全文获取类型
收费全文 | 808篇 |
免费 | 47篇 |
国内免费 | 43篇 |
专业分类
898篇 |
出版年
2023年 | 7篇 |
2022年 | 8篇 |
2021年 | 8篇 |
2020年 | 10篇 |
2019年 | 12篇 |
2018年 | 19篇 |
2017年 | 15篇 |
2016年 | 17篇 |
2015年 | 28篇 |
2014年 | 32篇 |
2013年 | 42篇 |
2012年 | 29篇 |
2011年 | 41篇 |
2010年 | 26篇 |
2009年 | 48篇 |
2008年 | 45篇 |
2007年 | 59篇 |
2006年 | 38篇 |
2005年 | 36篇 |
2004年 | 26篇 |
2003年 | 31篇 |
2002年 | 27篇 |
2001年 | 14篇 |
2000年 | 19篇 |
1999年 | 35篇 |
1998年 | 22篇 |
1997年 | 12篇 |
1996年 | 13篇 |
1995年 | 18篇 |
1994年 | 16篇 |
1993年 | 26篇 |
1992年 | 13篇 |
1991年 | 18篇 |
1990年 | 7篇 |
1989年 | 14篇 |
1988年 | 14篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1966年 | 1篇 |
1963年 | 1篇 |
排序方式: 共有898条查询结果,搜索用时 15 毫秒
81.
82.
S. Ejaz Ahmed 《Biometrical journal. Biometrische Zeitschrift》2000,42(8):973-979
Symmetric parallel‐line biological assays involve the estimation of (log) relative potencies. The class of p(≥ 2) combination of symmetric parallel line bioassays are considered in this study. A large sample test for the equality of the several potencies is developed. An estimator and a confidence interval are proposed for the common relative potency parameter. The asymptotic distribution of the proposed test‐statistic under the null hypothesis as well as under a contagious hypothesis is derived. 相似文献
83.
M. N. Katanbaf P. B. Siegel E. A. Dunnington 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1988,76(4):540-544
Summary Organ growth of male chickens selected for high and low 56-day body weight and their reciprocal F1 crosses was compared at a common age (56 days) or at a common body weight (180 g). Organs that differed at a common body weight included weights of proventriculus, small intestine, lungs, feathers and abdominal fat and length of esophagus. Organ weights that differed at a common age included esophagus, gizzard, heart, liver, lungs, breast, legs, feathers and abdominal fat, and lengths of shank, esophagus and small intestine. Heterosis for most organs was less than 15%. Those exhibiting heterosis greater than 30% included weights of fat depots and feathers, plus lengths of the esophagus, small intestine and shank. Heterosis for these traits, however, varied depending on whether comparisons were made at common body weight or age. These results imply that biological functions of organs at specific ages may not reflect the situations at common body weights and suggest differences in resource allocations among populations. 相似文献
84.
85.
We constructed mutant genes of Caldococcus noboribetus isocitrate dehydrogenase containing ancestral amino acid residues that were inferred using the maximal likelihood method and a composite phylogenetic tree of isocitrate dehydrogenase and 3-isopropylmalate dehydrogenase. The mutant genes were expressed in Escherichia coli and the protein products purified. Thermostabilities, reported as the half-inactivation temperatures, for the purified enzymes were determined and compared with that of the wild-type enzyme. Four of the five mutant enzymes have greater thermal stabilities than wild-type isocitrate dehydrogenase. The results are compatible with the hyperthermophilic universal ancestor (commonote) hypothesis. Incorporation of ancestral residues into a modern-day protein sequence can be used to improve protein thermostability. 相似文献
86.
《Dendrochronologia》2014,32(3):220-229
Pointer year analysis, simple correlations, and response functions were combined in a dendroecological study to evaluate climate–growth relationships over the last century in two Abies alba Mill. and Fagus sylvatica L. mixed stands in Southern Italy mountainous areas. Analyses revealed species-specific attributes at the two study sites, i.e. Molise and Basilicata. Growth divergence between the two species emerged based on three primary climatic drivers, including drought stress and spring warmer temperatures during the current growing season for F. sylvatica, and water availability in the previous growing season for A. alba. However, despite the microclimatic differences between the two study sites, F. sylvatica showed similar climate–growth patterns, while differences were indicated for A. alba, due to its minor susceptibility to drought stress during the current growing season at the Basilicata site. Indeed, at the southernmost geographic limits of A. alba drought avoidance mechanisms were confirmed, consistent with traits considered diagnostic for the species in the Mediterranean region. 相似文献
87.
