首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   456篇
  免费   21篇
林业   6篇
农学   2篇
  35篇
综合类   40篇
农作物   50篇
水产渔业   29篇
畜牧兽医   296篇
园艺   4篇
植物保护   15篇
  2023年   3篇
  2021年   5篇
  2020年   8篇
  2019年   11篇
  2018年   12篇
  2017年   5篇
  2016年   4篇
  2015年   9篇
  2014年   6篇
  2013年   19篇
  2012年   22篇
  2011年   27篇
  2010年   13篇
  2009年   14篇
  2008年   18篇
  2007年   15篇
  2006年   23篇
  2005年   21篇
  2004年   13篇
  2003年   17篇
  2002年   13篇
  2001年   20篇
  2000年   12篇
  1999年   9篇
  1997年   3篇
  1996年   5篇
  1995年   3篇
  1992年   9篇
  1991年   5篇
  1990年   15篇
  1989年   8篇
  1988年   6篇
  1987年   12篇
  1986年   7篇
  1985年   6篇
  1983年   3篇
  1980年   2篇
  1979年   4篇
  1973年   2篇
  1972年   2篇
  1971年   4篇
  1970年   4篇
  1969年   3篇
  1966年   2篇
  1947年   4篇
  1946年   8篇
  1945年   7篇
  1944年   6篇
  1943年   6篇
  1941年   3篇
排序方式: 共有477条查询结果,搜索用时 15 毫秒
51.
52.
Nickel activities were measured in 11 alkaline and slightly acidic soils using the chelate method. The measured activities ranged from 10?6.82 to 10?8.78 M and were inversely related to the pH of the soils. The measured Ni2+ activities in the 11 soils could be predicted using the following expression: (Ni2+) = 106.95 (H+)2. The solubility of Ni was plotted on a log (Ni2+) vs. pH solubility diagram for various Ni solid phase compounds. It is most likely that NiFe2O4 (trevorite) is the solid phase controlling Ni solubility in these soils. Identification of the presence of this mineral needs further investigation.  相似文献   
53.
Images of the DNA double helix in water   总被引:8,自引:0,他引:8  
The scanning tunneling microscope can image uncoated DNA submerged in water. The grooves of the double helix were clearly resolved in images of the 146-base pair fragment extracted from calf thymus nucleosome. In contrast to images obtained with dry DNA, the helix pitch varied only a small amount (36 +/- 5 angstroms). The path of the helix shows considerable variation. It is quite straight when the molecules are densely packed, but it curves and bends in isolated molecules.  相似文献   
54.
Methane was the most abundant hydrocarbon released during the 2010 Deepwater Horizon oil spill in the Gulf of Mexico. Beyond relevancy to this anthropogenic event, this methane release simulates a rapid and relatively short-term natural release from hydrates into deep water. Based on methane and oxygen distributions measured at 207 stations throughout the affected region, we find that within ~120 days from the onset of release ~3.0 × 10(10) to 3.9 × 10(10) moles of oxygen were respired, primarily by methanotrophs, and left behind a residual microbial community containing methanotrophic bacteria. We suggest that a vigorous deepwater bacterial bloom respired nearly all the released methane within this time, and that by analogy, large-scale releases of methane from hydrate in the deep ocean are likely to be met by a similarly rapid methanotrophic response.  相似文献   
55.
Stochastic on-off conductivity switching observed in phenylene-ethynylene oligomers has been explained in terms of changes in ring conformations, or electron localization, or both. We report the observation of stochastic on-off switching in the simplest of wired molecules: octanedithiol, decanedithiol, and dodecanedithiol bonded on an Au(111) surface. Stochastic switching was observed even when a top gold contact was pressed on by a conducting atomic force microscope tip at constant force. The rate of switching increased substantially at 60 degrees C, a temperature at which these films are commonly annealed. Because such switching in alkanethiols is unlikely to be caused by internal molecular electronic changes and cannot be fully accounted for by breaking of the top contact, we argue that the cause is the well-known mobility of molecules tethered to gold via a thiol linkage.  相似文献   
56.
