全文获取类型
收费全文 | 8520篇 |
免费 | 1131篇 |
国内免费 | 1011篇 |
出版年
2024年 | 28篇 |
2023年 | 232篇 |
2022年 | 246篇 |
2021年 | 303篇 |
2020年 | 327篇 |
2019年 | 386篇 |
2018年 | 350篇 |
2017年 | 369篇 |
2016年 | 353篇 |
2015年 | 315篇 |
2014年 | 421篇 |
2013年 | 725篇 |
2012年 | 299篇 |
2011年 | 439篇 |
2010年 | 344篇 |
2009年 | 518篇 |
2008年 | 506篇 |
2007年 | 457篇 |
2006年 | 453篇 |
2005年 | 405篇 |
2004年 | 361篇 |
2003年 | 342篇 |
2002年 | 316篇 |
2001年 | 233篇 |
2000年 | 171篇 |
1999年 | 215篇 |
1998年 | 175篇 |
1997年 | 161篇 |
1996年 | 144篇 |
1995年 | 146篇 |
1994年 | 128篇 |
1993年 | 108篇 |
1992年 | 89篇 |
1991年 | 91篇 |
1990年 | 64篇 |
1989年 | 39篇 |
1988年 | 52篇 |
1987年 | 42篇 |
1986年 | 37篇 |
1985年 | 41篇 |
1984年 | 53篇 |
1983年 | 26篇 |
1982年 | 55篇 |
1981年 | 27篇 |
1980年 | 13篇 |
1979年 | 20篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1973年 | 4篇 |
1971年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 718 毫秒
91.
Comparative study of two meromictic basins of Lake Banyoles (Spain) with sulphur phototrophic bacteria 总被引:1,自引:1,他引:0
The annual limnological dynamics of two meromictic basins of Lake Banyoles (C-III and C-IV) have been studied and compared
on the basis of their physical, chemical and biological characters. Stability values calculated for both basins gave 865 g
cm cm−2 and 495 g cm cm−2 for C-III and C-IV respectively. These values are in agreement with the fact that C-IV was almost completely mixed during
winter. In this basin, during stratification, the monimolimnion increased in thickness as the stability increased. Isolation
of the respective monimolimnia resulted in the development of anoxic conditions and the accumulation of sulphide in both C-III
and C-IV, which favoured the development of dense populations of sulfur phototrophic bacteria. The purple sulphur bacterium
Chromatium minus and the green sulphur bacterium Chlorobium phaeobacteroides were identified as the main components of these photosynthetic populations. The different depths at which the O2/H2S boundary was situated in both basins (and consequently the different light intensity reaching this zone) determined the
growth of these bacteria. Light intensities at the chemocline of C-IV reached values up to 5% of surface incident light. In
contrast, in C-III this variable was sensibly lower, with values depending on season and seldom reaching 1%. Phototrophic
bacteria were consequently found earlier in C-IV than in C-III, where no significant concentrations were found until August.
Finally stability is discussed as an important factor controlling chemical and biological dynamics in meromictic lakes. 相似文献
92.
93.
Abstract The global regulators agr and sar control expression of cell wall and extracellular proteins. Inactivation of either sar and/or agr in a typical heterogeneously methicillin-resistant Staphylococcus aureus resulted in a small but reproducible decrease in the number of cells in the subpopulation expressing high methicillin resistance. The amount of low affinity penicillin-binding protein PBP2', the prerequisite for methicillin resistance, was apparently not affected, however, a reduction in PBP1 and PBP3 production was observed, suggesting that these resident PBPs of the cells might be involved somehow together with PBP2' in high level methicillin resistance. 相似文献
94.
福建武夷山甜槠群落能量的研究 总被引:43,自引:0,他引:43
在生物量、生产力研究基础上,对武夷山甜槠(Castanopsis eyrei(Cham p.ex Benth.) Tutch.)群落各组分的热值、群落能量现存量、能量年净固定量以及太阳能转化效率进行了研究。结果表明:(1)甜槠群落各组分样品的干重热值具有一定的差异,树皮热值最高,细根热值最低。(2)甜槠群落的能量现存量达780584.1 kJ·m - 2,其中地上部分为678913.8 kJ·m - 2,占总量的86.98% ;地下部分为101670.3 kJ·m - 2,占13.02% 。(3)甜槠群落的能量年净固定量(1992年)为26856.2 kJ·m - 2·a- 1,林地太阳光合有效辐射能的转化效率为1.296% 。 相似文献
95.
96.
