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   3076篇
  免费   155篇
  国内免费   302篇
林业   242篇
农学   127篇
基础科学   132篇
  427篇
综合类   1012篇
农作物   137篇
水产渔业   198篇
畜牧兽医   939篇
园艺   171篇
植物保护   148篇
  2024年   15篇
  2023年   59篇
  2022年   87篇
  2021年   145篇
  2020年   107篇
  2019年   128篇
  2018年   87篇
  2017年   122篇
  2016年   132篇
  2015年   122篇
  2014年   164篇
  2013年   193篇
  2012年   241篇
  2011年   232篇
  2010年   166篇
  2009年   181篇
  2008年   148篇
  2007年   166篇
  2006年   150篇
  2005年   117篇
  2004年   90篇
  2003年   90篇
  2002年   87篇
  2001年   64篇
  2000年   56篇
  1999年   46篇
  1998年   28篇
  1997年   44篇
  1996年   29篇
  1995年   37篇
  1994年   27篇
  1993年   33篇
  1992年   21篇
  1991年   31篇
  1990年   22篇
  1989年   19篇
  1988年   12篇
  1987年   8篇
  1986年   4篇
  1985年   7篇
  1984年   6篇
  1983年   1篇
  1980年   3篇
  1979年   2篇
  1974年   1篇
  1956年   3篇
排序方式: 共有3533条查询结果,搜索用时 78 毫秒
991.
[Objective] This study aimed to investigate the combination effects of ligustrazine and cis-dichlorodiamine platinum (DDP) on anti-proliferation, cell cycle and apoptosis of SGC-7901 cell lines in vitro. [Methed] SGC-7901 cells were treat- ed with ligustrazine and DDP alone or combined for 48 h for morphology assay. Anti-proliferative effects with the same treatment for 24, 48 and 72 h were assayed by MTT method, respectively. Cell cycle distribution and apoptosis assay of treated cells were performed in flow cytometry. Zhengjun Jin's protocol was used to assay the effect of drug combination. [Result] Ligustrazine significantly increased the prolif- eration inhibition rate of DDP on SGC-7901 cells in combination with DDP, com- paring with the effects of ligustrazine or DDP alone, and exhibited synergistic antitu- mor effect. The combination drug treatment induced cell cycle arrest occurred in S and G2 phase of the cell cycle and increased the apoptosis rate significantly. [Conclusion] Our results indicate that ligustrazine, as a low-toxic and natural herbal component, can significantly increase the anti-proliferative effect and apoptosis rate of antitumor drug DDP on human gastric carcinoma SGC:.-7901 cells.  相似文献   
992.
Mitochondrial malate dehydrogenase (mMDH) and citrate synthase (CS) are sequential enzymes in Krebs cycle. mMDH, CS and the complex between mMDH and CS (mMDH+CS) were treated with nitric oxide solution. The roles of notric oxide (NO) on the secondary structures and the interactions between mMDH and CS were studied using circular diehroism (CD) and Fourier transform surface plasmon resonance (FT-SPR), respectivley. The effects of NO on the activities of mMDH, CS and mMDH+CS were also studied. And the regulations by NO on mMDH and CS were simulated by PyMOL software. The results of SPR conifrmed that strong interaction between mMDH and CS existed and NO could signiifcantly regulate the interaction between the two enzymes. NO reduced the mass percents ofα-helix and increased that of random in mMDH, CS and mMDH+CS. NO increased the activities of CS and mMDH+CS, and inhibited the activity of mMDH. Graphic simulation indicated that covalent bond was formed between NO and Asn242 in active site of CS. However, there was no direct bond between NO and mMDH. The increase in activity of mMDH+CS complex depended mostly on the interaction between NO and CS. All the results suggested that the regulations by NO on the activity and interaction between mMDH and CS were accord with the changes in mMDH, CS and mMDH+CS caused by NO.  相似文献   
993.
对妊娠的SPF级大、小鼠在临产前进行剖宫产手术,用隔离器内大、小鼠代乳并饲养,从而达到生物净化;统计分析手术成功率、仔鼠存活率以及离乳率等,并将净化后的动物繁殖,对其后代进行寄生虫、细菌和病毒检测。结果显示:(1)本实验室建立的剖宫产术平均手术成功率为83.0%±20.0%,仔鼠平均存活率为74.4%±22.8%,平均离乳率为97.7%±3.6%;(2)生物净化的后代寄生虫、细菌和病毒指标均符合SPF级动物国家标准;(3)原感染螺杆菌的小鼠经生物净化后,子代无螺杆菌感染现象。因此,用该剖宫产术净化SPF级动物可以达到生物净化的目的。  相似文献   
994.
规模养猪场粪便堆肥处理生命周期评价实例分析   总被引:2,自引:0,他引:2  
以北京郊区某规模化养猪场现有堆肥系统为例,运用生命周期评价方法,以养猪场现行堆肥工艺为参比,以4种不同翻堆频率条件下的条垛式堆肥工艺作为备选方案,对该养猪场粪便不同堆肥处理情景的环境影响进行系统对比分析。结果表明,每处理1 t新鲜猪粪的化石能源损耗潜力为80.8~221.5 MJ;堆肥氨挥发排放对系统总酸化效应和富营养化效应贡献率均达96%以上;CO2和N2O等温室气体排放对总温室效应的贡献分别为58%~88%和8%~35%。能源投入和气体排放是猪粪堆肥生命周期环境影响的关键因素,实践生产中应注重使用清洁能源,并结合工艺特点采用氨挥发与温室气体减排技术,减少堆肥生产过程的环境污染。  相似文献   
995.
Isolated environments are privileged settings to study transmission of infection. Montecristo is a small island where no wild or domestic carnivores are present. Invasive Black rats Rattus rattus (n = 78) were captured and tested by PCR for Leishmania infantum, Toxoplasma gondii and Neospora caninum. We wanted to test, for these parasites, the existence of a sylvatic cycle independent of reservoir or definitive hosts. None of the rats tested positive by PCR for either T. gondii or N. caninum. We recorded a 15.5% prevalence (CI95% 8–26%) of L. infantum in the rats and Phlebotomus mascittii was captured in Montecristo, leading us to identify it as possible vector of the parasite.  相似文献   
996.

