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   6124篇
  免费   316篇
  国内免费   16篇
  6456篇
  2024年   19篇
  2023年   92篇
  2022年   221篇
  2021年   316篇
  2020年   154篇
  2019年   168篇
  2018年   185篇
  2017年   160篇
  2016年   279篇
  2015年   310篇
  2014年   324篇
  2013年   485篇
  2012年   438篇
  2011年   467篇
  2010年   274篇
  2009年   249篇
  2008年   324篇
  2007年   299篇
  2006年   269篇
  2005年   226篇
  2004年   203篇
  2003年   173篇
  2002年   169篇
  2001年   50篇
  2000年   48篇
  1999年   40篇
  1998年   32篇
  1997年   34篇
  1996年   20篇
  1995年   31篇
  1994年   30篇
  1993年   24篇
  1992年   18篇
  1991年   16篇
  1990年   15篇
  1989年   19篇
  1988年   19篇
  1987年   19篇
  1985年   24篇
  1984年   22篇
  1983年   15篇
  1982年   28篇
  1981年   14篇
  1980年   16篇
  1979年   9篇
  1978年   14篇
  1977年   10篇
  1974年   7篇
  1973年   7篇
  1966年   9篇
排序方式: 共有6456条查询结果,搜索用时 0 毫秒
911.
912.
913.
Numerous laboratory investigations have compared Drosophila melanogaster and D. simulans for various life history traits and fitness related ecophysiological parameters. From presently available information, it is however difficult to get a general comparative pattern describing the divergence of their ecological niches and understanding their demographic success. Two environmental factors seem however to have played a major role: temperature and alcoholic resources. From an ecophysiological approach, D. simulans may be described as generally more sensitive to stresses; other results point to this species as more cold adapted than its sibling; in some cases, however, D. simulans may appear as better adapted to a warm environment. When investigated, ecophysiological traits show a lesser geographic variability in D. simulans than in D. melanogaster. Presently available information does not explain the ecological prevalence of D. simulans in many places with a mild temperate or subtropical climate. This is presumably due to the fact that most comparisons have been done at a single, standard temperature of 25 degrees C. Comparative studies should be undertaken, spanning the thermal ranges of the two species, and the phenotypic plasticity of ecophysiological traits should now be considered.  相似文献   
914.
915.
During a survey of the ciliate protozoal composition of the stomach contents of nine dromedary camels of Egypt, fourteen morphotypes of Entodinium ovumrajae, which has been considered as a species peculiar to camels, were found in six camels. Except for five morphotypes including one originally described as an independent species and its forms, these were newly detected. These morphotypes, divided into three groups, can be identified mainly by the morphology of their ectoplasmic processes. Each camel had on average, about five morphotypes of this species.  相似文献   
916.
Poaceae pollen is one of the most prevalent aeroallergens causing allergenic reactions. The aim of this study was to characterise the grass pollen season in Tetouan during the years 2008–2010, to analyse the effect of some meteorological parameters on the incidence of the airborne Poaceae pollen, and to establish forecasting variables for daily pollen concentrations. Aerobiological sampling was undertaken over three seasons using the volumetric method. The pollen season started in April and showed the highest pollen index in May and June, when the maximum temperature ranged from 23 to 27 °C, respectively. The annual pollen score recorded varied from year to year between 2,588 and 5,404. The main pollen season lasted 114–173 days, with peak days occurring mainly in May; the highest concentration reached 308 pollen grains/m3. Air temperature was the most important meteorological parameter and correlated positively to daily pollen concentration increase. An increase in relative humidity and precipitation was usually related to a decrease in airborne pollen content. External validation of the models performed using data from 2011 showed that Poaceae pollen concentration can be highly predicted (64.2–78.6 %) from the maximum temperature, its mean concentration for the same day in other years, and its concentration recorded on the previous day. Sensitive patients suffering allergy to Poaceae pollen are at moderate to highest risk of manifesting allergic symptoms to grass pollen over 33–42 days. The results obtained provide new information on the quantitative contribution of the Poaceae pollen to the airborne pollen of Tetouan and on its temporal distribution. Airborne pollen can be surveyed and forecast in order to warn the atopic population.  相似文献   
917.
918.

Background

Serum creatinine and cystatin C are used as markers of glomerular filtration rate (GFR). The performance of these GFR markers relative to exogenously measured GFR (mGFR) in HIV-positive individuals is not well established.

Methods

We assessed the performance of the chronic kidney disease epidemiology collaboration equations based on serum concentrations of creatinine (eGFRcr), cystatin C (eGFRcys) and both biomarkers combined (eGFRcr-cys) in 187 HIV-positive and 98 HIV-negative participants. Measured GFR was calculated by plasma iohexol clearance. Bias and accuracy were defined as the difference between eGFR and mGFR and the percentage of eGFR observations within 30% of mGFR, respectively. Activated CD4 and CD8 T-cells (CD38+ HLA-DR+) were measured by flow cytometry.

Results

The median mGFR was >100 ml/min/1.73 m2 in both groups. All equations tended to be less accurate in HIV-positive than in HIV-negative subjects, with eGFRcr-cys being the most accurate overall. In the HIV-positive group, eGFRcys was significantly less accurate and more biased than eGFRcr and eGFRcr_cys. Additionally eGFRcys bias and accuracy were strongly associated with use of antiretroviral therapy, HIV RNA suppression, and percentages of activated CD4 or CD8 T-cells. Hepatitis C seropositivity was associated with larger eGFRcys bias in both HIV-positive and HIV-negative groups. In contrast, eGFRcr accuracy and bias were not associated with HIV-related factors, T-cell activation, or hepatitis C.

Conclusions

The performance of eGFRcys relative to mGFR was strongly correlated with HIV treatment factors and markers of T-cell activation, which may limit its usefulness as a GFR marker in this population.  相似文献   
919.
A systematic revision for the species of Launaea Cass. in Egypt has revealed the presence of fourteen taxa belonging to 12 species, of which L. acanthodes (Boiss.) Kuntze is a new recorded. One new combination “L. fragilis subsp. tenuiloba (Boiss.) Zareh & Mohamed” is made. Achene diversity of the studied taxa of Launaea was studied using light and scanning electron microscopy. The taxonomic relationships between the studied taxa were discussed as well as SEM micrographs of achenes are provided. Synonyms and key for the identification of the investigated taxa based on morphological characters as well as notes on distribution are provided.  相似文献   
920.
Compsilura concinnata (Meigen) is one of the most famous, most polyphagous and most widely distributed tachinid flies (Diptera, Tachinidae) in the world. This species is well known as a biocontrol agent of some injurious pests of cultural and wild plants and has been introduced from Europe to the United States to control mainly the gypsy moth. Recently we found three new species very closely resembling C. concinnata from Southeast and East Asia: C. lobata sp. nov. (Japan and Thailand), C. malayana sp. nov. (Malaysia) and C. pauciseta sp. nov. (Japan and Taiwan). Additionally, C. samoaensis Malloch is treated as a junior synonym of C. concinnata based on the examination of the type specimen. The genetic differences in the mitochondrial COI gene data are examined to assess the accuracy of species delimitation of Compsilura. The male postabdominal characters of these species are illustrated. The piercing female postabdomen of C. concinnata is illustrated and compared to those of other members belonging to the Blondelia group including Blondelia Robineau-Desvoidy, Celatoria Coquillett, Eucelatoria Townsend and Vibrissina Rondani.  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