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   78809篇
  免费   4252篇
  国内免费   93篇
林业   3191篇
农学   2042篇
基础科学   461篇
  8567篇
综合类   15395篇
农作物   3124篇
水产渔业   3414篇
畜牧兽医   41674篇
园艺   847篇
植物保护   4439篇
  2019年   719篇
  2018年   1091篇
  2017年   1212篇
  2016年   1046篇
  2015年   903篇
  2014年   1188篇
  2013年   2620篇
  2012年   2110篇
  2011年   2572篇
  2010年   1615篇
  2009年   1726篇
  2008年   2593篇
  2007年   2402篇
  2006年   2283篇
  2005年   2135篇
  2004年   2074篇
  2003年   2136篇
  2002年   1922篇
  2001年   2384篇
  2000年   2438篇
  1999年   1869篇
  1998年   719篇
  1997年   711篇
  1996年   711篇
  1995年   806篇
  1993年   740篇
  1992年   1554篇
  1991年   1750篇
  1990年   1634篇
  1989年   1647篇
  1988年   1473篇
  1987年   1534篇
  1986年   1598篇
  1985年   1549篇
  1984年   1185篇
  1983年   1057篇
  1982年   704篇
  1979年   1152篇
  1978年   894篇
  1977年   755篇
  1975年   774篇
  1974年   1024篇
  1973年   1024篇
  1972年   1043篇
  1971年   993篇
  1970年   987篇
  1969年   903篇
  1968年   810篇
  1967年   799篇
  1966年   732篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Objective: To determine the effect of storage on ammonia concentration in canine packed red blood cell (pRBC) units.
Design: In vitro and in vivo study.
Setting: University Veterinary Teaching Hospital.
Interventions: Ammonia concentration was measured in 7 units of canine pRBC prepared in citrate-phosphate-dextrose (CPD) and Adsola on Days 1 and 35 of storage. Ammonia was measured in 4 additional units of canine pRBC on Days 0, 7, 14, 21, 28, and 35. Plasma ammonia was also determined in 5 anemic dogs receiving pRBC.
Measurements and Main Results: Ammonia concentration increased from 73 ± 15 mmol/L (mean ± SD) on Day 1 to 800 ± 275 mmpl/L on Day (p<0.001). When measured every 7 days in 4 units of canine pRBC, ammonia concentration increased from 23 ± 8 mmol/L on Day 0 to 179 ± 13 mmol/L (Day 7), 276 ± 56 mmol/L (Day 14). 383 ± 47 mmol/L (Day21), 466 ± 30 mmol/L (Day 28), and 562 ± 27 mmol/L (Day 35) (p<0.05 for all comparisons). In a preliminary study, plasma ammonia concentration measured in blood samples from 5 anemic dogs without primary liver disease immediately before and after transfusion with 5–10 ml/kg of stored pRBC remained in the normal reference range.
Conclusions: The ammonia concentration in stored canine pRBC increased markedly with time. In this preliminary study, ammonia concentrations in dogs without primary liver disease did not increase above the reference range after transfusion with pRBC.  相似文献   
992.
993.
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   
994.
Resistance in the tubers of potato clones with various levels of foliar multigenic resistance toPhytophthora infestans was measured to detect possible correlations between tuber and foliar resistance. A highly significant correlation was found using wound-healed tuber tissues. High levels of tuber resistance were detected in most foliar-resistant clones when inoculations with race 1,2,3,4 were made after 24–48 hours of wound periderm formation at 20 C. The resistance of wound-healed tissues increased as the time between wounding and inoculation increased. This increase in resistance was significantly greater for clones with foliar resistance than for clones with susceptible foliage.  相似文献   
995.
996.
