全文获取类型
收费全文 | 4174篇 |
免费 | 343篇 |
国内免费 | 3篇 |
出版年
2023年 | 18篇 |
2022年 | 35篇 |
2021年 | 69篇 |
2020年 | 36篇 |
2019年 | 64篇 |
2018年 | 105篇 |
2017年 | 97篇 |
2016年 | 125篇 |
2015年 | 187篇 |
2014年 | 242篇 |
2013年 | 295篇 |
2012年 | 347篇 |
2011年 | 309篇 |
2010年 | 200篇 |
2009年 | 185篇 |
2008年 | 264篇 |
2007年 | 243篇 |
2006年 | 255篇 |
2005年 | 227篇 |
2004年 | 237篇 |
2003年 | 214篇 |
2002年 | 194篇 |
2001年 | 54篇 |
2000年 | 37篇 |
1999年 | 52篇 |
1998年 | 45篇 |
1997年 | 32篇 |
1996年 | 30篇 |
1995年 | 34篇 |
1994年 | 23篇 |
1993年 | 28篇 |
1992年 | 20篇 |
1991年 | 19篇 |
1990年 | 17篇 |
1989年 | 26篇 |
1988年 | 17篇 |
1987年 | 6篇 |
1986年 | 10篇 |
1985年 | 11篇 |
1984年 | 10篇 |
1983年 | 8篇 |
1982年 | 10篇 |
1981年 | 8篇 |
1980年 | 13篇 |
1979年 | 13篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1976年 | 7篇 |
1974年 | 4篇 |
1960年 | 3篇 |
排序方式: 共有4520条查询结果,搜索用时 31 毫秒
21.
Life histories and habitat selection in Daphnia: divergent life histories of D. magna clones differing in phototactic behaviour 总被引:4,自引:0,他引:4
Luc De Meester 《Oecologia》1994,97(3):333-341
To test the hypothesis of co-adaptation of life histories and daytime vertical distribution (vertical migration behaviour) in Daphnia, life history characteristics were analysed for two positively, three negatively, and four intermediately phototactic Daphnia magna clones. Clones with different phototactic behaviour were found to have divergent life history strategies, with positively phototactic clones being good exploiters under the non-limiting conditions provided in the laboratory, i.e. low density (1 ind./1), high food concentration (6,5–7 105
Scenedesmus cells/ml, restored daily) and high temperature (20° C). They realized a high intrinsic rate of increase at a small adult body size through rapid development, at a cost of producing small neonates. Negatively and intermediately phototactic clones had larger adult body sizes, and produced larger neonates that were more starvation-resistant than those of positively phototactic clones. Selection for high intrinsic rate of increase in intermediately phototactic clones was mediated through the production of large clutches. 相似文献
22.
Olivier Bornet Gérard Lancelot Luc Chanteloup Nguyen T. Thuong Jean-Marie Beau 《Journal of biomolecular NMR》1994,4(4):575-580
Summary We present NMR studies of an intramolecular triple helix, the three strands of which have been linked by a hexaethylene glycol chain. To overcome the generally encountered difficulties of assignment in the homopyrimidine strands, the carbon C1 of the pyrimidines were selectively 13C-enriched. Assignments of the aromatic and sugar protons were obtained from NOESY-HMQC and TOCSY-HMQC spectra. We show that the recognition of a DNA duplex by a third strand via triplex formation is easily carried out in solution by observing the changes of the 1H1–13C1 connectivities as a function of pH. Furthermore, the conformation of the sugars has been found to be C2-endo, on the basis of the coupling constant values directly measured in an HSQC spectrum. 相似文献
23.
Bert Weckhuysen Luc Vriens Hubert Verachtert 《Applied microbiology and biotechnology》1993,39(3):395-399
Butanal is one of the odorous compounds produced in the animal-rendering and food-processing industries and also in sewage-treatment plants. It shows the necessity for complementing such plants with systems for off-gas treatment. Biofiltration using simple packing material was tested for the removal of butanal. Excellent results were obtained when the filters operated at optimal humidity and were supplemented with inorganic nutrients. Without nutrients, butyric acid was detected in the effluent gas, which may explain the lower efficiency of filters without nutrients. Under optimal conditions an elimination of around 90 g·m–3·h–1 was reached. 相似文献
24.
