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   336篇
  免费   21篇
林业   25篇
农学   23篇
基础科学   6篇
  52篇
综合类   56篇
农作物   18篇
水产渔业   16篇
畜牧兽医   121篇
园艺   11篇
植物保护   29篇
  2023年   2篇
  2021年   2篇
  2020年   5篇
  2019年   5篇
  2018年   7篇
  2017年   6篇
  2016年   5篇
  2015年   13篇
  2014年   5篇
  2013年   24篇
  2012年   11篇
  2011年   16篇
  2010年   15篇
  2009年   14篇
  2008年   17篇
  2007年   20篇
  2006年   18篇
  2005年   23篇
  2004年   21篇
  2003年   14篇
  2002年   21篇
  2000年   2篇
  1999年   3篇
  1997年   6篇
  1996年   3篇
  1995年   6篇
  1994年   4篇
  1993年   8篇
  1992年   3篇
  1991年   2篇
  1990年   3篇
  1989年   2篇
  1988年   4篇
  1987年   4篇
  1985年   2篇
  1984年   5篇
  1983年   3篇
  1982年   4篇
  1981年   4篇
  1980年   2篇
  1979年   2篇
  1978年   2篇
  1977年   2篇
  1974年   1篇
  1973年   2篇
  1972年   4篇
  1967年   1篇
  1960年   1篇
  1955年   1篇
  1945年   1篇
排序方式: 共有357条查询结果,搜索用时 15 毫秒
51.
Editing of CD1d-bound lipid antigens by endosomal lipid transfer proteins   总被引:3,自引:0,他引:3  
It is now established that CD1 molecules present lipid antigens to T cells, although it is not clear how the exchange of lipids between membrane compartments and the CD1 binding groove is assisted. We report that mice deficient in prosaposin, the precursor to a family of endosomal lipid transfer proteins (LTP), exhibit specific defects in CD1d-mediated antigen presentation and lack Valpha14 NKT cells. In vitro, saposins extracted monomeric lipids from membranes and from CD1, thereby promoting the loading as well as the editing of lipids on CD1. Transient complexes between CD1, lipid, and LTP suggested a "tug-of-war" model in which lipid exchange between CD1 and LTP is on the basis of their respective affinities for lipids. LTPs constitute a previously unknown link between lipid metabolism and immunity and are likely to exert a profound influence on the repertoire of self, tumor, and microbial lipid antigens.  相似文献   
52.
53.
Hybrid striped bass fingerlings were grown in replicated indoor tanks for 42 days to evaluate progeny performance of two growth-related traits. Analysis of variance demonstrated significant differences in length and weight of fingerlings among paternal and maternal half-sib families and high estimates of heritability for dams and sires. Based on significant sire and dam effects and high heritabilities, a substantial portion of the selection differential could be expected to be gained in offspring from selected pure-line parents. As demonstrated by these results, a breeding program could be used to select parental lines that produce faster growing hybrid offspring.  相似文献   
54.
Landscape Ecology - Animals must move to find food, shelter and mates, and escape predation and competition. Changes in landscape configuration and resource availability can disrupt natural...  相似文献   
55.
A 7-year-old, neutered male, domestic shorthair cat was presented for severe inspiratory dyspnea of 2 to 3 days' duration. Radiography and tracheobronchoscopy confirmed the diagnosis of primary extrathoracic tracheal collapse. The cat was treated with oxygen, dexamethasone, and terbutaline, but no improvement was seen. Surgical correction was performed using nine prosthetic tracheal ring implants. Clinical signs improved after surgery, and the cat continued to do well 11 months after surgery, despite development of unilateral laryngeal paralysis.  相似文献   
56.
Neodymium-142 data for rocks from the Nuvvuagittuq greenstone belt in northern Quebec, Canada, show that some rock types have lower 142Nd/144Nd ratios than the terrestrial standard (epsilon142Nd = -0.07 to -0.15). Within a mafic amphibolite unit, 142Nd/144Nd ratios correlate positively with Sm/Nd ratios and produce a 146Sm-142Nd isochron with an age of 4280(-81)(+53) million years. These rocks thus sample incompatible-element-enriched material formed shortly after Earth formation and may represent the oldest preserved crustal section on Earth.  相似文献   
57.
