全文获取类型
收费全文 | 18606篇 |
免费 | 1644篇 |
国内免费 | 3312篇 |
出版年
2023年 | 430篇 |
2022年 | 508篇 |
2021年 | 695篇 |
2020年 | 668篇 |
2019年 | 830篇 |
2018年 | 759篇 |
2017年 | 708篇 |
2016年 | 793篇 |
2015年 | 811篇 |
2014年 | 1114篇 |
2013年 | 1369篇 |
2012年 | 888篇 |
2011年 | 985篇 |
2010年 | 727篇 |
2009年 | 1008篇 |
2008年 | 1037篇 |
2007年 | 1004篇 |
2006年 | 920篇 |
2005年 | 895篇 |
2004年 | 794篇 |
2003年 | 701篇 |
2002年 | 607篇 |
2001年 | 466篇 |
2000年 | 451篇 |
1999年 | 382篇 |
1998年 | 369篇 |
1997年 | 358篇 |
1996年 | 310篇 |
1995年 | 308篇 |
1994年 | 260篇 |
1993年 | 241篇 |
1992年 | 215篇 |
1991年 | 217篇 |
1990年 | 190篇 |
1989年 | 158篇 |
1988年 | 113篇 |
1987年 | 121篇 |
1986年 | 125篇 |
1985年 | 146篇 |
1984年 | 132篇 |
1983年 | 92篇 |
1982年 | 127篇 |
1981年 | 97篇 |
1980年 | 71篇 |
1979年 | 61篇 |
1978年 | 41篇 |
1977年 | 43篇 |
1976年 | 36篇 |
1975年 | 30篇 |
1973年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
H.G. Ferrari C.A. Gobatto F.B. Manchado-Gobatto 《Biology of sport / Institute of Sport》2013,30(4):289-294
This study aimed to evaluate the leukocyte subset counts, serum immunoglobulin A, performance and upper respiratory symptoms (URS), as well as their interrelationships, of well-trained cyclists for a 29-week training season using monitored loads. The season was divided into three phases: preparatory (nine weeks), first competitive phase (nine weeks) and second competitive phase (11 weeks). The sample consisted of eight well-trained cyclists, aged 18 ± 2 years. Immunological parameters and performance were evaluated during weeks 1 (baseline), 10 (early first competitive phase), 19 (early second competitive phase) and 29 (end of the second competitive phase). The training loads (volume x rating of perceived exertion) were monitored daily while the monitoring of URS was performed every 15 days using the WURSS-44 questionnaire. The data were analyzed using a one-way ANOVA and a Pearson correlation test with the significance level set at p ≤ 0.05. No significant differences were found for training load, leukocyte subset counts or serum immunoglobulin A among the three phases. However, serum immunoglobulin A was 50.9% below the control group values. URS were significantly higher during the preparatory period, and there were significant correlations between URS and training load (strain) in the preparatory period (r = 0.72, p = 0.032) and second competitive phase (r = 0.73, p = 0.036). In conclusion, indicators of training load without a significant change throughout the season did not significantly affect immune parameters measured; however, the increase of strain can cause an increase of upper respiratory symptoms throughout the season, but without loss of performance. 相似文献
72.
Lidia Poplawska-Szczyglowska Jan Walewski Barbara Pienkowska-Grela Grzegorz Rymkiewicz Olga Mioduszewska 《Cancer immunology, immunotherapy : CII》1999,16(1):65-68
A 68-year-old man presented with hemiparesis, lymphocytosis, and cerebral lesions on MRI. Flow cytometry of blood, bone marrow
and cerebrospinal fluid showed B-CLL lymphocytes with bright CD20 expression, slg, and absence of CD23 antigen. Fluorescencein situ hybridisation showed trisomy 12 in 50% of analysed peripheral mononuclear cells. The patient died 6 months after the diagnosis.
Rapidly progressive and fatal course of the disease was consistent with known bad prognostic significance of CD20 bright expression
and trisomy 12. 相似文献
73.
74.
75.
