全文获取类型
收费全文 | 2994篇 |
免费 | 228篇 |
国内免费 | 386篇 |
专业分类
3608篇 |
出版年
2024年 | 16篇 |
2023年 | 83篇 |
2022年 | 78篇 |
2021年 | 101篇 |
2020年 | 120篇 |
2019年 | 196篇 |
2018年 | 175篇 |
2017年 | 154篇 |
2016年 | 136篇 |
2015年 | 88篇 |
2014年 | 174篇 |
2013年 | 243篇 |
2012年 | 118篇 |
2011年 | 165篇 |
2010年 | 121篇 |
2009年 | 147篇 |
2008年 | 134篇 |
2007年 | 151篇 |
2006年 | 122篇 |
2005年 | 96篇 |
2004年 | 117篇 |
2003年 | 103篇 |
2002年 | 102篇 |
2001年 | 55篇 |
2000年 | 52篇 |
1999年 | 64篇 |
1998年 | 42篇 |
1997年 | 34篇 |
1996年 | 42篇 |
1995年 | 32篇 |
1994年 | 36篇 |
1993年 | 33篇 |
1992年 | 35篇 |
1991年 | 26篇 |
1990年 | 22篇 |
1989年 | 20篇 |
1988年 | 19篇 |
1987年 | 17篇 |
1986年 | 13篇 |
1985年 | 17篇 |
1984年 | 21篇 |
1983年 | 18篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 9篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1976年 | 8篇 |
1973年 | 8篇 |
排序方式: 共有3608条查询结果,搜索用时 15 毫秒
91.
Parental contribution and coefficient of coancestry among maize inbreds: pedigree, RFLP, and SSR data 总被引:3,自引:0,他引:3
R. Bernardo J. Romero-Severson J. Ziegle J. Hauser L. Joe G. Hookstra R. W. Doerge 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》2000,100(3-4):552-556
The genetic relationship between inbreds i and j can be estimated from pedigree or from molecular marker data. The objectives of this study were to: (1) determine whether
pedigree, restriction fragment length polymorphism (RFLP), and simple sequence repeat (SSR) data give similar estimates of
parental contribution and coefficient of coancestry (f
ij
) among a set of maize (Zea mays L.) inbreds, and (2) compare the usefulness of RFLP and SSR markers for estimating genetic relationship. We studied 13 maize
inbreds with known pedigrees. The inbreds were genotyped using 124 RFLP and 195 SSR markers. For each type of marker, parental
contributions were estimated from marker similarity among an inbred and both of its parents, and were subsequently used to
estimate f
ij
. Estimates of parental contribution differed significantly (α<0.05) between pedigree data and either type of marker, but
not between the marker systems. The RFLP estimates of parental contribution failed to sum to 1.0, reflecting a higher frequency
of non-parental bands with RFLP than with SSR markers. The f
ij
estimated from pedigree, RFLP, and SSR data were highly correlated (r=0.87–0.97), although significant differences were found among the three sets of f
ij
estimates. We concluded that pedigree and marker data often lead to different estimates of parental contribution and f
ij
, and that SSR markers are superior to RFLP markers for estimating genetic relationship. A relevant question is whether or
not the inbreds previously genotyped with an older marker system (e.g., RFLP) need to be re-analyzed with a newer marker system
(e.g., SSR) for the purpose of estimating genetic relationship. Such re-analysis seems unnecessary if data for the same type
of marker are available for a given inbred and both of its parents.
Received: 2 June 1999 / Accepted: 30 July 1999 相似文献
92.
93.
94.
Autotaxin (ATX) is an attractive target for the anticancer therapeutics that inhibits angiogenesis, invasion and migration. ATX is an extracellular lysophospholipase D that hydrolyzes lysophosphatidylcholine to form the bioactive lipid lysophosphatidic acid. The aromatic phosphonate S32826 was the first described nanomolar inhibitor of ATX. However, the tridecylamide substituent on aromatic ring contributed to its poor solubility and bioavailability, severely limiting its utility in vivo. c Log P calculations revealed that the lipophilicity of S32826 could be lowered by shortening its hydrophobic chain and by introducing substituents alpha to the phosphonate. Herein, we describe the synthesis of a small set of α-substituted phosphonate analogs of S32826, and we show that shortening the chain and adding α-halo or α-hydroxy substituents increased solubility; however, ATX inhibition was reduced by most substitutions. An optimal compound was identified for examination of biological effects of ATX inhibition in vivo. 相似文献
95.
桃种质资源亲缘关系的研究进展 总被引:6,自引:0,他引:6
种质资源是现代育种和生物技术研究的物质基础。桃种质资源亲缘关系的研究将为探讨桃的起源、进化、分类、育种和资源利用提供科学依据。本文从形态学、细胞学、孢粉学、生物化学及DNA分子标记等几个方面综述了桃种质资源亲缘关系的研究进展,探讨了桃种质资源亲缘关系的研究现状、前景及尚需解决的问题,并就进一步开展桃种质资源亲缘关系的研究进行了分析,提出桃种质野生种、近缘野生种厦不同栽培品种群间的分子系统学关系是今后研究的重点。 相似文献
96.
