全文获取类型
收费全文 | 4669篇 |
免费 | 615篇 |
国内免费 | 653篇 |
出版年
2024年 | 4篇 |
2023年 | 114篇 |
2022年 | 97篇 |
2021年 | 180篇 |
2020年 | 213篇 |
2019年 | 239篇 |
2018年 | 192篇 |
2017年 | 203篇 |
2016年 | 208篇 |
2015年 | 257篇 |
2014年 | 245篇 |
2013年 | 297篇 |
2012年 | 229篇 |
2011年 | 262篇 |
2010年 | 172篇 |
2009年 | 270篇 |
2008年 | 283篇 |
2007年 | 280篇 |
2006年 | 252篇 |
2005年 | 211篇 |
2004年 | 204篇 |
2003年 | 173篇 |
2002年 | 169篇 |
2001年 | 139篇 |
2000年 | 135篇 |
1999年 | 128篇 |
1998年 | 111篇 |
1997年 | 73篇 |
1996年 | 89篇 |
1995年 | 66篇 |
1994年 | 62篇 |
1993年 | 51篇 |
1992年 | 57篇 |
1991年 | 55篇 |
1990年 | 28篇 |
1989年 | 24篇 |
1988年 | 29篇 |
1987年 | 30篇 |
1986年 | 17篇 |
1985年 | 16篇 |
1984年 | 11篇 |
1983年 | 10篇 |
1982年 | 13篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1975年 | 4篇 |
1974年 | 3篇 |
排序方式: 共有5937条查询结果,搜索用时 31 毫秒
61.
Zhi-Hui Su Osamu Tominaga Takeshi Ohama Eiji Kajiwara Ryoshuke Ishikawa Tokindo S. Okada Keiko Nakamura Syozo Osawa 《Journal of molecular evolution》1996,43(6):662-671
Molecular phylogenetic analyses using mitochondrial NADH dehydrogenase subunit 5 (ND5) gene sequences representing all 15 species and the majority of subspecies or races of theOhomopterus ground beetles from all over the Japanese archipelago have uncovered a remarkable evolutionary history. Clustering of the species in the molecular phylogenetic tree is linked to their geographic distribution and does not correlate with morphological characters. Taxonomically the same species or the members belonging to the same species-group fall out in more than two different places on the ND5 tree. Evidence has been presented against a possible participation of ancestral polymorphism and random lineage sorting or of hybrid individuals for the observed distribution of mitochondrial DNA haplotypes. The most plausible explanation of our results is that parallel evolution took place in different lineages. Most notably,O. dehaanii, O. yaconinus, andO. japonicus in a lineage reveal almost identical morphology with those of the same species (or subspecies) but belonging to the phylogenetically remote lineages.The nucleotide sequence data reported in this paper will appear in the DDBJ, EMBL, and GenBank nucleotide sequence databases with accession numbers D50711-DD-50733 and D87131-D87186. 相似文献
62.
We studied colony structure and body orientation behaviour of Schizolachnus pineti, a specialised aphid feeding on needles of Scots pine. Aphids feeding at the margin of a colony always face towards the centre of the colony, bounding the colony with their legs. This behaviour increased the defence success aginst the specialised parasitoid (Pauesia unilachni) and the specialised predator (Scymnus nigrinus). Both enemies had less success when attacking S. pineti from the abdominal site when compared to the front. We assume that the defence orientation displayed by marginally feeding aphids has an adaptive nature and is particularly effective when feeding in a row on linear plant structures such as needles. 相似文献
63.
Konrad Fiedler 《Entomologia Experimentalis et Applicata》1996,80(1):259-267
The Lycaenidae are the second-largest family of butterflies. From host-plant data collated for more than 1200 species worldwide, large-scale taxonomic, geographical and ecological patterns emerge which suggest that phytochemical similarities and barriers, coupled with phylogenetic conservatism and constraints are key factors governing hostplant use. More than two thirds of the lycaenid species are restricted to one plant family or genus. Affiliations with toxic plants are rare in the Lycaenidae, and excretion rather than sequestration of plant toxins appears to be their usual way of detoxifying host-plant compounds. Flavonoids are frequently sequestered by lycaenid larvae and are subsequently concentrated as pigments in the adults' wings, where they might play a role in visual communication. Mutualistic associations with ants occur in the larvae of more than 50% of the extant Lycaenidae species. Because of a conflict between the nutrient demands of the larvae and the proportion of plant-derived resources allocated to maintain the mutualism with ants, variation in resource quality often translates into variation of mutualistic capacities of the caterpillars, in particular under nutrient stress. 相似文献
64.
65.
Tansley Review No. 86 Accumulation of phytoalexins: defence mechanism and stimulus response system 总被引:1,自引:1,他引:0
C.J. SMITH 《The New phytologist》1996,132(1):1-45
66.
