全文获取类型
收费全文 | 436740篇 |
免费 | 45412篇 |
国内免费 | 206篇 |
出版年
2018年 | 3850篇 |
2016年 | 4912篇 |
2015年 | 6722篇 |
2014年 | 7788篇 |
2013年 | 11646篇 |
2012年 | 12793篇 |
2011年 | 13233篇 |
2010年 | 8936篇 |
2009年 | 8205篇 |
2008年 | 11690篇 |
2007年 | 12373篇 |
2006年 | 11618篇 |
2005年 | 11167篇 |
2004年 | 11186篇 |
2003年 | 10881篇 |
2002年 | 10695篇 |
2001年 | 17513篇 |
2000年 | 17510篇 |
1999年 | 14177篇 |
1998年 | 5050篇 |
1997年 | 5278篇 |
1996年 | 4986篇 |
1995年 | 4865篇 |
1994年 | 4763篇 |
1993年 | 4777篇 |
1992年 | 12396篇 |
1991年 | 12198篇 |
1990年 | 12184篇 |
1989年 | 11985篇 |
1988年 | 11295篇 |
1987年 | 10615篇 |
1986年 | 9920篇 |
1985年 | 10361篇 |
1984年 | 8564篇 |
1983年 | 7397篇 |
1982年 | 5778篇 |
1981年 | 5188篇 |
1980年 | 4821篇 |
1979年 | 8184篇 |
1978年 | 6420篇 |
1977年 | 6028篇 |
1976年 | 5762篇 |
1975年 | 6214篇 |
1974年 | 6779篇 |
1973年 | 6641篇 |
1972年 | 6179篇 |
1971年 | 5591篇 |
1970年 | 4818篇 |
1969年 | 4826篇 |
1968年 | 4501篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
I. G. Macintyre K. Rützler J. N. Norris K. P. Smith S. D. Cairns K. E. Bucher R. S. Steneck 《Coral reefs (Online)》1991,10(3):167-174
Submersible observations and collections reveal that a probable relict reef off the west coast of Barbados has a rich cover of sponges, along with algae and scattered corals, on a substrate of algal nodules in a muddy-sand matrix. The collections provide new data on the distributions of these fauna. This relict reef is about 20 km long, has a relief of up to 10 m, and is established at a depth of 80 m. Relict shallow-water features in other areas at similar depths along with data from core holes drilled off the south coast of Barbados suggest that this reef was probably established about 12,000 years ago and existed for no more than 2,000 years, during the Holocene sea-level transgression. 相似文献
102.
103.
104.
105.
Low-level inversion of the L component of pseudorabies virus is not dependent on sequence homology. 下载免费PDF全文
G F Rall S Kupershmidt X Q Lu T C Mettenleiter T Ben-Porat 《Journal of virology》1991,65(12):7016-7019
Pseudorabies virus has a class 2 genome in which the S component is found in two orientations relative to the L component. The L component is bracketed by sequences that are partially homologous; it is found mainly in one orientation, but a small proportion is inverted (J. M. DeMarchi, Z. Lu, G. Rall, S. Kuperschmidt, and T. Ben-Porat, J. Virol. 64:4968-4977, 1990). We have ascertained the role of the patchy homologous sequences bracketing the L component in its inversion. A viral mutant, vYa, from which the sequences at the right end of the L component were deleted was constructed. Despite the absence of homologous sequences bracketing the L component in vYa, its L component inverted to an extent similar to that of the L component in the wild-type virus. These results show the following. (i) The low-frequency inversion of the L component of PrV is not mediated by homologous sequences bracketing this component. (ii) Cleavage of concatemeric DNA at the internal junction between the S and L components is responsible for the appearance of the minority of genomes with an inverted L component in populations of pseudorabies virus. (iii) The signals present near or at the end of the S component are sufficient to allow low-frequency cleavage of concatemeric DNA; the sequences at the end of the L component are not essential for cleavage, although they enhance it considerably. 相似文献
106.
107.
The occurrence of microfloral components on medicated and non-medicated swine and chicken feed pellets and crumbles, produced in a Manitoba feed mill between June 1991 and October 1992, was determined. Addition of medicates to feeds generally decreased bacterial incidence and increased that of Eurotium spp. fungi. The effect was less pronounced when copper sulphate was added to medicated swine feeds.Contribution No. 1662. 相似文献
108.
Intrathalline and intracellular chlorophyll concentrations togetherwith thallus structure were studied in 12 species of the lichenfamily Umbilicariaceae in order to explore pigment limitationsfor light absorption and for maximum rates of net photosynthesis(Amax). Species from high light environments tended to haverelatively low chlorophyll concentrations and vice versa. Chlorophyllconcentration on a surface area basis of all species studiedwas sufficient to absorb 8099% of the incident light,which led us to believe that chlorophyll concentration was nota limiting factor for Amax. Amaxof Umbilicariaceae was smallerthan Amaxof leaves of vascular plants, both on a dry weightand on a surface area basis. These differences slightly decreasedbut did not disappear when referred to the chlorophyll concentrationof the photosynthetic tissues, but reference units had a confoundingeffect in the comparisons. Differences in Amaxon a dry weightbasis between Umbilicariaceae and vascular plants disappearedwhen comparisons were made with the whole plant and not simplywith the leaf. We found a significant, inverse relationshipbetween amount of alga and chlorophyll concentration withinthe algal cells in the lichens studied. This was interpretedas a mechanism to avoid pigment self-shading within the algallayer, distributing more efficiently the photosynthetic pigmentsover a given surface area, and is the first indication of sucha mechanism in lichens. It is suggested that thallus structureand physiology have a larger influence on the observed low Amaxofthe lichens studied than chlorophyll content. Algal cells; chlorophyll concentration; lichens; light harvesting; net photosynthesis; stereology; thallus structure; Umbilicariaceae 相似文献
109.
110.
Identifying differences in mRNA expression by representational difference analysis of cDNA. 总被引:55,自引:2,他引:53 下载免费PDF全文
Detection of differentially regulated genes has been severely hampered by technical limitations. In an effort to overcome these problems, the PCR-coupled subtractive process of representational difference analysis (RDA) [Lisitsyn, N. et al. (1993) Science 259, 946-951] has been adapted for use with cDNA. In a model system, RAG-1 and RAG-2, the genes responsible for activating V(D)J recombination, were identified in a genomic transfectant by cDNA RDA in a small fraction of the time taken by conventional means. The system was also modified to eliminate expected difference products to facilitate the identification of novel genes. Additional alterations to the conditions allowed isolation of differentially expressed fragments. Several caffeine up-regulated clones were obtained from the pre-B cell line 1-8, including IGF-1B, and a predicted homologue of the natural killer cell antigen, NKR-P1. The approach was found to be fast, extremely sensitive, reproducible, and predominantly lacked false positives. cDNA RDA has the capacity and adaptability to be applied to a wide range of biological problems, including the study of single gene disorders, characterization of mutant and complemented cell types, developmental or post-event expression time courses, and examination of pathogen-host interactions. 相似文献