全文获取类型
收费全文 | 3346篇 |
免费 | 319篇 |
国内免费 | 2篇 |
专业分类
3667篇 |
出版年
2022年 | 34篇 |
2021年 | 66篇 |
2020年 | 40篇 |
2019年 | 33篇 |
2018年 | 55篇 |
2017年 | 42篇 |
2016年 | 90篇 |
2015年 | 152篇 |
2014年 | 148篇 |
2013年 | 182篇 |
2012年 | 273篇 |
2011年 | 223篇 |
2010年 | 134篇 |
2009年 | 140篇 |
2008年 | 188篇 |
2007年 | 184篇 |
2006年 | 180篇 |
2005年 | 175篇 |
2004年 | 144篇 |
2003年 | 135篇 |
2002年 | 150篇 |
2001年 | 69篇 |
2000年 | 61篇 |
1999年 | 45篇 |
1998年 | 50篇 |
1997年 | 37篇 |
1996年 | 32篇 |
1995年 | 23篇 |
1994年 | 30篇 |
1993年 | 35篇 |
1992年 | 43篇 |
1991年 | 38篇 |
1990年 | 35篇 |
1989年 | 20篇 |
1988年 | 23篇 |
1987年 | 23篇 |
1986年 | 19篇 |
1985年 | 17篇 |
1984年 | 22篇 |
1983年 | 18篇 |
1981年 | 13篇 |
1980年 | 10篇 |
1979年 | 14篇 |
1978年 | 19篇 |
1976年 | 19篇 |
1975年 | 15篇 |
1974年 | 22篇 |
1972年 | 15篇 |
1971年 | 17篇 |
1969年 | 11篇 |
排序方式: 共有3667条查询结果,搜索用时 12 毫秒
81.
82.
Sophie Sanchez Vincent Dupret Paul Tafforeau Katherine M. Trinajstic Bettina Ryll Pierre-Jean Gouttenoire Lovisa Wretman Louise Zylberberg Fran?oise Peyrin Per E. Ahlberg 《PloS one》2013,8(2)
Background
Firm attachments binding muscles to skeleton are crucial mechanical components of the vertebrate body. These attachments (entheses) are complex three-dimensional structures, containing distinctive arrangements of cells and fibre systems embedded in the bone, which can be modified during ontogeny. Until recently it has only been possible to obtain 2D surface and thin section images of entheses, leaving their 3D histology largely unstudied except by extrapolation from 2D data. Entheses are frequently preserved in fossil bones, but sectioning is inappropriate for rare or unique fossil material.Methodology/Principal Findings
Here we present the first non-destructive 3D investigation, by propagation phase contrast synchrotron microtomography (PPC-SRµCT), of enthesis histology in extant and fossil vertebrates. We are able to identify entheses in the humerus of the salamander Desmognathus from the organization of bone-cell lacunae and extrinsic fibres. Statistical analysis of the lacunae differentiates types of attachments, and the orientation of the fibres, reflect the approximate alignment of the muscle. Similar histological structures, including ontogenetically related pattern changes, are perfectly preserved in two 380 million year old fossil vertebrates, the placoderm Compagopiscis croucheri and the sarcopterygian fish Eusthenopteron foordi.Conclusions/Significance
We are able to determine the position of entheses in fossil vertebrates, the approximate orientation of the attached muscles, and aspects of their ontogenetic histories, from PPC-SRµCT data. Sub-micron microtomography thus provides a powerful tool for studying the structure, development, evolution and palaeobiology of muscle attachments. 相似文献83.
Clemens Kittinger Michaela Lipp Bettina Folli Alexander Kirschner Rita Baumert Herbert Galler Andrea J. Grisold Josefa Luxner Melanie Weissenbacher Andreas H. Farnleitner Gernot Zarfel 《PloS one》2016,11(11)
In a clinical setting it seems to be normal these days that a relevant proportion or even the majority of different bacterial species has already one or more acquired antibiotic resistances. Unfortunately, the overuse of antibiotics for livestock breeding and medicine has also altered the wild-type resistance profiles of many bacterial species in different environmental settings. As a matter of fact, getting in contact with resistant bacteria is no longer restricted to hospitals. Beside food and food production, the aquatic environment might also play an important role as reservoir and carrier. The aim of this study was the assessment of the resistance patterns of Escherichia coli and Klebsiella spp. out of surface water without prior enrichment and under non-selective culture conditions (for antibiotic resistance). In addition, the presence of clinically important extended spectrum beta lactamase (ESBL) and carbapenmase harboring Enterobacteriaceae should be investigated. During Joint Danube Survey 3 (2013), water samples were taken over the total course of the River Danube. Resistance testing was performed for 21 different antibiotics. Samples were additionally screened for ESBL or carbapenmase harboring Enterobacteriaceae. 39% of all isolated Escherichia coli and 15% of all Klebsiella spp. from the river Danube had at least one acquired resistance. Resistance was found against all tested antibiotics except tigecycline. Taking a look on the whole stretch of the River Danube the proportion of multiresistances did not differ significantly. In total, 35 ESBL harboring Enterobacteriaceae, 17 Escherichia coli, 13 Klebsiella pneumoniae and five Enterobacter spp. were isolated. One Klebsiella pneumoniae harboring NMD-1 carbapenmases and two Enterobacteriaceae with KPC-2 could be identified. Human generated antibiotic resistance is very common in E. coli and Klebsiella spp. in the River Danube. Even isolates with resistance patterns normally associated with intensive care units are present. 相似文献
84.
