全文获取类型
收费全文 | 16560篇 |
免费 | 1422篇 |
国内免费 | 231篇 |
专业分类
18213篇 |
出版年
2023年 | 127篇 |
2022年 | 238篇 |
2021年 | 463篇 |
2020年 | 272篇 |
2019年 | 316篇 |
2018年 | 404篇 |
2017年 | 334篇 |
2016年 | 470篇 |
2015年 | 797篇 |
2014年 | 810篇 |
2013年 | 954篇 |
2012年 | 1210篇 |
2011年 | 1140篇 |
2010年 | 720篇 |
2009年 | 651篇 |
2008年 | 869篇 |
2007年 | 777篇 |
2006年 | 671篇 |
2005年 | 644篇 |
2004年 | 653篇 |
2003年 | 563篇 |
2002年 | 516篇 |
2001年 | 486篇 |
2000年 | 424篇 |
1999年 | 380篇 |
1998年 | 179篇 |
1997年 | 118篇 |
1996年 | 119篇 |
1995年 | 140篇 |
1994年 | 109篇 |
1993年 | 101篇 |
1992年 | 193篇 |
1991年 | 215篇 |
1990年 | 159篇 |
1989年 | 185篇 |
1988年 | 156篇 |
1987年 | 144篇 |
1986年 | 123篇 |
1985年 | 143篇 |
1984年 | 114篇 |
1983年 | 101篇 |
1982年 | 63篇 |
1981年 | 73篇 |
1980年 | 70篇 |
1979年 | 75篇 |
1978年 | 76篇 |
1977年 | 68篇 |
1975年 | 58篇 |
1974年 | 53篇 |
1973年 | 53篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Although tobacco appears highly addictive in humans, there has been persistent controversy about the ability of its psychoactive ingredient nicotine to induce self-administration behavior in laboratory animals, bringing into question nicotine's role in reinforcing tobacco smoking. Because of ethical difficulties in inducing nicotine dependence in na?ve human subjects, we explored reinforcing effects of nicotine in experimentally-naive non-human primates given access to nicotine for periods of time up to two years. Five squirrel monkeys with no experimental history were allowed to intravenously self-administer nicotine by pressing one of two levers. The number of presses on the active lever needed to obtain each injection was fixed (fixed-ratio schedule) or increased progressively with successive injections during the session (progressive-ratio schedule), allowing evaluation of both reinforcing and motivational effects of nicotine under conditions of increasing response cost. Over time, a progressive shift toward high rates of responding on the active lever, but not the inactive lever, developed. The monkeys' behavior was clearly directed toward nicotine self-administration, rather than presentation of environmental stimuli associated with nicotine injection. Both schedules of reinforcement revealed a high motivation to self-administer nicotine, with monkeys continuing to press the lever when up to 600 lever-presses were needed for each injection of nicotine. Thus, nicotine, by itself, in the absence of behavioral or drug-exposure history, is a robust and highly effective reinforcer of drug-taking behavior in a non-human primate model predictive of human behavior. This supports the use of nicotinic ligands for the treatment of smokers, and this novel preclinical model offers opportunities to test future medications for the treatment of nicotine dependence. 相似文献
962.
This article correlates colonization with parameters, such as chemotaxis, biofilm formation, and bacterial growth, that are
believed to be connected. We show here, by using two varieties of soybean plants that seeds axenically produced exudates,
induced a chemotactic response in Bacillus amyloliquefaciens, whereas root exudates did not, even when the exudates, also collected under axenic conditions, were concentrated up to 200-fold.
Root exudates did not support bacterial cell division, whereas seed exudates contain compounds that support active cell division
and high cell biomass at stationary phase. Seed exudates of the two soybean varieties also induced biofilm formation. B. amyloliquefaciens colonized both seeds and roots, and plant variety significantly affected bacterial root colonization, whereas it did not
affect seed colonization. Colonization of roots in B. amyloliquefaciens occurred despite the lack of chemotaxis and growth stimulation by root exudates. The data presented in this article suggest
that soybean seed colonization, but not root colonization, by B. amyloliquefaciens is influenced by chemotaxis, growth, and biofilm formation and that this may be caused by qualitative changes of the composition
of root exudates. 相似文献
963.
