全文获取类型
收费全文 | 49865篇 |
免费 | 4408篇 |
国内免费 | 4603篇 |
专业分类
58876篇 |
出版年
2024年 | 133篇 |
2023年 | 666篇 |
2022年 | 1576篇 |
2021年 | 2559篇 |
2020年 | 1849篇 |
2019年 | 2225篇 |
2018年 | 2164篇 |
2017年 | 1599篇 |
2016年 | 2229篇 |
2015年 | 3090篇 |
2014年 | 3686篇 |
2013年 | 3866篇 |
2012年 | 4663篇 |
2011年 | 4095篇 |
2010年 | 2493篇 |
2009年 | 2258篇 |
2008年 | 2596篇 |
2007年 | 2270篇 |
2006年 | 2001篇 |
2005年 | 1706篇 |
2004年 | 1526篇 |
2003年 | 1434篇 |
2002年 | 1222篇 |
2001年 | 1037篇 |
2000年 | 857篇 |
1999年 | 786篇 |
1998年 | 521篇 |
1997年 | 473篇 |
1996年 | 454篇 |
1995年 | 392篇 |
1994年 | 386篇 |
1993年 | 253篇 |
1992年 | 331篇 |
1991年 | 273篇 |
1990年 | 222篇 |
1989年 | 186篇 |
1988年 | 129篇 |
1987年 | 150篇 |
1986年 | 100篇 |
1985年 | 110篇 |
1984年 | 64篇 |
1983年 | 47篇 |
1982年 | 46篇 |
1981年 | 35篇 |
1980年 | 19篇 |
1979年 | 14篇 |
1978年 | 10篇 |
1976年 | 10篇 |
1975年 | 11篇 |
1973年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
1971年,N.Ulzijkhutag根据采自阿拉善戈壁(=Bordzon-Go-bi)的标本(1970-07-29,V.I.Grubov,N.Ulzijkhutag et G.Tserendalzhid,无号)发表新种Oxytrops grubovii,并以此建立新组Oxytropis subgen.Traganthoxytropis sect.Monan-thos;1987年,G.P.Yakovlev将O.grubovii归入海绵豆属Spongiocarpella;1988年,张振万同意N.Ulzijkhutag的意见,并根据刺垫棘豆之龙骨瓣无喙等特征将Ulzijkhutag的刺 相似文献
22.
23.
Estimation of whole-plant resistance to gaseous exchange independent of leaf temperature measurement 总被引:1,自引:1,他引:0
For studies into the uptake of mercury vapor by wheat (Triticum aestivum), a simple theory and plant chamber were employed to estimate total leaf resistance of whole plants to water vapor exchange. The estimates were independent of leaf temperature, for which mean values were indirectly determined. The approach involved the measurement, at steady-state conditions, of the net change in water vapor flux per unit of leaf surface (Δqv) in response to a small induced change in absolute humidity (ΔCa). Assuming that total leaf resistance (rl) was constant and that change in leaf temperature (Tl) was negligible, total leaf resistance was calculated from the equation, [Formula: see text] 相似文献
24.
Under physiological conditions, many polypeptide chains spontaneously fold into discrete and tightly packed three-dimensional structures. The folded polypeptide chain conformation is believed to represent a minimum Gibbs energy of the system, governed by the weak interactions that operate between the amino acid residues and between the residues and the solvent. A semiempirical molecular thermodynamic model is proposed to represent the Gibbs energy of folding of aqueous homopolypeptide systems. The model takes into consideration both the entropy contribution and the enthalpy contribution of folding homopolypeptide chains in aqueous solutions. The entropy contribution is derived from the Flory-Huggins expression for the entropy of mixing. It accounts for the entropy loss in folding a random-coiled polypeptide chain into a specific polypeptide conformation. The enthalpy contribution is derived from a molecular segment-based Non-Random Two Liquid (NRTL) local composition model [H. Renon and J. M. Prausnitz (1968) AIChE J., Vol. 14, pp. 135-142; C.-C. Chen and L. B. Evans (1986) AIChE J., Vol. 32, pp. 444-454], which takes into consideration of the residue-residue, residue-solvent, and solvent-solvent binary physical interactions along with the local compositions of amino acid residues in aqueous homopolypeptides. The UNIFAC group contribution method [A. Fredenslund, R. L. Jones, and J. M. Prausnitz (1975) AIChE J., 21, 1086-1099; A. Fredenslund, J. Gmehling, and P. Rasmussen (1977) Vapor-Liquid Equilibrium Using UNIFAC, Elsevier Scientific Publishing Company, Amsterdam], developed originally to estimate the excess Gibbs energy of solutions of small molecules, was used to estimate the NRTL binary interaction parameters. The model yields a hydrophobicity scale for the 20 amino acid side chains, which compares favorably with established scales [Y. Nozaki and C. Tanford (1971) Journal of Biological Chemistry, Vol. 46, pp. 2211-2217; E. B. Leodidis and T. A. Hatton (1990) Journal of Physical Chemistry, Vol. 94, pp. 6411-6420]. In addition, the model generates qualitatively correct thermodynamic constants and it accurately predicts thermodynamically favorable folding of a number of aqueous homopolypeptides from random-coiled states into alpha-helices. The model further facilitates estimation of the Zimm-Bragg helix growth parameter s and the nucleation parameter sigma for amino acid residues [B. H. Zimm and J. K. Bragg (1959) Journal of Chemical Physics, Vol. 31, pp. 526-535]. The calculated values of the two parameters fall into the ranges suggested by Zimm and Bragg. 相似文献
25.
