全文获取类型
收费全文 | 2202篇 |
免费 | 162篇 |
国内免费 | 1篇 |
专业分类
2365篇 |
出版年
2022年 | 16篇 |
2021年 | 31篇 |
2020年 | 19篇 |
2019年 | 25篇 |
2018年 | 26篇 |
2017年 | 16篇 |
2016年 | 47篇 |
2015年 | 62篇 |
2014年 | 86篇 |
2013年 | 100篇 |
2012年 | 132篇 |
2011年 | 119篇 |
2010年 | 70篇 |
2009年 | 76篇 |
2008年 | 93篇 |
2007年 | 74篇 |
2006年 | 75篇 |
2005年 | 74篇 |
2004年 | 82篇 |
2003年 | 70篇 |
2002年 | 80篇 |
2001年 | 51篇 |
2000年 | 59篇 |
1999年 | 44篇 |
1998年 | 19篇 |
1997年 | 24篇 |
1996年 | 15篇 |
1995年 | 16篇 |
1994年 | 18篇 |
1993年 | 17篇 |
1992年 | 35篇 |
1991年 | 29篇 |
1990年 | 19篇 |
1989年 | 26篇 |
1988年 | 17篇 |
1987年 | 16篇 |
1986年 | 24篇 |
1985年 | 20篇 |
1984年 | 21篇 |
1979年 | 26篇 |
1978年 | 18篇 |
1977年 | 19篇 |
1975年 | 23篇 |
1974年 | 16篇 |
1973年 | 25篇 |
1972年 | 19篇 |
1971年 | 24篇 |
1970年 | 30篇 |
1968年 | 14篇 |
1967年 | 21篇 |
排序方式: 共有2365条查询结果,搜索用时 0 毫秒
21.
Leilani M. Lehmann Sean E. Walker† & Matthew H. Persons 《Ethology : formerly Zeitschrift fur Tierpsychologie》2004,110(4):323-339
The wolf spider, Pardosa milvina, reduces activity in the presence of chemical cues (silk and excreta) from a larger predatory wolf spider, Hogna helluo. Hogna is sexually dimorphic in body size and males and females differ in their propensity to attack prey. Consequently, each sex may present different levels of risk to Pardosa. We measured predation risk of Pardosa in the presence of male or female Hogna. We also assessed Pardosa antipredator responses and survival in the presence or absence of previously deposited chemical cues from male or female Hogna. In the absence of predator chemical cues, Pardosa survived significantly longer in the presence of male Hogna compared with female Hogna. We then assessed Pardosa survival in the presence of chemical cues from each Hogna sex by placing Pardosa in containers previously occupied by a female Hogna, a male Hogna, or no Hogna (control). We then introduced a female Hogna into each container and measured predation latency. Pardosa survived significantly longer in the presence of female and male cues compared with the control treatment. Median survival time of Pardosa was over four times longer on substrates with female Hogna cues compared with male cues, but this difference was not statistically significant. We tested Pardosa activity levels in the presence of chemical cues from male or female Hogna. Both Hogna sexes were maintained in separate containers after which we placed an adult female Pardosa in one of the containers or a blank control container. Pardosa significantly decreased activity in the presence of chemical cues from either sex relative to the control. Activity was lowest on substrates with female Hogna cues, but not significantly lower than on substrates with male Hogna cues. Results suggest that chemical information from male or female Hogna significantly reduces Pardosa activity which results in increased survival. 相似文献
22.
Ralf W. Dittmann Peter M. Wehmeier Alexander Schacht Martin Lehmann Gerd Lehmkuhl 《Attention deficit and hyperactivity disorders》2009,1(2):187-200
To report on (1) psychometric properties of the Rosenberg Self-Esteem Scale (SES) studied in adolescents with ADHD, (2) correlations of SES with ADHD scale scores, and (3) change in patient-reported self-esteem with atomoxetine treatment. ADHD patients (12–17 years), treated in an open-label study for 24 weeks. Secondary analyses on ADHD symptoms (assessed with ADHD-RS, CGI, GIPD scales) and self-esteem (SES) were performed. One hundred and fifty-nine patients were treated. A dichotomous structure of the SES could be confirmed. Reliability and internal consistency were moderate to excellent. Highest coefficients were found for the correlation between SES and GIPD scores. Self-esteem significantly increased over time, accompanied by an improvement of ADHD symptoms and related perceived difficulties. The Rosenberg SES was shown to be internally consistent, reliable, and sensitive to treatment-related changes of self-esteem. According to these findings, self-esteem may be an important individual patient outcome beyond the core symptoms of ADHD. 相似文献
23.