Yoke-Kqueen Cheah Noorzaleha Awang Salleh Learn-Han Lee Son Radu Sabrina Sukardi Jiun-Horng Sim 《World journal of microbiology & biotechnology》2008,24(3):327-335
Salmonella enterica subsp. enterica (S.) serovar Weltevreden has emerged as a public health problem in many countries. Genomic DNA of S. Weltevreden from indigenous vegetables namely ‘selom’ (Oenanthe stolonifera), ‘pegaga’ (Centella asiatica), ‘kesum’ (Polygonum minus) and ‘kangkong’ (Ipomoea aquatica) were characterized by duplex-polymerase chain reaction (duplex-PCR), multiplex-polymerase chain reaction (multiplex-PCR),
random amplified polymorphic DNA (RAPD), enterobacterial repetitive intergenic consensus-polymerase chain reaction (ERIC-PCR)
and polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). The results demonstrated that a total of
four clusters and three single isolates were generated from ERIC-PCR with primers ERIC-1 and ERIC-2 whereas RAPD with arbitrary
primers OPAR2, OPAR17 and OPAR19 discriminated the S. Weltevreden into nine clusters and eight single isolates at a common 65% similarity level with discriminatory index (D) of 0.7443 and 0.9394 respectively. Composite analysis of banding profiles generated from RAPD-PCR and ERIC-PCR showed eight
clusters and six single isolates at 65% similarity level with the highest D value that is 0.9508. On the other hand, PCR-RFLP and duplex PCR data exhibited a consistent profile for S. Weltevreden. Multiplex-PCR targeting three different antibiotic resistance genes and a common Salmonella specific gene segment produced two distinguishing profiles among the S. Weltevreden examined. These results demonstrated that the combined analysis of RAPD-PCR and ERIC-PCR is a better tool for
characterizing S. Weltevreden than individual methods. 相似文献
964.
Sylvain Giroud Martine Perret Peter Stein Jo?lle Goudable Fabienne Aujard Caroline Gilbert Jean Patrice Robin Yvon Le Maho Alexandre Zahariev Stéphane Blanc Iman Momken 《PloS one》2010,5(1)
During moderate calorie restriction (CR) the heterotherm Microcebus murinus is able to maintain a stable energy balance whatever the season, even if only wintering animals enter into torpor. To understand its energy saving strategies to respond to food shortages, we assessed protein and energy metabolisms associated with wintering torpor expression or summering torpor avoidance. We investigated body composition, whole body protein turnover, and daily energy expenditure (DEE), during a graded (40 and 80%) 35-day CR in short-days (winter; SD40 and SD80, respectively) and long-days (summer; LD40 and LD80, respectively) acclimated animals. LD40 animals showed no change in fat mass (FM) but a 12% fat free mass (FFM) reduction. Protein balance being positive after CR, the FFM loss was early and rapid. The 25% DEE reduction, in LD40 group was mainly explained by FFM changes. LD80 animals showed a steady body mass loss and were excluded from the CR trial at day 22, reaching a survival-threatened body mass. No data were available for this group. SD40 animals significantly decreased their FM level by 21%, but maintained FFM. Protein sparing was achieved through a 35 and 39% decrease in protein synthesis and catabolism (protein turnover), respectively, overall maintaining nitrogen balance. The 21% reduction in energy requirement was explained by the 30% nitrogen flux drop but also by torpor as DEE FFM-adjusted remained 13% lower compared to ad-libitum. SD80 animals were unable to maintain energy and nitrogen balances, losing both FM and FFM. Thus summering mouse lemurs equilibrate energy balance by a rapid loss of active metabolic mass without using torpor, whereas wintering animals spare protein and energy through increased torpor expression. Both strategies have direct fitness implication: 1) to maintain activities at a lower body size during the mating season and 2) to preserve an optimal wintering muscle mass and function. 相似文献
965.
Arguedas Cortés D Dolz G Romero Zúñiga JJ Jiménez Rocha AE León Alán D 《Revista de biología tropical》2010,58(4):1453-1465
Centrocestusformosanus is a zoonotic trematode from Asia and has been mainly associated as cause of death of cultured fish. To identify pathogen trematode species in tilapia fry (Oreochromis niloticus) and to determine mollusks hosting these parasites, freshwater mollusks were collected from tilapia cultured ponds and experimental infections were carried out with tilapia fries and different mollusk species. A total of 907 freshwater mollusks were obtained from tilapia ponds and were identified to species level, four gastropods and one bivalve were determined: Melania tuberculata, Melanoides turricula, Pomacea flagellata, Haitia cubensis and Anodontiles luteola. For the first time, the presence of M. turricula and H. cubensis are reported in Costa Rica. Seven morphotypes of cercariae (Xifiodiocercaria, Equinostoma, Oftalmocercaria, Parapleurolofocercus, Cistocerca, Furcocercaria and Leptocercaria) parasitizing all five species of mollusks were found, all of distome type. Experimental exposure of tilapia fry to M. tuberculata demonstrated that the parapleurolofocercus morphotype found in the mollusk is in accordance with the finding of C. formosanus in tilapia fry. An abundance and mean intensity of 1018-1027 digeneans per gill in each exposed fish was determined. Centrocestus formosanus is reported for the first time in Costa Rica, for which the primary and secondary intermediate hosts were also determined. 相似文献
966.