57.
58.
A simple HPLC method is presented to quantify the low concentration of hydroxylysyl pyridinoline (PYD) cross-links in Atlantic salmon (Salmo salar L.) muscle. The method involved the extraction of tissue with NaOH prior to hydrolysis, which greatly reduced the amount of protein to be hydrolyzed and made downstream operations easier and more reproducible. The concentration of PYD was 426 pmol g(-)(1) dry mass muscle in post-rigor muscle stored at 0 degrees C and sampled 3 d after death. Hydroxproline (HYP) concentration was determined following NaOH extraction as a measure of collagen content. In post-rigor samples, the alkaline-insoluble HYP fraction comprised 18.3% of the total HYP. Scanning electron microscopy revealed shrinkage of muscle fibers and a retraction of the connective tissue matrix in smoked salmon. PYD concentration was relatively resistant to processing to the smoked product, decreasing by around 11.7%, as compared to a 22.2% decrease in HYP. There was a positive correlation between PYD concentration and the firmness of post-rigor muscle samples as measured by an instrumental texture analyzer, explaining 25% of the total variation. A weaker but still significant correlation was found between PYD concentration and firmness in the smoked product. There was no relationship between fillet firmness and total collagen concentration, although the correlation with HYP in the alkaline-insoluble fraction was significant at the 6% level (P = 0.057). Our results indicate that only 1-3% of collagen molecules are linked by nonreducible mature cross-links in harvest size farmed Atlantic salmon and that PYD concentration is an important raw material characteristic for flesh quality.  相似文献   
59.
Infrared spectra of material captured from comet 81P/Wild 2 by the Stardust spacecraft reveal indigenous aliphatic hydrocarbons similar to those in interplanetary dust particles thought to be derived from comets, but with longer chain lengths than those observed in the diffuse interstellar medium. Similarly, the Stardust samples contain abundant amorphous silicates in addition to crystalline silicates such as olivine and pyroxene. The presence of crystalline silicates in Wild 2 is consistent with mixing of solar system and interstellar matter. No hydrous silicates or carbonate minerals were detected, which suggests a lack of aqueous processing of Wild 2 dust.  相似文献   
60.
OBJECTIVE: To determine complications and neurologic outcomes associated with dexamethasone administration to dogs with surgically treated thoracolumbar intervertebral disk herniation, compared with dogs not receiving dexamethasone. DESIGN: Retrospective case series. ANIMALS: 161 dogs with surgically confirmed thoracolumbar disk herniation. PROCEDURES: Medical records from 2 hospitals were used to identify dogs that had received dexamethasone < 48 hours prior to admission (dexamethasone group dogs), dogs that received glucocorticoids other than dexamethasone < 48 hours prior to admission (other-glucocorticoid group dogs), and dogs that received no glucocorticoids (nontreatment group dogs). Signalment, neurologic injury grade, laboratory data, and complications were extracted from medical records. RESULTS: Dexamethasone group dogs were 3.4 times as likely to have a complication, compared with other-glucocorticoid or nontreatment group dogs. Dexamethasone group dogs were 11.4 times as likely to have a urinary tract infection and 3.5 times as likely to have diarrhea, compared with other-glucocorticoid or nontreatment group dogs. No differences in neurologic function at discharge or recheck evaluation were detected among groups. CONCLUSIONS AND CLINICAL RELEVANCE: Results indicated that treatment with dexamethasone before surgery is associated with more adverse effects, compared with treatment with glucocorticoids other than dexamethasone or no treatment with glucocorticoids, in dogs with thoracolumbar intervertebral disk herniation. In this study population, no difference in outcome was found among groups. These findings suggest that the value of dexamethasone administration before surgery in dogs with thoracolumbar disk herniation should be reconsidered.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号