Patterns of species richness on very small islands: the plants of the Aegean archipelago 总被引:4,自引:0,他引:4
Maria Panitsa Dimitrios Tzanoudakis Kostas A. Triantis Spyros Sfenthourakis 《Journal of Biogeography》2006,33(7):1223-1234
Aim To investigate the species–area relationship (SAR) of plants on very small islands, to examine the effect of other factors on species richness, and to check for a possible Small Island Effect (SIE). Location The study used data on the floral composition of 86 very small islands (all < 0.050 km2) of the Aegean archipelago (Greece). Methods We used standard techniques for linear and nonlinear regression in order to check several models of the SAR, and stepwise multiple regression to check for the effects of factors other than area on species richness (‘habitat diversity’, elevation, and distance from nearest large island), as well as the performance of the Choros model. We also checked for the SAR of certain taxonomic and ecological plant groups that are of special importance in eastern Mediterranean islands, such as halophytes, therophytes, Leguminosae and Gramineae. We used one‐way anova to check for differences in richness between grazed and non‐grazed islands, and we explored possible effects of nesting seabirds on the islands’ flora. Results Area explained a small percentage of total species richness variance in all cases. The linearized power model of the SAR provided the best fit for the total species list and several subgroups of species, while the semi‐log model provided better fits for grazed islands, grasses and therophytes. None of the nonlinear models explained more variance. The slope of the SAR was very high, mainly due to the contribution of non‐grazed islands. No significant SIE could be detected. The Choros model explained more variance than all SARs, although a large amount of variance of species richness still remained unexplained. Elevation was found to be the only important factor, other than area, to influence species richness. Habitat diversity did not seem important, although there were serious methodological problems in properly defining it, especially for plants. Grazing was an important factor influencing the flora of small islands. Grazed islands were richer than non‐grazed, but the response of their species richness to area was particularly low, indicating decreased floral heterogeneity among islands. We did not detect any important effects of the presence of nesting seabird colonies. Main conclusions Species richness on small islands may behave idiosyncratically, but this does not always lead to a typical SIE. Plants of Aegean islets conform to the classical Arrhenius model of the SAR, a result mainly due to the contribution of non‐grazed islands. At the same time, the factors examined explain a small portion of total variance in species richness, indicating the possible contribution of other, non‐standard factors, or even of stochastic effects. The proper definition of habitat diversity as pertaining to the taxon examined in each case is a recurrent problem in such studies. Nevertheless, the combined effect of area and a proxy for environmental heterogeneity is once again superior to area alone in explaining species richness. 相似文献
97.
98.
99.
基于结构方程模型的吴起县农业生态经济系统耦合关系 总被引:6,自引:0,他引:6
基于2008年吴起县农户调查资料,利用结构方程模型对吴起县农业生态经济系统耦合关系进行了分析.结果表明:具有相关关系的农业资源和产业态势对系统耦合有一定影响,其影响系数分别为0.14和1.00;农业资源与产业态势之间的相关系数为-0.11,说明吴起县的产业发展并非建立在对农业资源有效利用基础之上,农业资源对经济效益的变化没有直接作用;产业的发展主要建立在对农业资源之外的能量和物质的开发利用上,导致产业态势对经济效益的直接路径系数为-2.95,总的路径系数仅为0.23.从农业生态系统与经济系统的关系及其耦合的角度看,目前研究区农业系统建立在不稳定的、较少依赖于本系统资源的耦合态势下,即在农业生态经济系统良性耦合机制的建立与形成过程中潜伏了较大危机.为此,需要提高牧草等农业资源的利用率,发展林草及其相关产业. 相似文献
100.
Ryuji Ando 《Inorganica chimica acta》2004,357(3):817-823
The objective was to prepare mononuclear, binuclear, and insoluble polynuclear oxovanadium(IV)-Schiff base complexes and to use them for sulfoxidation and epoxidation of organic substrates. [VO(salen)] (complex 1) with tetradentate salen(salicylideneethylenediamine) being coordinated in the equatorial plane of oxovanadium(IV), [VO(salap)] (complex 2), and [(VO)2(sal2-dhdabp)] (complex 3) with tridentate salap(salicylideneorthoaminophenol) and sal2-dhdabp(salicylidene-3,3′-dihydroxy-4,4′-diaminobiphenyl) being bound, respectively, in the equatorial plane, of which polynuclear complexes were constituted as monomer units, were prepared and spectroscopically characterized. A sulfide and olefins were oxidized by use of complexes 1 and 2 (mononuclear), complex 3 (binuclear), and the polynuclear complexes (poly-1 and poly-3) synthesized with 1 and 3, respectively. The reaction rates for poly-1 and -3 were a little lower than those of the corresponding 1 and 3. On oxidation of sulfides, poly-3 exhibited lowering of activity by about 15% in three cycles, while poly-1 showed significant lose of activity with each use. Poly-3 was efficient for the oxidation of the olefins only in the first cycle. It was suggested that the loss of activity depends not only on the coordination geometry of the oxovanadium complex, but also on the kind of the substrate. 相似文献