全文获取类型
收费全文 | 6757篇 |
免费 | 656篇 |
国内免费 | 3篇 |
出版年
2022年 | 40篇 |
2021年 | 105篇 |
2020年 | 50篇 |
2019年 | 51篇 |
2018年 | 71篇 |
2017年 | 83篇 |
2016年 | 137篇 |
2015年 | 253篇 |
2014年 | 264篇 |
2013年 | 369篇 |
2012年 | 411篇 |
2011年 | 378篇 |
2010年 | 278篇 |
2009年 | 235篇 |
2008年 | 368篇 |
2007年 | 382篇 |
2006年 | 360篇 |
2005年 | 348篇 |
2004年 | 350篇 |
2003年 | 364篇 |
2002年 | 330篇 |
2001年 | 78篇 |
2000年 | 41篇 |
1999年 | 66篇 |
1998年 | 121篇 |
1997年 | 75篇 |
1996年 | 66篇 |
1995年 | 74篇 |
1994年 | 45篇 |
1993年 | 68篇 |
1992年 | 51篇 |
1991年 | 47篇 |
1990年 | 48篇 |
1989年 | 39篇 |
1988年 | 36篇 |
1987年 | 57篇 |
1986年 | 49篇 |
1985年 | 50篇 |
1984年 | 70篇 |
1983年 | 38篇 |
1982年 | 54篇 |
1981年 | 67篇 |
1980年 | 52篇 |
1978年 | 49篇 |
1977年 | 40篇 |
1976年 | 39篇 |
1974年 | 54篇 |
1973年 | 49篇 |
1972年 | 34篇 |
1961年 | 36篇 |
排序方式: 共有7416条查询结果,搜索用时 15 毫秒
111.
Enzymes for the methylation of 1hydroxycanthin-6-one and a series of coumarins have been isolated from Ailanthus altissima cell suspension cultures. The coumarin methyltransferases methylate aesculetin to scopoletin and isoscopoletin, but not scopoletin, to scoparone. Fraxetin was methylated to isofraxidine but not to fraxidine and only fraxidine was methylated to 6,7,8-trimethoxycoumarin. These enzymes were studied throughout the culture growth cycle with two cell lines: 1, which produced 1-methoxycanthin-6-one as the major alkaloid and 2, in which canthin-6-one was the major alkaloid.Abbreviations UV
ultraviolet
- DEAE
diethylaminoethyl
- dw
dry weight
- MS
mass spectrometry
- NMR
nuclear magnetic resonance
- TLC
thin layer chromatography 相似文献
112.
Size structure of the metazoan community in a Piedmont stream 总被引:1,自引:0,他引:1
N. LeRoy Poff Margaret A. Palmer Paul L. Angermeier Robert L. Vadas Jr. Christine C. Hakenkamp Alexa Bely Peter Arensburger Andrew P. Martin 《Oecologia》1993,95(2):202-209
We characterized the size structure of virtually the entire metazoan community in a fourth order, sandybottomed Piedmont stream during late summer. Our study, the first to sample across all habitat types and sizes of metazoans in an aquatic ecosystem, indicates that at the community level, stream size spectra may be bimodal for the benthos or trimodal when fish are included. Animals spanning 10 orders of magnitude in dry mass (from gastrotrichs to fish) were quantitatively collected from nine habitat types. The bimodal benthic size spectrum was characterized by a meiofaunal component (mostly oligochaetes and micro-crustacea) and a macrobenthic component (mostly the introduced asiatic clam, Corbicula fluminea). Insects contributed little to overall standing crop. Size-specific contribution to whole-community metabolism was assessed using allometric equations for respiration, and we found a distinctly bimodal distribution across the entire metazoan size range, with peaks in the meiofaunal and benthic macrofaunal size ranges. Our bimodal benthic size spectrum is similar to that observed for marine benthos but not to other freshwater benthic systems, possibly because the entire range of habitat types and/or animal sizes were not sampled in the latter. Numerous factors may influence size spectra in stream ecosystems, including local geomorphic (habitat) conditions, water level fluctuations, species introductions, and predation processes. 相似文献
113.
114.
