全文获取类型
收费全文 | 448篇 |
免费 | 88篇 |
专业分类
林业 | 8篇 |
农学 | 12篇 |
基础科学 | 1篇 |
58篇 | |
综合类 | 103篇 |
农作物 | 33篇 |
水产渔业 | 14篇 |
畜牧兽医 | 261篇 |
园艺 | 8篇 |
植物保护 | 38篇 |
出版年
2020年 | 6篇 |
2018年 | 10篇 |
2017年 | 12篇 |
2016年 | 13篇 |
2015年 | 9篇 |
2014年 | 8篇 |
2013年 | 45篇 |
2012年 | 16篇 |
2011年 | 19篇 |
2010年 | 13篇 |
2009年 | 10篇 |
2008年 | 10篇 |
2007年 | 15篇 |
2006年 | 13篇 |
2005年 | 17篇 |
2004年 | 13篇 |
2003年 | 9篇 |
2002年 | 11篇 |
2001年 | 7篇 |
2000年 | 8篇 |
1999年 | 10篇 |
1998年 | 7篇 |
1997年 | 8篇 |
1996年 | 11篇 |
1994年 | 5篇 |
1993年 | 7篇 |
1991年 | 5篇 |
1990年 | 14篇 |
1989年 | 7篇 |
1988年 | 11篇 |
1987年 | 11篇 |
1986年 | 5篇 |
1984年 | 9篇 |
1983年 | 4篇 |
1982年 | 7篇 |
1981年 | 8篇 |
1978年 | 5篇 |
1975年 | 6篇 |
1974年 | 10篇 |
1973年 | 8篇 |
1972年 | 4篇 |
1971年 | 13篇 |
1970年 | 8篇 |
1969年 | 5篇 |
1967年 | 4篇 |
1948年 | 5篇 |
1946年 | 3篇 |
1945年 | 4篇 |
1944年 | 6篇 |
1916年 | 3篇 |
排序方式: 共有536条查询结果,搜索用时 15 毫秒
31.
32.
The effect of salicylate on the relative cation permeability of a membrane was investigated in large, identified molluscan neurons, with the use of intracellular recording techniques. Salicylate caused a reversible, dose-dependent decrease in the permeability of rubidium, cesium, sodium, and lithium ions relative to that of potassium ions. The results suggest that the changes in cation selectivity result from the adsorption of salicylate anions to the membrane with a subsequent increase in the density and field strength of anionic sites in the membrane. 相似文献
33.
Functional transfer of mitochondrial genes to the nucleus is very common in some taxa but entirely lacking in others. Current evolutionary theories to account for this variation predict that outcrossing, which allows escape from Muller's ratchet and faster spread of beneficial mutations, should favor gene transfer. We find that functional gene transfer is more common in selfing or clonal plants than in outcrossing plants, a pattern opposite to prediction. We suggest that reproductive modes, such as selfing and vegetative reproduction, conserve adaptive mitonuclear gene combinations, allowing functional transfer, whereas outcrossing prevents transfer by breaking up these combinations. 相似文献
34.
Johnson TC Brown ET McManus J Barry S Barker P Gasse F 《Science (New York, N.Y.)》2002,296(5565):113-132
High-resolution profiles of the mass accumulation rate of biogenic silica and other geochemical proxies in two piston cores from northern Lake Malawi provide a climate signal for this part of tropical Africa spanning the past 25,000 years. The biogenic silica mass accumulation rate was low during the relatively dry late Pleistocene, when the river flux of silica to the lake was suppressed. Millennial-scale fluctuations, due to upwelling intensity, in the late Pleistocene climate of the Lake Malawi basin appear to have been closely linked to the Northern Hemisphere climate until 11 thousand years ago. Relatively cold conditions in the Northern Hemisphere coincided with more frequent north winds over the Malawi basin, perhaps resulting from a more southward migration of the Intertropical Convergence Zone. 相似文献
35.
36.
Self-Assembled Smectic Phases in Rod-Coil Block Copolymers 总被引:1,自引:0,他引:1
Rod-coil block copolymers are self-assembling polymers that combine the physics of orientational ordering of rodlike polymers and the microphase separation of coil-coil block copolymers. Several new solid-state morphologies were observed in a series of anionically synthesized model poly(hexyl isocyanate-b-styrene) rod-coil diblock copolymers examined by transmission electron microscopy and selected-area electron diffraction. The rod-coils formed smectic C-like and O-like morphologies with domain sizes ranging from tens of nanometers to almost 1 micrometer. Both structural and orientational changes were found for increasing rod volume fractions. In addition, some morphologies exhibited spontaneous long-range orientational order over many tens of micrometers. 相似文献
37.
