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   303篇
  免费   19篇
  国内免费   1篇
林业   71篇
农学   9篇
基础科学   2篇
  68篇
综合类   29篇
农作物   6篇
水产渔业   3篇
畜牧兽医   107篇
园艺   6篇
植物保护   22篇
  2021年   3篇
  2020年   3篇
  2019年   3篇
  2018年   4篇
  2017年   5篇
  2016年   10篇
  2015年   9篇
  2014年   8篇
  2013年   22篇
  2012年   5篇
  2011年   11篇
  2010年   9篇
  2009年   7篇
  2008年   12篇
  2007年   6篇
  2006年   9篇
  2005年   5篇
  2004年   5篇
  2003年   11篇
  2002年   6篇
  2001年   6篇
  2000年   5篇
  1999年   8篇
  1998年   6篇
  1997年   6篇
  1995年   4篇
  1994年   3篇
  1993年   4篇
  1992年   5篇
  1991年   6篇
  1990年   3篇
  1987年   3篇
  1981年   3篇
  1979年   3篇
  1978年   4篇
  1975年   3篇
  1972年   4篇
  1970年   3篇
  1955年   3篇
  1953年   5篇
  1948年   2篇
  1937年   5篇
  1936年   6篇
  1935年   4篇
  1934年   6篇
  1933年   8篇
  1932年   2篇
  1931年   2篇
  1929年   2篇
  1908年   2篇
排序方式: 共有323条查询结果,搜索用时 234 毫秒
91.
An 8-year-old intact male cat was presented with a subcutaneous mass in the region of the right jugular vein. Cytologic and histopathologic examinations revealed cells with multilobulated nuclei (flower cells). Immunochemistry using a panel of markers showed vimentin-positivity on cytologic specimens, and postive staining for CD79a and BLA36 on histologic specimens. The final diagnosis was lymphoma of B-cell origin. We have observed similar multilobulated cells in ascites fluid, thoracic fluid, and peripheral blood from dogs and cats with a variety of lymphoid and myeloid neoplasms. Cells with multilobulated nuclei that resemble flower petals also have been described in humans. These cells are infrequently observed in canine and feline cytology specimens and require immunochemistry to determine their cell of origin.  相似文献   
92.
Fine root distribution and turnover were investigated in ca. 40-year-old pure Norway spruce (Picea abies Karst.) stands in Germany, growing on four sites that differed in soil acidity (Eberg?tzen < Barbis < Fichtelgebirge = Harz). The density of fine root biomass and necromass in different soil horizons differed among the sites. At one of the most acidic sites (Harz), fine root density in the humus layer was more than twice that at the least acidic site (Eberg?tzen). At the two most acidic sites, Fichtelgebirge and Harz, the ratio of biomass to necromass was significantly lower than at Eberg?tzen and Barbis, particularly in the subsoil layer. In each stand, clear vertical gradients in fine root length density and root tip density were observed. Most of the roots and the root tips were in the humus layer and in the first mineral soil horizon (0-10 cm). There was a significantly different decrease in specific root length (cm gDM (-1)) and specific root tip density (root tips gDM (-1)) in the more acidified stands Fichtelgebirge and Harz compared with Eberg?tzen and Barbis. Fine root production estimated by ingrowth cores and a net method was approximately twice as high in the more acidic stands Fichtelgebirge and Harz compared with Eberg?tzen and Barbis. Rates of living fine root biomass turnover were higher at the Fichtelgebirge and Harz sites than at the Eberg?tzen site. Rates of necromass turnover were similar at all sites. The results suggest that the accumulation of necromass was not due to a slower disappearance at the more acid sites, but to earlier root death. Roots contributed 46% to root + needle litter and 32% to root + total aboveground litter at the Harz site in 1997.  相似文献   
93.
94.
Pure species and F1 hybrid families ofSalix viminalis andS. dasyclados were tested for resistance to four single uredinium isolates ofMelampsora rust in laboratory experiments using excised leaves. Rust isolates were derived from:S. viminalis, S. dasyclados, aS. viminalis x triandra hybrid, andS. daphnoides. Incidence of infection, number of uredinia per leaf, and numbers of spores per uredinium were measured. As expected, the isolate fromS. daphnoides did not infect any of the willow species or hybrids tested. For the other three rust isolates that were tested, the parent from which the isolate was derived was susceptible, the other parent was resistant, and hybrids were intermediate in resistance for incidence and uredinia per leaf. These patterns indicate additive inheritance of these resistance traits in hybrids. Numbers of spores per uredinium were similar on the hybrids and the susceptible parent for one rust isolate, suggesting dominant inheritance of this trait in the hybrids.  相似文献   
95.
Plant defenses     
Fritz R 《Science (New York, N.Y.)》1992,256(5057):680-681
  相似文献   
96.
Fossil medusoids identified as Cyclomedusa davidi?, Beltanelliformis brunsae, and cf. Sekwia excentrica are reported from Late Precambrian strata in the Wernecke Mountains. They are representatives of the Ediacaran fauna, the oldest assemblage of cosmopolitan metazoans, and are only the third such occurrence in Canada. In addition, specimens broadly resembling the problematic structure Rugoinfractus ovruchensis, previously known only from the Precambrian of the Ukraine, are reported from the Lower Cambrian of the nearby Mackenzie Mountains.  相似文献   
97.
美国航空静电喷雾系统的发展历史与中国应用现状   总被引:1,自引:5,他引:1  
航空静电喷雾系统是传统静电喷雾技术在空中操作平台上的专门应用,是航空植保技术的重要内容。美国在上个世纪60年代开始开展航空静电喷雾的技术研究,本文着重介绍了美国航空静电喷雾系统近50a的发展历史和相关的测试工作,包括航空静电喷雾系统的早期研究和持续改进,以及最终的系统模型确定。航空静电喷雾系统可有效减少施药液量和提高药液沉积量,但是仍然存在如何提高病虫害防治效果和减少下风处的喷雾飘移等方面的问题,航空静电喷雾系统针对不同作物和病虫害的适用范围和最佳作业条件还需要进一步的研究和探索。然而,航空静电喷雾系统的巨大优势已经奠定了其在现代农业航空发展中的地位,随着不断的试验研究和评估,现代航空静电喷雾技术必将继续支持农业航空产业的发展。  相似文献   
98.
99.
100.
Zusammenfassung Neben dem in einer früheren Darstellung behandelten Herunterfallen von L?usen, die sich beim Umherlaufen auf der Kleidungs-oberfl?che gegenseitig mit den Beinen verkrallen, kommt als Ursache für das L?usestreuen auch die K?ltestarre der L?use in Frage, die bei Temperaturen in der N?he von 0° eintritt. Die Beobachtung an verlausten Kleidungsstücken bei −6°C zeigte, da? bei Windst?rke 5–6 L?use im Zustand der K?ltestarre von dem Luftzug, bereits von den Stoffunterlagen abgerissen werden k?nnen.  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