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   284693篇
  免费   16492篇
  国内免费   658篇
林业   24332篇
农学   14214篇
基础科学   3009篇
  45385篇
综合类   30478篇
农作物   17436篇
水产渔业   17749篇
畜牧兽医   118004篇
园艺   6398篇
植物保护   24838篇
  2020年   2938篇
  2019年   3725篇
  2018年   4368篇
  2017年   4760篇
  2016年   5073篇
  2015年   4501篇
  2014年   5863篇
  2013年   16435篇
  2012年   7366篇
  2011年   9593篇
  2010年   7903篇
  2009年   8362篇
  2008年   8949篇
  2007年   8038篇
  2006年   8418篇
  2005年   7566篇
  2004年   7384篇
  2003年   7167篇
  2002年   6468篇
  2001年   7672篇
  2000年   7369篇
  1999年   6553篇
  1998年   4305篇
  1997年   4357篇
  1996年   4073篇
  1995年   4650篇
  1994年   4098篇
  1993年   3770篇
  1992年   5229篇
  1991年   5329篇
  1990年   5158篇
  1989年   5005篇
  1988年   4435篇
  1987年   4410篇
  1986年   4395篇
  1985年   4623篇
  1984年   4123篇
  1983年   3821篇
  1982年   3063篇
  1981年   2948篇
  1980年   2913篇
  1979年   3616篇
  1978年   3215篇
  1977年   2921篇
  1976年   2780篇
  1975年   2719篇
  1974年   2891篇
  1973年   2893篇
  1972年   2584篇
  1971年   2577篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Summary Acute febrile diseases are characterized by specific and non-specific symptoms. The non-specific responses are presented under the headings: fever, inflammation and pain, experimental models for investigating febrile reactions, haematologic changes, blood biochemical changes, cardiovascular effects, changes in gastric function, and the effects of fever upon pharmacokinetics of drugs. It was the purpose of this review to describe present concepts of thermoregulation and fever, the associated reactions produced by bacterial pyrogens and the mechanisms of these reactions. The available data illustrate, that many questions have not yet been clearly answered. However, the entire field of research involving endogenous substances, such as interleukin-1, is now moving ahead with great speed. Furthermore, there is some evidence which suggests that fever and the associated lower plasma zinc and iron levels act together as a co-ordinated non-specific host defence mechanism. Since experimental fever has a distinct effect upon the pharmacokinetics of drugs, more attention should be given to this aspect.  相似文献   
122.
Prevention of renal carriage of leptospirosis in dogs by vaccination   总被引:1,自引:0,他引:1  
Dogs vaccinated with a Leptospira interrogans vaccine containing serogroups Canicola and Icterohaemorrhagiae and prepared from cultures grown in a protein-free medium resisted challenge with heterologous representatives of these serogroups. In contrast, control dogs were pyrexic and leptospires were isolated from the blood for nine days following Canicola challenge and six days after Icterohaemorrhagiae challenge. Leptospires were isolated from the urine of controls throughout the post challenge period and from kidneys at post mortem examination of six out of six and four out of six Canicola and Icterohaemorrhagiae challenged controls, respectively. There have been no reports of hypersensitivity reactions to this vaccine since its introduction in 1980.  相似文献   
123.
GRAY  S. J.; PRICE  COLIN 《Forestry》1990,63(2):161-175
Traditional forest economics deals with regularly-ordered activities.For many reasons, however, choices must often be made amongconflicting operational priorities. In these circumstances,maximizing net discounted revenue per hectare does not maximizeglobal profit. Priorities for action should be chosen in orderof urgency index ratio, defined as the cost of delaying an operationdivided by the units of limited resource needed to undertakethe operation. Forest managers tend to use informal and sometimesconflicting rules to assign priorities among uses of limitedresources. These priorities often differ from those indicatedby urgency index ratio. Urgency index ratio can be determinedby simple calculations based on readily-available data. Theconcept can be extended to include multiple constraints.  相似文献   
124.
Mid-storage hydration-dehydration treatments with medium-vigour lettuce seed very effectively reduced physiological deterioration in storage and ultimately showed better field emergence and final yield. Soaking-drying treatment of high-vigour lettuce seed was not effective. Pre-sowing treatment of medium-vigour lettuce seed, though less effective than mid-storage treatment, gave better field performance, but high-vigour seed did not benefit from the treatment. For carrot, both mid-storage and pre-sowing seed treatments proved beneficial, with the former showing better performance than the latter. Unlike lettuce, well-preserved carrot seed responded positively to the pre-sowing treatment.  相似文献   
125.
Vanilla is a large genus of about 110 species in the orchid family (Orchidaceae), including the species Vanilla planifolia from which commercial vanilla flavoring is derived. Since most species of vanilla are considered rare and endangered there is an urgent need to conserve them through genetic analysis and propagation/conservation studies on this crop.The present study investigated the genetic diversity among nine leafy- and leaf-less Vanilla species employing 30 decamer RAPD primers and 10 ISSR primers. The species under study were diverse and displayed a range of variability (0–66% and 0–81% for RAPD and ISSR, respectively). A total of 154 RAPD polymorphic markers (83.24%, h = 0.378) and 93 ISSR polymorphic markers (86.11%, h = 0.363) were used to generate a genetic similarity matrix followed by the cluster analysis. Specific groupings were revealed by each cluster analysis with slight variation between two different markers. Among the nine species studied, V. planifolia, Vanilla aphylla and Vanilla tahitensis revealed very low level of variation within their collections, thus indicating a narrow genetic base. The large genetic distance of Vanilla andamanica from other species suggests its different origin. A close genetic affinity was observed between the pairs V. planifolia, V. tahitensis and Vanilla albida, V. aphylla. These are the first comparative results for RAPD and ISSR reporting inter-relationship among nine cultivated, wild and hybrid Vanilla species.  相似文献   
126.
127.
128.
Ohne Zusammenfassung  相似文献   
129.
130.
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