全文获取类型
收费全文 | 126854篇 |
免费 | 3545篇 |
国内免费 | 821篇 |
出版年
2021年 | 494篇 |
2020年 | 281篇 |
2019年 | 393篇 |
2018年 | 12189篇 |
2017年 | 10988篇 |
2016年 | 8063篇 |
2015年 | 1644篇 |
2014年 | 1520篇 |
2013年 | 1925篇 |
2012年 | 6117篇 |
2011年 | 14729篇 |
2010年 | 13143篇 |
2009年 | 9349篇 |
2008年 | 11461篇 |
2007年 | 13094篇 |
2006年 | 2034篇 |
2005年 | 2341篇 |
2004年 | 2703篇 |
2003年 | 2769篇 |
2002年 | 2444篇 |
2001年 | 661篇 |
2000年 | 393篇 |
1999年 | 408篇 |
1998年 | 492篇 |
1997年 | 369篇 |
1996年 | 366篇 |
1995年 | 285篇 |
1994年 | 343篇 |
1993年 | 337篇 |
1992年 | 309篇 |
1991年 | 301篇 |
1990年 | 268篇 |
1989年 | 239篇 |
1988年 | 274篇 |
1987年 | 257篇 |
1986年 | 222篇 |
1985年 | 305篇 |
1984年 | 395篇 |
1983年 | 332篇 |
1982年 | 375篇 |
1981年 | 436篇 |
1980年 | 416篇 |
1979年 | 252篇 |
1978年 | 259篇 |
1977年 | 276篇 |
1976年 | 263篇 |
1974年 | 234篇 |
1973年 | 192篇 |
1972年 | 371篇 |
1971年 | 371篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
HIF-1 expression in healing wounds: HIF-1alpha induction in primary inflammatory cells by TNF-alpha 总被引:12,自引:0,他引:12
13.
Background
Recent studies point to a great diversity of non-ribosomal peptide synthesis systems with major roles in amino acid and co-factor biosynthesis, secondary metabolism, and post-translational modifications of proteins by peptide tags. The least studied of these systems are those utilizing tRNAs or aminoacyl-tRNA synthetases (AAtRS) in non-ribosomal peptide ligation. 相似文献14.
15.
16.
17.
Background
Substituted catechols are important precursors for large-scale synthesis of pharmaceuticals and other industrial products. Most of the reported chemical synthesis methods are expensive and insufficient at industrial level. However, biological processes for production of substituted catechols could be highly selective and suitable for industrial purposes. 相似文献18.
Tuberculate mycorrhizae on Pinus contorta (lodgepole pine) have previously been shown to reduce acetylene, but an outstanding question has been to what degree these structures could meet the nitrogen requirements of the tree. We compared the growth, tissue nitrogen contents, and stable nitrogen isotope ratios of P. contorta growing in gravel pits to the same species growing on adjacent intact soil. Trees growing in severely nitrogen deficient gravel pits had virtually identical growth rates and tissue nitrogen contents to those growing on intact soil that had nitrogen levels typical for the area. δ15N values for trees in the gravel pits were substantially lower than δ15N values for trees on intact soil, and isotope ratios in vegetation were lower than the isotope ratios of the soil. The form of soil nitrogen in the gravel pits was almost exclusively nitrate, while ammonium predominated in the intact soil. Discrimination against 15N during plant uptake of soil nitrate in the highly N-deficient soil should be weak or nonexistent. Therefore, the low δ15N in the gravel pit trees suggests that trees growing in gravel pits were using another nitrogen source in addition to the soil. Precipitation-borne nitrogen in the study area is extremely low. In conjunction with our other work, these findings strongly suggests that P. contorta and its microbial symbionts or associates fix nitrogen in sufficient amounts to sustain vigorous tree growth on the most nitrogen-deficient soils. 相似文献
19.
20.
Food availability is important to the dynamics of animal social organizations or populations. However, the role of winter
food availability in animal population dynamics is still controversial. We carried out an experimental study to test Lack’s
hypothesis that reduced food in winter limits survival and spring numbers of breeding individuals of social groups, using
the Mongolian gerbil (Meriones unguiculatus) as model species. We established 24 gerbil social groups in 24, 10 × 10 m, pens in September 2008. We provided wheat seeds
as supplemental food in 12 enclosures from September 2008 to March 2009; the other 12 enclosures, not provided with supplemental
food, served as controls. We live-trapped gerbils at a 2-week interval from September to April. Supplemental food during winter
increased biweekly survival by 10% relative to that in control groups. Only four control social groups survived to the end
of our study whereas all 12 food-supplemented social groups survived through our study period. Supplemental food also increased
cumulative numbers of recruits and group sizes of gerbils. We conclude that winter food availability limits winter survival
and spring social groups or population sizes of Mongolian gerbils. 相似文献