Kazushige Morimoto Eisuke Tsuda Ahmed Abdu Said Eriko Uchida Satoshi Hatakeyama Masatsugu Ueda Takao Hayakawa 《Glycoconjugate journal》1996,13(6):1013-1020
Ten erythropoietin (EPO) fractions differing in sialic acid content, ranging from 9.5 to 13.8 mol mol–1 of EPO, were obtained from baby hamster kidney cell-derived recombinant human EPO by Mono Q column chromatography. The mean pI values of the EPO fractions determined by IEF-gel electrophoresis systematically shifted from 4.11 to 3.31, coinciding with the sialic acid content, without a change in the constitution of asialo N-linked oligosaccharides of each fraction. Although a linear relationship between thein vivo bioactivity and the sialic acid content of the fractionated, samples was observed until 12.1 mol mol–1 of EPO, there was no further increase in their activity over 12.4 mol mol–1 of EPO. On the other hand, an inverse relationship between thein vitro bioactivity and sialic acid content of EPO was observed. Also, we showed that thein vivo bioactivity of some fractions with low sialic acid contents was increased after treatment with 2,6-sialyltransferase, but thein vivo bioactivity of the other fractions with high sialic acid contents was either decreased or not affected.Abbreviations EPO
erythropoietin
- rHuEPO
recombinant human erythropoietin
- hCG
human chorionic gonadotropin
- BHK
baby hamster kidney
- CHO
Chinese hamster ovary
- NeuAc
N-acetyl neuraminic acid
- Gal
galactose
- HRCs
hemolyser-resistant cells
- WST-1
2-(4-iodophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium Na
- IEF
isoelectric focusing
- pI
isoelectric point 相似文献
97.
The intrinsic chlorophyll-protein CP 47 is a component of photosystem II which functions in both light-harvesting and oxygen evolution. Using site-directed mutagenesis we have produced the mutant W167S which lies in loop C of CP 47. This strain exhibited a 75% loss in oxygen evolution activity and grew extremely slowly in the absence of glucose. Examination of normalized oxygen evolution traces indicated that the mutant was susceptible to photoinactivation. Analysis of the variable fluorescence yield indicated that the mutant accumulated very few functional PS II reaction centers. This was confirmed by immunoblotting experiments. Interestingly, when W167S was grown in the presence of 20 M DCMU, the mutant continued to exhibit these defects. These results indicate that tryptophan 167 in loop C of CP 47 is important for the assembly and stability of the PS II reaction center. 相似文献
98.
Genetically stable cell lines of cucumber for the large-scale production of diploid somatic embryos 总被引:2,自引:0,他引:2
Marc Kreuger Wiert van der Meer Erik Postma Rob Abbestee Natasja Raaijmakers Gerrit-Jan van Holst 《Physiologia plantarum》1996,97(2):303-310
For the initiation of embryogenic cucumber ( Cucumis sativus L.) cell lines, from excised radicles, directly in liquid medium, the culture regime, explant density and type and concentration of hormones were adjusted so that pro-embryogenic masses (PEMs) were formed within about 8 weeks. The established cucumber cell lines were maintained tor several years without loss of embryogenic and genetic stability. The ploidy level of somatic embryos from different cucumber eell lines was either diploid or tetraploid and depended on the ploidy level of Ihe cell line. Cucumber cell lines that produced only diploid embryos were obtained by selecting completely diploid explant material and growing it in the dark during the initiation phase. Mixoploid explains could lead to tetraploid or mixoploid ceil lines. Isolation and additional selection and subculturing of single PEMs resulted in either completely diploid or tetraploid cell lines, indicating that all cells of individual PEMs are either diploid or tetraploid. The ernbryogenic cucumber cell Imes, differing only in ploidy level, were indistinguishable in growth rate and embryogetiic potential and were genetically stable over several years. 相似文献
99.
Estimation of the position and effect of a lethal factor locus on a molecular marker linkage map 总被引:3,自引:0,他引:3
R. Cheng A. Saito Y. Takano Y. Ukai 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1996,93(4):494-502
In the mapping of DNA markers the distortion of segregation of marker genotypes is often observed, which may be caused by a lethal factor acting in filial generations derived from distant crosses. A method is presented for estimating the recombination values between a lethal factor locus and neighboring molecular markers, and the relative viability or fertilization ability of gametes or zygotes affected by the lethal factor in an F2 population using the maximum likelihood method and the expectation conditional maximization (ECM) algorithm. Three selection models of gamete or zygote were considered, and the most likely one was determined by goodness of fit of the observed frequency of the phenotypes to the expected ones under the models. The method was applied to segregation data of molecular markers of an F2 population consisting of 144 individuals derived from a cross between an Indica and a Japonica rice variety. The presence of a lethal factor locus (L) located on chromosome III that caused partial gametic selection in both the male and female sides was suggested. The locus L was tightly linked to RFLP marker number 23 of the RFLP linkage map of Saito et al. (1991a), and the fertilization chance of a male or female gamete possessing the lethal factor was, on average, 41.5% of that of the normal gamete. 相似文献
100.
The evolution of phage lysis timing 总被引:17,自引:0,他引:17
Summary The effect of host quantity and host quality on the evolution of phage lysis timing is analysed using marginal value theorem of optimal foraging theory. Both factors have been shown to strongly influence the latent period. A high host density selects for short latent period, which is the same result as previous investigators have found. A good host quality also promotes a short latent period. However, elasticity analysis shows that these two factors exert their influences under different sets of conditions. When host density is low, the host density is more important in determining the length of latent period, whereas when host density is high, the host quality is more important. 相似文献