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   728篇
  免费   43篇
林业   73篇
农学   13篇
基础科学   1篇
  133篇
综合类   54篇
农作物   22篇
水产渔业   40篇
畜牧兽医   378篇
园艺   21篇
植物保护   36篇
  2023年   14篇
  2022年   15篇
  2021年   23篇
  2020年   31篇
  2019年   40篇
  2018年   26篇
  2017年   19篇
  2016年   24篇
  2015年   23篇
  2014年   31篇
  2013年   34篇
  2012年   51篇
  2011年   70篇
  2010年   35篇
  2009年   27篇
  2008年   40篇
  2007年   48篇
  2006年   49篇
  2005年   32篇
  2004年   35篇
  2003年   27篇
  2002年   23篇
  2001年   7篇
  2000年   7篇
  1999年   5篇
  1998年   2篇
  1997年   3篇
  1995年   1篇
  1994年   1篇
  1993年   4篇
  1992年   3篇
  1991年   5篇
  1990年   2篇
  1988年   1篇
  1987年   1篇
  1985年   1篇
  1984年   1篇
  1983年   3篇
  1982年   1篇
  1981年   2篇
  1975年   1篇
  1973年   1篇
  1970年   1篇
  1950年   1篇
排序方式: 共有771条查询结果,搜索用时 15 毫秒
71.
72.
73.
Cardio-facio-cutaneous (CFC) syndrome is a sporadic developmental disorder involving characteristic craniofacial features, cardiac defects, ectodermal abnormalities, and developmental delay. We demonstrate that heterogeneous de novo missense mutations in three genes within the mitogen-activated protein kinase (MAPK) pathway cause CFC syndrome. The majority of cases (18 out of 23) are caused by mutations in BRAF, a gene frequently mutated in cancer. Of the 11 mutations identified, two result in amino acid substitutions that occur in tumors, but most are unique and suggest previously unknown mechanisms of B-Raf activation. Furthermore, three of five individuals without BRAF mutations had missense mutations in either MEK1 or MEK2, downstream effectors of B-Raf. Our findings highlight the involvement of the MAPK pathway in human development and will provide a molecular diagnosis of CFC syndrome.  相似文献   
74.
Fishing, trophic cascades, and the process of grazing on coral reefs   总被引:1,自引:0,他引:1  
Since the mass mortality of the urchin Diadema antillarum in 1983, parrotfishes have become the dominant grazer on Caribbean reefs. The grazing capacity of these fishes could be impaired if marine reserves achieve their long-term goal of restoring large consumers, several of which prey on parrotfishes. Here we compare the negative impacts of enhanced predation with the positive impacts of reduced fishing mortality on parrotfishes inside reserves. Because large-bodied parrotfishes escape the risk of predation from a large piscivore (the Nassau grouper), the predation effect reduced grazing by only 4 to 8%. This impact was overwhelmed by the increase in density of large parrotfishes, resulting in a net doubling of grazing. Increased grazing caused a fourfold reduction in the cover of macroalgae, which, because they are the principal competitors of corals, highlights the potential importance of reserves for coral reef resilience.  相似文献   
75.
A high mortality outbreak of respiratory mycoplasmosis occurred in goats in Mexico. The clinicopathologic presentation resembled contagious caprine pleuropneumonia caused by Mycoplasma capricolum subspecies capripneumoniae. By using a battery of polymerase chain reaction assays, the mycoplasma associated with this outbreak was identified as Mycoplasma mycoides subsp. capri.  相似文献   
76.
Folic acid (pteroylmonoglutamic acid) is used in enriched foods; however, very little folic acid occurs naturally in fruits and vegetables. For the U.S. Department of Agriculture's National Food and Nutrient Analysis Program, a number of fruits and vegetables have been assayed for endogenous folates, by a liquid chromatography-mass spectrometry method, to evaluate the accuracy of existing data for total folate determined by standard microbiological analysis. Folate in red and green sweet peppers (Capsicum annuum) differed notably (70.2 and 20.7 microg/100 g, respectively) and exceeded existing values determined by microbiological assay (18 and 11 microg/100 g, respectively). 5-Methyltetrahydrofolate was the predominant vitamer, but a significant amount of 5-formyltetrahydrolfolate and some 10-formylfolate were present. These findings may assist in making dietary recommendations or developing research diets related to folate. The data from this study have been used to update the folate values in release 19 of the USDA Nutrient Database for Standard Reference.  相似文献   
77.
A seven-year-old female spayed Domestic Short-haired cat was brought for examination following a four-month history of conjunctivitis in the right eye. The superior palpebral conjunctiva was diffusely swollen and firm, displacing the right globe ventrally and preventing full closure of the eyelids. The right popliteal lymph node was enlarged as were several other peripheral lymph nodes. An incisional biopsy of the conjunctival swelling revealed a lymphoid neoplasm, the cellular make up of which suggested features of feline Hodgkin's-like lymphoma. Although the neoplasm was poorly responsive to standard chemotherapeutic protocols for lymphoma, radiation therapy rapidly reduced its size. Three years following the initial diagnosis, there has been no recurrence of the conjunctival neoplasm. To the authors' knowledge, there are no previous reports of extranodal or adnexal (conjunctiva, eyelid, orbit) Hodgkin's-like lymphoma in the veterinary literature.  相似文献   
78.
79.
80.
Atmospheric black carbon (BC) warms Earth's climate, and its reduction has been targeted for near-term climate change mitigation. Models that include forcing by BC assume internal mixing with non-BC aerosol components that enhance BC absorption, often by a factor of ~2; such model estimates have yet to be clearly validated through atmospheric observations. Here, direct in situ measurements of BC absorption enhancements (E(abs)) and mixing state are reported for two California regions. The observed E(abs) is small-6% on average at 532 nm-and increases weakly with photochemical aging. The E(abs) is less than predicted from observationally constrained theoretical calculations, suggesting that many climate models may overestimate warming by BC. These ambient observations stand in contrast to laboratory measurements that show substantial E(abs) for BC are possible.  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