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   3620篇
  免费   238篇
  国内免费   383篇
林业   342篇
农学   379篇
基础科学   168篇
  496篇
综合类   1128篇
农作物   312篇
水产渔业   362篇
畜牧兽医   758篇
园艺   143篇
植物保护   153篇
  2024年   8篇
  2023年   32篇
  2022年   70篇
  2021年   109篇
  2020年   124篇
  2019年   112篇
  2018年   122篇
  2017年   119篇
  2016年   130篇
  2015年   141篇
  2014年   139篇
  2013年   190篇
  2012年   226篇
  2011年   264篇
  2010年   229篇
  2009年   191篇
  2008年   182篇
  2007年   179篇
  2006年   162篇
  2005年   186篇
  2004年   129篇
  2003年   175篇
  2002年   205篇
  2001年   180篇
  2000年   139篇
  1999年   95篇
  1998年   66篇
  1997年   54篇
  1996年   32篇
  1995年   26篇
  1994年   34篇
  1993年   24篇
  1992年   29篇
  1991年   21篇
  1990年   23篇
  1989年   15篇
  1988年   21篇
  1987年   14篇
  1986年   6篇
  1985年   6篇
  1984年   6篇
  1983年   5篇
  1982年   3篇
  1981年   3篇
  1979年   2篇
  1977年   6篇
  1973年   1篇
  1967年   1篇
  1966年   1篇
  1964年   1篇
排序方式: 共有4241条查询结果,搜索用时 31 毫秒
91.
This study was performed to evaluate the effect of beta-tricalcium phosphate and poly L-lactide-co-glycolide-co-epsilon-caprolactone (TCP/PLGC) membrane in the repair of partial bone defects in canine proximal humerus. Three adult mixed-breed dogs were used during the experimental period. The length of the defect was quarter of the full length of humerus, and width of the defect was quarter of middle diameter of the lateral aspect of humerus. The humeri of each dog were divided into treatment (TCP/PLGC) and control groups. The defect was covered with TCP/PLGC membrane in treatment group. To evaluate regeneration of the bone, computerized tomography (CT) and histopathologic examination were performed. The radiopaque lines were appeared at the original defect sites in TCP/PLGC group but below the original site in control at 4th week. Radiopacity and thickness of the defect sites, and radiopaque lines were more increased at 8th week than those of 4th week. Histopathologic findings revealed fibrous connective tissue migration into the defect and the migration inhibited the structure of new cortex to be placed in the original level in control whereas new cortex growth was found in the level of original line in TCP/PLGC group. However, the new cortical bone in the TCP/PLGC group was thinner and less organized than the adjacent intact cortex, and the amount of new cancellous bones were also scanty. The result suggested that TCP/PLGC membrane is a good guided bone regeneration material to restore the original morphology of humerus in partial defect.  相似文献   
92.
AIM: To investigate the effects of constant magnetic field on apoptosis, secretion and expression of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) in human umbilical vein endothelial cells (HUVEC), and their adhesion rates with THP-1 monocytes induced by angiotensin Ⅱ (AngⅡ).METHODS: The third passage of cultured HUVEC was used.There were six groups: control group, Ang Ⅱ (10-6 mol/L) group, Ang Ⅱ with 1, 5, 10 or 20 gausses of constant magnetic field group.Samples were collected 24 h after incubation with or without magnetic field.Apoptosis was determined by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) and propidinm iodide staining with flow cytometry.Secretion and expression of ICAM-1 and VCAM-1 were detected by ELISA and immunocytochemistry, respectively.Adhesion rate between HUVEC and THP-1 was measured by counting method.RESULTS: Ang Ⅱ at concentration of 10-6mol/L induced apoptosis in HUVECs (P<0.05 vs control), whereas in 1, 5, 10 and 20 gausses group, apoptosis of HUVECs was significantly lower than that in Ang Ⅱ group (P<0.05).Ang Ⅱ at concentration of 10-6 mol/L significantly increased secretion and expression of ICAM-1 and VCAM-1 (P<0.05 vs control), whereas secretion and expression of ICAM-1 and VCAM-1 in 1, 5, 10 and 20 gausses group significantly decreased, compared with Ang Ⅱ group (P<0.05).The adhesion rates between HUVEC and THP-1 significantly increased 24 h after incubation of HUVEC with Ang Ⅱ (P<0.05 vs control), in contrast, the adhesion rates between HUVEC and THP-1 in 1, 5, 10 and 20 gausses group significantly decreased, compaed with Ang Ⅱ group (P<0.05).CONCLUSIONS: One gauss to 20 gausses of constant magnetic field antagonizes the effects of Ang Ⅱ on HUVEC, decreases apoptosis and expression of ICAM-1 and VCAM-1 in HUVEC, and also decreases the adhesion rates between HUVEC and monocytes induced by Ang Ⅱ.  相似文献   
93.