85.
86.
Willmar L. Leiser Marcus O. Olatoye H. Frederick W. Rattunde Günter Neumann Eva Weltzien Bettina I. G. Haussmann 《Plant and Soil》2016,409(1-2):51-64
Background and aims
Herbaspirillum seropedicae (Hs) Z67 a diazotrophic endophyte was genetically engineered for secretion of 2-keto-D-gluconic acid by heterologous expression of genes for pqq synthesis and gluconate dehydrogenase to study its beneficial effect on plants.Methods
Two plasmids, pJNK5, containing a 5.1 Kb pqq gene cluster of Acinetobacter calcoaceticus and pJNK6, carrying in addition the Pseudomonas putida KT2440 gluconate dehydrogenase (gad) operon were constructed in pUCPM18Gmr under Plac promoter. H. seropedicae Z67 transformants were monitored for P and K solubilization, cadmium (Cd) tolerance and rice growth promotion.Results
Hs (pJNK5) secreted 23.5 mM gluconic acid and Hs (pJNK6) secreted 3.79 mM gluconic acid and 15.8 mM 2-ketogluconic acid respectively. Under aerobic conditions, Hs (pJNK5) and Hs (pJNK6) solubilized 239.7 μM and 457.7 μM P on HEPES rock phosphate and, 76.7 μM and 222.7 μM K on HRPF (feldspar), respectively, in minimal medium containing 50 mM glucose. Under N free minimal medium, similar effects of P and K solubilization were obtained. Hs (pJNK5) and Hs (pJNK6) inoculation increased the biomass, N, P, K content of rice plants (Gujarat – 17). These plants also accumulated 0.73 ng/g PQQ, and had improved growth and tolerance to CdCl2.Conclusions
Incorporation of pqq and gad gene clusters in H. seropedicae Z67 imparted additional plant growth promoting traits of P and K solubilization and ability to alleviate Cd toxicity to the host plant.87.
88.
89.
Bertrand Brandt Munoz-Garay Carlos 《International journal of peptide research and therapeutics》2019,25(4):1441-1450
International Journal of Peptide Research and Therapeutics - Multidrug resistant pathogenic microorganisms have created a global health crisis. This resistance emerged due to the improper use of... 相似文献
90.
Sara García‐Condado Raúl Lpez‐Lozano Lorenzo Panarello Iacopo Cerrani Luigi Nisini Antonio Zucchini Marijn Van der Velde Bettina Baruth 《Global Change Biology Bioenergy》2019,11(6):809-831
This study assesses crop residues in the EU from major crops using empirical models to predict crop residues from yield statistics; furthermore it analyses the inter‐annual variability of those estimates over the period 1998‐2015, identifying its main drivers across Europe. The models were constructed based on an exhaustive collection of experimental data from scientific papers for the crops: wheat, barley, rye, oats, triticale, rice, maize, sorghum, rapeseed, sunflower, soybean, potato and sugarbeet. We discuss the assumptions on the relationship between yield and the harvest index, adopted by previous studies, to interpret the experimental data, quantify the uncertainties of these models, and establish the premises to implement them at regional scale –i.e., NUTS level 3– within the EU. To cope this, we created a consolidated sub‐national statistical data along with an algorithm able to aggregate (figures are provided at country level) and disaggregate (production at 25 km grid is provided assupplementary material) estimates. The total lignocellulosic biomass production in the EU28 over the review period, according to our models, is 419 Mt, from which wheat is the major contributor (155 Mt). Our results show that maize and rapeseed are the two crops with the highest residue yield, respectively 8.9 and 8.6 t ha‐1. The spatial analysis revealed that these three crops, which, according to our results, are feedstocks highly suitable a priori for second generation biofuels in the EU and are unevenly distributed across Europe. Weather fluctuation was identified as the major driver in residue production from cereals, while, in the case of starch crops and oilseeds – which are predominant in northern Europe – corresponded to the marked production trend likely influenced by the agricultural policies and agro‐management over the review period. Our results, among others, could help to understand and quantify the ecological boundaries of the bioeconomy from agriculture. 相似文献