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   848篇
  免费   199篇
  国内免费   63篇
  2023年   41篇
  2022年   31篇
  2021年   60篇
  2020年   62篇
  2019年   51篇
  2018年   56篇
  2017年   44篇
  2016年   41篇
  2015年   44篇
  2014年   56篇
  2013年   64篇
  2012年   36篇
  2011年   23篇
  2010年   39篇
  2009年   50篇
  2008年   32篇
  2007年   46篇
  2006年   23篇
  2005年   32篇
  2004年   30篇
  2003年   21篇
  2002年   20篇
  2001年   17篇
  2000年   15篇
  1999年   16篇
  1998年   18篇
  1997年   11篇
  1996年   13篇
  1995年   11篇
  1994年   9篇
  1993年   8篇
  1992年   8篇
  1991年   8篇
  1990年   5篇
  1989年   7篇
  1988年   3篇
  1987年   6篇
  1986年   8篇
  1985年   10篇
  1984年   2篇
  1983年   3篇
  1982年   5篇
  1980年   4篇
  1979年   4篇
  1977年   4篇
  1976年   2篇
  1975年   3篇
  1974年   2篇
  1972年   1篇
  1971年   2篇
排序方式: 共有1110条查询结果,搜索用时 15 毫秒
11.
Summary Two municipal sludges, one from a highly industrialized city, Chicago, Il, and another from a little industrialized, highly agricultural area, Tucson, AZ are compared for winter barley production on Pima c 1 (Typic torrifluvent). Both sludges were responsible for highly significant additions of Zn, Cu, Ni, Cd and P to the soil each year when applied at the rates of 100 mt/ha singly and 20 mt/ha each year for 4 years. Nitrogen responses for barley straw and grain were observed from both sludges. Tucson sludge appears to be attractive as a potential fertilizer, not only as an NPK source, but also for its organic matter and minimal amounts of heavy metals. The Chicago sludge with relatively high levels of heavy metals, particularly Cd, appears poorly suited as a fertilizer, if used for an extended period of time, because of the plant's tendency to take up elevated levels of certain heavy metals. Some parts of barley plants proved to be a better indicator of heavy metal uptake and concentration than others. The diagnostic-tissue test promises to be a useful tool to warn against undesirable accumulation of heavy metals. Fortunately, when compared with other plant parts, the heavy metal in grain was the least altered as a result of continued sewage sludge use on arid land. The soil's neutral to slightly alkaline pH and the presence of lime throughout the soil profile appeared to be critical factors in keeping plant uptake of heavy metals relatively low as compared with soils of other climates.  相似文献   
12.
Blood was collected from newborn calves before and after their first milk intake after birth; extracts of plasma were assayed by radioimmunoassay for the presence of beta-casomorphin-7 immunoreactive materials. No beta-casomorphin immunoreactivity was found in samples collected before milk ingestion; however, in samples collected after milk ingestion a beta-casomorphin-7 immunoreactive material was detected. Chromatographic characterization showed that this material was not identical with beta-casomorphin-7 but might rather represent a precursor thereof. The material proved resistant to enzymatic attack during a 30-min incubation period at 37 degrees C in the plasma of newborn calves, whereas beta-casomorphin-7 was degraded under these conditions. A physiological significance of beta-casomorphin-7 eventually cleaved from such a precursor material at any site in the newborn mammal is suggested.  相似文献   
13.
长白山劲松林场植物群落的分类和排序   总被引:7,自引:2,他引:5  
将长白山白河林业局劲松林场的54块样地用聚类分析法划分成6个植被类群,再用PC-VTAB程序中经过改进的Braun-Blanquet学派的植被排表分析法进行综合,产生了鉴别概要表,为各个等级的植被类群筛选出诊断种。此外,还用鉴别种地样地记录进行主成分分析,以验证诊断种的有效性,结果表明,PC-VTAB中的植被排表分析法是筛选鉴别种的有效方法,而鉴别种以显著地提高植被分析和排序的质量。  相似文献   
14.
