全文获取类型
收费全文 | 2558篇 |
免费 | 83篇 |
国内免费 | 1篇 |
出版年
2023年 | 23篇 |
2021年 | 22篇 |
2018年 | 69篇 |
2017年 | 69篇 |
2016年 | 72篇 |
2015年 | 89篇 |
2014年 | 98篇 |
2013年 | 125篇 |
2012年 | 129篇 |
2011年 | 117篇 |
2010年 | 110篇 |
2009年 | 71篇 |
2008年 | 39篇 |
2007年 | 39篇 |
2006年 | 37篇 |
2005年 | 143篇 |
2004年 | 108篇 |
2003年 | 83篇 |
2002年 | 24篇 |
2001年 | 31篇 |
1999年 | 21篇 |
1998年 | 21篇 |
1996年 | 27篇 |
1995年 | 29篇 |
1994年 | 21篇 |
1993年 | 25篇 |
1992年 | 22篇 |
1991年 | 34篇 |
1990年 | 25篇 |
1988年 | 28篇 |
1987年 | 24篇 |
1986年 | 25篇 |
1985年 | 28篇 |
1984年 | 37篇 |
1983年 | 40篇 |
1982年 | 28篇 |
1981年 | 31篇 |
1980年 | 32篇 |
1979年 | 59篇 |
1978年 | 31篇 |
1977年 | 24篇 |
1976年 | 24篇 |
1975年 | 35篇 |
1974年 | 33篇 |
1973年 | 31篇 |
1972年 | 31篇 |
1971年 | 21篇 |
1970年 | 24篇 |
1969年 | 35篇 |
1966年 | 23篇 |
排序方式: 共有2642条查询结果,搜索用时 15 毫秒
1.
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.
2.
The production of L-lysine fromDL-α-amino-ε-caprolactam (DL-ACL) by new strains producingL-α-amino-ε-caprolactamase and aminocaprolactam racemase is described. Optimal conditions for hydrolysis ofL-ACL byCryptococcus sp. and for racemization of ACL by cells of a strain isolated in nature and identified asPseudomonas sp. were determined. Synthesis ofL-α-amino-ε-caprolactamase is induced byDL-ACL orL-lysine with the same effectivity. A positive effect of phosphates (potassium salts) on reduction of the induction lag was
detected, the synthesis of this enzyme was found to be repressed by glucose and some possibilities of the reversion of this
repressive effect were demonstrated. Under conditions optimal for the production of both enzymes a quantitative theoretical
conversion of 10 % aqueousDL-ACL toL-lysine by a mixture of native cells in a mass ratio of 1: 2 (producer of ACL-hydrolase to producer of ACL-racemase) occurred
in 8 h at 40 °C and pH 8.0 相似文献
3.
Effect of precursors on biosynthesis of monensins A and B 总被引:1,自引:0,他引:1
Precursors of monensins (acetate, propionate, butyrate, isobutyrate) affect the total production and the relative proportion of monensins A and B. Addition of propionate into the fermentation medium causes a prevalence of monensin B whereas butyrate and isobutyrate stimulate the production of monensin A and suppress the production of monensin B. 相似文献
4.
Collections from high altitudes (3840–5100 m a.s.l.) have added several species to the Pannariaceae of Ecuador: Fuscopannaria praetermissa, Pannaria hookeri, Psoroma paleaceum and Psoroma tenue are new to the Andes, and phytogeographic consequences of these are discussed. The following new taxa are described: Parmeliella corallina P. M. Jørg. & Palice, Parmeliella psoromoides P. M. Jørg. & Palice, Psoroma cinnamomeum ssp. andinum P. M. Jørg. & Palice and Santessoniella macrospora P. M. Jørg. & Palice. They are part of a high‐andean element which is not well known and is indicative of a migration route between the polar regions. In addition, two species with associated cyanobacteria, as the primary or secondary symbiont, are reported for the first time from South America: Thelignya lignyota (Wahlenb.) P. M. Jørg. & Henssen and Pilophorus cereolus (Ach.) Th. Fr, both originating from the Northern Hemisphere. 相似文献
5.
Marie Lipoldová Helena Havelková Jana Badalová Jarmila Vojtíšková Lei Quan Magdaléna Krulová Yahya Sohrabi Alphons P. Stassen Peter Demant 《Cancer immunology, immunotherapy : CII》2010,59(2):203-213
Low infiltration of lymphocytes into cancers is associated with poor prognosis, but the reasons why some patients exhibit
a low and others a high infiltration of tumors are unknown. Previously we mapped four loci (Lynf1–Lynf4) controlling lymphocyte infiltration of mouse lung tumors. These loci do not encode any of the molecules that are involved
in traffic of lymphocytes. Here we report a genetic relationship between these loci and the control of production of IFNγ
in allogeneic mixed lymphocyte cultures (MLC). We found that IFNγ production by lymphocytes of O20/A mice is lower than by
lymphocytes of OcB-9/Dem mice (both H2
pz
) stimulated in MLC by irradiated splenocytes of C57BL/10SnPh (H2
b
) or BALB/cHeA (H2
d
) mice, or by ConA. IFNγ production in MLCs of individual (O20 × OcB-9)F2 mice stimulated by irradiated C57BL/10 splenocytes and genotyped for microsatellite markers revealed four IFNγ-controlling
loci (Cypr4-Cypr7), each of which is closely linked with one of the four Lynf loci and with a cluster of susceptibility genes for different tumors. This suggests that inherited differences in certain
lymphocyte responses may modify their propensity to infiltrate tumors and their capacity to affect tumor growth. 相似文献
6.
