全文获取类型
收费全文 | 945篇 |
免费 | 81篇 |
专业分类
林业 | 58篇 |
农学 | 21篇 |
基础科学 | 2篇 |
137篇 | |
综合类 | 159篇 |
农作物 | 42篇 |
水产渔业 | 58篇 |
畜牧兽医 | 494篇 |
园艺 | 21篇 |
植物保护 | 34篇 |
出版年
2024年 | 3篇 |
2023年 | 14篇 |
2022年 | 10篇 |
2021年 | 28篇 |
2020年 | 23篇 |
2019年 | 30篇 |
2018年 | 29篇 |
2017年 | 31篇 |
2016年 | 28篇 |
2015年 | 31篇 |
2014年 | 23篇 |
2013年 | 44篇 |
2012年 | 77篇 |
2011年 | 86篇 |
2010年 | 42篇 |
2009年 | 35篇 |
2008年 | 50篇 |
2007年 | 53篇 |
2006年 | 46篇 |
2005年 | 40篇 |
2004年 | 20篇 |
2003年 | 36篇 |
2002年 | 28篇 |
2001年 | 8篇 |
2000年 | 17篇 |
1999年 | 8篇 |
1998年 | 4篇 |
1996年 | 7篇 |
1995年 | 7篇 |
1993年 | 4篇 |
1992年 | 15篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 9篇 |
1988年 | 10篇 |
1987年 | 8篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 4篇 |
1983年 | 8篇 |
1982年 | 3篇 |
1978年 | 8篇 |
1976年 | 8篇 |
1974年 | 6篇 |
1973年 | 3篇 |
1971年 | 8篇 |
1970年 | 5篇 |
1969年 | 4篇 |
1967年 | 6篇 |
1966年 | 3篇 |
排序方式: 共有1026条查询结果,搜索用时 15 毫秒
81.
82.
83.
Examples of sympatric speciation in nature are rare and hotly debated. We describe the parallel speciation of finches on two small islands in the Tristan da Cunha archipelago in the South Atlantic Ocean. Nesospiza buntings are a classic example of a simple adaptive radiation, with two species on each island: an abundant small-billed dietary generalist and a scarce large-billed specialist. Their morphological diversity closely matches the available spectrum of seed sizes, and genetic evidence suggests that they evolved independently on each island. Speciation is complete on the smaller island, where there is a single habitat with strongly bimodal seed size abundance, but is incomplete on the larger island, where a greater diversity of habitats has resulted in three lineages. Our study suggests that the buntings have undergone parallel ecological speciation. 相似文献
84.
McConnell JR Edwards R Kok GL Flanner MG Zender CS Saltzman ES Banta JR Pasteris DR Carter MM Kahl JD 《Science (New York, N.Y.)》2007,317(5843):1381-1384
Black carbon (BC) from biomass and fossil fuel combustion alters chemical and physical properties of the atmosphere and snow albedo, yet little is known about its emission or deposition histories. Measurements of BC, vanillic acid, and non-sea-salt sulfur in ice cores indicate that sources and concentrations of BC in Greenland precipitation varied greatly since 1788 as a result of boreal forest fires and industrial activities. Beginning about 1850, industrial emissions resulted in a sevenfold increase in ice-core BC concentrations, with most change occurring in winter. BC concentrations after about 1951 were lower but increasing. At its maximum from 1906 to 1910, estimated surface climate forcing in early summer from BC in Arctic snow was about 3 watts per square meter, which is eight times the typical preindustrial forcing value. 相似文献
85.
Daly MJ Gaidamakova EK Matrosova VY Vasilenko A Zhai M Venkateswaran A Hess M Omelchenko MV Kostandarithes HM Makarova KS Wackett LP Fredrickson JK Ghosal D 《Science (New York, N.Y.)》2004,306(5698):1025-1028
Deinococcus radiodurans is extremely resistant to ionizing radiation. How this bacterium can grow under chronic gamma radiation [50 grays (Gy) per hour] or recover from acute doses greater than 10 kGy is unknown. We show that D. radiodurans accumulates very high intracellular manganese and low iron levels compared with radiation-sensitive bacteria and that resistance exhibits a concentration-dependent response to manganous chloride [Mn(II)]. Among the most radiation-resistant bacterial groups reported, Deinococcus, Enterococcus, Lactobacillus, and cyanobacteria accumulate Mn(II). In contrast, Shewanella oneidensis and Pseudomonas putida have high iron but low intracellular manganese concentrations and are very sensitive. We propose that Mn(II) accumulation facilitates recovery from radiation injury. 相似文献
86.