J. M. Morais H. N. Consolaro L. L. Bergamini V. Ferrero 《Plant biology (Stuttgart, Germany)》2020,22(5):872-880
- Enantiostyly is a floral polymorphism in which two floral forms in the same species differ in deflection of the stigma to right or left position. In monomorphic enantiostylous plants, flowers of the two morphs occur within the same individual, usually in the same proportion. In self‐compatible species the function of monomorphic enantiostyly is proposed to increase outcrossing rates and offer a reproductive advantage under pollination limitation. Enantiostylous species are usually self‐compatible and show heteranthery, with poricide anthers and pollen as pollinator reward; however, there are families, such as Vochysiaceae, that have different characteristics.
- We analysed the reproductive system and pollination biology of Qualea parviflora and Q. multiflora, two enantiostylous species from the Brazilian Cerrado that have specific morphological and physiological traits. For this, we characterized flower traits, performed hand pollinations and studied floral visitors.
- We found no differences between morphs in the proportion of flowers, nectar produced or its concentration, pollen quantity and fruit set. Both species were self‐incompatible and quite generalist regarding floral visitors.
- Enantiostyly in self‐incompatible plants seems to confer a reproductive advantage by reducing self‐interference resulting from stigma clogging. This novel result helps to expand our knowledge on this complex floral polymorphism and opens new avenues for future research on this topic.
76.
Gerald Ozanne 《Aerobiologia》1990,6(1):8-10
Summary Under the ?Bois plus? scheme, the CTBA is currently developing a third party certification, based on the Scrivener law, with the aim of defending
the consumers' interests. At this early stage, wood preservatives and treated wood are checked for the efficacy of treatments
in order to ensure the stability and security of works in buildings and civil engineering. According to the different classes
of risks in service, ?Bois plus? provides the most adeguate level of protection. In the close future, the industry is planning to incorporate, with the help
of the CTBA and experts, health criteria within the frame of the ?Bois plus? certification scheme. The means could beSafety indexes, which are objectives of quality for professionals.Safety indexes are concentrations of substances in wood which can be taken as safe for humans and the general environment, while still toxic
for the wood pests.
This attractive experience aims to make with the brandname ?Bois plus? a synonym of efficacy and safety. This approach meets the essential requirements 1, 3, 4 of the 89/106 EEC directive: mechanical
resistance and stability/hygiene, health and environment/safety in use. 相似文献
77.
Brain endothelial cells (BEC) of the vascular blood-brain barrier (BBB) interact with many different cell types in the brain, including microglia, the brain's resident immune cells. Physical associations of microglia with the BBB and the importance of these interactions in health and disease are an emerging area of study and likely involved in neuroimmune communication. In this mini-review, we consider how microglia and the BBB are intrinsically linked in the developing brain, discuss possible mechanisms that attract microglia to the vasculature in healthy physiological conditions, and examine the known microglial-vascular associated changes in systemic infection and various disease states. Our findings shed light on the complexities of microglial-vascular interactions and highlight the contributions of microglia to the functions of the neurovascular unit. 相似文献
78.
阿拉善高原2种荒漠植物根系构型及生态适应性特征 总被引:1,自引:0,他引:1
根系构型决定了植物对资源的吸收方式,根系构型的变化是植物对环境的生态适应和有效生存策略。在阿拉善高原西南缘红砂(Reaumuria soongarica)-珍珠猪毛菜(Salsola passerina)混生群落采用传统挖掘法收集两种植物根系,基于量化的根系形态指标,利用几何拓扑学及分形理论分析了根系构型特征,探讨了该地区2种植物对干旱生境的生态适应策略。结果表明:红砂和珍珠猪毛菜根系均以水平分布占优,根系浅层化分布明显,混生的两种植物占据不同的生态位;2种荒漠植物均具有较大的比根长(SRL)和比表面积(SRA),红砂SRL=21.3 cm/g,SRA=7.6 cm2/g,珍珠SRL=22.4 cm/g,SRA=6.5 cm2/g,有利于水分和养分的获取;红砂根系拓扑指数(TI)、修正拓扑参数(qa和qb)分别为0.86、0.52、0.49,珍珠猪毛菜对应参数分别为0.93、0.76、0.73,表明2种植物根系均趋向于鱼尾形分支结构;根系分形维数值(FD红=1.488、FD珍=1.422)较小,而分形丰度值(lgK红=1.855、lgK珍=1.774)较大,表明2种植物分支相对简单,但空间拓展能力强,有利于对营养空间的占有。上述特征可能是阿拉善西南缘红砂-珍珠猪毛菜群落2种荒漠植物植物对干旱贫瘠生境的重要生态适应策略。 相似文献
79.