Vladimír Křen Jiří Ludvík Olga Kofroňová Jaroslava Kozová Zdeněk Řeháček 《Applied microbiology and biotechnology》1987,26(3):219-226
Summary The 550-day semicontinuous cultivation of Claviceps fusiformis immobilized in calcium alginate is documented. The vegetative mycelium from seed or from early-production submerged culture is the best choice for immobilization. No extracellular glucans are produced by immobilized cells. Immobilized spores give low yields of clavine alkaloids. Alginate concentrations in a range of 2%–4% do not influence yield and spectrum of alkaloids. The cytoplasm of the immobilized cells becomes condensed (after 3 days), polysaccharides disappear, and centres of lipid synthesis are formed in the cytoplasm. After 60 days the cells harbour a great number of lipid particles, mitochondria are diminishing and their cristae partly disappear, indicating a decreased respiration capacity. After 350–500 days the volume of most cells is increased many times and the cells are filled with large oval bodies of electrondense material. Chloramphenicol protects immobilized cultures against bacterial contamination. 相似文献
7.
7-Methylguanine (m7Gua), N2,N2-dimethylguanosine (m2(2)Guo), and pseudouridine (psi) are degradation products from RNA turnover and can be used as markers for the whole-body turnover of mRNA-cap, tRNA, and rRNA (in healthy individuals, urinary excretion of these catabolites follows a regular pattern; the relative molar ratio of psi:m7Gua:m2(2)Guo is approximately 100:19:6). HPLC methods were developed to measure serum concentrations of these RNA catabolites after deproteinization of the samples by ultrafiltration through microcollodion bags with a nominal exclusion Mr of 12,400. For healthy adults the following values (mean +/- SD) were found: psi, 2760 +/- 460 nmol/liter (n = 10); m7Gua, 129.7 +/- 24.0 nmol/liter (n = 13); m2(2)Guo, 31.0 +/- 3.7 nmol/liter (n = 9). The relative molar ratio of these substances in serum derived from our data is approximately 100:4.7:1.1. 7-Methylguanosine (m7Guo) added to serum is to a large extent converted to the corresponding free base, m7Gua, the form which is excreted in urine. 相似文献
8.
During the search for compounds with insect juvenile hormone activity, the biotransformation of 2-(4-methoxybenzyl)-l-cyclohexanone,
of 2-(4-methoxybenzyl)-l-cyclohexanone ethylene acetal and of both isomers of 2-(4-methoxybenzyl)-l-cyclohexanol by plant
cells was examined. The compounds were metabolized by cell suspension culture of Solatium aviculare Forst. The reaction conditions
were optimized and the metabolic products isolated and identified. A scheme of biotransformation pathway has been proposed. 相似文献
9.
Marta Munzarová Daniela Zemanová Jan Kovařík Zdeněk Pačovský Aleš Rejthar Jiří Bártek 《Cancer immunology, immunotherapy : CII》1987,24(3):272-274
Summary Skin tests with autologous cholesteryl hemisuccinate (CHS)-treated and untreated cells were performed in ten metastatic melanoma patients. In the majority of cases evident reaction was noted with CHS-treated cells (9/10) while the reaction with untreated cells was mostly negative (7/10). Tumour cell suspensions used for skin tests were characterized for reactivity with monoclonal antibody TAL 1B5 detecting the HLA-DR alpha chain. There were no differences between CHS-treated and untreated cells with respect to HLA-DR expression and no correlation was found between grade of skin reaction to CHS-treated cells and the proportion of HLA-DR positive cells in the injected cell sample. 相似文献
10.
B. Koukalová J. Reich R. Matyášek V. Kuhrová M. Bezděk 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1989,78(1):77-80
Summary HRS60.1, a monomer unit (184 bp) of a highly repeated nuclear DNA sequence of Nicotiana tabacum, has been cloned and sequenced. Following BamHI digestion of tobacco DNA, Southern hybridization with HRS60.1 revealed a ladder of hybridization bands corresponding to multiples of the basic monomer unit. If the tobacco DNA was digested with restriction endonucleases which have no target site in HRS60.1, the larger part of DNA homologous to HRS60.1 remained as uncleaved relic DNA. These results suggest a tandem arrangement of this DNA repeat unit. Four other clones of tobacco nuclear DNA cross-hybridized with HRS60.1, thus forming a HRS60-family. Sequencing their inserts has shown their strong mutual homology. HRS60-family comprised about 2% of the nuclear genome of N. tabacum. Computer comparisons with other tandem plant-repeated DNA sequences could not detect any other homologous sequence. 相似文献