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   952篇
  免费   71篇
  国内免费   38篇
  2023年   9篇
  2022年   18篇
  2021年   43篇
  2020年   26篇
  2019年   34篇
  2018年   28篇
  2017年   26篇
  2016年   39篇
  2015年   60篇
  2014年   56篇
  2013年   72篇
  2012年   60篇
  2011年   69篇
  2010年   25篇
  2009年   39篇
  2008年   42篇
  2007年   36篇
  2006年   45篇
  2005年   31篇
  2004年   17篇
  2003年   27篇
  2002年   25篇
  2001年   21篇
  2000年   20篇
  1999年   20篇
  1998年   19篇
  1997年   17篇
  1996年   10篇
  1995年   7篇
  1994年   6篇
  1993年   4篇
  1992年   6篇
  1991年   6篇
  1990年   5篇
  1989年   7篇
  1988年   7篇
  1987年   7篇
  1986年   5篇
  1982年   4篇
  1981年   7篇
  1980年   5篇
  1979年   3篇
  1976年   4篇
  1975年   7篇
  1974年   3篇
  1973年   4篇
  1972年   4篇
  1971年   6篇
  1970年   3篇
  1968年   3篇
排序方式: 共有1061条查询结果,搜索用时 838 毫秒
911.
以新疆天山山脉从西到东不同经度3个地点(昭苏、天池和奇台)的天山云杉林为对象,沿海拔梯度采集不同地点的天山云杉球果,分析其种子性状(种鳞长、种鳞宽、种鳞长宽比、种翅长、种翅宽、种翅长宽比、种子长、种子宽和种子长宽比)的变化.结果表明: 随海拔的升高,天山云杉种子性状(种子宽除外)总体呈现下降的趋势;除种翅宽、种子宽和种子长宽比与海拔的相关性不显著外,种子其他性状与海拔均呈显著负相关;种鳞长、种鳞宽、种鳞长宽比与经度呈现显著正相关;种鳞长、种鳞长宽比、种翅长宽比与坡度呈显著负相关;除种翅宽外,种子其他性状与坡向相关性均不显著.海拔是影响天山云杉种子的种鳞长、种鳞长宽比和种翅长宽比的主要因子.  相似文献   
912.
A total of 973 isolates of endophytic fungi were recovered from 1144 tissue fragments of the six medicinal plant species belonging to 4 families collected in the Beijing Botanical Garden. Of these isolates 778 sporulated and were identified into 21 taxa by morphological characteristics. Among the taxa 11 belonged to Coelomycetes, 6 to Ascomycetes, and 4 to Hyphomycetes. Various numbers of endophytic fungi (5–8 taxa) were obtained from each plant. Alternaria alternata was the dominant species in the 6 plants, and Microsphaeropsis conielloides was also dominant in Eucommia ulmoides. There were high colonization rates (47.9%–63.1%) and isolation rates (0.7–0.93) of endophytic fungi, and they were conspicuously higher in twigs than those in leaves in the 6 plants examined. The colonization and isolation rates of endophytic fungi increased with the twig age. The results based on the analyses of cluster and Sorenson’s similarity coefficients indicated that some endophytic fungi showed a certain degree of host and tissue preference.  相似文献   
913.
914.
Amelogenesis imperfecta (AI) is a collection of diverse inherited disorders featuring dental-enamel defects in the absence of significant nondental symptoms. AI phenotypes vary and are categorized as hypoplastic, hypocalcified, and hypomaturation types. Phenotypic specificity to enamel has focused research on genes encoding enamel-matrix proteins. We studied two families with autosomal-dominant hypocalcified AI and have identified nonsense mutations (R325X and Q398X) in the FAM83H gene on chromosome 8q24.3. The mutations perfectly cosegregate with the disease phenotype and demonstrate that FAM83H is required for proper dental-enamel calcification.  相似文献   
915.