Synopsis Sex ratios of least brook lamprey,Lampetra aepyptera, larvae varied widely among 12 geographically-diverse streams of the eastern United States. The extremes were 29 and 71% male, and the proportion of males increased significantly with relative population density, which was estimated among the streams from the number of larvae collected per m2 of substrate. The skewed sex ratios were not likely due to differential mortality between the sexes or differential recruitment to the adult stock, since they were established at the time of gonadal differentiation (at ca. 2 years of age) and remained relatively constant over the subsequent 2–3 years of larval life. Furthermore, although females seemed to predominate in the oldest larval age class, thus appearing to metamorphose later than males, their numbers were small and were omitted from the overall sex ratio. Sex ratio did not vary significantly with water hardness, pH, annual thermal units, or latitude. The possible adaptive significance of density-dependent sex determination in lampreys, however, remains elusive. It has been proposed that growth-promoting conditions might yield female-biased sex ratios as a tactic for ensuring that relatively large individuals become females, thereby increasing their fecundity. As predicted, larval size at a given age was generally greater in low-density populations, but there was no relationship between sex ratio and larval size, and female larvae were not consistently larger than the males.  相似文献   
15.
A protein alignment scoring system sensitive at all evolutionary distances   总被引:1,自引:0,他引:1  
Summary Protein sequence alignments generally are constructed with the aid of a substitution matrix that specifies a score for aligning each pair of amino acids. Assuming a simple random protein model, it can be shown that any such matrix, when used for evaluating variable-length local alignments, is implicitly a log-odds matrix, with a specific probability distribution for amino acid pairs to which it is uniquely tailored. Given a model of protein evolution from which such distributions may be derived, a substitution matrix adapted to detecting relationships at any chosen evolutionary distance can be constructed. Because in a database search it generally is not known a priori what evolutionary distances will characterize the similarities found, it is necessary to employ an appropriate range of matrices in order not to overlook potential homologies. This paper formalizes this concept by defining a scoring system that is sensitive at all detectable evolutionary distances. The statistical behavior of this scoring system is analyzed, and it is shown that for a typical protein database search, estimating the originally unknown evolutionary distance appropriate to each alignment costs slightly over two bits of information, or somewhat less than a factor of five in statistical significance. A much greater cost may be incurred, however, if only a single substitution matrix, corresponding to the wrong evolutionary distance, is employed.  相似文献   
16.
Abstract An oligonucleotide primer (ChInt) was synthesised from the variable internally transcribed spacer (ITS) 1 region of ribosomal DNA (rDNA) of Cylindrocarpon heteronema . PCR with primers ChInt and ITS4 (from a conserved sequence of the rDNA) amplified a 470-bp fragment from several isolates of C. heteronema but not from various apple wood saprophytes. Amplification of this fragment was achieved from 1–2 pg of fungal DNA. These primers amplified a fragment of the same size from DNA extracted from cankered wood but only after impurities were removed from the DNA on a Qiagen tip-5 column. Southern hybridization analysis confirmed the 470-bp fragment from C. heteronema DNA and cankered wood to be identical.  相似文献   
17.
18.
摘要 目的:探讨泌尿系造影联合彩色多普勒对小儿先天性肾积水的诊断价值。方法:选取2018年11月~2021年11月在本院治疗的88例先天性肾积水患儿为研究对象,所有患儿均完善静脉肾盂造影及彩色多普勒超声检查,以病理诊断结果为金标准,对比两种检查方法对小儿先天性肾积水的诊断价值。结果:彩色多普勒超声检查结果显示,肾积水轻度、中度、重度患儿分别为10例、39例、39例,不同病情程度患儿比较,重度组收缩期峰值速度(PSV)、舒张期最小流速(EDV)均低于中度组和轻度组,重度组血流阻力指数(RI)高于中度组和轻度组(P<0.05),但轻度组与中度组PSV、EDV、RI比较差异无统计学意义(P>0.05)。与病理学诊断检查结果对比,彩色多普勒超声对中度、重度先天性肾积水患儿具有较高的诊断效能,其准确度分别为90.91%、93.18%,与病理诊断kappa值分别为0.795、0.862,具有较高的一致性;但对轻度肾积水诊断效能较低,kappa值为0.629,一致性一般。静脉肾盂造影对轻度先天性肾积水患儿具有较高的诊断效能,准确度为96.59%,与病理诊断kappa值为0.824,具有较高的一致性;但对中度、重度肾积水诊断效能较低,kappa值分别为0.583、0.565,一致性一般。彩色多普勒超声联合静脉肾盂造影诊断准确率高达94.32%,明显高于两检查方法单独应用(P<0.05)。结论:不同病情程度的先天性肾积水患儿具有不同超声征象,彩色多普勒超声对中、重度肾积水患儿具有较好的诊断价值,而静脉肾盂造影诊断轻度肾积水患儿的效能较好,将二者联合可提高对先天性肾积水的诊断准确率。  相似文献   
19.
