全文获取类型
收费全文 | 13413篇 |
免费 | 1158篇 |
国内免费 | 10篇 |
专业分类
14581篇 |
出版年
2023年 | 48篇 |
2022年 | 126篇 |
2021年 | 227篇 |
2020年 | 140篇 |
2019年 | 184篇 |
2018年 | 205篇 |
2017年 | 171篇 |
2016年 | 328篇 |
2015年 | 562篇 |
2014年 | 585篇 |
2013年 | 789篇 |
2012年 | 952篇 |
2011年 | 971篇 |
2010年 | 631篇 |
2009年 | 557篇 |
2008年 | 827篇 |
2007年 | 865篇 |
2006年 | 776篇 |
2005年 | 746篇 |
2004年 | 765篇 |
2003年 | 712篇 |
2002年 | 652篇 |
2001年 | 142篇 |
2000年 | 89篇 |
1999年 | 144篇 |
1998年 | 165篇 |
1997年 | 122篇 |
1996年 | 114篇 |
1995年 | 102篇 |
1994年 | 93篇 |
1993年 | 107篇 |
1992年 | 99篇 |
1991年 | 67篇 |
1990年 | 78篇 |
1989年 | 59篇 |
1988年 | 74篇 |
1987年 | 74篇 |
1986年 | 56篇 |
1985年 | 78篇 |
1984年 | 79篇 |
1983年 | 78篇 |
1982年 | 104篇 |
1981年 | 94篇 |
1980年 | 88篇 |
1979年 | 54篇 |
1978年 | 52篇 |
1977年 | 53篇 |
1976年 | 54篇 |
1974年 | 49篇 |
1973年 | 56篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
An empirical projection method is described to predict the magnitudes and signs of two-bond 13C-13C spin-coupling constants (2JCC) in aldopyranosyl rings. The method has been applied primarily to the interpretation of 2JCCC values, although the behavior of 2JCOC has also been examined in light of the new approach, producing results which may prove useful in the conformational analysis of O-glycosidic linkages in oligosaccharides. High-level ab initio calculations of 2JCC values in model compounds were found to be in agreement with the predictions. 相似文献
92.
Wesley Hicks Robert Ward David Edelstein Leon Hall Anthony Albino Robert Hard Bonnie Asch 《Cell biology international》1995,19(4):301-306
The cytokertatins in respiratory epithelial cells (REC) of human nasal polyps and turbinates were analyzed by immunohistochemistry. Cytokeratin 19 (CK19) was present in all REC, CK5 and 14 were expressed primarily in basal cells, and CK7, 8, and 18 were found in suprabasal cells. Differences in cytoplasmic locations were also apparent among the individual cytokeratins. CK13 was not detected in any REC of these tissues. The results indicate the profile of cytokeratins in REC of human nasal polyps and turbinates is essentially identical to that of REC in the more distal respiratory tract. 相似文献
93.
Formaldehyde Solution Effectively Inactivates Spores of Bacillus anthracis on the Scottish Island of Gruinard 下载免费PDF全文
Richard J. Manchee Malcolm G. Broster Anthony J. Stagg Stephen E. Hibbs 《Applied microbiology》1994,60(11):4167-4171
Gruinard Island was heavily contaminated with the spores of virulent Bacillus anthracis during biological weapons trials in World War II. However, an extensive survey in 1979 showed that most of the island was not contaminated. In the early 1980s, a more intensive survey revealed that the contamination was largely confined to the top 8 cm of the soil in a 2.6-ha area of the 211-ha island. Small-scale tests showed that the spores could be inactivated by drenching the soil with fluid biocides. A solution of 5% formaldehyde in seawater applied by surface spray to each square meter of ground was shown to be the most effective treatment and was utilized for large-scale decontamination of the affected areas. Following this treatment, extensive sampling revealed that most of the spores of B. anthracis had been inactivated. Isolated pockets of surviving spores were treated further. A flock of sheep was then allowed to graze over the entire island for 5 months; none contracted anthrax. 相似文献
94.
Nicholas A. Ashbaugh Anthony A. Echelle Alice F. Echelle 《Journal of fish biology》1994,45(2):291-302
Starch gel electrophoresis of proteins was used to study geographic variation at 26 gene loci in the Red River pupfish ( Cyprinodon rubrofluviatílís ), a species restricted to west Texas and Oklahoma. Marked differences were detected between populations in the Red and Brazos river drainages, with fixed or nearly fixed differences occurring at five gene loci. In addition, mean heterozygosity was uniformly high for the Red River form ( = 0·076–0·101) while samples of the Brazos River form were genetically depauperate ( =0·00–0·017). Introduced populations in the South Canadian and Colorado river drainages appear to have been derived from the Red River drainage. The presence of alleles diagnostic of the Red and Brazos river forms supports the suggestion from previous work that they may represent cryptic species. Regardless of taxonomy, however, the presence of two genetically distinct forms must be taken into consideration by those concerned with maintenance of biotic diversity. 相似文献
95.
96.
Felicity Z. Watts Neil Butt Philip Layfield Jesse Machuka Julian F. Burke Anthony L. Moore 《Plant molecular biology》1994,26(1):445-451
An Arabidopsis thaliana gene (UBC6) encoding a homologue to ubiquitin-conjugating enzymes has been isolated which is capable of encoding a protein of 183 amino acids of ca. 21 kDa. Northern analysis indicates that the gene is expressed in flowers, seeds and, to a somewhat lesser extent, in 10-day seedlings but not in mature leaves, callus and pre-flowering plants. This pattern of expression is confirmed using transgenic Arabidopsis plants containing a UBC6 promoter-GUS gene fusion construct. These plants displey GUS activity in mature anthers prior to dehiscence, in developing embryos, sepals and the style after pollination. 相似文献
97.
98.
We present the results of a 5-year examination of variation in the stable carbon isotope composition () of three C3 graminoid species from a Sandhills prairie: Agropyron smithii, Carex heliophila and Stipa comata. Although consistent species-specific patterns for mean were seen, there was also significant and substantial among-year and within-season variation in . A smaller contribution to variation in came from topographic variation among sampling sites. Effects of species, year, season and topography contribute to variation in in an additive manner. An association between climate and exists that is consistent with previous work suggesting that reflects the interplay between photosynthetic gas exchange and plant water relations. Within the growing season, the time over which integrates plant response to the environment ranges from days to months. 相似文献
99.
100.
C. Caroline Blackwell Donald M. Weir Anthony Busuttil Abdulrahman T. Saadi Steven D. Essery Mohammed W. Raza V.S. James D.A.C. Mackenzie 《FEMS immunology and medical microbiology》1994,9(2):91-100
Abstract Epidemiological factors associated with susceptibility to respiratory infections are similar to those associated with Sudden Infant Death Syndrome. Here we review the evidence that respiratory pathogens might be involved in some cases of Sudden Infact Death Syndrome in the context of factors identified in epidemiological studies of cot deaths: the age range affected; mother's smoking; respiratory viral infections; immunisation status. Both laboratory and epidimiological evidence suggests that vulnerability of infants to infectious agents depends on interactions between genetic, developmental and environmental factors that contribute to colonisation by microorganisms, the inflammatory and specific immune responses and the infants' physiological responses to inflammatory mediators. A model is proposed to explain how microorganisms might trigger a series of events resulting in some of these unexpected deaths and discusses how the present recommendations regarding child care practices might help reduce the numbers of Sudden Infant Death Syndrome cases associated with infectious agents. 相似文献