OBJECTIVE: To compare the effects of postoperative pain after ovariohysterectomy by harmonic scalpel-assisted laparoscopy (HALO) and traditional ovariohysterectomy (OVH) in dogs. STUDY DESIGN: A randomized, blinded, prospective study. SAMPLE POPULATION: Sixteen, purpose-bred, intact female, Beagle dogs. METHODS: Dogs were divided into 2 groups: Group 1 (8 dogs), which had OVH by HALO, and Group 2 (8 dogs), which had traditional OVH. Physiologic data, abdominal nociceptive threshold scores, and University of Melbourne pain scores (UMPS) were recorded at 2, 6, 12, 24, 48, and 72 hours after surgery. Blood samples for measurement of plasma cortisol, glucose, and creatine phosphokinase (CPK) concentrations were collected at the time of the incision, and 2, 6, 12, 24, 48, and 72 hours after surgery. RESULTS: No significant surgical complications occurred. The HALO mean surgical time was significantly longer (55.7 minutes) than traditional OVH (31.7 minutes). No significant differences were observed between groups for the pain measures of heart rate, respiratory rate, temperature, CPK, and glucose concentrations. The OVH group had significantly higher mean plasma cortisol levels at hour 2 after surgery than the HALO group (P=.0001). The mean UMPS were significantly higher in OVH than the HALO group at all postoperative times (P=.0001). The mean nociceptive threshold measurements revealed significantly higher tolerated palpation pressures in HALO than OVH at all postoperative times, except hour 72 (P=.0002). CONCLUSIONS: Dogs appeared to be in less pain with HALO than OVH. The harmonic scalpel coagulated ovarian and uterine vessels completely with minimal collateral damage to surrounding tissues. CLINICAL RELEVANCE: HALO is a safe alternative to OVH and offers a minimally invasive and less painful method of surgery.  相似文献   
58.
Grain compositional components impacting barley (Hordeum vulgare L.) use in food, feed and fuel products, must be combined with improved agronomic traits to produce a commercially viable barley cultivar. Little current information is available on grain composition and variability among winter barley genotypes. This study was conducted to determine the variability among modern hulled and hulless winter barley genotypes in grain composition. Barley types varied significantly in grain and kernel weight, starch, beta-glucan, oil and ash content, but not in protein concentration. Hulless barley had significantly higher grain test weight and starch concentration than hulled and malting types, and significantly higher beta-glucan than malting barley. Hulless barley had significantly lower kernel weights, oil, and ash concentrations than hulled and malting types. Higher starch and lower fiber and ash in grain of hulless barley versus hulled feed or malting type barley are characteristics that increase hulless barley desirability. Selection for high starch concentration among all barley types is feasible and will facilitate development of barley cultivars better suited for use in feed, malt, and ethanol production.  相似文献   
59.
以非洲菊(Gerbera jamesonii L.)品种Amaretto'为试材,研究外源水杨酸(SA)对缓解非洲菊生殖生长期因盐胁迫引起的过氧化伤害作用.在开花前分别用0、0.5和1.0mmol L-1SA对非洲菊植株进行预处理,3d后用100mmol L-1NaCl灌溉,持续2周;对照植株未经NaCl和SA处理.结果表明:盐胁迫处理后,叶片的超氧化物歧化酶(SOD)和过氧化物酶(POD)活性增加,丙二醛(MDA)含量、电解质外渗率和脯氨酸含量提高;外源SA预处理显著降低了盐胁迫植株叶片的MDA含量和电解质外渗率;在外源0.5mmol L-1SA预处理后,植株SOD和POD活性分别是对照的1.4倍和2.4倍;SA预处理后与植株耐盐性相关的生理生化指标呈显著相关,以0.5mmol L-1SA处理增强植株盐胁迫耐性的效果更好.说明外源SA处理能够诱导盐胁迫下非洲菊植株的抗氧化反应,水杨酸将是一种提高园艺作物耐盐性的有效物质.  相似文献   
60.
Several remote observations have indicated that water ice may be presented in permanently shadowed craters of the Moon. The Lunar Crater Observation and Sensing Satellite (LCROSS) mission was designed to provide direct evidence. On 9 October 2009, a spent Centaur rocket struck the persistently shadowed region within the lunar south pole crater Cabeus, ejecting debris, dust, and vapor. This material was observed by a second "shepherding" spacecraft, which carried nine instruments, including cameras, spectrometers, and a radiometer. Near-infrared absorbance attributed to water vapor and ice and ultraviolet emissions attributable to hydroxyl radicals support the presence of water in the debris. The maximum total water vapor and water ice within the instrument field of view was 155 ± 12 kilograms. Given the estimated total excavated mass of regolith that reached sunlight, and hence was observable, the concentration of water ice in the regolith at the LCROSS impact site is estimated to be 5.6 ± 2.9% by mass. In addition to water, spectral bands of a number of other volatile compounds were observed, including light hydrocarbons, sulfur-bearing species, and carbon dioxide.  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