摘要 目的:探讨不同容量对胸段经椎间孔硬膜外注射(TFEI)药液扩散范围和镇痛效果的影响以及胸段TFEI用于诊断性阻滞的可行性。方法:选择2021年1月至2022年12月南京大学医学院附属鼓楼医院收治的胸段带状疱疹相关疼痛患者140例,随机分为4组,实施单次TFEI并分别注入不同容量含造影剂局麻药(A组:0.2 mL;B组:0.5 mL;C组:1.0 mL;D组:2 mL),CT扫描并观察造影剂在硬膜外向头侧、尾侧及总扩散节段,造影剂在椎间孔、同侧椎旁间隙、同侧及对侧硬膜外间隙扩散情况,判断是否为选择性神经根阻滞。评估注射前、注射后30分钟及24小时视觉模拟评分(VAS)。结果:头侧和尾侧扩散节段以及总扩散节段数,D组最多,A组最少(P<0.05);C组、D组造影剂扩散≥3个节段发生率明显高于B组(P<0.05);C组、D组病例造影剂扩散至同侧椎旁间隙和对侧硬膜外间隙的发生率明显高于A组、B组(P<0.05),仅A组37.1%的病例实现选择性神经根阻滞,其余各组均无选择性阻滞病例。注射后30分钟,C、D组VAS评分显著低于A、B组(P<0.05);注射后24小时,D组VAS评分显著低于A、B组(P<0.05)。结论:胸段带状疱疹相关疼痛患者TFEI药液扩散范围随注射容量的增加而扩大,且在硬膜外倾向于头侧扩散,2 mL容量单次TFEI可阻滞3个以上的神经节段,获得良好的镇痛效果。胸段TFEI行诊断性阻滞的可行性较差。  相似文献   
20.
摘要 目的:探讨经阴道三维超声联合糖类抗原125(CA125)、糖类抗原199(CA199)、中性粒细胞/淋巴细胞比值(NLR)及血小板/淋巴细胞比值(PLR)检测对绝经后女性子宫内膜癌(EC)的诊断效能。方法:回顾性分析本院2019年5月至2022年5月收治的因绝经后出血就诊的92例疑似子宫内膜肿瘤患者的病历资料,经手术病理证实,EC患者48例(EC组)、子宫内膜良性病变患者44例(良性组)。比较两组CA125、CA199、NLR及PLR水平,对比两组子宫内膜血管指数(VI)、血流指数(FI)、血管-血流指数(VFI)、子宫内膜容积(EV)变化及血流信号分布情况。绘制受试者工作特征(ROC)曲线分析经阴道三维超声联合CA125、CA199、NLR及PLR检测对绝经后EC的诊断效能。结果:EC组CA125、CA199、NLR及PLR水平均高于良性组(P<0.05)。EC组三维超声参数VI、FI、VFI及EV均高于良性组(P<0.05)。EC组血流信号Ⅱ级占比25.00%,Ⅲ级占比70.83%;良性组血流信号Ⅱ级占比88.64%,Ⅲ级占比2.27%。EC组血流信号Ⅱ级占比低于良性组,血流信号Ⅲ级占比高于良性组(P<0.05)。两组血流信号0级、Ⅰ级比较无差异(P>0.05)。ROC结果显示五项检查联合检测曲线下面积(AUC)值为0.944(95%Cl:0.902~0.985),具有更优的诊断效能(敏感度为89.10%、特异度为91.80%)。结论:CA125、CA199、NLR及PLR检测联合经阴道三维超声检查诊断绝经后EC效果显著,可为临床诊断提供参考。  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