全文获取类型
收费全文 | 194591篇 |
免费 | 6931篇 |
国内免费 | 900篇 |
专业分类
202422篇 |
出版年
2022年 | 781篇 |
2021年 | 1564篇 |
2020年 | 1281篇 |
2019年 | 1447篇 |
2018年 | 13733篇 |
2017年 | 12371篇 |
2016年 | 10316篇 |
2015年 | 4876篇 |
2014年 | 4899篇 |
2013年 | 6263篇 |
2012年 | 10905篇 |
2011年 | 18419篇 |
2010年 | 15508篇 |
2009年 | 11214篇 |
2008年 | 14329篇 |
2007年 | 15674篇 |
2006年 | 4982篇 |
2005年 | 4513篇 |
2004年 | 4994篇 |
2003年 | 4567篇 |
2002年 | 4109篇 |
2001年 | 2975篇 |
2000年 | 2682篇 |
1999年 | 2106篇 |
1998年 | 1055篇 |
1997年 | 794篇 |
1996年 | 794篇 |
1995年 | 739篇 |
1994年 | 664篇 |
1993年 | 623篇 |
1992年 | 1267篇 |
1991年 | 1199篇 |
1990年 | 1121篇 |
1989年 | 1184篇 |
1988年 | 990篇 |
1987年 | 1008篇 |
1986年 | 909篇 |
1985年 | 958篇 |
1984年 | 792篇 |
1983年 | 722篇 |
1979年 | 795篇 |
1978年 | 600篇 |
1975年 | 700篇 |
1974年 | 754篇 |
1973年 | 717篇 |
1972年 | 900篇 |
1971年 | 851篇 |
1970年 | 626篇 |
1969年 | 687篇 |
1968年 | 646篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
S E Andersson C Zackrisson A Hemsén J M Lundberg 《Biochemical and biophysical research communications》1992,188(3):1116-1121
Intratracheal instillation of Sephadex beads induced a long-lasting inflammation in the rat lung as seen by an increase in lung weights. Repeated instillation enhanced this reaction and increased lung endothelin-1 content 3.5 times. Budesonide given s.c. abolished these effects and even reduced basal endothelin-1 content by 72%. The tissue content of the sensory neuropeptide neurokinin A were unaffected by both treatments. Endothelin has been proposed to play a part in the pathogenesis of bronchial asthma. If it is so, the ability of budesonide to reduce endothelin-1 content could thus be added to the list of beneficial effects of glucocorticosteroids in these conditions. 相似文献
82.
Calcium channel blockers, verapamil, nitrendipin and nifedipin, and cyclosporin A inhibited growth of colonies ofBotrytis cinerea in a concentration-dependent manner and simultaneously induced morphological changes of its hyphal tips. Exogenous calcium
at the concentration of 100 mmol/L decreased the growth-inhibitory effects of channel blockers and cyclosporin A; however,
at the concentration of 500 mmol/L Ca2+ their inhibitory effects were increased. At the latter concentration, calcium partly reversed the morphogenic effects of
the blockers but not of cyclosporin A. 相似文献
83.
The involvement of gibberellins in the control of flowering of sunflower was studied by direct application of GA3 to the apex of the plants, analysis of the endogenous levels of gibberellin-like substances at different plant ages, and indirectly by the application of paclobutrazol, an inhibitor of gibberellin synthesis. GA3 speeded-up flower initiation and floral apex development. The time of GA3 application was more critical than the amount of GA3 applied. The endogenous levels of gibberellin-like compounds increased significantly by day 15 after sowing. The application of paclobutrazol markedly delayed floral initiation and this effect was also depedent on plant age. Both GA3 and paclobutrazol had their greatest effects between 10 and 20 days after sowing suggesting that an increase in gibberellins in that time period plays a role in floral initiation. 相似文献
84.
Se-Chan Kim Alexander Ghanem Heidi Stapel Klaus Tiemann Pascal Knuefermann Andreas Hoeft Rainer Meyer Christian Grohé Anne A Knowlton Georg Baumgarten 《BMC physiology》2007,7(1):5
Backgound
It has been reported that Toll-like receptor 4 (TLR4) deficiency reduces infarct size after myocardial ischemia/reperfusion (MI/R). However, measurement of MI/R injury was limited and did not include cardiac function. In a chronic closed-chest model we assessed whether cardiac function is preserved in TLR4-deficient mice (C3H/HeJ) following MI/R, and whether myocardial and systemic cytokine expression differed compared to wild type (WT). 相似文献85.
86.