Ferguson SM Brasnjo G Hayashi M Wölfel M Collesi C Giovedi S Raimondi A Gong LW Ariel P Paradise S O'toole E Flavell R Cremona O Miesenböck G Ryan TA De Camilli P 《Science (New York, N.Y.)》2007,316(5824):570-574
Dynamin 1 is a neuron-specific guanosine triphosphatase thought to be critically required for the fission reaction of synaptic vesicle endocytosis. Unexpectedly, mice lacking dynamin 1 were able to form functional synapses, even though their postnatal viability was limited. However, during spontaneous network activity, branched, tubular plasma membrane invaginations accumulated, capped by clathrin-coated pits, in synapses of dynamin 1-knockout mice. Synaptic vesicle endocytosis was severely impaired during strong exogenous stimulation but resumed efficiently when the stimulus was terminated. Thus, dynamin 1-independent mechanisms can support limited synaptic vesicle endocytosis, but dynamin 1 is needed during high levels of neuronal activity. 相似文献
87.
Evolutionary social psychology and family homicide 总被引:2,自引:0,他引:2
Homicide is an extreme manifestation of interpersonal conflict with minimal reporting bias and can thus be used as a conflict "assay." Evolutionary models of social motives predict that genetic relationship will be associated with mitigation of conflict, and various analyses of homicide data support this prediction. Most "family" homicides are spousal homicides, fueled by male sexual proprietariness. In the case of parent-offspring conflict, an evolutionary model predicts variations in the risk of violence as a function of the ages, sexes, and other characteristics of protagonists, and these predictions are upheld in tests with data on infanticides, parricides, and filicides. 相似文献
88.
Angiotensin I: metabolism by plasma membrane of lung 总被引:8,自引:0,他引:8
(8-L-[(14)C] phenylalanine) angiotensin I is metabolized in one passage through blood-free lungs. Approximately 20 percent of the radioactivity emerges as angiotensin 11, the remainder as lower homologs. Radioactivity is not retained by the lungs but has the same volume of distribution and mean transit time as blue dextran, a compound unlikely to leave the intravascular space. Plasma membrane fractions of lung are capable of converting angiotensin I to angiotensin II. These data, taken together, indicate the circulating angiotensin I is metabolized by enzymes of the luminal surface of pulmonary endothelial cells. 相似文献
89.
Ryan EP Heuberger AL Weir TL Barnett B Broeckling CD Prenni JE 《Journal of agricultural and food chemistry》2011,59(5):1862-1870
Emerging evidence supporting chronic disease fighting properties of rice bran has advanced the development of stabilized rice bran for human use as a functional food and dietary supplement. A global and targeted metabolomic investigation of stabilized rice bran fermented with Saccharomyces boulardii was performed in three rice varieties. Metabolites from S. boulardii-fermented rice bran were detected by gas chromatography-mass spectrometry (GC-MS) and assessed for bioactivity compared to nonfermented rice bran in normal and malignant lymphocytes. Global metabolite profiling revealed significant differences in the metabolome that led to discovery of candidate compounds modulated by S. boulardii fermentation. Fermented rice bran extracts from three rice varieties reduced growth of human B lymphomas compared to each variety's nonfermented control and revealed that fermentation differentially altered bioactive compounds. These data support that integration of global and targeted metabolite analysis can be utilized for assessing health properties of rice bran phytochemicals that are enhanced by yeast fermentation and that differ across rice varieties. 相似文献
90.
Vitamin D and sterol composition of 10 types of mushrooms from retail suppliers in the United States
Phillips KM Ruggio DM Horst RL Minor B Simon RR Feeney MJ Byrdwell WC Haytowitz DB 《Journal of agricultural and food chemistry》2011,59(14):7841-7853
Vitamin D(2) (ergocalciferol) and sterols were analyzed in mushrooms sampled nationwide in the United States to update the USDA Nutrient Database for Standard Reference. Vitamin D(2) was assayed using HPLC with [(3)H]-vitamin D(3) internal standard and sterols by GC-FID mass spectrometric (MS) confirmation. Vitamin D(2) was low (0.1-0.3 μg/100 g) in Agaricus bisporus (white button, crimini, portabella) and enoki, moderate in shiitake and oyster (0.4-0.7 μg/100 g), and high in morel, chanterelle, maitake (5.2-28.1 μg/100 g) and UV-treated portabella (3.4-20.9 μg/100 g), with significant variability among composites for some types. Ergosterol (mg/100 g) was highest in maitake and shiitake (79.2, 84.9) and lowest in morel and enoki (26.3, 35.5); the range was <10 mg/100 g among white button composites but 12-50 mg/100 g among samples of other types. All mushrooms contained ergosta-5,7-dienol (22,23-dihydroergosterol) (3.53-18.0 mg/100 g) and (except morel) ergosta-7-enol. Only morel contained brassicasterol (28.6 mg/100 g) and campesterol (1.23-4.54 mg/100 g) and no ergosta-7,22-dienol. MS was critical in distinguishing campesterol from ergosta-7,22-dienol. 相似文献