全文获取类型
收费全文 | 3148篇 |
免费 | 283篇 |
国内免费 | 1篇 |
出版年
2022年 | 37篇 |
2021年 | 59篇 |
2020年 | 50篇 |
2019年 | 58篇 |
2018年 | 74篇 |
2017年 | 72篇 |
2016年 | 110篇 |
2015年 | 127篇 |
2014年 | 143篇 |
2013年 | 175篇 |
2012年 | 202篇 |
2011年 | 168篇 |
2010年 | 137篇 |
2009年 | 140篇 |
2008年 | 157篇 |
2007年 | 181篇 |
2006年 | 167篇 |
2005年 | 138篇 |
2004年 | 157篇 |
2003年 | 133篇 |
2002年 | 134篇 |
2001年 | 66篇 |
2000年 | 62篇 |
1999年 | 54篇 |
1998年 | 43篇 |
1997年 | 30篇 |
1996年 | 30篇 |
1995年 | 25篇 |
1994年 | 26篇 |
1993年 | 28篇 |
1992年 | 41篇 |
1991年 | 43篇 |
1990年 | 22篇 |
1989年 | 35篇 |
1988年 | 21篇 |
1987年 | 20篇 |
1986年 | 29篇 |
1985年 | 15篇 |
1984年 | 14篇 |
1983年 | 18篇 |
1982年 | 16篇 |
1981年 | 16篇 |
1980年 | 13篇 |
1979年 | 18篇 |
1978年 | 10篇 |
1977年 | 10篇 |
1973年 | 8篇 |
1972年 | 12篇 |
1971年 | 8篇 |
1967年 | 8篇 |
排序方式: 共有3432条查询结果,搜索用时 15 毫秒
1.
2.
Allopatric isolation in glacial refugia has caused differentiation and speciation in many taxa globally. In this study, we investigated the nuclear and mitochondrial genetic differentiation of the long fingered bat, Myotis capaccinii during the ice ages in south-eastern Europe and Anatolia. The mitochondrial DNA (mtDNA) analyses indicated a suture zone similar to those recorded in other animal species, including bats, suggesting the association of more than one refugium with the region. Contrary to most of the other species where a suture zone was seen in Anatolia, for M. capaccinii the geographical location of the genetic break was in south-eastern Europe. This mitochondrial differentiation was not reflected in the nuclear microsatellites, however, suggesting that the lack of contact during the ice ages did not result in reproductive isolation. Hence taxonomically, the two mitochondrial clades cannot be treated as separate species. 相似文献
3.
4.
5.
Marcia Divoll David J. Greenblatt 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1981,222(1):125-128
A simple and sensitive high-performance liquid chromatographic assay of methotrexate (MTX) and its two active metabolites, 7-hydroxymethotrexate (7-OH-MTX) and 2,4-di-amino-N10-methylpteroic acid (APA) in plasma, saliva and urine was developed. The method involved deproteinization with acetonitrile followed by addition of isoamyl alcohol and ethyl acetate. After extraction the sample was chromatographed on a cation-exchange column and monitored at 313 nm. The retention times were 5, 7 and 9 min and detection limits 20, 10 and 5 ng/ml for 7-OH-MTX, MTX and APA, respectively. For concentrations greater than 100 ng/ml one-step deproteinization of 0.1 ml sample with 0.25 ml acetonitrile was satisfactory for sample preparation. The method has been evaluated in samples from patients and rabbits receiving MTX. 相似文献
6.
D Hernández S Guerrero M Morales 《Comparative biochemistry and physiology. A, Comparative physiology》1987,87(3):649-656
1. The cardiac pacemaker cells of the frog Caudiverbera caudiverbera are centrally located in the sinus venosus. These cells are rounded, smaller than contractile fibres and have large nuclei. 2. Intracellular recording confirmed the existence of primary and transitional pacemaker cells. 3. Action potentials from primary cells were resistant to blockade by tetrodotoxin (TTX), but were abolished by verapamil suggesting that their bioelectric activity is dependent on a slow inward current. 4. Transitional cells appeared to have two different inward currents contributing to the upstroke: a fast TTX-sensitive and a slow verapamil-sensitive current. 相似文献
7.
Christian Alvarez Glaucia Gon?alves Barbosa Raquel de Vasconcellos Carvalhaes de Oliveira Bernardina Penarrieta Morales Bodo Wanke Márcia dos Santos Lazéra 《Memórias do Instituto Oswaldo Cruz》2013,108(1):126-129
In this study, we evaluated several techniques for the detection of the yeast form of Cryptococcus in decaying wood and measured the viability of these fungi in environmental samples stored in the laboratory. Samples were collected from a tree known to be positive for Cryptococcus and were each inoculated on 10 Niger seed agar (NSA) plates. The conventional technique (CT) yielded a greater number of positive samples and indicated a higher fungal density [in colony forming units per gram of wood (CFU.g-1) ] compared to the humid swab technique (ST). However, the difference in positive and false negative results between the CT-ST was not significant. The threshold of detection for the CT was 0.05.103 CFU.g-1, while the threshold for the ST was greater than 0.1.103 CFU-1. No colonies were recovered using the dry swab technique. We also determined the viability of Cryptococcus in wood samples stored for 45 days at 25ºC using the CT and ST and found that samples not only continued to yield a positive response, but also exhibited an increase in CFU.g-1, suggesting that Cryptococcus is able to grow in stored environmental samples. The ST.1, in which samples collected with swabs were immediately plated on NSA medium, was more efficient and less laborious than either the CT or ST and required approximately 10 min to perform; however, additional studies are needed to validate this technique. 相似文献
8.
9.
The frequency distribution of female Haemonchus contortus bahiensis Grisi, 1974 in sheep from Venezuelan arid zones is 15.32% for the type with vulvar flap, 51.6% for the vulvar-knob and 33.07% for smooth type. A Shannon-Weaver diversity index corresponding to 1.44 bits was calculated for these forms with similarities in the general size, egg-size and in the number of the longitudinal cuticular ridges. An aggregated kind of distribution in the host population according to the K parameter of a negative binomial distribution was recorded for male and female worms. A complicated interaction was observed between the abundance, aggregation and prevalence of this ecotype and the importance of these findings is discussed with regard to host-parasite relationships. 相似文献
10.
Indole compounds secreted byFrankia sp. HFPArI3 in defined culture medium were identified with gas chromatography-mass spectrometry (GC-MS). WhenFrankia was grown in the presence of13C(ring-labelled)-L-tryptophan,13C-labelled indole-3-acetic acid (IAA), indole-3-ethanol (IEtOH), indole-3-lactic acid (ILA), and indole-3-methanol (IMeOH) were identified.High performance liquid chromatography (HPLC) and GC-MS with selected ion monitoring were used to quantify levels of IAA and IEtOH inFrankia culture medium. IEtOH was present in greater abundance than IAA in every experiment. When no exogenous trp was supplied, no or only low levels of indole compounds were detected.Seedling roots ofAlnus rubra incubated in axenic conditions in the presence of indole-3-ethanol formed more lateral roots than untreated plants, indicating that IEtOH is utilized by the host plant, with physiological effects that modify patterns of root primordium initiation. 相似文献