全文获取类型
收费全文 | 76318篇 |
免费 | 2596篇 |
国内免费 | 12443篇 |
出版年
2022年 | 770篇 |
2021年 | 1023篇 |
2020年 | 1056篇 |
2019年 | 1868篇 |
2018年 | 1966篇 |
2017年 | 1284篇 |
2016年 | 1322篇 |
2015年 | 1496篇 |
2014年 | 2234篇 |
2013年 | 3224篇 |
2012年 | 2152篇 |
2011年 | 1863篇 |
2010年 | 1463篇 |
2009年 | 2162篇 |
2008年 | 2340篇 |
2007年 | 3075篇 |
2006年 | 3235篇 |
2005年 | 3196篇 |
2004年 | 3304篇 |
2003年 | 3020篇 |
2002年 | 2940篇 |
2001年 | 2618篇 |
2000年 | 2010篇 |
1999年 | 2261篇 |
1998年 | 1961篇 |
1997年 | 1505篇 |
1996年 | 1459篇 |
1995年 | 1496篇 |
1994年 | 1570篇 |
1993年 | 1398篇 |
1992年 | 1214篇 |
1991年 | 1205篇 |
1990年 | 992篇 |
1989年 | 848篇 |
1988年 | 869篇 |
1987年 | 689篇 |
1986年 | 618篇 |
1985年 | 2087篇 |
1984年 | 3103篇 |
1983年 | 2038篇 |
1982年 | 2440篇 |
1981年 | 2056篇 |
1980年 | 1920篇 |
1979年 | 1697篇 |
1978年 | 1558篇 |
1977年 | 1418篇 |
1976年 | 1305篇 |
1975年 | 1102篇 |
1974年 | 1088篇 |
1973年 | 1024篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
1,3-Propanediol (PAD) was fed to rats for 15 weeks, and its effects on hepatic and testicular DNA were studied. The control rats were fed a casein-based diet that contained 10% tocopherol-stripped corn oil with 30 IU of d,l-α-tocopherol acetate/kg; the experimental rats were fed the same diet with 500 ppm of PAD. Homogenates prepared from the livers of each group of rats converted 1,3-propanediol to malondialdehyde (MDA) with equal efficacy, but homogenates of testes did not catalyze this conversion. After 10–15 weeks of feeding the diets, the hepatic DNA of the rats fed PAD had less template activity, more bound tryptophan and more DNA-protein and interstrand DNA cross-links than that of the control rats. As measured by template activity and bound tryptophan, testicular DNA of the experimental rats was not different from that of the control rats; however, there was slightly more cross-linking in the testicular DNA of experimental rats than in that of control rats. Testes of the experimental rats contained more lipid-soluble fluorophores than did those of the control rats. The results are consistent with the conclusion that PAD was converted to MDA in vivo and that MDA is the reactive species that caused the observed biological damage. 相似文献
42.
George John Acton 《Phytochemistry》1974,13(8):1303-1310
The light-dependent increment in RNase activity (which is ribosome bound in cell extracts) is distributed as a gradient increasing from base to hook of lupin hypocotyls. No evidence was found of non-specific or of specific activation of pre-formed enzyme molecules following isolation, either before or after (latent activity) destruction of particles. The autodegradation capacity of ribosomes isolated from irradiated cells was almost double that of ribosomes from etiolated tissue. It is proposed that association between the bulk of the light-controlled RNase fraction and lupin ribosomes results from binding of soluble protein. It is not clear whether binding is specific or an artifact of isolation. 相似文献
43.
Volatiles from the epicuticular wax of watercress were collected by ether washing and examined using gas chromatographic and mass spectrometric analysi 相似文献
44.
Nabiel A.M. Saleh Zeinab A.R. El-karemy Ragaa M.A. Mansour Abdel-Aziz A. Fayed 《Phytochemistry》1983,22(11):2501-2505
The flavonoids of five Geranium, fourteen Erodium and four Monsonia species were studied. Quercetin was the most common aglycone with lesse 相似文献
45.