The influence of different phosphorus and nitrogen sources on Lactococcus lactis subsp. lactis NIZO 22186 growth and nisin production was studied in batch fermentations using a complex medium. KH2PO4 was found to be the best phosphorus source for nisin production. Increasing initial phosphate concentrations from 0 to 5% KH2PO4 exerted a double effect, creating favourable pH conditions and particularly stimulating the nisin production levels, which were highest at 5% KH2PO4. Up to now, no such high initial phosphate concentrations have been reported for the production of other antibiotics or bacteriocins. Nisin, a lanthionine-containing peptide antibiotic with bacteriocin properties, clearly behaved as a primary metabolite, since its formation was linked with active growth and was not suppressed by phosphate concentrations up to 5%. A complex medium supplemented with cotton seed meal as nitrogen source also gave very high nisin yields.
Correspondence to: L. De Vuyst 相似文献
25.
Molecular characterization of salt-stress-associated protein in citrus: protein and cDNA sequence homology to mammalian glutathione peroxidases 总被引:16,自引:0,他引:16
Doron Holland Gozal Ben-Hayyim Zehava Faltin Luc Camoin A. Donny Strosberg Yuval Eshdat 《Plant molecular biology》1993,21(5):923-927
A gene encoding for a citrus salt-stress-associated protein (Cit-SAP) was cloned from Citrus sinensis salt-treated cell suspension. The gene, designated csa, was isolated from a cDNA expression library. The partial amino acid sequence of the protein, as well as that deduced from the nucleotide sequence of csa, revealed a considerable homology to mammalian glutathione peroxidase (GP), and to clone 6P229 from tobacco protoplasts. The increased expression of Cit-SAP in NaCl-treated cultured citrus cells and in citrus plants irrigated with saline water, and its similarity to GP, raise the possibility that one of the effects of salt stress in plants may be the increase of the level of free radicals. 相似文献
26.
27.
We compare the primary sex ratio (proportion of haploid eggs laid by queens) and the secondary sex ratio (proportion of male pupae produced) in the Argentine ant Iridomyrmex humilis with the aim of investigating whether workers control the secondary sex ratio by selectively eliminating male brood. The proportion of haploid eggs produced by queens was close to 0.5 in late winter, decreased to less than 0.3 in spring and summer, and increased again to a value close to 0.5 in fall. Laboratory experiments indicate that temperture is a proximate factor influencing the primary sex ratio with a higher proportion of haploid eggs being laid at colder temperatures. Production of queen pupae ceased in mid-June, about three weeks before that of male pupae. After this time only worker pupae were produced. During the period of production of sexuals, the proportion of male pupae ranged from 0.30 to 0.38. Outside this period no males were reared although haploid eggs were produced all the year round by queens. Workers thus exert a control on the secondary sex ratio by eliminating a proportion of the male brood during the period of sexual production and eliminating all the males during the remainder of the cycle. These data are consistent with workers preferring a more female-biased sex ratio than queens. The evolutionary significance of the production of male eggs by queens all the year round is as yet unclear. It may be a mechanism allowing queen replacement in the case of the death of the queens in the colony. 相似文献
28.
M. Maes F. De Meyer D. Peeters H. Meltzer C. Schotte S. Scharpe P. Cosyns 《International journal of biometeorology》1993,37(2):78-82
Recently, some investigators have established a seasonal pattern in normal human psychology, physiology and behaviour, and in the incidence of psychiatric psychopathology. In an attempt to elucidate the chronopsy and meteotropism in the latter, we have examined the chronograms of, and the biometeorological relationships to bed occupancy of the psychiatric ward of the Antwerp University Hospital during three consecutive calendar years (1987–1989). Weather data for the vicinity were provided by a local meteorological station and comprise mean atmospheric pressure, air temperature, relative humidity, wind speed and minutes of sunlight and precipitation/day. The number of psychiatric beds occupied during the study period exhibited a significant seasonal variation. Peaks in bed occupancy were observed in March and November, with lows in August. An important part of the variability in the number of beds occupied could be explained by the composite effects of weather variables of the preceding weeks. Our results suggest that short-term fluctuations in atmospheric activity may dictate some of the periodicities in psychiatric psychopathology. 相似文献
29.
A. Maes A. Hilali E. D. Léonard A. Léonard L. Verschaeve 《Radiation and environmental biophysics》1993,32(4):319-324
A thorough cytogenetic analysis using G-banding was performed on 100 peripheral blood lymphocytes from an individual who had been accidentally exposed to radiation more than 25 years previously. More than 60% of the analysed cells were found to possess one or more stable chromosome aberrations (e.g. reciprocal translocations). Chromosomes 1 and 11 were more involved in these aberrations than would be expected from the relative chromosome lengths. No identical stable aberrations were found, suggesting that, 25 years after nearlethal exposure, haemopoietic stem cells display substantial diversity. 相似文献