Margaret C. Gawienowski Daniel Szymanski Imara Y. Perera Raymond E. Zielinski 《Plant molecular biology》1993,22(2):215-225
Three new, unique cDNA sequences encoding isoforms of calmodulin (CaM) were isolated from an Arabidopsis cDNA library cloned in gt10. These sequences (ACaM-4, -5, and -6) represent members of the Arabidopsis CaM gene family distinct from the three DNA sequences previously reported. ACaM-4 and -6 encode full-length copies of CaM mRNAs of ca. 0.75 kb. The ACaM-5 sequence encodes a partial length copy of CaM mRNA that is lacking sequences encoding the amino-terminal 10 amino acids of mature CaM and the initiator methionine. The derived amino acid sequence of ACaM-5 is identical to the sequences encoded by two of the previously characterized ACaM cDNAs, and is identical to TCH-1 mRNA, whose accumulation was increased by touch stimulation. The polypeptides encoded by ACaM-4 and -6 differ from that encoded by ACaM-5 by six and two amino acid substititions, respectively. Most of the deduced amino acid sequence substitutions in the Arabidopsis CaM isoforms occurred in the fourth Ca2+-binding domain. Polymerase chain reaction amplification assays of ACaM-4, -5 and -6 mRNA sequences indicated that each accumulated in Arabidopsis leaf RNA fractions, but only ACaM-4 and -5 mRNAs were detected in silique total RNA. The six different CaM cDNA sequences each hybridize with unique Eco RI restriction fragments in genomic Southern blots of Arabidopsis DNA, indicating that these sequences were derived from distinct structural genes. Our results suggest that CaM isoforms in Arabidopsis may have evolved to optimize the interaction of this Ca2+-receptor protein with specific subsets of response elements. 相似文献
115.
116.
Asim K. Dutta-Roy Margaret J. Gordon Derek J. Leishman Brian J. Paterson Garry G. Duthie William P. T. James 《Molecular and cellular biochemistry》1993,123(1-2):139-144
An -tocopherol-binding protein has been isolated and purified from rabbit heart cytosol. The purified protein had an apparent molecular mass of 14,200, as derived from SDS-PAGE. The content of the protein in rabbit heart was around 11.8 g per g of tissue. The binding of -tocopherol to the purified protein was rapid, reversible, and saturable. Neither nor tocopherol could displace the bound -tocopherol from the protein, suggesting a high specificity for -tocopherol. -Tocopherol-binding protein did not bind oleate. Transfer of -tocopherol from liposomes to mitochodria was stimulated 8-fold in the presence of the binding protein, suggesting that this protein may be involved in the intracellular transport of -tocopherol in the heart. 相似文献
117.
Carles Ibáñez Nuno Caiola José Barquín Oscar Belmar Xavier Benito-Granell Frederic Casals Siobhan Fennessy Jocelyne Hughes Margaret Palmer Josep Peñuelas Estela Romero Jordi Sardans Michael Williams 《Global Change Biology》2023,29(5):1248-1266
Trends and ecological consequences of phosphorus (P) decline and increasing nitrogen (N) to phosphorus (N:P) ratios in rivers and estuaries are reviewed and discussed. Results suggest that re-oligotrophication is a dominant trend in rivers and estuaries of high-income countries in the last two–three decades, while in low-income countries widespread eutrophication occurs. The decline in P is well documented in hundreds of rivers of United States and the European Union, but the biotic response of rivers and estuaries besides phytoplankton decline such as trends in phytoplankton composition, changes in primary production, ecosystem shifts, cascading effects, changes in ecosystem metabolism, etc., have not been sufficiently monitored and investigated, neither the effects of N:P imbalance. N:P imbalance has significant ecological effects that need to be further investigated. There is a growing number of cases in which phytoplankton biomass have been shown to decrease due to re-oligotrophication, but the potential regime shift from phytoplankton to macrophyte dominance described in shallow lakes has been documented only in a few rivers and estuaries yet. The main reasons why regime shifts are rarely described in rivers and estuaries are, from one hand the scarcity of data on macrophyte cover trends, and from the other hand physical factors such as peak flows or high turbidity that could prevent a general spread of submerged macrophytes as observed in shallow lakes. Moreover, re-oligotrophication effects on rivers may be different compared to lakes (e.g., lower dominance of macrophytes) or estuaries (e.g., limitation of primary production by N instead of P) or may be dependent on river/estuary type. We conclude that river and estuary re-oligotrophication effects are complex, diverse and still little known, and in some cases are equivalent to those described in shallow lakes, but the regime shift is more likely to occur in mid to high-order rivers and shallow estuaries. 相似文献
118.
