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   21900篇
  免费   3996篇
  国内免费   1871篇
林业   1884篇
农学   2904篇
基础科学   1245篇
  3548篇
综合类   5155篇
农作物   1153篇
水产渔业   2977篇
畜牧兽医   6206篇
园艺   501篇
植物保护   2194篇
  2024年   40篇
  2023年   147篇
  2022年   388篇
  2021年   640篇
  2020年   985篇
  2019年   1504篇
  2018年   1247篇
  2017年   1410篇
  2016年   1450篇
  2015年   1442篇
  2014年   1439篇
  2013年   1728篇
  2012年   1316篇
  2011年   1386篇
  2010年   1455篇
  2009年   1054篇
  2008年   987篇
  2007年   898篇
  2006年   883篇
  2005年   839篇
  2004年   508篇
  2003年   709篇
  2002年   1058篇
  2001年   869篇
  2000年   673篇
  1999年   513篇
  1998年   320篇
  1997年   295篇
  1996年   221篇
  1995年   224篇
  1994年   224篇
  1993年   185篇
  1992年   171篇
  1991年   117篇
  1990年   99篇
  1989年   112篇
  1988年   85篇
  1987年   39篇
  1986年   21篇
  1985年   17篇
  1984年   11篇
  1983年   6篇
  1982年   8篇
  1981年   4篇
  1980年   7篇
  1979年   3篇
  1978年   6篇
  1977年   5篇
  1975年   3篇
  1970年   4篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
The cervical patency of six domestic female cats was monitored under sedation by infusion of contrast medium (Omnipaque) into the cranial vagina during early oestrus, mid‐oestrus, late oestrus and interoestrus or a radiopharmaceutical (99mTc‐HSA) during mid‐ and interoestrus in a non‐ovulatory oestrous cycle. The transport of the contrast medium or the radiopharmaceutical through the cervix and within the uterine horns was observed under fluoroscopy and with the aid of scintigraphy. In three of the queens, transcervical transport of contrast medium was demonstrated in all stages of oestrus, in one queen during mid‐oestrus, late oestrus and 1 day after oestrus, and in two queens only during late oestrus. The relations between the cervical patency to the contrast medium and the oestrous behaviour, cornification of the vaginal cells and the serum oestradiol‐17β concentration were evaluated, and a relationship was found between the cervical patency and the degree of vaginal cornification. Transcervical transport of the radiopharmaceutical was observed in three queens during mid‐oestrus. When the cervix was open, hysterography under a fluoroscope and hysteroscintigraphy were performed. The fluoroscopic and scintigraphic recordings revealed the patterns of the uterine contractions during oestrus in both ascending and descending directions, and the movement of the uterine contents back and forth between the uterine horns. The hysterograms were classified according to the shape of the uterine horns and the appearance of the endometrial lining. Spiral‐shaped uterine horns with a smooth inner contour were observed in two queens, and a corkscrew appearance with irregular filling defects in the uterine lumen was shown in two queens that had developed subclinical cystic endometrial hyperplasia. These findings demonstrated that fluids or particles deposited in the cranial vagina of the cat can be transported into the uterus during some stages of the oestrous cycle. The fluoroscopic and scintigraphic techniques developed in this study may be further modified to permit more detailed studies of uterine contractile patterns and sperm transport in the feline female reproductive tract. Hysterography proved useful to diagnose uterine disease. The information on cervical patency is of value also for the development of techniques for artificial insemination in this species, and should be studied also in the ovulatory cycle.  相似文献   
2.
Abstract – The different reproductive roles of the sexes can predict the direction and magnitude of sexual dimorphism of external and internal morphology. Males should have enlarged structures that enhance the acquisition of mating opportunities, whereas females are predicted to have enlarged organs that are associated with the production of eggs. We tested these predictions in male and female lake whitefish, a species in which both sexes have similar overall body size and shape. After controlling for body size, male lake whitefish had significantly longer jaws and pectoral and pelvic fins, larger hearts, and more muscle than females. Sexual dimorphism in relative muscle mass may be one of the most fundamental morphological differences between males and females. Females had relatively heavier livers than males. Because the liver is important for the breakdown of fats and vitellogenesis, selection should favour an enlarged liver in females for the processing of energy and the production of large numbers of eggs.  相似文献   
3.
中国的枸橼     
枸橼在我国有11个类型(变种、品种或杂种),根据它们的地理分布和植物学性状,认为枸橼的原发中心在云南的西南部和西部;野生的沧源香橼是枸橼的原始类型,由其演化为小香橼、小果香橼和栽培的大香橼等;佛手是由枸橙演化而来。  相似文献   
4.