Christelle Hély Kelly K. Caylor Peter Dowty Samuel Alleaume Robert J. Swap Hank H. Shugart Christopher O. Justice 《Ecosystems》2007,10(7):1116-1132
Abstract
We present a regional fuel load model (1 km2 spatial resolution) applied in the southern African savanna region. The model is based on a patch-scale production efficiency
model (PEM) scaled up to the regional level using empirical relationships between patch-scale behavior and multi-source remote
sensing data (spatio-temporal variability of vegetation and climatic variables). The model requires the spatial distribution
of woody vegetation cover, which is used to determine separate respiration rates for tree and grass. Net primary production,
grass and tree leaf death, and herbivory are also taken into account in this mechanistic modeling approach. The fuel load
model has been calibrated and validated from independent measurements taken from savanna vegetation in Africa southward from
the equator. A sensitivity analysis on the effect of climate variables (incoming radiation, air temperature, and precipitation)
has been conducted to demonstrate the strong role that water availability has in determining productivity and subsequent fuel
load over the southern African region. The model performance has been tested in four different areas representative of a regional
increasing rainfall gradient—Etosha National Park, Namibia, Mongu and Kasama, Zambia, as well as in Kruger National Park,
South Africa. Within each area, we analyze model output from three different magnitudes of canopy coverage (<5, 30, and 50%).
We find that fuel load ranges predicted by the model are globally in agreement with field measurements for the same year.
High rainfall sustains green herbaceous production late in the dry season and delays tree leaf litter production. Effect of
water on production varies across the rainfall gradient with delayed start of green material production in more arid regions. 相似文献
87.
88.
The experimental study of the relationship between biodiversity and ecosystem function has mainly addressed the effect of species and number of functional groups. In theory, this approach has mainly focused on how extinction affects function, whereas dispersal limitation of ecosystem function has been rarely discussed. A handful of seed introduction experiments, as well as numerous observations of the effects of long‐distance dispersal of alien species, indicate that ecosystem function may be strongly determined by dispersal limitation at the local, regional and/or global scales. We suggest that it is time to replace biodiversity manipulation experiments, based on random draw of species, with those addressing realistic scenarios of either extinction or dispersal. Experiments disentangling the dispersal limitation of ecosystem function should have to take into account the probability of arrival. The latter is defined as the probability that a propagule of a particular species will arrive at a particular community. Arrival probability depends on the dispersal ability and the number of propagules of a species, the distance a species needs to travel, and the permeability of the matrix landscape. Current databases, in particular those in northwestern and central Europe now enable robust estimation of arrival probability in plant communities. We suggest a general hypothesis claiming that dispersal limitation according to arrival probability will have ecosystem‐level effects different from those arising due to random arrival. This hypothesis may be rendered more region‐, landscape‐ or ecosystem‐specific by estimating arrival probabilities for different background conditions. 相似文献
89.
Tapas K. Nandi Hridoy R. Bairagya Bishnu P. Mukhopadhyay K. Sekar Dipankar Sukul Asim K. Bera 《Journal of biosciences》2009,34(1):27-34
The role of invariant water molecules in the activity of plant cysteine protease is ubiquitous in nature. On analysing the
11 different Protein DataBank (PDB) structures of plant thiol proteases, the two invariant water molecules W1 and W2 (W220
and W222 in the template 1PPN structure) were observed to form H-bonds with the Ob atom of Asn 175. Extensive energy minimization and molecular dynamics simulation studies up to 2 ns on all the PDB and solvated
structures clearly revealed the involvement of the H-bonding association of the two water molecules in fixing the orientation
of the asparagine residue of the catalytic triad. From this study, it is suggested that H-bonding of the water molecule at
the W1 invariant site better stabilizes the Asn residue at the active site of the catalytic triad. 相似文献
90.
H Mohelská O Parízek V Hyncica E Hartlová 《Journal of hygiene, epidemiology, microbiology, and immunology》1988,32(1):7-16
Ten Swiss albino ICR SPF female mice 110 days old (weight about 30 g) were exposed for 48 hours to a solution of plutonium-238 nitrate (spec. act. 5 MBq/1 m1, pH 2.7) injected in amounts of 0.01 ml into the popliteal area of the right femur, each thus receiving about 500 kBq per 30 g body weight. Of the injected activity, 50% was retained in the right femur, 2% in the left femur and approximately 2-3% in the excrements collected separately from each animal during the whole exposure period. Ultrastructurally, electron micrographs revealed a variety of changes, including hypertrophy and destruction of endosteal cell organelles (primary damage), deformation and hypertrophy of osteocytes (secondary damage) and the irregularities in the osteocyte self-burial process leading to an abnormal formation of bone tissue structure (tertiary damage). Qualitatively, these changes in the irradiated bone ultrastructure were analogous to those occurring with age. This was confirmed by comparing two groups of control mice 110 and 330 days old. Assessed quantitatively, changes due to irradiation were more pronounced than those associated with aging. 相似文献