全文获取类型
收费全文 | 59608篇 |
免费 | 5706篇 |
国内免费 | 22篇 |
专业分类
65336篇 |
出版年
2023年 | 244篇 |
2022年 | 505篇 |
2021年 | 1009篇 |
2020年 | 633篇 |
2019年 | 808篇 |
2018年 | 993篇 |
2017年 | 868篇 |
2016年 | 1479篇 |
2015年 | 2447篇 |
2014年 | 2710篇 |
2013年 | 3162篇 |
2012年 | 4216篇 |
2011年 | 4078篇 |
2010年 | 2582篇 |
2009年 | 2347篇 |
2008年 | 3387篇 |
2007年 | 3438篇 |
2006年 | 3266篇 |
2005年 | 3096篇 |
2004年 | 3033篇 |
2003年 | 2774篇 |
2002年 | 2706篇 |
2001年 | 872篇 |
2000年 | 715篇 |
1999年 | 816篇 |
1998年 | 854篇 |
1997年 | 591篇 |
1996年 | 513篇 |
1995年 | 471篇 |
1994年 | 501篇 |
1993年 | 494篇 |
1992年 | 625篇 |
1991年 | 501篇 |
1990年 | 487篇 |
1989年 | 490篇 |
1988年 | 441篇 |
1987年 | 409篇 |
1986年 | 403篇 |
1985年 | 378篇 |
1984年 | 435篇 |
1983年 | 399篇 |
1982年 | 419篇 |
1981年 | 383篇 |
1980年 | 395篇 |
1979年 | 312篇 |
1978年 | 299篇 |
1977年 | 264篇 |
1976年 | 258篇 |
1974年 | 234篇 |
1973年 | 209篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
Both neonatal humans and mice are exquisitely susceptible to severe HSV infection. We have now documented a profound defect in the ability of neonatal C57BL/6 mice to produce anti-HSV ADCC antibody. This ability is acquired over the first 2 to 4 wk of life. Reconstitution of neonatal mice by i.p. injection of peritoneal cells from adult nonimmune syngeneic mice both affords dose-dependent protection against lethal HSV infection and reconstitutes the antibody-production defect. By cell-separation techniques (adherence, nylon wool column purification, B cell panning) and cell ablation techniques (silica treatment, irradiation, anti-T cell, anti-Ia, anti-Lyt-1.2 and anti-Lyt-2.2 monoclonal antibodies plus complement treatment) the subpopulations involved in the antibody production reconstitution of neonatal mice by adult cells were identified. These include both an Ia+, radioresistant, adherent, silica-sensitive macrophage population and a nylon wool column-purified, radiosensitive, anti-T, anti-Lyt-1.2-sensitive helper T cell population. The latter cell may be substituted for by concanavalin A-stimulated lymphokine-containing spleen cell supernatants or human recombinant IL 2. In addition to reconstitution of ADCC antibody production, the same cell populations, or cells plus lymphokine-containing supernatants or IL 2, protected the newborn mice from lethal HSV infection. Further characterization of this system and of soluble replacement factors has implications for therapy or immunoprophylaxis of human neonates with, or at risk of, HSV infection. 相似文献
62.
63.
We are investigating the mechanisms for producing or avoiding errors during DNA synthesis catalyzed by DNA replication and repair proteins purified from eukaryotic sources. Using assays that monitor the fidelity of a single round of DNA synthesis in vitro, we have defined the error frequency and mutational specificity of the four classes of animal cell DNA polymerases (alpha, beta, delta, gamma), and the fidelity of an SV40 origin-dependent DNA replication complex in extracts of HeLa cells. 相似文献
64.
William S. Bowie Thomas K. Hunt Hubert A. Allen Jr 《The Western journal of medicine》1988,149(2):172-177
This study was presented in part at the annual meeting of the Wilderness Medical Society at Aspen, Colorado, in August 1986 and at “Mountain Medicine 1987,” Leavenworth, Washington, in November 1987.We questioned 220 injured rock climbers or their partners seen consecutively at the Yosemite (California) Medical Clinic over 3 ½ years regarding details of their accidents. Injury type and location were extracted from medical records and severity quantified. The National Park Service rescued 27% of the climbers. Injured climbers were characteristically male (88%) and experienced (mean 5.9 years) and typically fell while leading climbs (66%). Among 451 injuries, 50% were to the skin or subcutaneous tissues, while 28% involved the lower extremity and were predominantly fractures. In terms of each climber''s most severe injury (n = 220), 45% involved the lower extremities (30% from the ankle alone). Head injury or hypothermia caused 12 of 13 fatalities, showing the lowest case-fatality rate reported to date among injured climbers (6%). Rescue personnel successfully managed airways in victims of head injuries, anticipated and treated complications of hypothermia, and stabilized fractures. Victims requiring immediate extensive surgical intervention or blood transfusion usually died before rescue could be effected. 相似文献
65.
