’s 1997–2003 condition assessment exercise. Generally, sites in unfavourable condition tend to be larger in area, located at higher elevations, but with higher human population density and are more spatially isolated from units of the same habitat. However, despite the range of different parameters included in our models, the extent to which the condition of any given site could be predicted was low. Our results have implications for the delivery of PSA targets, funding allocation, and the location of new protected areas.  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