The Human Development Index (HDI) based on life expectancy, education and per-capita income, is one of the most used indicators of human development. However, undeniable problems in data collection limit between-countries comparisons reducing the practical applicability of the HDI in official statistics. Elvidge et al. (2012) proposed an alternative index of human development (the so called Night Light Development Index, NLDI) derived from nighttime satellite imagery and population density, with improved comparability over time and space. The NLDI assesses inequality in the spatial distribution of night light among resident inhabitants and has proven to correlate with the HDI at the country scale. However, the NLDI presents some drawbacks, since similar NLDI values may indicate very different levels of human development. A modified NLDI overcoming such a drawback is proposed and applied to assessment of human development at 3 spatial scales (the entire country, 5 geographical divisions and 20 administrative regions) in Italy, a country with relevant territorial disparities in various socioeconomic dimensions. The original and modified NLDI were correlated with 5 independent indicators of economic growth, sustainable development and environmental quality. The spatial distribution of the original and modified NLDI is not coherent with the level of human development in Italy being indeed associated with various indexes of environmental quality. Further investigation is required to identify in which socioeconomic context (and at which spatial scale) the NDLI approach correctly estimates the level of human development in affluent countries. 相似文献
80.
《植物生态学报》2015,39(8):816
Aims Fractal root system is phenotypic plasticity result of plant root architecture to respond to environmental heterogeneity, may reflect the growth strategy of plants to adapt to environmental conditions. Our objective was to explore the relationship between root fractal dimension and fractal abundance of fractal root system of Melica przewalskyi population in response to aspect variation in the northwest of China. Methods The study site was located in a degraded alpine grassland on the northern slope in Qilian Mountains, Gansu Province, China. Survey and sampling were carried out at 40 plots which were set up along four slope aspects transects with 20 m distance between adjacent plots. Handheld GPS was used to determine the elevation, longitude and latitude of each plot. ArcGIS was used to set up digital elevation model (DEM). Community traits were investigated and six individuals roots of M. przewalskyi were collected randomly at each plot. The samples were cleaned and divided into different organs, then scanning the root with the Win-RHIZO for measurements of fractal dimension and fractal abundance in laboratory, and their biomass were then measured after being dried at 80 °C in an oven. Important findings With the slope aspect turned from north to east, west, and south, the density, height and soil moisture content of the plant community displayed a pattern of initial decline, the height, density, root fractal abundance of M. przewalskyi increased and the root fractal dimension decreased. The root fractal dimension was negatively associated with the fractal abundance in all aspects, but the relationship varied along the slope aspects gradient; there was a highly significant negative correlation (p < 0.01) between the root fractal dimension and fractal abundance at north slope and south slope aspect, whereas the correlation only reached a significant level (p < 0.05) at the east slope aspect and west slope aspect; indicating that there is a trade-off between the root fractal dimension and fractal abundance. In addition, when the slope aspect changed from north to east, west and south, the standardized major axis (SMA) slope of the regression equation in the scaling relationships between root fractal dimension and fractal abundance increased (p < 0.05), indicating that the roots of M. przewalskyi at the droughty southern slope have less branch and more sparse in the same soil volume of root exploitation and utilization. Consequently, the resource allocation pattern on reasonable trade-off between root fractal dimension and fractal abundance in different slope aspect of M. przewalskyi, reflects the relationship between the income and the cost of construction of plant root architecture. 相似文献