全文获取类型
收费全文 | 8685篇 |
免费 | 716篇 |
国内免费 | 697篇 |
出版年
2023年 | 117篇 |
2022年 | 129篇 |
2021年 | 189篇 |
2020年 | 251篇 |
2019年 | 318篇 |
2018年 | 327篇 |
2017年 | 274篇 |
2016年 | 251篇 |
2015年 | 314篇 |
2014年 | 433篇 |
2013年 | 679篇 |
2012年 | 323篇 |
2011年 | 415篇 |
2010年 | 328篇 |
2009年 | 407篇 |
2008年 | 362篇 |
2007年 | 403篇 |
2006年 | 341篇 |
2005年 | 316篇 |
2004年 | 288篇 |
2003年 | 249篇 |
2002年 | 242篇 |
2001年 | 211篇 |
2000年 | 216篇 |
1999年 | 185篇 |
1998年 | 191篇 |
1997年 | 164篇 |
1996年 | 148篇 |
1995年 | 158篇 |
1994年 | 151篇 |
1993年 | 145篇 |
1992年 | 123篇 |
1991年 | 122篇 |
1990年 | 115篇 |
1989年 | 87篇 |
1988年 | 109篇 |
1987年 | 90篇 |
1986年 | 73篇 |
1985年 | 119篇 |
1984年 | 132篇 |
1983年 | 90篇 |
1982年 | 89篇 |
1981年 | 80篇 |
1980年 | 58篇 |
1979年 | 42篇 |
1978年 | 41篇 |
1977年 | 41篇 |
1975年 | 33篇 |
1974年 | 31篇 |
1973年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 194 毫秒
11.
This research presents the results of constructing and parameterizing an individual-based model of spatiotemporal dynamics of mixed forest stands. The model facilitates computerized experiments with forest stands having different combinations of species and age structures. These forest stands grow on temperate areas where light is the main system-forming factor that shapes and develops forest ecosystems. The model TEMFORM (TEMperate FORests Model) is developed with few equations and parameters, most of which can be estimated using standard forest inventory data. Parameterization of the model used the growth tables of a set of basic forest-forming species in Far East Russia. Simulation results of the development of the natural single- and mixed-species stands and the effects of different types of disturbances on the stand dynamics and compositions are presented. 相似文献
12.
13.
14.
Yuichi Esaki Kenji Obayashi Keigo Saeki Kiyoshi Fujita Nakao Iwata Tsuyoshi Kitajima 《Chronobiology international》2020,37(6):887-896
ABSTRACT Previous studies have found that keeping the room dark at night was associated with a decrease in manic symptoms for patients with bipolar disorder (BD). However, the association between light at night of real-life conditions and manic symptoms is unclear. We investigated the association between bedroom light exposure at night and manic symptoms in BD patients. One-hundred and eighty-four outpatients with BD participated in this cross-sectional study. The average light intensity at night during sleep was evaluated using a portable photometer for seven consecutive nights. Manic symptoms were assessed using the Young Mania Rating Scale (YMRS), and scores ≥5 were treated as a “hypomanic state.” The median (interquartile range) YMRS score was 2.0 (0–5.0), and 52 (28.2%) participants were in a hypomanic state. The prevalence of a hypomanic state was significantly higher in the participants with an average light intensity at night exposure of ≥3 lux than in those with <3 lux (36.7% versus 21.9%; P = .02). In multivariable logistic regression analysis adjusted for BD type, depressive symptoms, sleep duration, and daytime physical activity, the odds ratio (OR) for a hypomanic state was significantly higher for the participants with an average light intensity at night exposure of ≥3 lux than for those with <3 lux (OR: 2.15, 95% confidence interval: 1.09–4.22, P = .02). This association remained significant at the cutoff value of YMRS score ≥6 (OR: 2.51, 95% confidence interval: 1.15–5.46; P = .02). The findings of this study indicate bedroom light exposure at night is significantly associated with manic symptoms in BD patients. Although the results of this cross-sectional investigation do not necessarily imply causality, they may serve to inform beneficial nonpharmacological intervention and personalized treatment of BD patients. 相似文献
15.
16.
Makoto Tajima Nobuko Sekiguchi Masao Fujimaki 《Bioscience, biotechnology, and biochemistry》2013,77(2):319-320
High phosphate accumulating bacteria were isolated by autoradiography. One isoate, Arthrobacter globiformis PAB-6 accumulated phosphate intracellularly at 20% of dry cell mass in a simple synthetic medium. This amount was 3~7 times higher than type cultures examined. Almost no phosphate was released into the medium after cessation of growth. Fifty percent of total intracellular phosphate was fractionated as nucleic acids, while 20% each was recovered from cold PCA soluble fractions and polyphosphate fractions. The large content of nucleic acids in this bacterium appeared due to increased RNA content, specifically 4 S RNA fraction. 相似文献
17.
ABSTRACT With countless “natural” experiments triggered by the COVID-19-associated physical distancing, one key question comes from chronobiology: “When confined to homes, how does the reduced exposure to natural daylight arising from the interruption of usual outdoor activities plus lost temporal organization ordinarily provided from workplaces and schools affect the circadian timing system (the internal 24 h clock) and, consequently, health of children and adults of all ages?” Herein, we discuss some ethical and scientific facets of exploring such natural experiments by offering a hypothetical case study of circadian biology. 相似文献
18.
Abstract Close monitoring of the lepidopteran leafroller Cnephasia jactatana under laboratory colonisation revealed few distinct effects of successive rearing on artificial diet on the life cycle. The second laboratory generation had a prolonged development time and altered sex synchronism in pupation and eclosion patterns. Some deleterious changes were observed in later generations, including decreases in fertility, egg hatch and sperm motility, failure of mating adults to separate, and pupal and adult malformations. These changes were not adaptive, but were due to incompatibility with the general purpose diet (GPD) used; they were absent under sub-colonisation on a sheepnut-bean based diet (SBD). Success in the laboratory colonisation of C. jactatana is attributed to a random mating protocol, choice of environmental conditions representing the wild habitat, and a rapid rate of population growth. 相似文献
19.
Aorta smooth myosin contains two types of light chain, LC20 and LC17, which fold together with the N-terminal region of each heavy chain to form the globular head region of myosin. We demonstrate an altered conformation of LC20 after its separation from heavy chain by high concentrations of urea, on the basis of the following evidende: 1) A polyclonal antibody against LC20 was not able to recognize this conformationally altered form; 2) Myosin reconstituted from heavy chains and urea-dissociated light chains exhibited extremely low ATPase activity. Circular dichroism unfolding profiles showed that light chains dissociated from heavy chains by SDS appeared to be more stable than those generated by urea dissociation. 相似文献
20.