甜樱桃(Prunus avium L.)属配子体型自交不亲和(Gametophyticself-incompatibility,GSI),遗传上由染色体上具有复等位基因构成的单一位点或基因座控制(称为S基因座)。在一个S基因座上,植物种群内可含有多个S等位基因,称为S基因。甜樱桃品种大多数自交不亲和,在生产中必须配置授  相似文献   
94.
以吉林省通榆县为例,构建了县域农业可持续发展的生态安全评价指标体系,采用均方差法确定了评价指标的权重,对研究区1985、1990、1995、1998、2001、2004年6个时段的农业生态安全进行了评价.从评价结果可以看出,近20年,通榆县农业生态安全呈现下降态势,安全等级处于Ⅲ~Ⅴ级,安全状态从敏感态下降到风险态.1998年是研究区农业生态安全变化的转折点,从1998年起,农业生态安全有所回升.在分析变化原因的基础上,提出农业可持续发展的生态安全建设对策.  相似文献   
95.
对青海省大通县退耕还林还草地及周边农田鼠类群落组成进行调查研究,在退耕还林还草地共捕获鼠类6种728只,以根田鼠和高原鼢鼠为优势种;农田中共捕获各种鼠类3种75只,以长尾仓鼠为优势种。比较发现,退耕还林还草后,鼠类群落组成发生了明显的变化,与之相适应,鼠害治理应针对不同的鼠类种群和危害程度采取相应的控制措施。  相似文献   
96.
97.
Enzyme-linked immunosorbent assays (ELISAs) were developed to measure IgE antibodies specific for Dermatophagoides farinae in dogs and cats. Although higher levels were detected in atopic dogs and cats than in normal animals without skin disease, the differences were not statistically significant. On the other hand, levels in dogs and cats that were reared under laboratory conditions, and thus presumably not exposed to house dust mites, were either very low or undetectable. IgE antibodies were induced in 10 laboratory-reared cats using low-dose antigenic stimulation in aluminium hydroxide. All cats developed detectable IgE, but not all developed positive skin tests. However, serum from those cats with positive skin tests were able to give positive Prausnitz–Küstner (PK) tests. The canine data, together with previous work on basophil histamine release, suggests that the distinction between atopic and normal dogs may result from a heterogeneity of either IgE or of the high-affinity mast cell receptor. The feline data can only be explained by the existence of a heterogeneity of IgE.  相似文献   
98.
易程程  艾东 《农业工程》2017,7(4):113-119
近几年,一些村集体开始有意愿投入土地整治中,并规避政府主导的方式,以自主筹资并主导整治的整治行为出现。村集体是土地整治密切的主体,针对村集体的主导整治行为,以北京市丰台区C镇Z村为例,利用利益相关者理论,结合传统政府主导下土地整治模式,对Z村土地复垦案例中各个利益相关者的角色、权责和利益3方面进行对比分析,在此基础上说明村集体主导下的土地整治模式下存在的冲突,并以此为村集体主导的土地整治模式发展提出明晰利益相关者政府与村集体的定位,减少利益相关者对土地的依赖等相关的建议与思考。   相似文献   
99.
