全文获取类型
收费全文 | 7924篇 |
免费 | 264篇 |
国内免费 | 431篇 |
出版年
2023年 | 57篇 |
2022年 | 91篇 |
2021年 | 164篇 |
2020年 | 119篇 |
2019年 | 161篇 |
2018年 | 151篇 |
2017年 | 145篇 |
2016年 | 169篇 |
2015年 | 283篇 |
2014年 | 394篇 |
2013年 | 394篇 |
2012年 | 295篇 |
2011年 | 344篇 |
2010年 | 325篇 |
2009年 | 494篇 |
2008年 | 504篇 |
2007年 | 523篇 |
2006年 | 361篇 |
2005年 | 342篇 |
2004年 | 286篇 |
2003年 | 232篇 |
2002年 | 171篇 |
2001年 | 188篇 |
2000年 | 197篇 |
1999年 | 238篇 |
1998年 | 204篇 |
1997年 | 173篇 |
1996年 | 136篇 |
1995年 | 180篇 |
1994年 | 148篇 |
1993年 | 133篇 |
1992年 | 137篇 |
1991年 | 102篇 |
1990年 | 80篇 |
1989年 | 87篇 |
1988年 | 77篇 |
1987年 | 96篇 |
1986年 | 63篇 |
1985年 | 60篇 |
1984年 | 54篇 |
1983年 | 28篇 |
1982年 | 35篇 |
1981年 | 41篇 |
1980年 | 31篇 |
1979年 | 21篇 |
1978年 | 36篇 |
1977年 | 12篇 |
1976年 | 13篇 |
1975年 | 9篇 |
1973年 | 8篇 |
排序方式: 共有8619条查询结果,搜索用时 640 毫秒
1.
Gian Maria Rossolini Patrizia Muscas Alessandra Chiesurin Giuseppe Satta 《FEMS microbiology letters》1994,119(3):321-328
Abstract Analysis of the Salmonella chromosomal region located upstream of the fimA gene (coding for the major type 1 fimbrial subunit) showed a close linkage of this gene to the folD gene (coding for the enzyme 5,10-methylenetetrahydrofolate dehydrogenase/5, 10-methenyltetrahydrofolate cyclohydrolase), indicating that the fim gene cluster of Salmonella , unlike that of Escherichia coli , has no regulatory genes located upstream of fimA and apparently terminates with this gene. The respective locations of the fim and folD genes in the E. coli and Salmonella genetic maps suggests that the fimA-folD intergenic region of Salmonella encompasses a junctional site of a genetic rearrangement that probably originated from the different chromosomal location of the fim genes in these species. 相似文献
2.
3.
4.
Levels of genetic diversity at different stages of the domestication cycle of interior spruce in British Columbia 总被引:3,自引:0,他引:3
M. U. Stoehr Y. A. El-Kassaby 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1997,94(1):83-90
Concerns over the reductionist nature of the domestication of forest-tree species focus on the possibility of potential genetic
erosion during this process. To address these concerns, genetic diversity assessments in a breeding zone the Province of British
Columbia “interior” spruce (Picea glauca×engelmanni) program was conducted using allozyme markers. Genetic-variation comparisons were made between natural and production (seed
orchard) populations as well as seed and seedling crops produced from the same breeding zone’s seed orchard. The natural population
sample consisted of a total of 360 trees representing three stands within each of three watersheds present in the Shuswap-Adams
low-elevation zone of interior British Columbia. Small amounts of genetic differentiation were observed among the nine natural
populations (4%) and this was attributable to extensive gene flow Consequently, the sum of these nine populations was considered as a baseline for the genetic variation present in the breeding
zone. The comparisons between the seed orchard and the breeding zone produced a similar percentage of polymorphic loci while the expected hetrozygosity (0.207 vs 0.210) and the average number of alleles per locus (2.7 vs 2.4) were slightly lower in the seed orchard. A total
of seven natural populations’ rare alleles were not present in the orchard population, while one allele was unique to the orchard. The %P increased to 70.6% in the seedlot, but dropped to the natural populations level (64.7%) in the plantation. The observed increase
in %P was a result of pollen contamination in the orchard. It is suspected that the reduction in the plantation was caused by an
unintentional selection in the nursery. Simulated roguing in the orchard did not drastically reduce even if up to 50% of the orchard’s clones were rogued. However, roguing was associated with a reduction in the average number
of alleles per locus (i.e., sampling effect).
Received: 2 January 1996 / Accepted: 24 May 1996 相似文献
5.
6.
Eleutheronema tetradactylum is an economically important fish species in China water. To investigate the genetic diversity and describe population structure of it, an 1151 base pair (bp) fragment of the mitochondrial DNA Cytb sequence was analyzed in 120 individuals from four populations in the East China Sea and the South China Sea. A total of 16 haplotypes were defined by 24 variable nucleotide sites. High level of haplotype diversity and low nucleotide diversity were observed in all populations. The results of AMOVA detected that 89.44% of the genetic variation occurred within populations. Significant genetic differentiations were detected among populations (0.05097, P < 0.05), but no large-scale regional differences were detected. Analysis of neutral evolution and mismatch distribution suggested no recent population expansion happened. The present results provided new information for genetic assessment, fishery management and conservation of this species. 相似文献
7.
A robust method for selection of variables with the greatest discriminatory power is presented in the paper. The method deals with the two groups of data problem. An application of the method to some respiratory disease data and comparisons with classical procedures are given, also. 相似文献
8.
This article is part of a Special Issue Energy Balance. 相似文献
9.
The advantage of using terrestrial isopods in toxicity studies is that a battery of parameters can be tested at different levels of biological complexity. Feeding parameters for example link organism level response to potential ecological consequences but a problem with using feeding parameters in toxicity tests with terrestrial isopods is their high variability. The aim of our study was to set benchmark values for feeding parameters for isopod Porcellio
scaber (Isopoda, Crustacea) in laboratory-controlled experiments. In the work presented here, the daily feeding rate of the central 50% of the control population of Porcellio
scaber and a correlation between feeding rate and isopod weight were set. Values outside these ranges need additional evaluation to increase the relevance of test outcomes. We suggest using benchmark values for feeding parameters as well as the coefficient of variation (a) to identify animals with altered feeding parameters with respect to controls, and (b) to assess the data quality in each experiment. 相似文献
10.
Post-mortem human subjects (PMHS) are frequently used to characterize biomechanical response and injury tolerance of humans to various types of loading by means of instrumentation installed directly on the skeleton. Data extracted from such tests are often used to develop and validate anthropomorphic test devices (ATDs), which function as human surrogates in tests for injury assessment. Given that the location and orientation of installed instrumentation differs between subjects, nominally similar measurements made on different PMHS must be transformed to standardized, skeletal-based local coordinate systems (LCS) before appropriate data comparisons can be made. Standardized PMHS LCS that correspond to ATD instrumentation locations and orientations have not previously been published. This paper introduces anatomically-defined PMHS LCS for body regions in which kinematic measurements are made using ATDs. These LCS include the head, sternum, single vertebrae, pelvis, femurs (distal and proximal), and tibiae (distal and proximal) based upon skeletal landmarks extracted from whole body CT scans. The proposed LCS provide a means to standardize the reporting of PMHS data, and facilitate both the comparison of PMHS impact data across institutions and the application of PMHS data to the development and validation of ATDs. 相似文献