全文获取类型
收费全文 | 13681篇 |
免费 | 1819篇 |
国内免费 | 1966篇 |
出版年
2024年 | 89篇 |
2023年 | 486篇 |
2022年 | 426篇 |
2021年 | 639篇 |
2020年 | 739篇 |
2019年 | 847篇 |
2018年 | 712篇 |
2017年 | 665篇 |
2016年 | 663篇 |
2015年 | 611篇 |
2014年 | 681篇 |
2013年 | 873篇 |
2012年 | 565篇 |
2011年 | 619篇 |
2010年 | 467篇 |
2009年 | 660篇 |
2008年 | 720篇 |
2007年 | 750篇 |
2006年 | 698篇 |
2005年 | 628篇 |
2004年 | 572篇 |
2003年 | 492篇 |
2002年 | 439篇 |
2001年 | 356篇 |
2000年 | 355篇 |
1999年 | 279篇 |
1998年 | 281篇 |
1997年 | 195篇 |
1996年 | 201篇 |
1995年 | 176篇 |
1994年 | 154篇 |
1993年 | 116篇 |
1992年 | 130篇 |
1991年 | 140篇 |
1990年 | 94篇 |
1989年 | 85篇 |
1988年 | 90篇 |
1987年 | 95篇 |
1986年 | 51篇 |
1985年 | 90篇 |
1984年 | 94篇 |
1983年 | 53篇 |
1982年 | 86篇 |
1981年 | 65篇 |
1980年 | 72篇 |
1979年 | 51篇 |
1978年 | 25篇 |
1977年 | 20篇 |
1976年 | 22篇 |
1975年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
Peter B. Oelrichs Michael W. Hill Peter J. Vallely John K. MacLeod Tadeusz F. Molinski 《Phytochemistry》1983,22(2):531-534
Four new tetranortriterpenes, meliatoxins A1, A2, B1 and B2 have been isolated and identified as toxic constituents of the fruit of Melia azedarach L. var. australasica. Toxicity and pathological results confirm that the meliatoxins are responsible for most but not all of the symptoms resulting from the ingestion of whole fruit. 相似文献
132.
Bonnie J. Reger Maurice S.B. Ku Jeffrey W. Pottert John J. Evans 《Phytochemistry》1983,22(5):1127-1132
The ribulose-1,5-bisphosphate carboxylase/oxygenase purified from maize (a C4 monocot) to homogeneity has a MW of532 000 and sedimentation coeffici 相似文献
133.
Structural proteins and the characteristics of infectious flacherie virus (IFV) purified from the silkworm, Bombyx mori, are described. The purified IFV had four major structural proteins, which were detected only in high concentration gels of sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and a few minor ones. Molecular weights of the major proteins were 35,200 (VP 1), 33,000 (VP 2), 31,200 (VP 3), and 11,600 (VP 4), and numbers per virion were 62, 57, 54, and 31, respectively. Amino acid compositions of VP 1, VP 2, and VP 3 were similar to each other but that of VP 4 was somewhat different. By isoelectric focusing and two-dimensional electrophoresis, high resolution of the structural proteins was obtained with silver staining. The isoelectric points of the four major proteins were determined as 7.7(VP 1), 6.7(VP 2), 4.8(VP 3), and 5.5(VP 4). This work is the first report on insect picornaviruses that presents some discriminative properties of each viral protein that was compared to those of mammalian picornaviruses. 相似文献
134.
La possibilité de nourrir la punaiseMacrolophus caliginosus Wagner à l'aide de milieux artificiels a été testée. En l'absence de végétal, les témoins nourris avec des ?ufs d'Ephestia kuehniella Zeller ont une survie médiocre et fournisent 33% d'adultes. Avec le meilleur milieu artificiel la survie est comparable à celle des témoins et 21 % d'adultes sont obtenus. Avec ce même milieu, mais en présence de feuille de géranium, la production d'adultes atteint 62%. Les compositions en acides aminés des punaises élevées avec le milieu montrent des écarts inférieurs à 20% par rapport aux témoins. Le développement complet deM. caliginosus nourri à l'aide de milieu artificiel a été obtenu. Le végétal joue un rôle important dans la biologie de ce prédateur. 相似文献
135.
