全文获取类型
收费全文 | 16163篇 |
免费 | 1660篇 |
国内免费 | 2篇 |
出版年
2021年 | 235篇 |
2020年 | 143篇 |
2019年 | 165篇 |
2018年 | 176篇 |
2017年 | 200篇 |
2016年 | 286篇 |
2015年 | 499篇 |
2014年 | 602篇 |
2013年 | 755篇 |
2012年 | 829篇 |
2011年 | 831篇 |
2010年 | 545篇 |
2009年 | 442篇 |
2008年 | 678篇 |
2007年 | 768篇 |
2006年 | 651篇 |
2005年 | 672篇 |
2004年 | 633篇 |
2003年 | 567篇 |
2002年 | 600篇 |
2001年 | 443篇 |
2000年 | 422篇 |
1999年 | 377篇 |
1998年 | 215篇 |
1997年 | 177篇 |
1996年 | 173篇 |
1995年 | 150篇 |
1994年 | 159篇 |
1993年 | 147篇 |
1992年 | 251篇 |
1991年 | 253篇 |
1990年 | 256篇 |
1989年 | 226篇 |
1988年 | 234篇 |
1987年 | 249篇 |
1986年 | 191篇 |
1985年 | 217篇 |
1984年 | 195篇 |
1983年 | 181篇 |
1982年 | 152篇 |
1981年 | 139篇 |
1980年 | 126篇 |
1979年 | 178篇 |
1978年 | 171篇 |
1977年 | 129篇 |
1976年 | 132篇 |
1975年 | 140篇 |
1974年 | 149篇 |
1973年 | 124篇 |
1972年 | 149篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
Frank J. Turano Kenneth Glade Wilson 《In vitro cellular & developmental biology. Plant》1985,21(3):135-139
Summary The feasibility of using plant cell culture to measure toxicity was determined by investigating the toxicological effects
of three chemical compounds, allyl alcohol, propargylglycine, and cadmium chloride, on cell cultures ofCatharanthus roseus G. Don (Madagascar periwinkle). Suspension cultures ofC. roseus were maintained in modified B5 medium and transferred every 5 d. Five-day-old cell cultures were exposed to various concentrations
(10,3,1,0.3,0.1,0.03,0.01,0.003,0.001,0.0003,0.0001, 0.00003, and 0.0 mM) of the toxicants in both acute and chronic toxicity tests.
In the acute test, cells were exposed to the toxicant for 24 h, washed three times with sterile medium, and plated in petri
plates with an equal volume of 1.4% agar medium. Cells in the chronic test were plated with an equal volume of 1.4% agar medium
containing various concentrations of the toxicant. Cells were incubated 28 d at 30°C in the dark. The colonies were counted
and the results plotted as percent survival versus toxicant concentration. The results indicate, at the concentrations tested,
thatC. roseus assay may be feasible in that it fulfills the criteria for a practical assay (e.g., rapid, simple, quantifiable, and reproducible).
This work was submitted to the faculty of Miami University in partial fulfillment of the requirements for the degree of Master
of Environmental Science, Institute of Environmental Sciences. 相似文献
4.
J. Bastow Wilson 《植被学杂志》2007,18(3):451-452
The recent Forum contribution by Grime (2006) contrasts the MacArthur/Diamond assembly‐rule approach to studying plant communities with the study of environmental trait gradients. Both are valid and useful. In doing so, Grime declares that the assembly rules model, in which negative interactions between plants act with limiting similarity to cause local trait divergence, is “not supported by empirical study of plant communities”. This is, he says, the agony of community ecology. I show that there is now abundant evidence for assembly rules, and no agony. 相似文献
5.
6.
The localization of latent brain hexokinase on synaptosomal mitochondria 总被引:10,自引:0,他引:10
J E Wilson 《Archives of biochemistry and biophysics》1972,150(1):96-104
7.
8.
9.
Carlo L. Seifert Martin Volf Leonardo R. Jorge Tomokazu Abe Grace Carscallen Pavel Drozd Rajesh Kumar Greg P.A. Lamarre Martin Libra Maria E. Losada Scott E. Miller Masashi Murakami Geoffrey Nichols Petr Pyszko Martin igut David L. Wagner Vojtch Novotný 《Ecology and evolution》2020,10(24):14137
- Assemblages of insect herbivores are structured by plant traits such as nutrient content, secondary metabolites, physical traits, and phenology. Many of these traits are phylogenetically conserved, implying a decrease in trait similarity with increasing phylogenetic distance of the host plant taxa. Thus, a metric of phylogenetic distances and relationships can be considered a proxy for phylogenetically conserved plant traits and used to predict variation in herbivorous insect assemblages among co‐occurring plant species.
- Using a Holarctic dataset of exposed‐feeding and shelter‐building caterpillars, we aimed at showing how phylogenetic relationships among host plants explain compositional changes and characteristics of herbivore assemblages.
- Our plant–caterpillar network data derived from plot‐based samplings at three different continents included >28,000 individual caterpillar–plant interactions. We tested whether increasing phylogenetic distance of the host plants leads to a decrease in caterpillar assemblage overlap. We further investigated to what degree phylogenetic isolation of a host tree species within the local community explains abundance, density, richness, and mean specialization of its associated caterpillar assemblage.
- The overlap of caterpillar assemblages decreased with increasing phylogenetic distance among the host tree species. Phylogenetic isolation of a host plant within the local plant community was correlated with lower richness and mean specialization of the associated caterpillar assemblages. Phylogenetic isolation had no effect on caterpillar abundance or density. The effects of plant phylogeny were consistent across exposed‐feeding and shelter‐building caterpillars.
- Our study reveals that distance metrics obtained from host plant phylogeny are useful predictors to explain compositional turnover among hosts and host‐specific variations in richness and mean specialization of associated insect herbivore assemblages in temperate broadleaf forests. As phylogenetic information of plant communities is becoming increasingly available, further large‐scale studies are needed to investigate to what degree plant phylogeny structures herbivore assemblages in other biomes and ecosystems.
10.
Role of a hisU gene in the control of stable RNA synthesis in Salmonella typhimurium 总被引:3,自引:0,他引:3
We have previously reported a fivefold reduction in expression of the ilvGEDA operon in a hisU mutant (hisU1820) originally isolated as a histidine regulatory mutant that exhibited derepressed (deattenuated) expression of the his operon. More recently, we have reported that a unitary explanation of the effect of this mutant on amino acid control is complicated by the observation of relaxed control of stable RNA synthesis during carbon/energy source downshifts. In the present study, we report the results of an analysis of the relaxation in control of RNA synthesis in relation to the accumulation of the guanosine polyphosphates, ppGpp and pppGpp. Unexpectedly, we observed that, despite the inability to restrict RNA accumulation upon carbon/energy downshifts, this mutant formed ppGpp at the normal rate. Further, the evidence clearly indicates that the defective control of RNA in this hisU mutant is not owing to an alteration in the spoT gene and that the relA-mediated RNA control is unaltered. However, relaxed RNA synthesis in hisU is suppressed by hyper-elevated levels of ppGpp; thus, an inverse correlation between RNA accumulation and ppGpp level during carbon/energy downshifts is still demonstrable in the hisU mutant. These data led us to the observation that the increased accumulation of stable RNA upon a carbon/energy downshift is apparently the consequence of a hisU-conferred increase in RNA stability. 相似文献