Steven I. Higgins Robert B. O’Hara Olga Bykova Michael D. Cramer Isabelle Chuine Eva‐Maria Gerstner Thomas Hickler Xavier Morin Michael R. Kearney Guy F. Midgley Simon Scheiter 《Journal of Biogeography》2012,39(12):2132-2145
Aim To develop a physiologically based model of the plant niche for use in species distribution modelling. Location Europe. Methods We link the Thornley transport resistance (TTR) model with functions which describe how the TTR’s model parameters are influenced by abiotic environmental factors. The TTR model considers how carbon and nutrient uptake, and the allocation of these assimilates, influence growth. We use indirect statistical methods to estimate the model parameters from a high resolution data set on tree distribution for 22 European tree species. Results We infer, from distribution data and abiotic forcing data, the physiological niche dimensions of 22 European tree species. We found that the model fits were reasonable (AUC: 0.79–0.964). The projected distributions were characterized by a false positive rate of 0.19 and a false negative rate 0.12. The fitted models are used to generate projections of the environmental factors that limit the range boundaries of the study species. Main conclusions We show that physiological models can be used to derive physiological niche dimensions from species distribution data. Future work should focus on including prior information on physiological rates into the parameter estimation process. Application of the TTR model to species distribution modelling suggests new avenues for establishing explicit links between distribution and physiology, and for generating hypotheses about how ecophysiological processes influence the distribution of plants. 相似文献
66.
67.
Thomas Bell Michael B. Bonsall Angus Buckling Andrew S. Whiteley Timothy Goodall Robert I. Griffiths 《Biology letters》2010,6(5):639-642
Productivity and predation are thought to be crucial drivers of bacterial diversity. We tested how the productivity–diversity of a natural bacterial community is modified by the presence of protist predators with different feeding preferences. In the absence of predators, there was a unimodal relationship between bacterial diversity and productivity. We found that three protist species (Bodo, Spumella and Cyclidium) had widely divergent effects on bacterial diversity across the productivity gradient. Bodo and Cyclidium had little effect on the shape of the productivity–diversity gradient, while Spumella flattened the relationship. We explain these results in terms of the feeding preferences of these predators. 相似文献
68.
Mutations in Aprataxin cause the neurodegenerative syndrome ataxia oculomotor apraxia type 1. Aprataxin catalyzes removal of adenosine monophosphate (AMP) from the 5′ end of a DNA strand, which results from an aborted attempt to ligate a strand break containing a damaged end. To gain insight into which DNA lesions are substrates for Aprataxin action in vivo, we deleted the Saccharomyces cerevisiae HNT3 gene, which encodes the Aprataxin homolog, in combination with known DNA repair genes. While hnt3Δ single mutants were not sensitive to DNA damaging agents, loss of HNT3 caused synergistic sensitivity to H2O2 in backgrounds that accumulate strand breaks with blocked termini, including apn1Δ apn2Δ tpp1Δ and ntg1Δ ntg2Δ ogg1Δ. Loss of HNT3 in rad27Δ cells, which are deficient in long-patch base excision repair (LP-BER), resulted in synergistic sensitivity to H2O2 and MMS, indicating that Hnt3 and LP-BER provide parallel pathways for processing 5′ AMPs. Loss of HNT3 also increased the sister chromatid exchange frequency. Surprisingly, HNT3 deletion partially rescued H2O2 sensitivity in recombination-deficient rad51Δ and rad52Δ cells, suggesting that Hnt3 promotes formation of a repair intermediate that is resolved by recombination. 相似文献
69.
70.
Chromosome numbers were determined for 86 Anthurium species. Fifty-one of these were newly determined with counts ranging from 2n = 24 to 66 and 30 being the most common. All known Anthurium chromosome numbers were summarized, and 43 taxonomic changes were made in the previous reports to reflect current taxonomy. In terms of somatic chromosome numbers, the numbers form four polyploid series of 20–40–60, 24–30–48–84, 28–56 and 30–60–90–ca. 124. Paleoaneuploidy, polyploidy and B-chromosomes are basic features of the genus, but subsequent recent aneuploidy is not. The exact nature of chromosome evolution in Anthurium remains to be elucidated. 相似文献