A six-year-old cat presented with clinical signs consistent with distal aortic thromboembolism while clinical signs of cardiovascular disease were absent. Diagnostics, including thoracic radiographs, electrocardiography, and echocardiography revealed no cardiovascular anomalies. Thoracic radiographs revealed multifocal pulmonary lesions consistent with neoplasia. Complete blood cell count demonstrated a marked thrombocytosis, leukopenia, and neutropenia. Histopathology of the pulmonary lesions confirmed multiple bronchoalveolar carcinomas. Myelodysplasia with megakaryocytic hyperplasia and ineffective myelopoiesis was noted on bone-marrow histopathology from multiple sites. The absence of other causes suggested a paraneoplastic thrombocytosis. The diagnosis of paraneoplastic thrombocytosis-induced thromboembolism was made due to the lack of underlying cardiac disease and the presence of a marked thrombocytosis. The presence of thrombocytosis and thromboembolism associated with neoplasia is discussed.  相似文献   
997.
Microscopic evidence of multifocal mineralizations (psammoma bodies) were seen in brains of 33/53 (62%) raccoons (Procyon lotor) necropsied on Parramore Island, Virginia. Most mineralized foci had concentric laminations and were present in small capillaries of meninges of the brain (15/33), in choroid plexus (3/33), or at both these sites (13/33). In 2 raccoons, the lesions were confined to the meninges of the proximal cervical spinal cord. In most cases, the affected vessels appeared to have been completely occluded. However, no evidence of ischemic changes in the brain parenchyma was seen, and none of the raccoons had abnormal neurologic signs prior to euthanasia. The condition appears to be a common incidental histopathologic finding in raccoons from the eastern United States. Although the exact cause of this condition is not known, a primary vascular insult with resultant dystrophic mineralization of the affected vessels is suspected.  相似文献   
998.
The effects of no‐till versus conventional farming practices were evaluated on soft wheat functional and nutritional characteristics, including kernel physical properties, whole wheat composition, antioxidant activity, and end‐product quality. Soft white winter wheat cultivar ORCF 102 was evaluated over a two‐year period from three long‐term replicated no‐till versus conventional tillage studies in Oregon. Wheat from the no‐till cropping systems generally had greater test weight, kernel diameter, and kernel weight and had softer kernels compared with wheat from the conventional tillage systems. Compared with the conventional systems, no‐till whole wheat flour had lower protein and SDS sedimentation volume. Ash content as well as most minerals measured (calcium, copper, iron, magnesium, and zinc), except for manganese and phosphorus, were generally slightly lower in no‐till than in conventional wheat. Whole wheat flour from the no‐till cropping systems generally had slightly lower total phenolic content and total antioxidant capacity. Milling properties, including flour yield, break flour yield, and mill score, were not affected by tillage systems. Refined flour from no‐till systems had lower protein, SDS sedimentation volume, and lactic acid and sucrose solvent retention capacities compared with flour from conventional tillage. No‐till wheat generally had greater sugar‐snap cookie diameter than conventionally tilled wheat. In conclusion, no‐till soft white winter wheat generally had slightly reduced nutritional properties (protein, ash, most minerals, and total antioxidant content) compared with wheat from conventionally tilled systems, and it had equivalent or sometimes superior functional properties for baking cookie‐type products.  相似文献   
999.
Nectria ascospores discharged from a Beech bark diseased tree were monitored by a spore trap, and prevailing meteorological conditions were recorded. Forest and glasshouse experiments are described in which necrosis of beech bark followed wound inoculation with Nectria isolates during dormancy. Perithecia were produced in vitro and spore dimensions were measured. Linear growth rate of the fungus in culture was compared over a range of temperatures.  相似文献   
1000.
This case series describes a rare entity, nasal angiofibroma, in 13 dogs that were presented to the University of Wisconsin, School of Veterinary Medicine from 1988 to 2000. All dogs in this case series presented with clinical signs and radiographic changes that were strongly suggestive of a locally invasive neoplasm. However, histopathology completed on transnostral core biopsy samples revealed benign appearing vascular proliferation with secondary lymphosuppurative inflammation was established despite cytologic criteria of malignancy present in five dogs. On the basis of the outcomes in this case series, nasal angiofibroma should be considered a differential for dogs presenting with clinical signs consistent with a malignant nasal tumour.  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