QTLs for salt-tolerance(ST)related traits at the seedling and tillering stages were identified using 99 BC2F8 introgression lines(IL)derived from a cross between IR64(indica)as a recurrent parent and Binam(japonica)from Iran as the donor parent.Thirteen QTLs affecting survival days of seedlings(SDS), score of salt toxicity of leaves(SST),shoot K concentration(SKC)and shoot Na concentration(SNC) at the seedling stage and 22 QTLs underlying fresh weight of shoots(FW),tiller number per plant(TN) and plant height(PH)at the tillering stage were identified.Most QTLs detected at the tillering stage showed obvious differential expression to salt stress and were classified into three types based on their differential behaviors.Type I included 11 QTLs which were expressed only under the non-stress condition.Type II included five QTLs expressed in the control and the salt stress conditions,and three of them(QPh5,QPh8 and QTn9)had similar quantity and the same direction of gene effect,suggesting their expression was less influenced by salt stress.Type III included six QTLs which were detectable only under salt stress,suggesting that these QTLs were apparently induced by the stress.Thirteen QTLs affecting trait difference or trait stability of ILs between the stress and non-stress conditions were identified and the Binam alleles at all loci except QPh4,QTn2 and QFw2a decreased trait difference.The three QTLs less influenced by the stress and 13 QTLs affecting trait stability were considered as ST QTLs which contributed to ST.Comparing the distribution of QTLs detected at the seedling and tillering stages,most(69%)of them were genetically independent.Only four were the same or adjacent regions on chromosomes 1,2,8 and 11 harboring ST QTLs detected at the two stages,suggesting that partial genetic overlap of ST across the two stages occurs.It is likely,therefore,to develop ST rice variety for both stages by pyramiding of ST QTLs of different stages or selection against the overlapping QTLs between the two stages via marker-assisted selection(MAS).  相似文献   
916.
Objective: To investigate the familial aggregation of metabolic syndrome (MetS) and its components in the Chinese. Methods and Procedures: A total of 17,954 subjects from 5,224 families with multiple siblings aged 25–64 years old (mean age 45.8 years, 51.6% male) were enrolled from a rural area of Anhui Province of China during 2004–2005. Anthropometric measurement, body composition, blood pressure, plasma lipids, and fasting glucose and insulin, as well as a questionnaire interview, were obtained from each participant. Results: Significant correlations among siblings were observed in all the traits examined, including BMI, waist circumference, total body and abdominal fat percentage, fasting plasma glucose (FPG) and insulin, insulin resistance index of homeostatic model assessment (HOMA‐IR), total cholesterol, triglyceride, high‐density lipoprotein cholesterol (HDL‐C), low‐density lipoprotein cholesterol, and blood pressure, after adjustment for age, gender and some other covariates. The correlation coefficients varied from 0.18 for FPG to 0.42 for HDL‐C. In stratified analyses, we found siblings with a smaller age gap among them had higher intraclass correlation coefficients (ICCs) for most of the above phenotypes than those with a greater age difference, and the correlation of systolic blood pressure (SBP) was stronger in male siblings than that in female. If the eldest sibling is affected by MetS or any of its components, younger siblings bear a twofold to threefold higher risk for developing MetS or any of its components than those with a healthy eldest sibling. Discussion: Our study demonstrated a significant familial resemblance as regards MetS and its components among the Chinese. Further studies are warranted to investigate specific genetic and environmental factors related to MetS in this population.  相似文献   
917.
早在20世纪80年代, 我国就引入IUCN红色名录原理, 对我国生物物种的濒危状况开展评估工作。但是随着经济发展和气候变化, 一些物种的数量和分布区发生了变化, 加之在以往的评估中存在一些不足, 亟需对我国生物物种的濒危状况开展一次全面的评估。2008年, 环境保护部联合中国科学院启动了《中国生物多样性红色名录》的编制工作, 《中国生物多样性红色名录——高等植物卷》和《中国生物多样性红色名录——脊椎动物卷》分别于2013年9月和2015年5月正式对外发布。本文回顾了《中国生物多样性红色名录》的编制背景、过程和取得的成果。《中国生物多样性红色名录》完成了对我国34,450种高等植物和除海洋鱼类外的4,357种脊椎动物受威胁状况的评估, 是迄今为止对象最广、信息最全、参与专家人数最多的一次评估。在评估中取得了一系列成果: 统计了中国已知高等植物和脊椎动物物种数, 确定了物种丰富度在世界上的排名; 完善了国际上所使用的IUCN红色名录评估等级标准体系; 评估分析了我国已知高等植物和脊椎动物的受威胁程度及分布差异; 评估分析了高等植物和脊椎动物濒危灭绝的原因, 其中人类活动导致的生境丧失和退化是首要因素。这些成果将对我国生物多样性保护和管理工作产生积极的影响。  相似文献   
918.