Julie M. Grossman Brendan E. O’Neill Siu Mui Tsai Biqing Liang Eduardo Neves Johannes Lehmann Janice E. Thies 《Microbial ecology》2010,60(1):192-205
We compared the microbial community composition in soils from the Brazilian Amazon with two contrasting histories; anthrosols
and their adjacent non-anthrosol soils of the same mineralogy. The anthrosols, also known as the Amazonian Dark Earths or
terra preta, were managed by the indigenous pre-Colombian Indians between 500 and 8,700 years before present and are characterized
by unusually high cation exchange capacity, phosphorus (P), and calcium (Ca) contents, and soil carbon pools that contain
a high proportion of incompletely combusted biomass as biochar or black carbon (BC). We sampled paired anthrosol and unmodified
soils from four locations in the Manaus, Brazil, region that differed in their current land use and soil type. Community DNA
was extracted from sampled soils and characterized by use of denaturing gradient gel electrophoresis (DGGE) and terminal restriction
fragment length polymorphism. DNA bands of interest from Bacteria and Archaea DGGE gels were cloned and sequenced. In cluster
analyses of the DNA fingerprints, microbial communities from the anthrosols grouped together regardless of current land use
or soil type and were distinct from those in their respective, paired adjacent soils. For the Archaea, the anthrosol communities
diverged from the adjacent soils by over 90%. A greater overall richness was observed for Bacteria sequences as compared with
those of the Archaea. Most of the sequences obtained were novel and matched those in databases at less than 98% similarity.
Several sequences obtained only from the anthrosols grouped at 93% similarity with the Verrucomicrobia, a genus commonly found in rice paddies in the tropics. Sequences closely related to Proteobacteria and Cyanobacteria sp. were recovered only from adjacent soil samples. Sequences related to Pseudomonas, Acidobacteria, and Flexibacter sp. were recovered from both anthrosols and adjacent soils. The strong similarities among the microbial communities present
in the anthrosols for both the Bacteria and Archaea suggests that the microbial community composition in these soils is controlled
more strongly by their historical soil management than by soil type or current land use. The anthrosols had consistently higher
concentrations of incompletely combusted organic black carbon material (BC), higher soil pH, and higher concentrations of
P and Ca compared to their respective adjacent soils. Such characteristics may help to explain the longevity and distinctiveness
of the anthrosols in the Amazonian landscape and guide us in recreating soils with sustained high fertility in otherwise nutrient-poor
soils in modern times. 相似文献
24.
Vera Jankowski Markus T?lle Thi Nguyet Anh Tran Markus van der Giet Mirjam Schuchardt Kerstin Lehmann Doreen Janke Burkhard Flick Alberto Arduan Ortiz Ni?o Maria Dolores Sanchez Martin Tepel Walter Zidek Joachim Jankowski 《PloS one》2013,8(7)
The secretion of angiogenic factors by vascular endothelial cells is one of the key mechanisms of angiogenesis. Here we report on the isolation of a new potent angiogenic factor, diuridine tetraphosphate (Up4U) from the secretome of human endothelial cells. The angiogenic effect of the endothelial secretome was partially reduced after incubation with alkaline phosphatase and abolished in the presence of suramin. In one fraction, purified to homogeneity by reversed phase and affinity chromatography, Up4U was identified by MALDI-LIFT-fragment-mass-spectrometry, enzymatic cleavage analysis and retention-time comparison. Beside a strong angiogenic effect on the yolk sac membrane and the developing rat embryo itself, Up4U increased the proliferation rate of endothelial cells and, in the presence of PDGF, of vascular smooth muscle cells. Up4U stimulated the migration rate of endothelial cells via P2Y2-receptors, increased the ability of endothelial cells to form capillary-like tubes and acts as a potent inducer of sprouting angiogenesis originating from gel-embedded EC spheroids. Endothelial cells released Up4U after stimulation with shear stress. Mean total plasma Up4U concentrations of healthy subjects (N = 6) were sufficient to induce angiogenic and proliferative effects (1.34±0.26 nmol L-1). In conclusion, Up4U is a novel strong human endothelium-derived angiogenic factor. 相似文献
25.