Feral and laboratory flocks of rock doves () show a pattern of grouped sequential exploitation when simultaneously presented with two dispersed, depleting patches of seed. This behavior contrasts with the ideal free distribution pattern shown when patches are small and concentrated. Grouped sequential exploitation consists of two phases: all pigeons first land together and feed at one patch, then leave one by one for the other patch. Departure times of individuals for the second patch are correlated with feeding rate at patch 1, which is in turn correlated with position in the dominance hierarchy. The decision to switch from patch 1 to patch 2 improves individual feeding rates in all cases, but is done slightly later than it should according to optimal foraging theory. 相似文献
46.
47.
Shigeo Koyasu Makoto Asada Akio Fukuda Yoshimi Okada 《Journal of molecular biology》1981,153(2):471-475
The mode of polymerization of two species of flagellins, flagellin A and flagellin B, in polar flagella of Caulobacter crescentus was examined. By immunological staining we found that 1 to 1.2 μm of the portion of the flagellar filament proximal to the cell was composed of flagellin B, whereas about 5 μm of the distal portion was composed of flagellin A. This result, together with the previous observation that a flagellin B-less mutant cannot form normal flagella but instead forms stubs in spite of their high level of flagellin A synthesis, indicates that flagellin B is very important for the formation of complete flagella and/or for the initiation of filament formation from the hook. 相似文献
48.
Lens includes L.
culinaris subsp. culinaris (the cultivated lentil) and several wild species distributed from the Mediterranean region to western Asia. We compared sequence
variation in the ITS region among species of Lens in an effort to end persisting uncertainty regarding the phylogeny of the genus. The parsimony analysis revealed a single
minimum-length tree with a topology congruent with patterns derived by previous studies of nuclear and chloroplast DNA RFLPs.
The basal and highly divergent status of the L. nigricans clade is depicted, and the progenitor-derivative relationship between L. culinaris subsp. orientalis and L. culinaris subsp. culinaris is reaffirmed. Resolution in the tree was improved by combining the ITS data set with a pre-existing set of chloroplast DNA
restriction site data obtained from the same group of samples.
Received May 8, 2000 Accepted October 26, 2001 相似文献
49.
Using a number of drugs that increase cellular cAMP levels, alterations in the amount of cell surface fibronectin and other transformation parameters were studied in Chinese hamster ovary (CHO) cells. The drugs include db-cAMP, different methylxanthines (theophylline, aminophylline, methyl isobutyl xanthine (MIX), caffeine and theobromine), papaverine and cholera toxin. Methylxanthines that have a methyl group at the seventh position lack reverse transforming potential; those that lack a methyl group at the seventh position induced reverse transformation in CHO cells, causing an increase in surface fibronectin, cell substratum adhesive strength and anchorage dependence for growth. Further, as methyl xanthines are substituted in other positions different from the seventh position, the more efficient they become in restoring normal phenotypic properties; the later agents also induced low saturation density via a cytostatic state causing accumulation of cells in the S and G2 phases of the cycle in contrast to the G1 arrest of normal cells at low saturation density. db-cAMP and cholera toxin induced cell elongation but like caffeine and theobromine, did not induce surface fibronectin. The non-methylxanthine phosphodiesterase inhibitor papaverine induced neither cell elongation nor surface fibronectin but produced a cytostatic effect similar to aminophylline and MIX. These studies suggest that the reverse transformation properties fall into two groups: (a) Differentiation-related properties including cell morphology, parallel alignment and surface matrix fibronectin, etc.; (b) cell cycle-related properties-low saturation density, cell arrest at G1 phase and anchorage-dependent growth. Phosphodiesterase inhibitors reversibly eliminate indefinite division potential of CHO cells by inducing a cytostatic situation and not by inducing a G1-specific arrest. 相似文献
50.
Delays in the development of exploratory and locomotor behavior in neonatal male rats (up to 21 days of age) are shown to accrue as a consequence of low level lead exposure. Cross fostering experiments indicate that these delays are primarily due to prenatal exposure. These Pb induced behavioral modifications appear to be associated with the delays in synaptogenesis and biochemical development of the cerebral cortex reported previously (4, 18). A new behavioral bioassay for detecting delays in brain development is described. 相似文献