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   31134篇
  免费   2593篇
  国内免费   3713篇
  37440篇
  2024年   121篇
  2023年   682篇
  2022年   815篇
  2021年   985篇
  2020年   1064篇
  2019年   1411篇
  2018年   1099篇
  2017年   1162篇
  2016年   1213篇
  2015年   1289篇
  2014年   1652篇
  2013年   2211篇
  2012年   1263篇
  2011年   1506篇
  2010年   1231篇
  2009年   1611篇
  2008年   1681篇
  2007年   1663篇
  2006年   1546篇
  2005年   1431篇
  2004年   1276篇
  2003年   1135篇
  2002年   1030篇
  2001年   811篇
  2000年   693篇
  1999年   747篇
  1998年   585篇
  1997年   546篇
  1996年   500篇
  1995年   499篇
  1994年   485篇
  1993年   373篇
  1992年   348篇
  1991年   339篇
  1990年   269篇
  1989年   248篇
  1988年   228篇
  1987年   211篇
  1986年   168篇
  1985年   195篇
  1984年   204篇
  1983年   140篇
  1982年   167篇
  1981年   117篇
  1980年   119篇
  1979年   103篇
  1978年   81篇
  1977年   45篇
  1976年   40篇
  1974年   25篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
841.
Feeding behavior and scar production of male and female F. occidentalis (Pergande) (Thysanoptera: Thripidae) were studied in relation to transmission of tomato spotted wilt tospovirus (TSWV). Electrical penetration graph (EPG) analysis showed that females feed more frequently and intensively than males. The feeding intensity, reflected by silvery scar production and studied by an image analysis system, demonstrated that females induced more numerous scars than males. At the same time, males transmitted TSWV with a higher efficiency than females, indicating that TSWV transmission and scar production are not positively correlated. Furthermore, males produced significantly more local lesions of TSWV than females. These quantitative differences in scar production and transmission of TSWV can be explained by the lower mobility and higher consumption rate of females. The influence of the sex-ratio on crop damage and virus transmission, and thus to the spread of TSWV, is emphasized.  相似文献   
842.
A chimeric enzyme (GST121) of the human α-glutathione S-transferases GST1-1 and GST2-2, which has improved catalytic efficiency and thermostability from its wild-type parent proteins, has been crystallized in a space group that is isomorphous with that reported for crystals of GST1-1. However, a single-site (G82R) mutant of GST121, which exhibits a significant reduction both in vitro and in vivo in protein thermostability, forms crystals that are not isomorphous with GST1-1. The mutant protein crystallizes in space group P212121, with cell dimensions a = 49.5, b = 92.9, c = 115.9 Å, and one dimer per asymmetric unit. Preliminary crystallographic results show that a mutation of the surface residue Gly 82 from a neutral to a charged residue causes new salt bridges to be formed among the GST dimers, suggesting that the G82R mutant might aggregate more readily than does GST121 in solution resulting in a change of its solution properties. © 1994 Wiley-Liss, Inc.  相似文献   
843.
本文通过建立图象分析方法对免疫组织化学反应结果进行定量,检测观察H-ras在口腔颊粘膜上皮在正常(N)、慢性炎症(IF)、癌旁上皮(EAC)和鳞癌(SCC)的变化过程中的表达并进行分析。结果显示H-ras在SCC组中,以中等分化的SCC无论是H-ras表达的量还是细胞阳性率都较高。此外,组织学观察显示,H-ras在处于分化末期但尚未角化的正常上皮细胞中有较高的表达。本文结果显示了H-ras的过表达与上皮细胞的会化程度密切相关。本研究还显示,所采用的阳性区域透光值、平均总透光值及阳性反应区域与阴性反应区域比值可靠并有相关性。这进一步说明了用免疫组化定量方法检测H-ras癌基因表达的精确和可靠性。  相似文献   
844.
We have recently presented models to estimate parentage in breeding systems with multiple mating and incomplete sampling of the candidate parents. Here we provide formulas to calculate the statistical confidence and the optimal trade-off between the number of loci and offspring. These calculations allow an understanding of the statistical significance of the parentage estimates as well as the appropriate sampling regime required to obtain a desired level of confidence. We show that the trade-off generally depends on the parentage of the putative parents. When parentage is low, sampling effort should concentrate on increasing the number of loci. Otherwise, there are similar benefits from increasing the number of loci or offspring. We demonstrate these methods using genetic data from a nest of the bluegill sunfish (Lepomis macrochirus).  相似文献   
845.
诸葛菜茎叶中黄酮类化合物的研究   总被引:4,自引:0,他引:4  
诸葛菜茎叶乙醇提取物用Mg+HCl,Zn+HCl,1%FeCl3-乙醇液,1%NaOH进行显色反应,呈现黄酮类化合物性质特征颜色。又以槲皮素,山柰酚,异鼠李素为对照品,采用HPLC法分析测定了其茎叶中黄酮醇的含量。结果表明干品中总黄酮含量(以甙元计)为0.568%。  相似文献   
846.
