全文获取类型
收费全文 | 98529篇 |
免费 | 829篇 |
国内免费 | 884篇 |
专业分类
100242篇 |
出版年
2022年 | 52篇 |
2021年 | 105篇 |
2020年 | 48篇 |
2019年 | 54篇 |
2018年 | 11895篇 |
2017年 | 10716篇 |
2016年 | 7547篇 |
2015年 | 795篇 |
2014年 | 515篇 |
2013年 | 574篇 |
2012年 | 4553篇 |
2011年 | 13169篇 |
2010年 | 12207篇 |
2009年 | 8437篇 |
2008年 | 10059篇 |
2007年 | 11657篇 |
2006年 | 517篇 |
2005年 | 810篇 |
2004年 | 1264篇 |
2003年 | 1273篇 |
2002年 | 1052篇 |
2001年 | 334篇 |
2000年 | 223篇 |
1999年 | 122篇 |
1998年 | 100篇 |
1997年 | 85篇 |
1996年 | 58篇 |
1995年 | 47篇 |
1994年 | 63篇 |
1993年 | 90篇 |
1992年 | 74篇 |
1991年 | 79篇 |
1990年 | 80篇 |
1989年 | 58篇 |
1988年 | 69篇 |
1987年 | 55篇 |
1986年 | 58篇 |
1985年 | 46篇 |
1984年 | 49篇 |
1983年 | 51篇 |
1982年 | 37篇 |
1981年 | 38篇 |
1980年 | 34篇 |
1979年 | 41篇 |
1978年 | 39篇 |
1976年 | 47篇 |
1975年 | 37篇 |
1973年 | 43篇 |
1972年 | 277篇 |
1971年 | 299篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
In the context of pandemic influenza, the prompt and effective implementation of control measures is of great concern for
public health officials around the world. In particular, the role of vaccination should be considered as part of any pandemic
preparedness plan. The timely production and efficient distribution of pandemic influenza vaccines are important factors to
consider in mitigating the morbidity and mortality impact of an influenza pandemic, particularly for those individuals at
highest risk of developing severe disease. In this paper, we use a mathematical model that incorporates age-structured transmission
dynamics of influenza to evaluate optimal vaccination strategies in the epidemiological context of the Spring 2009 A (H1N1)
pandemic in Mexico. We extend previous work on age-specific vaccination strategies to time-dependent optimal vaccination policies
by solving an optimal control problem with the aim of minimizing the number of infected individuals over the course of a single
pandemic wave. Optimal vaccination policies are computed and analyzed under different vaccination coverages (21%–77%) and
different transmissibility levels (R0\mathcal{R}_{0} in the range of 1.8–3). The results suggest that the optimal vaccination can be achieved by allocating most vaccines to young
adults (20–39 yr) followed by school age children (6–12 yr) when the vaccination coverage does not exceed 30%. For higher
R0\mathcal{R}_{0} levels ($\mathcal{R}_{0}>=2.4$\mathcal{R}_{0}>=2.4), or a time delay in the implementation of vaccination (>90 days), a quick and substantial decrease in the pool of susceptibles
would require the implementation of an intensive vaccination protocol within a shorter period of time. Our results indicate
that optimal age-specific vaccination rates are significantly associated with R0\mathcal{R}_{0}, the amount of vaccines available and the timing of vaccination. 相似文献
972.
Nicla Contran Mariagrazia Tonelli Paolo Crosti Raffaella Cerana Massimo Malerba 《Acta Physiologiae Plantarum》2012,34(2):617-629
Reactive oxygen species (ROS) have pleiotropic effects in plants. ROS can lead to cellular damage and death or play key roles
in control and regulation of biological processes, such as programmed cell death (PCD). This dual role of ROS, as toxic or
signalling molecules, is possible because plant antioxidant system (AS) is able to achieve a tight control over ROS cellular
levels, balancing properly their production and scavenging. AS response in plant PCD has been clearly described only in the
hypersensitive response in incompatible plant–pathogen interactions and in the senescence process and has not been completely
unravelled. In sycamore (Acer pseudoplatanus L.) cultured cells PCD can be induced by Fusicoccin (Fc), Tunicamycin (Tu), and Brefeldin A (Ba). These chemicals induce
comparable PCD time course and extent, while H2O2 production is detectable only in Fc- and, to a lesser extent, in Ba-treated cells. In this paper the AS has been investigated
during PCD of sycamore cells, measuring the effects of the three inducers on the cellular levels of non-enzymatic and enzymatic
antioxidants. Results show that the AS behaviour is different in the PCD induced by the three chemicals. In Fc-treated cells
AS is mainly devoted to decrease the concentration of toxic intracellular H2O2 levels. On the contrary, in cells treated with Tu and Ba, the cell redox state is shifted to a more reduced state and the
enzymatic AS is partially down-regulated, allowing ROS to act as signalling molecules. 相似文献
973.
974.