Background

Previous studies have reported a seasonal increased risk for leptospirosis, but there is no consistent seasonality reported across regions in the United States.

Objectives

To evaluate and compare seasonal patterns in seropositivity for leptospirosis in dogs for 4 US regions (northeast [NE], midwest [MW], south‐central [SC], and California‐southern west coast [CS]).

Animals

Forty four thousand nine hundred and sixteen canine serum samples submitted to a commercial laboratory for microscopic agglutination tests (MAT) from 2000 through 2010.

Methods

In this retrospective study, positive cases were defined as MAT titers ≥1 : 3,200 for at least one of 7 tested serovars. Four geographic regions were defined, and MAT results were included in regional analyses based on hospital zipcode. A seasonal‐trend decomposition method for times series was utilized for the analysis. Monthly variation in the seropositive rate was evaluated using a seasonal cycle subseries plot and logistic regression.

Results

Two thousand and twelve of 44,916 (4.48%) samples were seropositive. Compared to seropositive rates for February, significantly higher monthly rates occurred during the 2nd half of the year in the MW (OR 3.92–6.35) and NE (OR 2.03–4.80) regions, and only in January (OR 2.34) and December (OR 1.74) in the SC region. Monthly seropositive rates indicative of seasonality were observed earlier in the calendar year for both CS and SC regions.

Conclusions and Clinical Importance

Seasonal patterns for seropositivity to leptospires differed by geographic region. Although risk of infection in dogs can occur year round, knowledge of seasonal trends can assist veterinarians in formulating differential diagnoses and evaluation of exposure risk.  相似文献   
997.
This report describes a spontaneous nephroblastoma with lung metastasis in a 10-week-old male Crl:CD(SD) rat. Macroscopically, a white mass in the kidney and two white masses in the lung were observed. Histopathologically, the renal mass was located in the cortex of a kidney, and it caused pressure on the surrounding renal parenchyma. Three components could be distinguished in the tumor: blastemal, epithelial (primitive glomerular/tubular structures) and mesenchymal (neoplastic connective tissues) elements. Immunohistochemically, the tumor cells were positive for Wilms tumor 1 protein (WT1) and vimentin. Metastasis was found in the lung. Thus, the case was diagnosed as a nephroblastoma with lung metastasis.  相似文献   
998.
999.
Oligodendroglioma is a rare tumor originating from oligodendrocytes found mainly in the cerebrum in aged rats. Only a few reports have shown spontaneous occurrence of this tumor in the spinal cord, and no report has mentioned its occurrence in young rats. We encountered a case of spontaneous oligodendroglioma in the lumbar portion of the spinal cord in a young (9 weeks old) female BrlHan:WIST@Jcl (GALAS) rat. Here we report the detailed histopathological and immunohistochemical characteristics of this case. No clinical signs, no gross lesions at necropsy, and no specific changes in hematology or blood biochemistry were observed. The tumor was located in the dorsal funiculus in the lumbar portion of the spinal cord and widely spread to the dorsal root nerve. The neoplastic cells showed a sheet-like growth pattern and had small round nuclei, clear cytoplasm and distinct cell borders that resulted in a honeycomb pattern. No mitotic figures or other histological lesions were observed. The neoplastic cells were positively stained for Olig2 and PCNA. The present case was considered to be a low-grade oligodendroglioma based on the histological and immunohistochemical features. To our knowledge, our case is considered to be extremely rare and the first report in a young rat.  相似文献   
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