In this study we measured the exogenous production and the intracellular content of elemental sulfur (S0) in the thermophilic sulfur-oxidizing bacteriaHydrogenobacter spp. andBacillus schlegelii during mixolithoautotrophic growth on hydrogen and thiosulfate. Under these conditions, all strains produced and released white-yellow hydrophilic S0 particles into the growth medium. Hydrophilic S0 was separated from cells by a differential low-speed centrifugation procedure. The S0 pellets were dried, and the S0 was purified by column chromatography and by thin-layer chromatography (TLC). The S0 TLC-band could be stained with triphenyltetrazolium chloride and piperidine procedure. Determination of intracellular S0 content was performed from fresh cells absolutely free of exogenous S0 particles. quantitation of S0 was performed by high-performance liquid chromatography, colorimetric thiocyanate procedure, and by UV-spectra analyses. All the strains studied, in particularB. schlegelii strains, released significant quantities of S0 into the growth media. In contrast, the intracellular S0 content was very low. Significant rhodanese activity in the presence of thiosulfate was measured with toluenepermeabilized cells and cell-free extracts ofB. schlegelii (type strain) andHydrogenobacter spp. (strain T3). 相似文献
119.
Margaret B. Fleming Lauren Stanley Robyn Zallen Matthew T. Chansler Lars A. Brudvig David B. Lowry Marjorie Weber Frank W. Telewski 《American journal of botany》2023,110(11):e16250
Premise
In 1879, Dr. William Beal buried 20 glass bottles filled with seeds and sand at a single site at Michigan State University. The goal of the experiment was to understand seed longevity in the soil, a topic of general importance in ecology, restoration, conservation, and agriculture, by periodically assaying germinability of these seeds over 100 years. The interval between germination assays has been extended and the experiment will now end after 221 years, in 2100.Methods
We dug up the 16th bottle in April 2021 and attempted to germinate the 141-year-old seeds it contained. We grew germinants to maturity and identified these to species by vegetative and reproductive phenotypes. For the first time in the history of this experiment, genomic DNA was sequenced to confirm species identities.Results
Twenty seeds germinated over the 244-day assay. Eight germinated in the first 11 days. All 20 belonged to the Verbascum genus: Nineteen were V. blattaria according to phenotype and ITS2 genotype; and one had a hybrid V. blattaria × V. thapsus phenotype and ITS2 genotype. In total, 20/50 (40%) of the original Verbascum seeds in the bottle germinated in year 141.Conclusions
While most species in the Beal experiment lost all seed viability in the first 60 years, a high percentage of Verbascum seeds can still germinate after 141 years in the soil. Long-term experiments such as this one are rare and invaluable for studying seed viability in natural soil conditions. 相似文献120.
Margaret Skinner B. L. Parker W. E. Wallner T. M. Odell Diantha Howard J. Aleong 《BioControl》1993,38(1):15-29
A 2-year study was conducted on the distribution of parasitoids of gypsy moth,Lymantria dispar (L.) (Lep.: Lymantriidae), in mesic and adjacent higher elevation transition and xeric forest habitats in Vermont (U.S.A.). In both years, overall parasitism ranged from 12–18% in each habitat. When analyzed according to the life stage at which the host was collected, parasitism rates of greater than 40% were obtained among the late instars.Parasetigena silvestris (Robineau-Desvoidy) andPhobocampe disparis (Viereck) were recovered most commonly from the mesic habitat, andCotesia melanoscelus (Ratzeburg) andBlepharipa pratensis (= Sturmia scutellata) (Meigen) were most common in collections from the xeric area. Parasitism byCompsilura concinnata (Meigen) occurred at similar levels in all three habitats, and this species was responsible for the highest parasitism rates on the site, reaching 40% among the late instars in 1985. Percent parasitism byC. concinnata increased three-four-fold from 1984 to 1985, while parasitism by other species declined. 相似文献