Joumana El-Turk Omorefe Asemota Juliette Leymarie Christophe Sallaud Stéphane Mesnage Colette Breda Dominique Buffard Adam Kondorosi Robert Esnault 《Gene》1996,170(2):213-216
We constructed an alfalfa cDNA library from mRNA extracted from leaves after infection with Pseudomonas syringae (incompatible interaction). Screening with oligodeoxyribonucleotides designed from regions conserved in all known peroxidases allowed the isolation of four cDNAs (MsprxlA, 1B, 1C and 2). Sequence analysis revealed the presence of open reading frames of 351, 355, 358 and 323 amino acids, respectively, with the characteristic consensus sequences of plant peroxidases. Sequence comparison showed that the Msprx2 product is significantly different from the others and, particularly, lacks a C-terminal propeptide which might be required for vacuolar targeting. 相似文献
67.
A meta-analysis of leaf gas exchange and nitrogen in trees grown under elevated carbon dioxide 总被引:26,自引:7,他引:19
P. S. CURTIS 《Plant, cell & environment》1996,19(2):127-137
The response of trees to rising atmospheric CO2 concentration ([CO2]) is of concern to forest ecologists and global carbon modellers and is the focus of an increasing body of research work. I review studies published up to May 1994, and several unpublished works, which reported at least one of the following: net CO2 assimilation (A), stomatal conductance (gs), leaf dark respiration (Rd) leaf nitrogen or specific leaf area (SLA) in woody plants grown at <400 μmol mol?1 CO2 or at 600–800 μmol mol?1 CO2. The resulting data from 41 species were categorized according to growth conditions (unstressed versus stressed), length of CO2 exposure, pot size and exposure facility [growth chamber (GC), greenhouse (GH), or open-top chamber (OTC)] and interpreted using meta-analytic methods. Overall, A showed a large and significant increase at elevated [CO2] but length of CO2 exposure and the exposure facility were important modifiers of this response. Plants exposed for < 50 d had a significantly greater response, and those from GCs had a significantly lower response than plants from longer exposures or from OTC studies. Negative acclimation of A was significant and general among stressed plants, but in unstressed plants was influenced by length of CO2 exposure, the exposure facility and/or pot size. Growth at elevated [CO2] resulted in moderate reductions in gs in unstressed plants, but there was no significant effect of CO2 on gs in stressed plants. Leaf dark respiration (mass or area basis) was reduced strongly by growth at high [CO2] > while leaf N was reduced only when expressed on a mass basis. This review is the first meta-analysis of elevated CO2 studies and provides statistical confirmation of several general responses of trees to elevated [CO2]. It also highlights important areas of continued uncertainty in our understanding of these responses. 相似文献
68.
69.
Isabel Allona Carmen Collada Rosa Casado Javier Paz-Ares Cipriano Aragoncillo 《Plant molecular biology》1996,32(6):1171-1176
Ch3, an endochitinase of 32 kDa present in Castanea sativa cotyledons, showed in vitro antifungal properties when assayed against Trichoderma viride. The characterization of a cDNA clone corresponding to this protein indicated that Ch3 is a class Ib endochitinase that is synthesized as a preprotein with a signal sequence preceding the mature polypeptide. Bacterial expression of mature Ch3 fused to the leader peptide of the periplasmic protein ompT resulted in active Ch3 enzyme. A plate assay was adapted for semi-quantitative determination of chitinase activity secreted from cultured bacteria, which should facilitate the identification of mutants with altered capacity to hydrolyse chitin. 相似文献
70.
The interactive effects of increased carbon dioxide (CO2) concentration and ultraviolet-B (UV-B, 280–320 nm) radiation on Acacia karroo Hayne, a C3 tree, and Themeda triandra Forsk., a C4 grass, were investigated. We tested the hypothesis that A. karroo would show greater CO2-induced growth stimulation than T. triandra, which would partially explain current encroachment of A. karroo into C4 grasslands, but that increased UV-B could mitigate this advantage. Seedlings were grown in open-top chambers in a greenhouse in ambient (360 μmol mol-1) and elevated (650 μmol mol-1) CO2, combined with ambient (1.56 to 8.66 kJ m-2 day-1) or increased (2.22 to 11.93 kJ m-2 day-1) biologically effective (weighted) UV-B irradiances. After 30 weeks, elevated CO2 had no effect on biomass of A. karroo, despite increased net CO2 assimilation rates. Interaction between UV-B and CO2 on stomatal conductance was found, with conductances decreasing only where elevated CO2 and UV-B were supplied separately. Increases in water use efficiencies, foliar starch concentrations, root nodule numbers and total nodule mass were measured in elevated CO2. Elevated UV-B caused only an increase in foliar carbon concentrations. In T. triandra, net CO2 assimilation rates were unaffected in elevated CO2, but stomatal conductances and foliar nitrogen concentrations decreased, and water use efficiencies increased. Biomass of all vegetative fractions, particularly leaf sheaths, was increased in elevated CO2. and was accompanied by increased leaf blade lengths and individual leaf and leaf sheath masses. However, tiller numbers were reduced in elevated CO2. Significantly moderating effects of elevated UV-B were apparent only in individual masses of leaf blades and sheaths, and in total sheath and shoot biomass. The direct CO2-induced growth responses of the species therefore do not support the hypothesis of CO2-driven woody encroachment of C4 grasslands. Rather, differential changes in resource use efficiency between grass and woody species, or morphological responses of grass species, could alter the competitive balance. Increased UV-B radiation is unlikely to substantially alter the CO2 response of these species. 相似文献