In the present study, we evaluated whether reproductive condition affects female reproductive behaviour in the induced ovulator
Ctenomys talarum. We also explored the effect of the interaction with a male on the reproductive condition of females. To evaluate this, we
arranged mating trials and evaluated female reproductive behaviour. Reproductive status of females was evaluated using a combined
approach of vaginal smears, urinary progesterone and oestradiol, and ovarian histology. Behaviours denoting attraction (‘male
sniff’ and ‘mount attempts’) and mutual courtship behaviours (‘spin’ and copula) were correlated with vaginal cytology before
and oestradiol and progesterone levels in urine 12 h after male–female encounter. After 24 h of the interaction, oestradiol
levels and vaginal epithelization increased while progesterone levels decreased in soliciting females. C. talarum females’ reproductive behaviour was related to its physiological reproductive state and vaginal cytology. The kind of male
interaction, whether couples copulated or remained indifferent affected the later status of females. Females are induced ovulators
by mating but male presence and interaction also affected other components of their reproductive physiology such as ovarian
hormones and vaginal cytology. 相似文献
975.
The preferred conformations of the glycerol region of a phospholipid have been explored using replica exchange molecular dynamics
(MD) simulations and compared with the results of standard MD approaches and with experiment. We found that due to isomerization
rates in key torsions that are slow on the timescale of atomistic MD simulations, standard MD is not able to produce accurate
equilibrium conformer distributions from reasonable trajectory lengths (e.g., on the 100 ns) timescale. Replica exchange MD,
however, results in quite efficient sampling due to the rapid increase in isomerization rate with temperature. The equilibrium
distributions obtained from replica exchange MD have been compared with the results of experimental nuclear magnetic resonance
observations. This comparison suggests that the sampling approach demonstrated here is a valuable tool that can be used in
evaluating force fields for molecular simulation of lipids. 相似文献
976.
The effectiveness of hybrid vigor, which can counteract deleterious effects of inbreeding in flightless Harmonia axyridis Pallas (Coleoptera: Coccinellidae), was investigated. First, we performed the reciprocal cross between two isofemale lines
and compared survival and reproductive characteristics between isofemale and hybrid lines under laboratory conditions. The
survival of one of the hybrid lines was significantly higher than that of the two isofemale lines. Early fecundity tended
to be higher in the two hybrid lines than in the two isofemale lines. Second, we compared the effectiveness of control of
Aphis gossypii Glover (Homoptera: Aphididae) between isofemale and hybrid lines by release experiments in greenhouses. The number of A. gossypii was suppressed in treatments in which two hybrid lines were released compared with those in which two isofemale lines were
released. These results suggest that hybrid vigor is effective as a method for assuring the quality of flightless H. axyridis. 相似文献
977.
Seasonal resource availability may act as a constraint on plant phenology and thereby influence the range of growth responses
observed among populations of annual species, especially those occupying a wide range of environments. We compared a mesic
and a xeric population of the non-native, annual grass, Bromus tectorum, to examine phenology in response to interspecific competition and water availability. Using a target-neighborhood approach,
we assessed how phenological patterns of the two populations affected morphological and growth responses to enhanced resource
availability represented by late-season soil moisture. The xeric population exhibited a highly constrained phenology and was
unable to extend the growing season despite available soil resources. Because of the low phenotypic variation, allocation
to reproduction was similar across resource conditions. In contrast, the mesic population flowered later and showed a more
opportunistic phenology in response to late-season water availability. The mesic population was not able to maintain consistent
reproductive allocation at low resource levels. The responses of the two populations to late-season water availability were
not affected by the density of neighboring plants. We suggest that post-introduction selection pressure on B. tectorum in the xeric habitat has resulted in a more fixed phenology which limits opportunistic response to unpredictable, particularly
late-season resource availability. Opportunistic and fixed responses represent contrasting strategies for optimizing fitness
in temporally varying environments and, while both play important roles for ensuring reproductive success, these results suggest
that local adaptation to temporal resource variation may reflect a balance between flexible and inflexible phenology. 相似文献
978.
X Liu J Fang M Zhang X Wang W Wang Y Gong X Xi M Li 《World journal of microbiology & biotechnology》2012,28(3):1013-1020
Contamination of Cronobacter spp. (Enterobacter sakazakii) in infant formulas and other food products is a severe problem. Here a loop-mediated isothermal amplification (LAMP) assay
was developed for rapidly detecting Cronobacter spp. in powdered infant formula. Sequences of 16S/23S rDNA internal intergenic spacer of Cronobacter spp. were used as the target template to design LAMP primers. The detection outcome can be evaluated by the white precipitate
or the fluorescence intensity under ultraviolet irradiation, both visible to naked eyes. The sensitivity and specificity of
the LAMP assay was further analyzed in comparison with that of regular PCR and real time quantitative PCR. The results showed
that all of Cronobacter spp. strains display positive reaction to the detections while all of the non-Cronobacter spp. strains were negative, and that the LAMP assay exhibits a high sensitivity of 9.1 fg/μL (The sensitivity of regular
PCR and real time quantitative PCR is 91 and 9.1 pg/μL, respectively.). The amplified reaction could be accomplished in about
1 h, with the results visible to naked eyes. Hence, the LAMP assay developed by this study can provide a rapid and simple
approach for the detection of Cronobacter spp. in infant formula. 相似文献
979.
980.
Liebman KA Stoddard ST Morrison AC Rocha C Minnick S Sihuincha M Russell KL Olson JG Blair PJ Watts DM Kochel T Scott TW 《PLoS neglected tropical diseases》2012,6(2):e1472