根据已有的基础资料, 采用IUCN评估等级和标准, 对中国目前已鉴定的1,443种内陆鱼类受威胁现状进行了评估。评估结果显示, 1,443种内陆鱼类中, 灭绝3种、区域灭绝1种、极危65种、濒危101种、易危129种、近危101种、无危454种和数据缺乏589种。同已有的IUCN评估结果相比, 本次被评估的物种数目多, 受威胁物种大幅度增加, 其数目达295种, 占已知中国内陆鱼类总数的20.44%, 低于全球平均值(29%)。属于灭绝等级的鱼类是大鳞白鱼(Anabarilius macrolepis)、异龙鲤(Cyprinus yilongensis)和茶卡高原鳅(Triplophysa cakaensis); 属于区域灭绝等级的鱼类是长颌北鲑(Stenodus nelma)。鲤科是受威胁物种数最多的科, 其中裂腹鱼亚科和鲤亚科的种类受威胁程度最高。长江上游和珠江上游受威胁物种最多, 是受威胁最严重的地区。中国内陆鱼类受威胁的主要因素为河流筑坝、生境退化或丧失、酷渔滥捕和引进外来种。列入数据缺乏等级的鱼类较多, 占中国内陆鱼类的40.82%, 表明对中国内陆鱼类物种多样性了解不充分, 需要加强野外调查以积累基础资料。  相似文献   
919.
依据中国哺乳类野生种群与生境现状, 我们利用IUCN Red List Categories and Criteria (Version 3.1), Guidelines for Using the IUCN Red List Categories and CriteriaGuidelines for Application of IUCN Red List Criteria at Regional and National Levels (Version 4.0), 评价了中国所有已知的673种哺乳动物的濒危状况。本次评估了71种《IUCN濒危物种红色名录(2015)》没有评估的哺乳动物, 还评估了60种《IUCN濒危物种红色名录(2015)》误认为中国没有分布的哺乳动物。发现中国有3种哺乳动物“野外灭绝”, 3种“区域灭绝”。受威胁中国哺乳动物共计178种, 约占评估物种总数的26.4%, 高于IUCN濒危物种红色名录的物种平均受威胁率(21.8%)。中国哺乳动物1/4的特有种属于受威胁物种。受威胁比例最高的目是灵长目、食肉目与鲸偶蹄目。多数省区的受威胁哺乳动物物种占本省区哺乳动物总数的20-30%。中国哺乳动物种类多分布在中国第二级地理阶梯。生活在高海拔地区的哺乳动物虽然种类少, 但是受威胁哺乳动物的种类比例高。过度利用、生境丧失和人类干扰名列受威胁哺乳动物致危因子的前3位。自从1989年《中华人民共和国野生动物保护法》实施以来, 一些中国濒危哺乳动物的生存状况得到了改善。然而, 鉴于中国哺乳动物区系的独特性和多样性, 以及中国地形地貌的复杂性, 如何拯救这些濒危物种仍是中国生物多样性保护的一项艰巨任务。  相似文献   
920.
A novel recombinant dual human stem cell factor (rdhSCF) gene which consisted of a full-length hSCF(1-165aa) cDNA and a truncated hSCF (1-145aa) cDNA, linked by a peptide (GGGGSGGGGSGG) coding region, was constructed and cloned into Escherichia coli expression vector pET-22b. The rdhSCF was expressed at high level in E. coli BL21(DE3) and existed mainly as inclusion bodies. The inclusion bodies were solubilized in urea and refolded by ion-exchange chromatography. After renaturation, the purity of the yielded rdhSCF was up to 90%. Cell proliferation assay showed that the specific activity of the rdhSCF was 2.86x10(5) U/mg, about 1.66 times as high as that of monomer rhSCF expressed in E. coli.  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