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   90827篇
  免费   5391篇
  国内免费   52篇
林业   3169篇
农学   2915篇
基础科学   568篇
  10525篇
综合类   16448篇
农作物   3806篇
水产渔业   4457篇
畜牧兽医   47240篇
园艺   1180篇
植物保护   5962篇
  2019年   856篇
  2018年   1262篇
  2017年   1455篇
  2016年   1360篇
  2015年   1191篇
  2014年   1425篇
  2013年   3623篇
  2012年   2697篇
  2011年   3219篇
  2010年   2117篇
  2009年   2090篇
  2008年   3109篇
  2007年   2967篇
  2006年   2824篇
  2005年   2667篇
  2004年   2508篇
  2003年   2525篇
  2002年   2402篇
  2001年   2992篇
  2000年   2868篇
  1999年   2343篇
  1998年   1018篇
  1997年   918篇
  1996年   821篇
  1995年   982篇
  1994年   902篇
  1993年   898篇
  1992年   1873篇
  1991年   1868篇
  1990年   1812篇
  1989年   1867篇
  1988年   1601篇
  1987年   1724篇
  1986年   1762篇
  1985年   1709篇
  1984年   1358篇
  1983年   1210篇
  1982年   884篇
  1981年   753篇
  1979年   1142篇
  1978年   937篇
  1977年   814篇
  1976年   771篇
  1975年   836篇
  1974年   1031篇
  1973年   1065篇
  1972年   1026篇
  1971年   977篇
  1970年   908篇
  1969年   846篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Multilineage potential of adult human mesenchymal stem cells   总被引:7,自引:0,他引:7  
Human mesenchymal stem cells are thought to be multipotent cells, which are present in adult marrow, that can replicate as undifferentiated cells and that have the potential to differentiate to lineages of mesenchymal tissues, including bone, cartilage, fat, tendon, muscle, and marrow stroma. Cells that have the characteristics of human mesenchymal stem cells were isolated from marrow aspirates of volunteer donors. These cells displayed a stable phenotype and remained as a monolayer in vitro. These adult stem cells could be induced to differentiate exclusively into the adipocytic, chondrocytic, or osteocytic lineages. Individual stem cells were identified that, when expanded to colonies, retained their multilineage potential.  相似文献   
992.
993.
An 8-year-old, male castrated golden retriever presented for cough and increased respiratory effort. Radiographs revealed an alveolar pattern in the right caudal lung lobe and an opacity at the carina suspected to be enlarged tracheobronchial lymph nodes. The disease progressed to involve the right middle lung lobe. Cytopathology of a fine-needle aspirate and bronchoalveolar lavage fluid were nondiagnostic. Surgical removal of the right caudal lung lobe and biopsy of the perihilar lymph nodes revealed pulmonary thromboembolism and reactive lymph nodes. The dog died several days postoperatively, and necropsy revealed diffuse pulmonary thromboembolism. Additionally, Blastomyces dermatitis organisms were identified in a pyogranulomatous mass surrounding the trachea near the carina. In an extensive literature search, no reports of pulmonary thromboembolism associated with blastomycosis were identified. It is suspected that the inflammation secondary to blastomycosis caused the thromboembolism.  相似文献   
994.
995.
996.
Temporohyoid osteoarthropathy (THO) is a well‐recognised clinical entity in horses. Temporohyoid osteoarthropathy is characterised by progressive osseous proliferation of the proximal portion of the stylohyoid and petrous temporal bones, and is generally described as a disease of middle‐aged horses. More recent reports also attribute a variety of clinical syndromes in young horses to THO, requiring a diversification of approaches to confirm the diagnosis and its significance, and to guide conservative and surgical treatment options. This commentary will review the purported aetiology, clinical syndromes, available diagnostic tests and treatment options of equine THO.  相似文献   
997.
998.
999.
THOMAS  R. C.; MILLER  H. G 《Forestry》1994,67(4):329-341
In an 11-year-old stand of Sitka spruce (Picea sitchensis [Bong.]Carr.) application of nitrogen fertilizers, at a rate of 10kgN ha–1 month–1, increased mean diameter incrementby 12 per cent, while the further addition of phosphorus, at5 kg ha–1 month–1, resulted in a 23 per cent increase.An attack by the green spruce aphid (Elatobium albietinum Walker)occurred during the period of fertilizer addition. The mostseverely affected trees showed a reduction in diameter growthof 50 to 56 per cent but the severity of the attack betweentrees was unrelated to the treatments applied. However, fertilizerapplication did hasten the recovery of diameter growth afterdefoliation.  相似文献   
1000.
Xanthomonas campestris pv. campestris (Xcc) is the causal agent of black rot in Brassicaceae. It is widespread in Italy and severe outbreaks occur under conditions that favour disease development. In this study a multilocus sequence typing approach (MLST) based on the partial sequence of seven loci was applied to a selection of strains representative of the main areas of cultivation and hosts. The aim was to investigate whether the long tradition of brassica crops in Italy has influenced the evolution of different Xcc populations. All loci were polymorphic; 14 allelic profiles were identified of which 13 were unique to Italian strains. Based on the seven loci, the most common genotype within the Italian Xcc strains (AP1) was also the most representative genotype found in worldwide Xcc strains. This genotype was included in a new clonal complex in addition to three other clonal complexes already identified in Xcc populations. The phylogenetic reconstruction using a concatenated dataset of four conserved protein-coding genes, dnaK, fuyA, gyrB and rpoD, showed that the Italian strains belonged to two genetic groups. Physiological races were also investigated for the first time in Italy. The race structure of Xcc was determined by inoculating eight differential Brassica lines belonging to five species and showed that, in Italy, race 4 is the most widespread, followed by races 1 and 6. No correlation was found between allelic profiles, host of isolation, geographical origin and races, although a prevalent race was identified within the same clonal complex.  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