We aimed to investigate the effects of inoculating Panax ginseng C.A. Meyer seedlings with arbuscular mycorrhizal fungi (AMF) by examining the root colonization, plant nutrition uptake, growth characteristics, and soil aggregation of P. ginseng seedlings inoculated at the time of transplantation. At 16 weeks, the AMF spore density per 30 g of fresh mycorrhizosphere in seedlings inoculated with AMF (AMF+ seedlings) was 256.8 and that in seedlings not inoculated with AMF (AMF− seedlings) was 186.3, respectively. The colonization rate of AMF in the lateral roots of AMF+ seedlings was approximately 19% higher than that in the lateral roots of AMF− seedlings. The patterns of AMF colonization in ginseng roots were similar to those of the Paris-type mycorrhizal association. Plant growth characteristics, such as plant height, root length, leaf area, number of lateral roots, fresh weight of shoots and roots, and chlorophyll content, were significantly enhanced in AMF+ seedlings compared to AMF− seedlings. The macronutrient content (P, K, and Ca) and micronutrient content (Cu, Fe, and Zn) of both shoots and roots were also significantly higher in AMF+ seedlings compared to AMF− seedlings. Furthermore, glomalin content and soil aggregation were significantly enhanced in AMF inoculated areas. Our results indicate that AMF inoculation may enhance the growth of ginseng seedlings by improving the uptake of mineral nutrients and the soil structure in mycorrhizosphere.  相似文献   
100.
Feng  Liu  Qinghui  Ai  Kangsen  Mai  Beiping  Tan  Hongming  Ma  Wei  Xu  Wenbing  Zhang  Zhiguo  LiuFu 《Journal of the World Aquaculture Society》2008,39(4):500-509
A 30‐d feeding experiment was conducted in tanks to investigate the effects of four different binders (2% in microdiet) on the survival, growth performance, and specific activity of digestive enzymes in tongue sole, Cynoglossus semilaevis Günther, postlarvae (24 d after hatching, with initial average wet weight of 22.4 ± 6.65 mg [mean ± SD]). Five approximately isonitrogenous and isoenergetic microbound diets were formulated with carrageenan (Car), sodium carboxymethylcellulose (CMS), sodium alginate (SA), and gelatin (Gel) as binders, and a diet with no special binder as a control. A commercial diet (RQ Com., manufactured by Marubeni Nisshin Feed Co., Ltd., Chita, Japan) was used as another tested diet, and Artemia nauplii was used as live prey control. The results showed that the survival of the fish fed SA diet (31.3%) was significantly higher than that of fish fed CMS (21.8%), Car (10.8%), and control (21.8%) diets (P < 0.05), but significantly lower than that of the fish fed the live prey (54.5%) diet (P < 0.05). The results of growth followed the similar pattern as that of survival, although there were no significant differences in specific growth rate (SGR) among fish fed SA, CMS, Gel, the control, and RQ diets (7.2, 6.8, 7.0, 6.9, and 7.3% per day; P > 0.05). However, fish fed Car diet had a significantly lower SGR (5.9% per day) than fish fed other diets. Percentages of secreted amylase in fish fed artificial diets were not significantly different, but fish fed artificial diets had significantly higher (P < 0.05) secreted amylase than live prey group. Percentage of secreted trypsin in fish fed SA diet (54.9%) was relatively higher compared with other binders, and not significantly different from RQ Com. and live prey treatments. However, trypsin in fish fed Car diets (29.3%) was significantly lower than in other groups. Specific activities of alkaline phosphatase (AP) in the intestine and AP and leucine aminopeptidase N in brush border membranes in fish fed SA diet (264.6, 1882.8, and 187.2 mU/mg protein) were relatively higher compared with other binders. These results indicate that SA is one of suitable binders compared with carrageenan, CMS, and gelatin in microdiet of postlarval tongue sole.  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