Using probe genotypes to dissect QTL × environment interactions for grain yield components in winter wheat 总被引:1,自引:0,他引:1
967.
968.
Tahereh Makiabadi Audrey Bouvrée Victor Le Nader Hélène Terrisse Guy Louarn 《Plasmonics (Norwell, Mass.)》2010,5(1):21-29
Generally, the immobilization of two-dimensional nanoparticles in immersion procedures is time-consuming (over 24 h). In this
paper, we report a very effective and simple method to fabricate two-dimensional gold nanoparticle patterns over large areas
with high regularity for surface-enhanced Raman scattering (SERS). We achieved a highly sensitive SERS colloid surface by
optimizing temperature and immersion time. The surfaces were characterized by X-ray photoelectron spectroscopy, UV–Vis, atomic
force microscopy, and scanning electron microscopy. The SERS activity of surfaces was compared by using two techniques: “dip”
and “dip and dry” in an aqueous solution of 10−6 M crystal violet. The influence of the morphology of the surface was investigated with both the dip and dip and dry techniques. 相似文献
969.
Nam Jin Noh Yowhan Son Jin Woo Koo Kyung Won Seo Rae Hyun Kim Yoon Young Lee Kyung Seun Yoo 《Journal of Plant Biology》2010,53(1):61-69
The nitrogenase activity, root nodule biomass, and rates of nitrogen (N) fixation were measured in 25-year-old pure north-
and south-facing Robinia pseudoacacia stands in an urban forest of Seoul (Kkachisan Mountain) in central Korea. The nitrogenase activity was estimated using an
acetylene reduction (AR) assay, which showed an increasing trend during the early growing season, with sustained high rates
from June through to September with a decrease thereafter. July had the highest nitrogenase activity rate (micromoles C2H4 per gram dry nodule per hour), averaging 95.8 and 115.1 for the north- and south-facing stands, respectively. The maximum
root nodule biomass (kilograms per hectare) was 45.7 and 9.1 for the north- and south-facing stands in July, respectively.
The AR rate appeared to be strongly correlated to the soil temperature (r
2 = 0.68, P < 0.001) and soil pH (r
2 = 0.59, P < 0.001) while root nodule biomass was correlated to the soil temperature (r
2 = 0.36, P < 0.01) and water content (r
2 = 0.35, P < 0.05). The soil temperature showed clear differences between seasons, while there was a significant difference in soil pH,
organic matter, total N concentrations, and available phosphorus between the north- and south-facing stands. The N2 fixation rates during the growing season varied from 0.1 to 37.5 kg N ha−1 month−1 depending on the sampling location and time. The annual N2 fixation rate (kg N per hectare per year) was 112.3 and 23.2 for the north- and south-facing stands, respectively. The differences
in N2 fixation rate between the two stands were due mainly to the differences in total nodule biomass. 相似文献
970.
Le Gouis J Devaux P Werner K Hariri D Bahrman N Béghin D Ordon F 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》2004,108(8):1521-1525
Breeding for resistant cultivars is the only way to prevent high yield loss in barley caused by the soil-borne barley mild mosaic virus (BaMMV) complex. We have characterized the BaMMV resistance of barley cv. Chikurin Ibaraki 1. Doubled haploid lines were obtained from the F1 between the susceptible six-rowed winter barley cultivar, Plaisant, and Chikurin Ibaraki 1. Each line was tested for reaction to BaMMV by mechanical inoculation followed by DAS-ELISA. Of 44 microsatellites that covered the genome, 22 polymorphic markers were tested on one susceptible and one resistant bulk, each comprising 30 lines. Differential markers and additional microsatellite markers in the same region were then tested on the whole population. A bootstrap analysis was used to compute confidence intervals of distances and to test the orders of the resistance gene and the closest markers. A segregation of 84 resistant/98 susceptible lines fitted a 1:1 ratio (2=1.08, P=0.30), which corresponds to a single gene in this DH lines population. The resistance gene was flanked by two markers near the centromeric region of chromosome 6HS—Bmag0173, at 0.6±1.2 cM, and EBmac0874, at 5.8 ± 3.4 cM. We propose to name this new resistance gene rym15. This resistance gene and associated markers will increase the possibilities to breed efficiently for new cultivars resistant to the barley mosaic disease.Communicated by P. Langridge 相似文献