全文获取类型
收费全文 | 40898篇 |
免费 | 16581篇 |
国内免费 | 8篇 |
专业分类
57487篇 |
出版年
2023年 | 67篇 |
2022年 | 212篇 |
2021年 | 616篇 |
2020年 | 2311篇 |
2019年 | 3863篇 |
2018年 | 4006篇 |
2017年 | 4250篇 |
2016年 | 4360篇 |
2015年 | 4557篇 |
2014年 | 4270篇 |
2013年 | 4910篇 |
2012年 | 2790篇 |
2011年 | 2418篇 |
2010年 | 3649篇 |
2009年 | 2415篇 |
2008年 | 1526篇 |
2007年 | 1159篇 |
2006年 | 1087篇 |
2005年 | 1150篇 |
2004年 | 1093篇 |
2003年 | 1140篇 |
2002年 | 1082篇 |
2001年 | 400篇 |
2000年 | 310篇 |
1999年 | 324篇 |
1998年 | 256篇 |
1997年 | 189篇 |
1996年 | 166篇 |
1995年 | 171篇 |
1994年 | 143篇 |
1993年 | 115篇 |
1992年 | 155篇 |
1991年 | 125篇 |
1990年 | 109篇 |
1989年 | 110篇 |
1988年 | 85篇 |
1987年 | 97篇 |
1986年 | 84篇 |
1985年 | 103篇 |
1984年 | 118篇 |
1983年 | 91篇 |
1982年 | 113篇 |
1981年 | 126篇 |
1980年 | 94篇 |
1979年 | 102篇 |
1978年 | 95篇 |
1977年 | 89篇 |
1976年 | 96篇 |
1974年 | 76篇 |
1973年 | 73篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Spores of Aspergillus niger obtained by solid state fermentation on buckwheat seeds produced gluconic acid from glucose with a high yield, near 1.06 g gluconic acid/g glucose, close to the stoichiometric value. The reaction itself could be carried out either with purified biocatalyst or with the whole buckwheat medium resulting from spore production process. 200 g gluconic acid/L were obtained in 200 h with sequential feedings of glucose up to 190 g/L. 相似文献
52.
53.
Judy L. Jakobek Jean C. Stutz Vicki H. Bess Ralph A. Backhaus 《Plant Cell, Tissue and Organ Culture》1993,35(3):297-299
Both Cowania subintegra Kearney and C. stansburiana Torr. were successfully propagated in vitro. Shoot proliferation occurred from shoot tips of green-house grown C. subintegra using a modified Murashige and Skoog medium supplemented with 4.4 M 6-benzyladenine and 0.5 M indole butyric acid. Excised microshoots (1.5–3.0 cm long) of both species were rooted using a two-step process in which they were cultured for 3 days in a root initiation medium with 2.7 M naphthaleneacetic acid and then transferred to a low nitrogen root elongation medium without auxin. Plantlets were successfully transferred to soilless potting mix. 相似文献
54.
Although semi-free-ranging Barbary macaque females are able to outrank older females from lower-ranking matrilines (matrilineal rank acquisition), they do not systematically outrank their older sisters, as is known to be the case for semi-free-ranging rhesus monkeys (Macaca mulatta) and Japanese macaques (Macaca fuscata). We test the hypothesis that differences in the support received by younger sisters against their older sisters and against older lower-ranking females might account for this interspecific difference. Thirty-one sister dyads, members of a group of 109 Barbary macaques living at La Montagne des Singes, France, were observed during 16 months. The results indicate that (1) all females were dominant to their younger sisters, and the latter were never observed to challenge their older sisters; (2) younger sisters received as much kin support against their older sisters as against older lower-ranking females; (3) only very young females received support from their kin against their older sisters; (4) younger sisters received much more support from nonkin females against lower-ranking females than against their older sisters; and (5) Barbary macaque females appear to be supported against their older sisters less frequently than rhesus macaque females are. We conclude that the lack of nonkin support is the main factor accounting for the failure of younger sisters to outrank their older sisters in Barbary macaques. Initially this might result from kin support not being sufficient to induce younger sisters to challenge and to solicit support against their older sisters. 相似文献
55.
Weng Naidong Jean W. Lee James D. Hulse 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1994,654(2)
An isocratic HPLC method was developed and validated for the quantitation of methocarbamol in human plasma. Methocarbamol and internal standard in 200 μl of human plasma were extracted with ethyl acetate, evaporated to dryness and reconstituted in water. Separation was achieved on a reversed-phase C18 column with a mobile phase of methanol—0.1 M potassium phosphate monobasic—water (35:10:55, v/v/v). The detection was by ultraviolet at 272 nm. Linearity was established at 1–100 μg/ml (r > 0.999). The limit of quantitation was designed as 1 μg/ml to suit pharmacokinetic studies. Inter-day precision and accuracy of the calibration standards were 1.0 to 3.6% coefficients of variance (C.V.) and −2.0 to +1.6% relative error (R.E.). Quality controls of 3, 20 and 70 μg/ml showed inter-day precision and accuracy of 2.5 to 3.6% C.V. and −0.9 to −0.4% R.E. Recovery of methocarbamol was 91.4–100.3% in five different lots of plasma. The method was shown to be applicable on different brands of C18 columns. 相似文献
56.
The complex [Eu(tpy)3](ClO4)3 where TPY=2,2′; 6,2″-terpyridine, has been prepared and reexamined. The complex appears to be stable in acetonitrile solution with respect to decomplexation of the ligands but the addition of water does cause partial replacement of tpy. Analogous complexes have been prepared with 3,3′; 5,3″-polymethylene bridged derivatives of tpy having two or three carbons in the bridge. The bridging enforces a cisoid geometry of the ligand and prohibits its replacement by added water. An X-ray determination was carried out for [Eu(3b)3](ClO4)3, where 3b=3,3′; 5,3″-dimethylene tpy, which crystallizes in the monoclinic space group P21/c with a=11.908(4), b=15.768(5), c=29.513(9) Å, β=93.60(2)°, μ=13.5 cm−1 and Z=4. The complex forms a tricapped trigonal prism with each of the ligands adopting the same dl conformation. Variable temperature NMR analysis of the bridged ligand complexes indicates that conformational inversion of the bound ligand is not a concerted process and barriers for inversion of individual methylene units can be estimated from coalescence of the signals from the geminal methylene protons. The luminescence properties of the bridged tpy complexes are similar to the parent unbridged system. 相似文献
57.
Parthenogenesis has been observed in several elasmobranch species, primarily in public aquaria. The majority of cases of parthenogenesis have occurred either when females were held without males or once a male was removed from a female's habitat. Here we report a second instance of parthenogenesis in a zebra shark female that was housed with conspecific mature males. This study calls into question the conditions under which elasmobranch females undergo parthenogenesis. 相似文献
58.
Colonies of the African stink ant Paltothyreus tarsatuslocated in the forest have nests with shorter horizontal galleries and a smaller total foraging surface than colonies located in open areas. Each solitary worker specializes on the same central or peripheral hunting zone but she does not specialize on a particular sector during group-retrieving. The search for prey is characterized by a wandering walk with spatial parameters varying in two ways. Capture of a termite releases a path characterized by sinuosity and a decrease in speed of movement. In contrast, a failure in the course of an attempted capture releases an increase in both sinuosity and speed of movement corresponding to a socalled reserve behavior. Each worker shortens her retrieving trip in comparison with her search trip and the straightness of the homing paths depends on the size and shape of the prey. Our data show that behavioral flexibility at the individual level in P. tarsatusis important in determining spatial foraging strategy at the colony level. 相似文献
59.
60.