全文获取类型
收费全文 | 101557篇 |
免费 | 1182篇 |
国内免费 | 892篇 |
专业分类
103631篇 |
出版年
2022年 | 71篇 |
2021年 | 132篇 |
2020年 | 80篇 |
2019年 | 121篇 |
2018年 | 11936篇 |
2017年 | 10760篇 |
2016年 | 7624篇 |
2015年 | 937篇 |
2014年 | 654篇 |
2013年 | 719篇 |
2012年 | 4741篇 |
2011年 | 13278篇 |
2010年 | 12292篇 |
2009年 | 8540篇 |
2008年 | 10193篇 |
2007年 | 11784篇 |
2006年 | 693篇 |
2005年 | 968篇 |
2004年 | 1372篇 |
2003年 | 1399篇 |
2002年 | 1165篇 |
2001年 | 421篇 |
2000年 | 300篇 |
1999年 | 164篇 |
1998年 | 129篇 |
1997年 | 114篇 |
1996年 | 92篇 |
1995年 | 96篇 |
1994年 | 103篇 |
1993年 | 113篇 |
1992年 | 128篇 |
1991年 | 128篇 |
1990年 | 137篇 |
1989年 | 115篇 |
1988年 | 96篇 |
1987年 | 118篇 |
1986年 | 70篇 |
1985年 | 74篇 |
1984年 | 86篇 |
1983年 | 79篇 |
1982年 | 65篇 |
1981年 | 60篇 |
1980年 | 50篇 |
1979年 | 64篇 |
1978年 | 51篇 |
1977年 | 44篇 |
1976年 | 46篇 |
1973年 | 58篇 |
1972年 | 299篇 |
1971年 | 318篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
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
2.
3.
4.
5.
6.
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
7.
8.
Summary The physicochemical properties of the interactions of RNA polymerase (RPase) with promoter and nonspecific DNA sequences have been investigated. These show that nonspecific binding is principally an ionic interaction and that promoter binding is more complex, involving nonionic interactions. Nonspecific binding has been shown to be very important in the promoter search, and one-dimensional diffusion can account for the rate at which RPase finds the promoter. Significant differences have been reported in the binding process for various promoters and in the effects of regulatory proteins. Further investigation of these differences will lead to a better understanding of the selectivity and regulation of the initiation process.The pathways of the initiation process have been outlined, by recent studies and considerable progress has been made in determining the rates of interconversion of the intermediate states. A number of questions remain about the detail of initiation and the effects of various parameters on the reactions. Of particular importance is the identification of the point at which the enzyme becomes truly processive. In addition, the step which is rate limiting has not been identified in either the productive or nonproductive process. The mechanistic features of the steps after bond formation are just beginning to yield to investigation.Use of substrate analogs with RPase has led to a picture of the polymerization site according to the ability of the enzyme to incorporate analogs. Base specificity appears to be determined primarily by interaction with the template rather than the enzyme, but the ribose moiety must interact with the site quite specifically. The orientation of the phosphate residues has been determined by NMR, which has also proved to be a valuable probe of the initiation site. At this site base specificity is resident in the enzyme and expressed through the interaction of the base and intrinsic metal, as shown by studies with the Cobalt substituted enzyme. In both initiation and polymerization, the reaction has been shown to proceed by inversion of configuration. Techniques similar to those used for initiation will probably be applied to the polymerization reaction as well, which has not recently received as much attention with respect to mechanism. Functional phenomena such as pausing make the polymerization process particularly promising for producing insight into RPase reactions. 相似文献
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