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   237491篇
  免费   14882篇
  国内免费   647篇
林业   23324篇
农学   13929篇
基础科学   2942篇
  41756篇
综合类   19811篇
农作物   16905篇
水产渔业   16498篇
畜牧兽医   87983篇
园艺   6302篇
植物保护   23570篇
  2021年   2451篇
  2020年   2863篇
  2019年   3667篇
  2018年   3954篇
  2017年   4254篇
  2016年   4761篇
  2015年   4296篇
  2014年   5672篇
  2013年   16261篇
  2012年   5878篇
  2011年   7730篇
  2010年   7205篇
  2009年   7793篇
  2008年   7057篇
  2007年   6094篇
  2006年   6764篇
  2005年   5946篇
  2004年   5859篇
  2003年   5661篇
  2002年   4971篇
  2001年   5280篇
  2000年   4914篇
  1999年   4823篇
  1998年   4082篇
  1997年   4137篇
  1996年   3886篇
  1995年   4445篇
  1994年   3827篇
  1993年   3518篇
  1992年   3933篇
  1991年   4148篇
  1990年   3746篇
  1989年   3765篇
  1988年   3340篇
  1987年   3347篇
  1986年   3302篇
  1985年   3674篇
  1984年   3452篇
  1983年   3299篇
  1982年   2764篇
  1981年   2690篇
  1980年   2692篇
  1979年   2981篇
  1978年   2709篇
  1977年   2563篇
  1976年   2415篇
  1975年   2216篇
  1974年   2355篇
  1973年   2271篇
  1972年   1996篇
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
61.
Landscape Ecology - The effects of changing land use, and especially urbanisation, on species and functional diversity are of global concern. Eco-estates are a form of urban housing development...  相似文献   
62.
A new cytotoxic thiodepsipeptide, verrucosamide (1), was isolated along with the known, related cyclic peptide thiocoraline, from the extract of a marine-derived actinomycete, a Verrucosispora sp., our strain CNX-026. The new peptide, which is composed of two rare seven-membered 1,4-thiazepane rings, was elucidated by a combination of spectral methods and the absolute configuration was determined by a single X-ray diffraction study. Verrucosamide (1) showed moderate cytotoxicity and selectivity in the NCI 60 cell line bioassay. The most susceptible cell lines were MDA-MB-468 breast carcinoma with an LD50 of 1.26 µM, and COLO 205 colon adenocarcinoma with an LD50 of 1.4 µM. Also isolated along with verrucosamide were three small 3-hydroxy(alkoxy)-quinaldic acid derivatives that appear to be products of the same biosynthetic pathway.  相似文献   
63.
Landscape Ecology - Several case studies investigated the role of ecosystem services in participatory planning processes. However, no systematic study exists that cuts across a large number of...  相似文献   
64.
65.
Osteomyelitis caused by Cryptococcus neoformans was diagnosed in a three-year-old male Irish wolfhound. The case details and radiological findings of radiolucency and periosteal reaction of the left third metatarsal bone had lead to an initial diagnosis of a malignant bone tumour. Biopsy revealed a fungal granuloma and culture identified the causal organism. A four month course of ketaconazole was instigated and the lameness resolved. Twelve months after the end of treatment there was no recurrence of the problem.  相似文献   
66.
Findings are presented on the precision of a clinical biochemistry analyser designed specifically for veterinary use. Twenty biochemical analytes have been examined in detail for variation within and between runs. The results indicate that the analyser can provide high precision for all the analytes with the possible exception of calcium, and suggest that the instrument can be used with confidence in the practice laboratory to aid diagnosis and to monitor biochemical changes in animals receiving treatment.  相似文献   
67.
68.
Bovine spongiform encephalopathy: epidemiological features 1985 to 1990.   总被引:4,自引:0,他引:4  
Following the identification of bovine spongiform encephalopathy (BSE) in the British cattle population in 1986 epidemiological studies were launched. This paper provides an updated account of the epidemiological features of BSE from 1985, when the first cases, based on clinical histories, occurred, until 1990. The number of cases up to December 1989 represents an annual incidence of 3.9 confirmed cases per 1000 adult animals in Great Britain. Many more dairy herds were affected than beef suckler herds, a difference attributable to the difference in feeding practices between the two herd types. The geographical variation in incidence previously described has persisted with the highest incidence in the south and east of England. Other features of the epidemiology, including the low within-herd incidence, remained unaltered from the earlier findings. The results support the previously suggested hypothesis that the outbreak of BSE was due to the sudden exposure of the cattle population to a scrapie-like agent in 1981/82. There was no evidence of direct transmission between cattle during the period considered.  相似文献   
69.
Differences in gas exchange parameters i.e. carbon exchange rate (CER), transpiration (Tr), water vapor conductance (g) were investigated, under a controlled environment, in a semi-dwarf (SD) Triticum aestivum line, its tall (TL) near isoline and the Payne (PA) variety characterized by small leaves. The plants were maintained in: (a) optimal substrate moisture condition (CTR); (b) salinized by watering with a 0.12 m NaCl solution (SLT); (c) water stressed by withholding waterings during a period of six days (STR); (d) stressed and salinized during the same period and with the same saline solution (S + S). CER and Tr were negatively affected by the stresses; SLT and S + S treatments had a higher WUE compared to their respective controls CTR and STR.
SD line had a better performance in terms of CER and WUE, particularly when water and salt stresses interacted. Leaf anatomy and assimilation/internal CO2 concentration curves contributed to explaining the SD performance.  相似文献   
70.
Acemannan, a complex carbohydrate shown to stimulate interleukin-1, tumor necrosis factor alpha and prostaglandin E2 production by macrophages, has also demonstrated antiviral activity in vitro against human immunodeficiency virus, Newcastle disease virus and influenza virus. A pilot study was undertaken to determine acemannan's effect in 49 feline immunodeficiency virus (FIV) infected cats with clinical signs of disease (Stage 3, 4 or 5), 23 of which had severe lymphopenia. Cats received acemannan either by intravenous (Group 1) or subcutaneous (Group 2) injection once weekly for 12 weeks, or by daily oral (Group 3) administration for 12 weeks. Upon entry into the study, cats were randomly assigned to one of the three groups. Laboratory analyses were performed at the beginning of the study and at Weeks 6 and 12. Cats were allowed to continue with a predetermined maintenance regimen of acemannan after completing the 12-week study. Thirteen cats died during the course of treatment. Upon necropsy, the most frequent histopathologic findings were neoplastic, kidney and pancreatic disease. Friedman's two-way ANOVA test showed no significant differences in efficacy among groups administered acemannan by the different routes. Therefore, groups were combined and a signed-ranks test was used to determine changes over time. A significant increase was seen in lymphocyte counts (P < 0.001). Neutrophil counts decreased significantly (P = 0.007), as did incidence of sepsis (P = 0.008). When cats entering with lymphopenia were analyzed separately, a much greater increase in lymphocyte counts was noted (235%) compared with non-lymphopenic cats (42%). A survival rate of 75% was found for all three groups. Thirty-six of 49 animals are alive 5-19 months post-entry. These results suggest that acemannan therapy may be of significant benefit in FIV-infected cats exhibiting clinical signs of disease.  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