全文获取类型
收费全文 | 30734篇 |
免费 | 2927篇 |
国内免费 | 12篇 |
出版年
2023年 | 121篇 |
2022年 | 289篇 |
2021年 | 685篇 |
2020年 | 413篇 |
2019年 | 479篇 |
2018年 | 566篇 |
2017年 | 539篇 |
2016年 | 848篇 |
2015年 | 1377篇 |
2014年 | 1589篇 |
2013年 | 1863篇 |
2012年 | 2391篇 |
2011年 | 2432篇 |
2010年 | 1561篇 |
2009年 | 1279篇 |
2008年 | 1914篇 |
2007年 | 1835篇 |
2006年 | 1658篇 |
2005年 | 1639篇 |
2004年 | 1627篇 |
2003年 | 1417篇 |
2002年 | 1269篇 |
2001年 | 359篇 |
2000年 | 315篇 |
1999年 | 334篇 |
1998年 | 332篇 |
1997年 | 230篇 |
1996年 | 237篇 |
1995年 | 195篇 |
1994年 | 206篇 |
1993年 | 183篇 |
1992年 | 228篇 |
1991年 | 187篇 |
1990年 | 150篇 |
1989年 | 152篇 |
1988年 | 179篇 |
1987年 | 177篇 |
1986年 | 134篇 |
1985年 | 144篇 |
1984年 | 174篇 |
1983年 | 170篇 |
1982年 | 128篇 |
1981年 | 130篇 |
1980年 | 122篇 |
1979年 | 120篇 |
1978年 | 101篇 |
1977年 | 81篇 |
1976年 | 98篇 |
1975年 | 65篇 |
1974年 | 85篇 |
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
1.
Christopher T. Clarke 《Restoration Ecology》1997,5(3):256-264
No guidelines are currently available that advise which soil properties of arable land can be used to suggest suitable locations for heathland reconstruction. This paper reviews studies comparing soil properties of heathland or semi-natural grassland with those of adjacent arable fields, investigations in the autecology of the dominant heathland plant, Calluna vulgaris (common heather), and long-term experiments of fertilizer inputs on arable soils. Three properties must be assessed before the suitability of a field can be determined: extractable phosphorus, exchangeable calcium, and pH. A number of other nutrients may also be important, but evidence is currently insufficient to substantiate this. Natural changes in levels of extractable phosphorus, exchangeable calcium, and pH appear to be very slow, so nutrient stripping and acidification will be necessary where recommended levels are exceeded to successfully restore heathland vegetation. 相似文献
2.
Wendy A. Douglass Robert H. Hyland Christopher D. Buckley Aymen Al-Shamkhani Jacqueline M. Shaw Sarah L. Scarth David L. Simmons S.K.Alex Law 《FEBS letters》1998,440(3):125
The cysteine-rich region (CRR) of the β2 integrin subunit was replaced by that of β1 to give the chimera β2NV1. β2NV1 can combine with αL to form a variant leukocyte-function-associated antigen (LFA)-1 on COS cell surface, suggesting that the specificity of the β2 interaction with αL does not lie in the CRR. Unlike those expressing wild-type LFA-1, COS cells expressing αLβ2NV1 are constitutively active in intercellular adhesion molecule (ICAM)-1 adhesion. These results suggest that activation of LFA-1 involves the release of an intramolecular constraint, which is maintained, in part, by the authentic β2 CRR. 相似文献
3.
Wei CHEN Liqing PENG Lichun JIANG David A.PIKE Christopher R.FRIESEN Gregory BROWN 《亚洲两栖爬行动物研究(英文版)》2018,(1)
Environmental unpredictability can influence strategies of maternal investment among eggs within a clutch. Models predict that breeding females should adopt a diversified bet-hedging strategy in unpredictable environments, but empirical field evidence from Asia is scarce. Here we tested this hypothesis by exploring spatial patterns in egg size along an altitudinal gradient in a frog species(Rana kukunoris) inhabiting the Tibetan Plateau. Within-clutch variability in egg size increased as the environment became variable(e.g., lower mean monthly temperature and mean monthly rainfall at higher altitudes), and populations in environments with more unpredictable rainfall produced eggs that were smaller and more variable in size. We provide support for a diversified bet-hedging strategy in high-altitude environments, which experience dynamic weather patterns and therefore are of unpredictable environmental quality. This strategy may be an adaptive response to lower environmental quality and higher unpredictable environmental variance. Such a strategy should increase the likelihood of breeding success and maximize maternal lifetime fitness by producing offspring that are adapted to current environmental conditions. We speculate that in high-altitude environments prone to physical disturbance, breeding females are unable to consistently produce the optimal egg size due to physiological constraints imposed by environmental conditions(e.g., duration of the active season, food availability). Species and populations whose breeding strategies are adapted to cope with uncertain environmental conditions by adjusting offspring size and therefore quality show a remarkable degree of ability to cope with future climatic changes. 相似文献
4.
5.
6.
7.
8.
9.
Jo-Anne M.D. Murray Catherine Dunnett Meriel J.S. Moore-Colyer Annette C. Longland 《Animal Feed Science and Technology》2012,171(2-4):192-204
A series of in vitro experiments were conducted to assess three fibrolytic enzyme preparations as potential feed additives in equine diets. The three fibrolytic enzyme preparations were a concentrated cellulase (E1), an acid cellulase (E2) and a concentrated xylanase (E3). The enzymes were evaluated on their ability to modify the cell wall fraction of high-temperature dried lucerne (HTL) under various experimental conditions including differences in temperature, pH, incubation period, substrate levels and particle size to enable selection of the enzyme preparation most effective in the hydrolysis of lucerne. Results showed enzyme activities (as measured by reducing sugar assays) to be greatest at 50 °C, pH 5 and over an incubation period of greater than 20 h. E1 exhibited the greatest effect on total monosaccharide release from the HTL compared to E2 and E3. Moreover, dry matter (DM) and total non-starch polysaccharide (TNSP) losses were also greater in HTL treated with E1 compared to E2 and E3. Therefore, since the cell wall fraction of HTL contained substantial amounts of cellulose, the enzyme with the highest cellulase activity (Enzyme 1) was most effective in hydrolysing the cell walls of HTL. Consequently, it would appear that the application of exogenous fibrolytic enzyme preparations to forages requires the chemical characterisation of the target forage to enable selection of enzymes that are (a) most suitable to degrade the cell wall components of the candidate forage and (b) effective under field conditions. 相似文献
10.
Mandana Nikpour Dafna D Gladman Dominique Ibanez Paula J Harvey Murray B Urowitz 《Arthritis research & therapy》2010,12(3):R125