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   97084篇
  免费   5947篇
  国内免费   1309篇
林业   4068篇
农学   3817篇
基础科学   1066篇
  12155篇
综合类   18596篇
农作物   3601篇
水产渔业   4634篇
畜牧兽医   48924篇
园艺   1383篇
植物保护   6096篇
  2021年   942篇
  2020年   1049篇
  2019年   1272篇
  2018年   1699篇
  2017年   1923篇
  2016年   1756篇
  2015年   1700篇
  2014年   1862篇
  2013年   3535篇
  2012年   3404篇
  2011年   4074篇
  2010年   2871篇
  2009年   2687篇
  2008年   3666篇
  2007年   3618篇
  2006年   3245篇
  2005年   3180篇
  2004年   2705篇
  2003年   2792篇
  2002年   2587篇
  2001年   3205篇
  2000年   3244篇
  1999年   2561篇
  1998年   1032篇
  1997年   913篇
  1996年   832篇
  1995年   946篇
  1994年   850篇
  1993年   835篇
  1992年   1858篇
  1991年   2058篇
  1990年   1910篇
  1989年   1845篇
  1988年   1736篇
  1987年   1824篇
  1986年   1791篇
  1985年   1573篇
  1984年   1348篇
  1983年   1142篇
  1982年   757篇
  1979年   1172篇
  1978年   912篇
  1976年   756篇
  1975年   814篇
  1974年   1072篇
  1973年   1042篇
  1972年   994篇
  1971年   935篇
  1970年   904篇
  1969年   865篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
182.
Telomere shortening in normal somatic cells has been proposed as a major barrier to unlimited cellular proliferation. Telomerase is an enzyme capable of maintaining telomere length, and thus bypassing this barrier. In human beings, telomerase activity is restricted to cancer cells and cells of stem or germ cell lineages. Dogs represent a potentially useful clinical model for the development of telomerase‐based therapies because telomerase activity is also restricted to cancer cells and stem cells in this species. We examined the ability of telomestatin to inhibit telomerase activity in telomerase‐positive D17 and CMT7 canine cancer cell lines. At a concentration of 2 μM, telomestatin treatment resulted in a decrease in telomerase activity, telomere shortening, growth inhibition and apoptosis in telomerase‐positive cancer cells. These effects were not seen in telomerase‐negative skin fibroblasts or negative controls. These results confirm that telomestatin specifically inhibits telomerase activity in canine cancer cells and strengthens the usefulness of dogs as a model for testing telomerase‐based therapies.  相似文献   
183.
Tuta absoluta (Meyrick) is an invasive pest of tomato native to South America, where it is responsible for extensive damage. It rapidly spread into several European countries, becoming a key pest. Knowledge of its main biological traits can be used to develop effective plant protection management strategies. It is a multivoltine species with a homodynamous behaviour, and the length of its life cycle depends on environmental conditions, particularly temperature. The larvae feed and develop inside tomato leaves, stems and fruits throughout the entire growing cycle. The adults have crepuscular habits, and in Mediterranean conditions they can be easily detected throughout the year. Damage is directly related to the reduction of plants' photosynthetic capacity and of production levels in both protected and open‐field tomato crops; indirect damage can be also caused by secondary infections, with pathogens developing on the infested plant and fruit tissues.  相似文献   
184.
A bilateral and symmetrical neuronal vacuolation associated with spinal cord white matter degeneration and laryngeal neuropathy was observed in a 12-week-old male mixed-breed dog with a history of progressive pelvic limbs ataxia. On clinical examination, signs included inspiratory stridor, spinal ataxia, tetraparesis, and proprioceptive deficits more severe in the pelvic limbs. Examination of the larynx showed bilateral laryngeal paralysis and electromyography revealed fibrillation potentials restricted to the intrinsic laryngeal muscles. Clinical and pathological findings resembled the syndrome of neuronal vacuolation and spinocerebellar degeneration described in Rottweiler dogs. This is the first report of a similar disorder in a dog different from Rottweiler.  相似文献   
185.
186.
Diospyros kaki Thunb. originated in Eastern Asia, as evidenced of its culture in China as early as several centuries B.C. In the seventh century, persimmon was introduced to Japan and later, in the fourteenth century to Korea. There is no information about persimmon culture in Europe until the seventeenth century, with the spread over the world occurring in the eighteenth century. The genus Diospyros contains more than 400 species, with levels of ploidy ranging from diploid (2n = 2x = 30) up to nonaploid (2n = 9x = 135). The primary economic crop species is Diospyros kaki Thunb., which is mainly hexaploid (2n = 6x = 90) and includes hundreds of cultivars. Although a relatively recent introduction in Europe, the species has adapted well, and the genetic diversity have been expanded with culture and selection for the past 200 years in the Mediterranean basin. These locally adapted cultivars were evaluated with cultivars from Asian origin in a germplasm collection established at IVIA in Valencia, Spain. In this paper 27 cultivars from the IVIA collection were studied by multivariate analysis, and 37 variables were analyzed using a Principal Components Analysis and cluster analysis following the method UPGMA. Studies on correlations and significance among variables identified the most relevant ones, and thus provided information for a future core collection.  相似文献   
187.
188.
189.
190.
Five microsatellites, IDVGA-2, IDVGA-27, IDVGA-46, IDVGA-55 and TGLA-44,were analyzed for polymorphisms in beef cattle. The number of alleles and polymorphism information content (PIC) values were 12/0. 82, 5/0. 58, 8/0. 70, 6/0. 57 and 11/0. 86 respectively. Each microstellite was typed on a half-sib family in order to verify the segregation of the alleles.  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