A. Minerva Arce-Ibarra Anthony Charles 《Human ecology: an interdisciplinary journal》2008,36(6):853-860
This study explores the management of inland fisheries on common lands (ejidos) within the Mayan Zone of Quintana Roo, Mexico. Qualitative survey data are used to examine whether there are (a) ongoing
fishery conservation activities; (b) local caretakers of fishing sites; (c) restrictions on where fishing can take place;
and (d) particular groups of individuals who lack the right to fish within the ejidos. Results indicate that in contrast to other local resources, fisheries in the ejidos have less defined use rights, being viewed as largely “open access”. In particular, most feel that outsiders can fish in
an ejido if they abide by local rules (notably not damaging the local water bodies, asking consent to fish, and having a friend or
relative in the community). However, there are within-community differences in views concerning management and conservation,
with those more dependent on fishing being more likely to support exclusion of outsiders. 相似文献
88.
In order to genotype hybrid genomes of distant asymmetric somatic hybrids, we synthesized hybrid calli and plants via PEG-mediated
protoplast fusion between recipient tall fescue (Festuca. arundinacea Schreb.) and donor wheat (Triticum aestivum L.). Seventeen and 25 putative hybrid clones were produced from the fusion combinations I and II, each with the donor wheat
protoplast treated by UV light for 30 s and 1 min, respectively. Isozyme and RAPD profiles confirmed that ten hybrid clones
were obtained from combination I and 19 from combination II. Out of the 29 hybrids, 12 regenerated hybrid plants with tall
fescue phenotype. Composition and methylation-variation of the nuclear and cytoplasmic genomes of some hybrids, either with
or without regenerative ability, were compared by genomic in situ hybridization, restriction fragment length polymorphism,
and DNA methylation-sensitive amplification polymorphism. Our results indicated that these selected hybrids all contained
introgressed nuclear and cytoplasmic DNA as well as obvious methylation variations compared to both parents. However, there
were no differences either in nuclear/cytoplasmic DNA or methylation degree between the regenerable and non-regenerable hybrid
clones. We conclude that both regeneration complementation and genetic material balance are crucial for hybrid plant regeneration. 相似文献
89.
The one-third law of evolutionary dynamics [Nowak et al. 2004. Emergence of cooperation and evolutionary stability in finite populations. Nature 428, 246-650] describes a robustness criterion for evolution in a finite population: If at an A-frequency of 1/3, the fitness of an A player is greater (smaller) than the fitness of a B player, then a single A mutant that appears in a population of otherwise all B has a fixation probability greater (smaller) than the neutral threshold 1/N, the inverse population size. We examine the case where at an A-frequency of 1/3, the fitness of an A player is exactly equal to the fitness of a B player. We find that in this case the relative magnitude of the cross payoffs matters: If the payoff of A against B is larger (smaller) than the payoff of B against A, then a single A mutant has a fixation probability larger (smaller) than 1/N. If the cross payoffs coincide, we are in the special case of a partnership game, where the deviation cost from an inefficient equilibrium is exactly balanced by the potential gain of switching to the payoff dominant equilibrium. We show that in this case the fixation probability of A is lower than 1/N. Finally, we illustrate our findings by a language game with differentiated costs of signals. 相似文献
90.
Abdelaal Shamseldin 《World journal of microbiology & biotechnology》2008,24(8):1603-1606
Plasmid profile analysis is useful to characterize Rhizobium strains within the same species. Among the 16 Rhizobium strains examined, 14 had distinct plasmid profiles. The size of plasmids ranged from 40 to 650 kb, and three plasmids of
650, 510 and 390 kb were common to several strains. Plasmid analysis revealed that Rhizobium etli contained a mega-plasmid, similar in size to Rhizobium tropici. All the salt-tolerant strains examined had a plasmid of 250 kb, except for strain EBRI 29. This suggests that this plasmid
may play an important adaptive role under salt stress conditions. 相似文献