Véronique Bergougnoux David Zalabák Michaela Jandová Ond?ej Novák Anika Wiese-Klinkenberg Martin Fellner 《PloS one》2012,7(9)
Light is one of the most important factor influencing plant growth and development all through their life cycle. One of the well-known light-regulated processes is de-etiolation, i.e. the switch from skotomorphogenesis to photomorphogenesis. The hormones cytokinins (CKs) play an important role during the establishment of photomorphogenesis as exogenous CKs induced photomorphogenesis of dark-grown seedlings. Most of the studies are conducted on the plant model Arabidopsis, but no or few information are available for important crop species, such as tomato (Solanum lycopersicum L.). In our study, we analyzed for the first time the endogenous CKs content in tomato hypocotyls during skotomorphogenesis, photomorphogenesis and de-etiolation. For this purpose, two tomato genotypes were used: cv. Rutgers (wild-type; WT) and its corresponding mutant (7B-1) affected in its responses to blue light (BL). Using physiological and molecular approaches, we identified that the skotomorphogenesis is characterized by an endoreduplication-mediated cell expansion, which is inhibited upon BL exposure as seen by the accumulation of trancripts encoding CycD3, key regulators of the cell cycle. Our study showed for the first time that iP (isopentenyladenine) is the CK accumulated in the tomato hypocotyl upon BL exposure, suggesting its specific role in photomorphogenesis. This result was supported by physiological experiments and gene expression data. We propose a common model to explain the role and the relationship between CKs, namely iP, and endoreduplication during de-etiolation and photomorphogenesis. 相似文献
26.
Helen Chung Ming Ye Chris Hanson Oluwaseun Oladokun Michael J. Campbell Gordon Kramer Ordan J. Lehmann 《PloS one》2012,7(11)
Background
It is widely recognised that significant discrepancies exist between the health of indigenous and non-indigenous populations. Whilst the reasons are incompletely defined, one potential cause is that indigenous communities do not access healthcare to the same extent. We investigated healthcare utilisation rates in the Canadian Aboriginal population to elucidate the contribution of this fundamental social determinant for health to such disparities.Methods
Healthcare utilisation data over a nine-year period were analysed for a cohort of nearly two million individuals to determine the rates at which Aboriginal and non-Aboriginal populations utilised two specialties (Cardiology and Ophthalmology) in Alberta, Canada. Unadjusted and adjusted healthcare utilisation rates obtained by mixed linear and Poisson regressions, respectively, were compared amongst three population groups - federally registered Aboriginals, individuals receiving welfare, and other Albertans.Results
Healthcare utilisation rates for Aboriginals were substantially lower than those of non-Aboriginals and welfare recipients at each time point and subspecialty studied [e.g. During 2005/06, unadjusted Cardiology utilisation rates were 0.28% (Aboriginal, n = 97,080), 0.93% (non-Aboriginal, n = 1,720,041) and 1.37% (Welfare, n = 52,514), p = <0.001]. The age distribution of the Aboriginal population was markedly different [2.7%≥65 years of age, non-Aboriginal 10.7%], and comparable utilisation rates were obtained after adjustment for fiscal year and estimated life expectancy [Cardiology: Incidence Rate Ratio 0.66, Ophthalmology: IRR 0.85].Discussion
The analysis revealed that Aboriginal people utilised subspecialty healthcare at a consistently lower rate than either comparatively economically disadvantaged groups or the general population. Notably, the differences were relatively invariant between the major provincial centres and over a nine year period. Addressing the causes of these discrepancies is essential for reducing marked health disparities, and so improving the health of Aboriginal people. 相似文献27.
28.
29.
Heiner Lehmann 《Plant biology (Stuttgart, Germany)》1971,84(10):615-627
In wachsenden Kulturen der blaugrünen Alge Microcystis aeruginosa nimmt die Anzahl der Gasvakuolen pro Zelle ab und erreicht bei einer Zellzahl von 1,2 × 107 pro ml eine Zahl von 4,5 × 103 Vakuolen pro Zelle. Die Gasvakuolen können in den Zellen durch Ultraschall zerstört werden, danach regenerieren sie. In beschallten Kulturen wurde die Zeitabhängigkeit der Gasvakuolenbildung untersucht, indem die Anzahl und die Größe der neugebildeten Vakuolen bestimmt wurde. Innerhalb der ersten halben Stunde nach Ultraschallbehandlung werden 120 Gasvakuolen pro Zelle gebildet, nach 24 Stunden enthält jede Zelle durchschnittlich 2300 Vakuolen. Die Größe der Gasvakuolen nimmt ebenfalls mit der Zeit zu. In der ersten halben Stunde erreichen die bis dahin isodiametrischen Vakuolen eine Größe von 70 nm, dann wachsen sie nur noch in die Länge und haben 24 Stunden nach Ultraschallbehandlung eine durchschnittliche Länge von 286 nm. Rifampin und Chloramphenicol bewirken eine teilweise Hemmung der Gasvakuolenbildung. DCMU und Hydroxylamin blockieren die Vakuolenbildung völlig. Auch im Dunkeln werden keine Gasvakuolen gebildet 相似文献
30.
Christiane Fueldner Anja Mittag Jens Knauer Maria Biskop Pierre Hepp Roger Scholz Ulf Wagner Ulrich Sack Frank Emmrich Attila Tárnok Joerg Lehmann 《Arthritis research & therapy》2012,14(1):R8-9