为了解采后果品电特性变化机理,并探索基于电特性识别果品品质的方法,研究了桃采后电特性和生理特性的变化。结果表明,随着桃新鲜度下降,相对介电常数呈余弦规律变化,损耗角正切逐渐减小,呼吸高峰时相对介电常数最大。分析了电特性变化的原因,进而建立了采后桃电特性与生理特性之间的关系模型。以相对介电常数和损耗角正切为输入特征参数,应用BP神经网络技术识别桃的新鲜度等级,平均识别率为82%。  相似文献   
5.
提出了卡环制造过程中磨削回弹的计算模型。根据卡环的精度和啮合力要求,设计了特殊制造工艺。考虑弯曲残余应力,对磨削工序进行力学分析,提出了磨削回弹弯矩、磨削回弹后搭叠长度和啮合力变化的计算模型;开发了卡环制造辅助工艺系统——Shuttle,对卡环磨削工序进行了模拟,并结合实验讨论了磨削前后曲率半径、啮合力和搭叠长度随余量的变化规律。研究表明:随着磨削余量的增加,试样磨削前的啮合力增加,而磨削回弹后的啮合力呈下降趋势;试样磨削前后的搭叠长度均呈下降趋势,并且试样截面惯性矩越大,磨削回弹后试样的搭叠长度下降速度越慢;在各工序阶段,试样的曲率半径有不同程度的增加。  相似文献   
6.
为解决除草剂防除困难的问题,采用室内盆栽试验测定了PX、PM和Mn2+三种代谢酶抑制剂对精喹禾灵、莠去津、烟嘧磺隆、硝磺草酮和烯禾啶防除稗草Echinochloa crus-galli的增效作用,测定不同用量代谢酶抑制剂PX对这5种除草剂的增效作用及其与莠去津混用后稗草体内谷胱甘肽转移酶(glutathione S-transferase,GST)和细胞色素 P450(cytochrome P450,CYP450)活性的变化。结果显示,在3种代谢酶抑制剂中,代谢酶抑制剂PX对精喹禾灵、莠去津、烟嘧磺隆、硝磺草酮和烯禾啶5种除草剂的增效作用最显著。不同用量代谢酶抑制剂PX对不同除草剂的增效作用不同,其中75 g (a.i.)/hm2代谢酶抑制剂PX对莠去津的增效最佳,株防除效果和鲜重防除效果分别增加了35.33个百分点和37.33个百分点。莠去津与代谢酶抑制剂PX混用后,稗草体内GST和CYP450活性均较对照显著降低。表明除草剂与代谢酶抑制剂PX混用能增加除草剂对稗草的防除效果,可以达到减施除草剂的目的。  相似文献   
7.
8.
利用16S rRNA全长序列鉴定植物乳杆菌   总被引:4,自引:1,他引:4  
从标明含有嗜酸乳杆菌的活菌胶囊中分离到一株乳杆菌Lal菌株,利用16S rRNA全序列分析法和传统分类法对Lal菌株进行分类鉴定表明:Lal菌株属于植物乳杆菌种新株系,该菌株现在已经被中国科学院北京微生物所菌种保藏中心收录保存,编号为ASl.2986。  相似文献   
9.
Anatid herpesvirus 1 (AHV-1) CH virulent strain was first isolated from an infected duck and it was found that this virus strain could induce cytopathic effect (CPE) in duck embryo fibroblast (DEF). Following AHV-1 infection, DEF showed morphological changes such as cell rounding, improved refractivity and detachment from the culture surface. However, its pathological characteristics were not adequately known. Related studies were performed and the results showed that syncytium formation could be observed as the other type of CPE in AHV-1 infection. Hematoxylin-eosin staining and 4’, 6-diamidino-2-phenylindole (DAPI) staining of infected DEF were each used to visualize the shape and distribution of chromatin within nuclei and nuclear fragmentation was observed. Chromatin condensation and margination, as well as formation of apoptotic bodies were observed by transmission electron microscopy (TEM). DNA ladder formation was detected in AHV-1 infected cells and apoptosis of the infected DEF was also detected by flow cytometry analysis of Annexin V-FITC/PI staining method. Therefore, it was suggested that AHV-1 virulent strain can induce syncytium and apoptosis in DEF. Syncytium formation and apoptosis observed in this study may contribute to the elucidation of AHV-1 pathogenesis.  相似文献   
10.
利用京、津、冀区域内所辖 6 5个气象观测站 1 95 4 - 2 0 0 2年的观测资料 ,分析了扬沙天气的气候变化特征。结果表明 :京、津、冀区域绝大部分地区是扬沙天气的主要影响区。高原、平原和丘陵地区是扬沙天气的多发区和易发区 ,宁晋、邢台一带是扬沙天气的高频区。扬沙的发生时间具有明显的季节变化特征 ;此区域内的扬沙呈现整体减少趋势 ,各分区域的变化趋势具有相似性 ,只是在减少幅度和年代际减少趋势上略有不同  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