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   218090篇
  免费   12022篇
  国内免费   9077篇
林业   14818篇
农学   16648篇
基础科学   7930篇
  30388篇
综合类   46241篇
农作物   9482篇
水产渔业   9897篇
畜牧兽医   84250篇
园艺   5257篇
植物保护   14278篇
  2023年   1189篇
  2022年   2601篇
  2021年   4165篇
  2020年   3998篇
  2019年   4076篇
  2018年   4209篇
  2017年   5404篇
  2016年   4992篇
  2015年   5838篇
  2014年   6170篇
  2013年   10170篇
  2012年   9880篇
  2011年   11062篇
  2010年   9388篇
  2009年   8932篇
  2008年   10093篇
  2007年   10051篇
  2006年   9396篇
  2005年   8465篇
  2004年   6121篇
  2003年   6102篇
  2002年   5495篇
  2001年   6442篇
  2000年   6371篇
  1999年   5694篇
  1998年   3177篇
  1997年   2984篇
  1996年   2739篇
  1995年   2860篇
  1994年   2405篇
  1993年   2468篇
  1992年   3632篇
  1991年   3633篇
  1990年   3478篇
  1989年   3385篇
  1988年   2990篇
  1987年   2818篇
  1986年   2664篇
  1985年   2463篇
  1984年   1999篇
  1983年   1754篇
  1982年   1202篇
  1979年   1661篇
  1978年   1285篇
  1975年   1170篇
  1974年   1359篇
  1973年   1449篇
  1972年   1380篇
  1971年   1197篇
  1970年   1250篇
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
991.
Landscape-ecological mapping of the Netherlands   总被引:1,自引:0,他引:1  
The Landscape-ecological Mapping of the Netherlands project (LMN project) started in 1983 with the aim of establishing a landscape-ecological database for use in developing and evaluating national land-use plans. The project, working with grid cells of 1 km2, has four working objectives: a) development of mapping potential for basic landscape-ecological data, b) assessment of susceptibility to interventions, c) evaluation of significance for nature conservation and d) production of vulnerability maps, as a combination of susceptibility and significance. In addition to information on soil, groundwater, ecotopes, flora and fauna, the database also incorporates information on physiographical features and entire landscapes. The resulting database is a geographic information system (GIS). This article describes the second phase of the project (1985–1989), covering the Randstad area, and focusses on the methods and the applications potential of the database.  相似文献   
992.
In dogs with experimentally induced heartworm infection, the onset of caval syndrome (CS) was characterized by a murmur, loudest over the tricuspid valve, and a large worm mass in the right ventricular lumen detectable during diastole by use of M-mode echocardiography. Two-dimensional echocardiography indicated that the worm mass was located in the right atrium and venae cavae and was "flowing" into the right ventricle during rapid diastolic filling. Paradoxical septal motion and vigorous right ventricular cranial wall motion also were observed. Other echocardiographic changes included decreased size of the left atrium and ventricle, aortic root, and ratio of left-to-right ventricular diastolic luminal diameter, compared with values obtained 6 months after experimentally induced heartworm infection. Right ventricular end diastolic diameter increased considerably. Most echocardiographic indices of left ventricular function (fractional shortening, velocity of circumferential fiber shortening, ejection fraction, and preejection period) were not altered appreciably, but estimates of cardiac index and stroke volume were markedly decreased. Electrocardiography revealed ventricular and supraventricular premature complexes in 7 of the 8 dogs studied, evidence of right ventricular enlargement in 6 of the 8 dogs studied, and increased mean heart rate, compared with that measured 6 months after inoculation of infective larvae, before the onset of CS. Cardiac catheterization was performed in 3 days at the onset of CS. Severe pulmonary arterial and right ventricular hypertension and decreased cardiac index (compared with values obtained before inoculation) were observed. Evidence of right ventricular inflow obstruction was not detected. Mean aortic blood pressure decreased with the onset of CS, but right ventricular end diastolic pressure increased.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
993.
994.
Thirteen adult cats were each fed consecutively five test diets: firstly a complete canned diet, and then four commercially available dry cat diets (New Formula Biscats, Cat Chow, Go-Cat and Whiskettes) for 7,13, 5, 4 and 4 weeks respectively. Urine samples were tested for pH and presence of struvite crystals. Body weight was recorded weekly. Urine pH was ranked from acid to alkaline as follows: Whiskettes (pH 5.8), canned diet (pH 5.9), Biscats (pH 6.1), Cat Chow (pH 6.45) and Go-Cat (pH 6.5). The pH of urine when cats were fed Whiskettes, Biscats and canned diet was not significantly different. Biscats produced a urine pH significantly higher than Whiskettes but, in each case, these three diets produced a significantly lower urine pH than Cat Chow and Go-Cat. Struvite crystals occurred spontaneously in 9% of urine samples from cats when fed Whiskettes and Biscats, 71% when fed Cat Chow and 38% when fed Go-Cat: spontaneous struvite was not seen in any cat fed CX9. Haematuria was seen in 2.9% (2/69) urine samples from cats fed Cat Chow. One tom cat had a blocked urethra after 24 days on Go-Cat. No abnormalities were observed in any cat when fed canned diet, Biscats or Whiskettes. It was concluded that of the four dry cat diets tested only Biscats and Whiskettes produce urine with an acceptably low rate of spontaneous struvite crystalluria.  相似文献   
995.
A one-year-old domestic long-haired cat was referred to the New York State College of Veterinary Medicine because of acute onset of paraparesis and hyperesthesia associated with trauma. Myelography and cerebrospinal fluid analysis revealed severe hydromyelia and myelitis, respectively. The definitive diagnosis of feline infectious peritonitis was made by histological examination at necropsy. Lesions were confined exclusively to the brain and spinal cord. Partial occlusion of the third and fourth ventricles with pyogranulomatous debris caused hydrocephalus and subsequent hydromyelia. The hydromyelia may have been the primary means of compensation for the hydrocephalus, thus masking subclinical disease.  相似文献   
996.
997.
Sheep affected by broken mouth periodontal disease (P.D.) were examined over a twelve month period for different clinical parameters. It is suggested that P.D. in sheep is an episodic phenomenon similar to human P.D., and that only a few animals with signs of P.D. may undergo clinically significant destruction over a yearly period. No single parameter could reliably predict future deterioration in other parameters.  相似文献   
998.
999.
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