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   244409篇
  免费   14730篇
  国内免费   693篇
林业   23182篇
农学   13990篇
基础科学   2962篇
  41959篇
综合类   24863篇
农作物   17051篇
水产渔业   16260篇
畜牧兽医   89910篇
园艺   6318篇
植物保护   23337篇
  2021年   2457篇
  2020年   2894篇
  2019年   3679篇
  2018年   3815篇
  2017年   4155篇
  2016年   4684篇
  2015年   4267篇
  2014年   5630篇
  2013年   16222篇
  2012年   5649篇
  2011年   7432篇
  2010年   7059篇
  2009年   7654篇
  2008年   6823篇
  2007年   5857篇
  2006年   6656篇
  2005年   5932篇
  2004年   5818篇
  2003年   5604篇
  2002年   5028篇
  2001年   5044篇
  2000年   4759篇
  1999年   4651篇
  1998年   4058篇
  1997年   4116篇
  1996年   3869篇
  1995年   4405篇
  1994年   3825篇
  1993年   3540篇
  1992年   4029篇
  1991年   4166篇
  1990年   3935篇
  1989年   3961篇
  1988年   3517篇
  1987年   3539篇
  1986年   3439篇
  1985年   3809篇
  1984年   3563篇
  1983年   3384篇
  1982年   2853篇
  1981年   2788篇
  1980年   2743篇
  1979年   3149篇
  1978年   2839篇
  1977年   2646篇
  1976年   2500篇
  1975年   2367篇
  1974年   2604篇
  1973年   2508篇
  1972年   2376篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
给8头生后3d的哺乳仔猪经口感染猪流行性腹泻病毒(PEDV)“吉”毒株,于感染后18、30、45和96h各扑杀2头,以透射电镜和扫描电镜观察了小肠粘膜上皮细胞及肠系膜淋巴结的超微结构。结果表明,小肠上皮细胞的病变因感染时间不同而有明显差异。上皮细胞的脱落和残留上皮细胞超微结构的破坏,以感染后30h最严重,病毒在这些上皮细胞内的增殖最显著。感染后45h,见有大量新生上皮细胞修补损伤的肠绒毛。感染后96h,小肠绒毛短缩、粗大乃至发生融合。实验仔猪肠系膜淋巴结内巨噬细胞和淋巴细胞的超微结构均遭到破坏,在巨噬细胞内见有PED冠状病毒粒子。  相似文献   
992.
Equine polymorphonuclear (PMN) and mononuclear (MN) leucocytes were separated on Percoll gradients and used to study the chemoattractant properties of the polar ether-linked phospholipid, platelet activating factor (PAF). Six concentrations of PAF ranging from 1 ng/ml to 100 g/ml were studied in each of two in vitro assay systems, the agarose microdroplet and a microfilter technique. Very significant (p<0.01) increases in the movement of both PMN and MN cells were obtained with most concentrations of PAF. In two instances there was no apparent concentration-response relationship, although the action of PAF was approximately bell-shaped in two others. The possible significance of these findings for equine inflammatory conditions is discussed.  相似文献   
993.
In conscious pigs the influence of intravenous infusion of live E. coli (7×108/kg), of the equivalent amount of endotoxin (20 g/kg) or of a high dose of endotoxin (2.5 mg/kg) on the hemodynamic, clinical and pathological parameters and on survival rate was studied. E. coli and endotoxin infusion resulted in pulmonary hypertension, systemic arterial hypotension, a decrease in cardiac output and an increase in heart rate. Clinical signs were characterized by respiratory and nervous disturbances, whereas necropsy revealed hemorrhages and edema in several organs. Although these findings were similar in the three groups, a marked difference in lethality was observed. Infusion of E. coli or of the high dose of endotoxin resulted in a significant mortality, whereas all pigs survived the infusion of the low dose of endotoxin. This suggests that the lethal pathophysiological mechanisms may only become activated when a sufficient amount of endotoxin is released into the circulation.  相似文献   
994.
Bovine and canine acute phase proteins   总被引:1,自引:0,他引:1  
Acute phase proteins are serum proteins which increase in concentration during the acute phase response to inflammation or infection. The response occurs in all animals, but in different species the response of individual proteins can be significantly different. Of the numerous acute phase proteins which have been identified in humans, a number have been examined in cattle and dogs but usually on an individual basis with little reference to their part in the acute phase response. Biochemical, physiological and clinical investigations into haptoglobin, fibrinogen, 1-proteinase inhibitor, ceruloplasmin, seromucoid and C-reactive protein of cattle and dogs have therefore been reviewed with the emphasis on their role in this response to tissue damage.  相似文献   
995.
996.
997.
998.
999.
Effects of Acid rain on freshwater ecosystems   总被引:1,自引:0,他引:1  
Acid-vulnerable areas are more numerous and widespread than believed 7 years ago. Lakes and streams in acid-vulnerable areas of northeastern North America have suffered substantial declines in acid-neutralizing capacity, the worst cases resulting in biological damage. Many invertebrates are very sensitive to acidification, with some disappearing at pH values as high as 6.0. However, the recent rate of acidification of lakes is slower than once predicted, in part the result of decreases in sulfur oxide emissions. A discussion of some of the processes that have contributed to the acidification of lakes as well as those that have protected acid-sensitive freshwaters is presented. The author is in the Department of Fisheries and Oceans, Freshwater Institute, 501 University Crescent, Winnipeg, Manitoba R3T 2N6, Canada.  相似文献   
1000.
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