Transgenic plants of the aromatic shrub Lavandula latifolia (Lamiaceae) were produced using Agrobacterium tumefaciens-mediated gene transfer. Leaf and hypocotyl explants from 35–40-day old lavender seedlings were inoculated with the EHA105 strain carrying the nptII gene, as selectable marker, and the reporter gusA gene with an intron. Some of the factors influencing T-DNA transfer to L. latifolia explants were assessed. Optimal transformation rates (6.0 ± 1.6% in three different experiments) were obtained when leaf explants precultured for 1 day on regeneration medium were subcultured on selection medium after a 24 h co-cultivation with Agrobacterium. Evidence for stable integration was obtained by GUS assay, PCR and Southern hybridisation. More than 250 transgenic plants were obtained from 37 independent transformation events. Twenty-four transgenic plants from 7 of those events were successfully established in soil. -glucuronidase activity and kanamycin resistance assays in greenhouse-grown plants from two independent transgenic lines confirmed the stable expression of both gusA and nptII genes two years after the initial transformation. Evidence from PCR data, GUS assays and regeneration in the presence of kanamycin demonstrated a 1:15 Mendelian segregation of both transgenes among seedlings of the T1 progeny of two plants from one transgenic L. latifolia line.  相似文献   
847.
Embodied water in a socio-economic system refers to the hidden water contained in products traded from one region or one sector to another and has been the center of concern for water management in recent years. However, most models developed for water system analysis ignore cycling and indirect flows, making it difficult to explain the effects of structure on these factors among sectors. Therefore, those models fail to examine the water utilization efficiency from an integral view. In this study, we investigate an embodied socio-economic water system using network analysis developed originally for ecological systems. In this manner, we identify structural and throughflow indicators, such as Finn Cycling Index, Indirect effects ratio, and aggradation, to show the efficiency of water utilization. The three indicators show different perspectives of the system's efficiency change over time, indicating that only the combination of these three indicators can provide a holistic portray about efficiency. Results showed that the structure influenced the cycling and indirect flows, and from a throughflow perspective, the system depends on large boundary inputs of fresh water. Furthermore, the values of Cycling Index and Indirect effect ratio are much lower than for natural food webs, implying that the policies that led to the structural change and reduction of boundary fresh water inputs do not lead to positive water utilization seen in natural systems. This study provides a novel perspective and methodology for assessing the structure and efficiency of water utilization system with a whole perspective.  相似文献   
848.
普通鸡冠花序中黄酮类化合物的研究   总被引:19,自引:0,他引:19  
红色普通鸡冠(Celosia argentea L.,red flower)花序乙醇提取物用Mg+HCl,Zn+HCl,1%FeCl3-乙醇液,2%AlCl3-乙醇液,1%NaOH进行显色反应,呈现黄酮类化合物性质特征颜色。又以槲皮素、山奈酚、异鼠李素为对照品,采用HPLC法测定分析了不同花期花序中黄酮醇的含量。结果表明,晚期花序干品中总黄酮含量(以甙元计)为0.761%。  相似文献   
849.
人类活动对普氏原羚分布的影响   总被引:15,自引:1,他引:15  
根据卫星遥感(TM)影像与野外——GPS(全球定位系统)定位的实地考察资料,结合数字化的地形图、植被图与土地利用图、交通图,利用地理信息系统软件——生态信息系统(EIS)建立了地理信息系统模型,采用生境评价方法HEP就人类活动对青海湖地区普氏原羚生境的影响进行了分析。影响普氏原羚生境适合度的因素主要为基质类型、坡度、水源远近、植被类型与人类活动。当不考虑人类活动影响时,普氏原羚活动区的面积为7955km 2,最佳生境为典型草原,其次为灌丛沙地,折合适宜生境面积4664km 2。由于交通、居民点、农业用地等土地利用的影响,普氏原羚仅分布在草地沙地边缘,其适宜分布区迅速减小,生境的适合度等级出现明显变化,最适生境缺损,折合最适生境面积511km 2。只占原来适合生境面积11% 。典型草原围栏与人为放牧影响,普氏原羚实际分布区面积只有350km 2,讨论了普氏原羚的保护对策。  相似文献   
850.
Mountain lions (Puma concolor) are often difficult to monitor because of their low capture probabilities, extensive movements, and large territories. Methods for estimating the abundance of this species are needed to assess population status, determine harvest levels, evaluate the impacts of management actions on populations, and derive conservation and management strategies. Traditional mark–recapture methods do not explicitly account for differences in individual capture probabilities due to the spatial distribution of individuals in relation to survey effort (or trap locations). However, recent advances in the analysis of capture–recapture data have produced methods estimating abundance and density of animals from spatially explicit capture–recapture data that account for heterogeneity in capture probabilities due to the spatial organization of individuals and traps. We adapt recently developed spatial capture–recapture models to estimate density and abundance of mountain lions in western Montana. Volunteers and state agency personnel collected mountain lion DNA samples in portions of the Blackfoot drainage (7,908 km2) in west-central Montana using 2 methods: snow back-tracking mountain lion tracks to collect hair samples and biopsy darting treed mountain lions to obtain tissue samples. Overall, we recorded 72 individual capture events, including captures both with and without tissue sample collection and hair samples resulting in the identification of 50 individual mountain lions (30 females, 19 males, and 1 unknown sex individual). We estimated lion densities from 8 models containing effects of distance, sex, and survey effort on detection probability. Our population density estimates ranged from a minimum of 3.7 mountain lions/100 km2 (95% CI 2.3–5.7) under the distance only model (including only an effect of distance on detection probability) to 6.7 (95% CI 3.1–11.0) under the full model (including effects of distance, sex, survey effort, and distance × sex on detection probability). These numbers translate to a total estimate of 293 mountain lions (95% CI 182–451) to 529 (95% CI 245–870) within the Blackfoot drainage. Results from the distance model are similar to previous estimates of 3.6 mountain lions/100 km2 for the study area; however, results from all other models indicated greater numbers of mountain lions. Our results indicate that unstructured spatial sampling combined with spatial capture–recapture analysis can be an effective method for estimating large carnivore densities. Published 2012. This article is a U.S. Government work and is in the public domain in the USA.  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