The fatty acid composition of ER, Golgi and peribacteroid membrane (PBM) from root nodules formed on Glycine max after infection with different strains of Bradyrhizobium japonicum has been analysed by gas chromatography. In each plant-microsymbiont combination the fatty acid composition (FAC) of the PBM is distinct from ER and Golgi. The similarity between ER and PBM fatty acid composition is significantly stronger than between Golgi and PBM. In addition the fatty acid composition of all membrane systems in nodules is affected by the microsymbiont strain. A comparison of four strains of Bradyrhizobium japonicum grown in agar surface culture and isolated as the symbiotic bacteroids reveals a decrease in oleic acid during bacteroid differentiation. 相似文献
136.
137.
Cores were collected from dominant pondcypress trees growing in a swamp that had received sewage effluent for 7 yr and a nearby control swamp to determine the combined effects of changes in nutrient supply and hydrologic regime on tree growth. The cores were used to measure two indices of tree growth: basal area increment (BAI) and relative basal area increment (RBAI, which accounts for differences in growth due to the size of teh tree) between 1970–1983 while one swamp remained untreated and the other received weekly additions of sewage effluent from 1974–1981. Throughout the whole period, the mean BAI and RBAI of pond-cypress trees in the untreated swamp remained unchanged, ranging between 5.55–6.38 cm2 yr–1 and 1.09–1.27% yr–1, respectively. In contrast, trees in the treated swamp increased their BAI approximately two-fold from 7.40 cm2 yr–1 prior to treatment to 14.83 cm2 yr–1 after the onset of treatment and maintained this rate of growth in the 2 yr period after cessation of treatment. Relative basal area increment showed a similar response, but the proportional increase due to treatment was less (1.5-fold factor) than for BAI. The response of pondcypress trees to the sewage effluent differed depending upon whether the trees were located in the deep or shallow water zones. Trees in the deep zone of the treated swamp had lower BAIs and RBAIs than those in the shallow zone during the treatment period, whereas in pre- and post-treatment periods growth indices were equal in both zones. No significant differences in growth between deep and shallow zones were observed during all three time periods in the control swamp. 相似文献
138.
Summary We present compositional statistics, a new method of phylogenetic inference, which is an extension of evolutionary parsimony. Compositional statistics takes account of the base composition of the compared sequences by using nucleotide positions that evolutionary parsimony ignores. It shares with evolutionary parsimony the features of rate invariance and the fundamental distinction between transitions and transversions. Of the presently available methods of phylogenetic inference, compositional statistics is based on the fewest and mildest assumptions about the mode of DNA sequence evolution. It is therefore applicable to phylogenetic studies of the most distantly related organisms or molecules. This was illustrated by analyzing conservative positions in the DNA sequences of the large subunit of RNA polymerase from three archaebacterial groups, a eubacterium, a chloroplast, and the three eukaryotic polymerases. Internally consistent results, which are in accord with our knowledge of organelle origin and archaebacterial physiology, were achieved. 相似文献
139.
Summary The protein sequences of seven 3-aminoglycoside phosphotransferases falling into the six identified types and three 6-aminoglycoside phosphotransferases were analyzed to give a rooted phylogenetic tree. This tree supports the origin of these groups of enzymes in an ancestor closely related to the actinomycetes, and that horizontal transfer of the resistance genes occurred, possibly via transposons. The implications for genetic engineering of a novel antibiotic are discussed. 相似文献
140.
Summary A few years ago we presented a stationary Markov model of gene evolution according to which only homologous genes from not
too divergent species obeying the condition of being stationary may behave as reliable molecular clocks. A compartmentalized
model of the nuclear genome in which the genes are distributed in compartments, the isochores, defined by their G+C content
has been proposed recently. We have found that only homologous gene pairs that are stationary, and belong to the same isochore,
can be used consistently for the determination of phylogeny and base substitution rate. In particular, for the rodent-human
couple, only about half of the homologous gene pairs are stationary. Stationary genes evolve at the third silent codon position
with the same velocity independent of the genes and base composition. By contrast, nonstationary genes display apparent rate
values (pseudovelocities) that are significantly higher. Our results cast doubt upon recent claims of a large acceleration
in the rate of molecular evolution in rodents. 相似文献