Thornalley DJ Barker S Broecker WS Elderfield H McCave IN 《Science (New York, N.Y.)》2011,331(6014):202-205
Deepwater formation in the North Atlantic by open-ocean convection is an essential component of the overturning circulation of the Atlantic Ocean, which helps regulate global climate. We use water-column radiocarbon reconstructions to examine changes in northeast Atlantic convection since the Last Glacial Maximum. During cold intervals, we infer a reduction in open-ocean convection and an associated incursion of an extremely radiocarbon ((14)C)-depleted water mass, interpreted to be Antarctic Intermediate Water. Comparing the timing of deep convection changes in the northeast and northwest Atlantic, we suggest that, despite a strong control on Greenland temperature by northeast Atlantic convection, reduced open-ocean convection in both the northwest and northeast Atlantic is necessary to account for contemporaneous perturbations in atmospheric circulation. 相似文献
38.
Chromosome 17 deletions and p53 gene mutations in colorectal carcinomas 总被引:175,自引:0,他引:175
S J Baker E R Fearon J M Nigro S R Hamilton A C Preisinger J M Jessup P vanTuinen D H Ledbetter D F Barker Y Nakamura R White B Vogelstein 《Science (New York, N.Y.)》1989,244(4901):217-221
Previous studies have demonstrated that allelic deletions of the short arm of chromosome 17 occur in over 75% of colorectal carcinomas. Twenty chromosome 17p markers were used to localize the common region of deletion in these tumors to a region contained within bands 17p12 to 17p13.3. This region contains the gene for the transformation-associated protein p53. Southern and Northern blot hybridization experiments provided no evidence for gross alterations of the p53 gene or surrounding sequences. As a more rigorous test of the possibility that p53 was a target of the deletions, the p53 coding regions from two tumors were analyzed; these two tumors, like most colorectal carcinomas, had allelic deletions of chromosome 17p and expressed considerable amounts of p53 messenger RNA from the remaining allele. The remaining p53 allele was mutated in both tumors, with an alanine substituted for valine at codon 143 of one tumor and a histidine substituted for arginine at codon 175 of the second tumor. Both mutations occurred in a highly conserved region of the p53 gene that was previously found to be mutated in murine p53 oncogenes. The data suggest that p53 gene mutations may be involved in colorectal neoplasia, perhaps through inactivation of a tumor suppressor function of the wild-type p53 gene. 相似文献
39.
Supraopticneurosecretory cells: autonomic modulation 总被引:1,自引:0,他引:1
Neurosecretory cells in the supraoptic nuclei of anesthetized cats were antidromically identified by electrical stimulation of the posterior pituitary. The responses of these units to afferent volleys from vagal and carotid sinus nerves were examined with a computer of average transients. Synaptic excitation of neurosecretory cells by stimulation of these pathways demonstrates the existence of excitatory inputs from vagal and carotid sinus nerve afferents. Since these pathways are involved in the release of antidiuretic hormone, the results support the hypothesis that its release is related to an increase in discharge frequency of supraoptic neurosecretory cells. 相似文献
40.
Analysis of fenbendazole residues in bovine milk by ELISA 总被引:1,自引:0,他引:1
Brandon DL Bates AH Binder RG Montague WC Whitehand LC Barker SA 《Journal of agricultural and food chemistry》2002,50(21):5791-5796
Fenbendazole residues in bovine milk were analyzed by ELISAs using two monoclonal antibodies. One monoclonal antibody (MAb 587) bound the major benzimidazole anthelmintic drugs, including fenbendazole, oxfendazole, and fenbendazole sulfone. The other (MAb 591) was more specific for fenbendazole, with 13% cross-reactivity with the sulfone and no significant binding to the sulfoxide metabolite. The limit of detection of the ELISA method in the milk matrix was 7 ppb for MAb 587 and 3 ppb for MAb 591. Fenbendazole was administered in feed, drench, and paste form to three groups of dairy cattle. Milk was collected immediately before dosing and then every 12 h for 5 days. The ELISA indicated that residue levels varied widely among individual cows in each group. Fenbendazole levels peaked at approximately 12-24 h and declined rapidly thereafter. Metabolites were detected at much higher levels than the parent compound, peaked at approximately 24-36 h, and declined gradually. Residue levels were undetectable by 72 h. The ELISA data correlated well with the total residues determined by chromatographic analysis, but the use of the two separate ELISAs did not afford an advantage over ELISA with the single, broadly reactive MAb 587. The ELISA method could be used to flag high-residue samples in on-site monitoring of fenbendazole in milk and is a potential tool for studying drug pharmacokinetics. 相似文献