Distribution and behaviour of Acetes sibogae Hansen (Decapoda, Crustacea) in an estuary in relation to tidal and diel environmental changes 总被引:1,自引:0,他引:1
Quantitative samples of Acetes sibogae were collected at 2 hintervals for 48 h at three sites across the axis of a tidalestuary to examine their distribution within the water bodyover tidal and diel cycles, and to assess the role of behaviourin maintaining population distribution in estuarine/coastalwaters in relation to selected environmental factors. Watertemperature, salinity, tidal height and light intensity wereconcurrently measured. Distribution of the shrimp across theestuary was uniform and consistent between daylight or darkperiods, and among flood or ebb tides. Changes of A.sibogaeabundance were related to light and tidal cycles at each sitewith higher catches in dark periods and during flood tides.Acetes sibogae also exhibited both nocturnal and tidal verticalmovements in the water body, with greater numbers being onlyfound near-surface rather than near-bottom during flood tidesand at night. No significant differences in the distributionof size groups were found between any sampled levels of anysite. Acetes sibogae was highly aggregated in the water body.It is suggested that aggregating behaviour and tidal and nocturnalvertical movements act to facilitate population maintenancein estuarine/coastal waters. 相似文献
26.
Stage-specific ribosomal RNA expression switches during sporozoite invasion of hepatocytes 总被引:2,自引:0,他引:2
J D Zhu A P Waters A Appiah T F McCutchan A A Lal M R Hollingdale 《The Journal of biological chemistry》1990,265(21):12740-12744
27.
28.
Weixiao Lei Zefu Wang Man Cao Hui Zhu Min Wang Yi Zou Yunchun Han Dandan Wang Zeyu Zheng Ying Li Bingbing Liu Dafu Ru 《DNA research》2022,29(3)
Sophora japonica is a medium-size deciduous tree belonging to Leguminosae family and famous for its high ecological, economic and medicinal value. Here, we reveal a draft genome of S. japonica, which was ∼511.49 Mb long (contig N50 size of 17.34 Mb) based on Illumina, Nanopore and Hi-C data. We reliably assembled 110 contigs into 14 chromosomes, representing 91.62% of the total genome, with an improved N50 size of 31.32 Mb based on Hi-C data. Further investigation identified 271.76 Mb (53.13%) of repetitive sequences and 31,000 protein-coding genes, of which 30,721 (99.1%) were functionally annotated. Phylogenetic analysis indicates that S. japonica separated from Arabidopsis thaliana and Glycine max ∼107.53 and 61.24 million years ago, respectively. We detected evidence of species-specific and common-legume whole-genome duplication events in S. japonica. We further found that multiple TF families (e.g. BBX and PAL) have expanded in S. japonica, which might have led to its enhanced tolerance to abiotic stress. In addition, S. japonica harbours more genes involved in the lignin and cellulose biosynthesis pathways than the other two species. Finally, population genomic analyses revealed no obvious differentiation among geographical groups and the effective population size continuously declined since 2 Ma. Our genomic data provide a powerful comparative framework to study the adaptation, evolution and active ingredients biosynthesis in S. japonica. More importantly, our high-quality S. japonica genome is important for elucidating the biosynthesis of its main bioactive components, and improving its production and/or processing. 相似文献
29.
30.