全文获取类型
收费全文 | 5777篇 |
免费 | 687篇 |
国内免费 | 3篇 |
专业分类
6467篇 |
出版年
2021年 | 79篇 |
2020年 | 42篇 |
2019年 | 53篇 |
2018年 | 73篇 |
2017年 | 79篇 |
2016年 | 95篇 |
2015年 | 146篇 |
2014年 | 194篇 |
2013年 | 279篇 |
2012年 | 273篇 |
2011年 | 282篇 |
2010年 | 195篇 |
2009年 | 157篇 |
2008年 | 216篇 |
2007年 | 250篇 |
2006年 | 226篇 |
2005年 | 211篇 |
2004年 | 202篇 |
2003年 | 194篇 |
2002年 | 192篇 |
2001年 | 137篇 |
2000年 | 163篇 |
1999年 | 137篇 |
1998年 | 94篇 |
1997年 | 69篇 |
1996年 | 78篇 |
1995年 | 67篇 |
1994年 | 57篇 |
1993年 | 59篇 |
1992年 | 109篇 |
1991年 | 102篇 |
1990年 | 83篇 |
1989年 | 88篇 |
1988年 | 107篇 |
1987年 | 104篇 |
1986年 | 81篇 |
1985年 | 83篇 |
1984年 | 98篇 |
1983年 | 85篇 |
1982年 | 54篇 |
1981年 | 67篇 |
1980年 | 58篇 |
1979年 | 79篇 |
1978年 | 51篇 |
1977年 | 46篇 |
1976年 | 55篇 |
1975年 | 43篇 |
1974年 | 55篇 |
1973年 | 45篇 |
1969年 | 44篇 |
排序方式: 共有6467条查询结果,搜索用时 15 毫秒
1.
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
2.
Mandana Nikpour Dafna D Gladman Dominique Ibanez Paula J Harvey Murray B Urowitz 《Arthritis research & therapy》2010,12(3):R125
Introduction
Total cholesterol (TC) and blood pressure (BP) are likely to take a dynamic course over time in patients with systemic lupus erythematosus (SLE). This would have important implications in terms of using single-point-in-time measurements of these variables to assess coronary artery disease (CAD) risk. The objective of this study was to describe and quantify variability over time of TC and BP among patients with SLE and to determine their correlates. 相似文献3.
Fire has historically been an important ecological component of forests in the Intermountain Region of the northwestern United States. This study is set in a small biogeographically disjunct mountain range. Our research objectives were to (1) investigate the historical frequency, severity, size, and spatial pattern of fire; (2) determine if and how fire regimes have changed since Euro-American settlement; and (3) compare how fire regimes of a small isolated range compare to nearby, but considerably larger, mountain agglomerations. Our findings suggest that this mountain range has historically supported fires typified by small size and high frequency, resulting in a high degree of spatial pattern complexity compared to mountain agglomerations. We also found disparity in size and burn severity solely within the study area based on the bisecting Continental Divide. Since the advent of Euro-American settlement in the 1870s, fire frequency and sizes of individual fires in the West Big Hole Range have significantly decreased resulting in an estimated 87% reduction in area burned. We discuss potential relationships of mountain range isolation and fire regimes in the Intermountain Region. Furthermore, we suggest that the relative small size of this mountain range predisposes it to greater anthropogenic effects upon fire occurrence. 相似文献
4.
5.
6.
The study tested the proposition that relationship involvement influences the implicit responses of women to high- and low-status professions. It was hypothesized that when a high-involvement context was primed, women would have more positive implicit associations with high-status occupations than when a low-involvement context was primed. In contrast, when a high-involvement context was primed, women would have more negative associations with low-status occupations than when a low-involvement context was primed. To test the hypothesis, 123 female participants received a high or low relationship involvement prime. Then the participants completed a single category implicit associations test designed to measure the participants' associations with either high- or low-status occupations. As predicted, the relationship involvement prime influenced the positivity of associations made with high- and low-status occupations. The study pointed to the possibility that persons possess evolutionary-based implicit associations. 相似文献
7.
8.
9.
Draud Matthew; Macias-Ordonez Rogelio; Verga Jack; Itzkowitz Murray 《Behavioral ecology》2004,15(1):102-108
Selection usually acts differently on males and females duringintrasexual competition for resources and/or mates. Nevertheless,agonistic behavior has been examined both theoretically andempirically mostly in males. Our research questions whethermales and females follow the same rules of engagement in intrasexualcontests as predicted by the sequential assessment model (SAM).The SAM predicts negative correlations between contest intensityand duration and the magnitude of asymmetry in resource holdingpower (RHP) between the contestants, such that the most escalatedcontests are those between similarly endowed individuals. Westaged male and female intrasexual contests with varying degreesof body size asymmetry under a round robin design using themonogamous Texas cichlid fish (Herichthys cyanoguttatum) asa study case. We used Mantel's matrix analysis to compare howthe behavioral content, duration, structure, and outcome ofmale and female contests were affected by the relative bodysize of the contestants. In the case of males, relative sizein each contest predicted outcome, duration, and frequency ofconventional and escalated behaviors according to prevailingtheory. Female contest structure and outcome, however, werenot predicted by the relative size of contestants. We discussour results in terms of other asymmetries that might be importantin structuring female contests, and we propose potential approachesto study femalefemale aggression. 相似文献
10.