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   20767篇
  免费   1945篇
  国内免费   1128篇
  2024年   29篇
  2023年   542篇
  2022年   534篇
  2021年   960篇
  2020年   894篇
  2019年   1073篇
  2018年   946篇
  2017年   727篇
  2016年   820篇
  2015年   1001篇
  2014年   1317篇
  2013年   1734篇
  2012年   967篇
  2011年   1109篇
  2010年   812篇
  2009年   973篇
  2008年   969篇
  2007年   901篇
  2006年   867篇
  2005年   832篇
  2004年   747篇
  2003年   579篇
  2002年   550篇
  2001年   444篇
  2000年   356篇
  1999年   316篇
  1998年   326篇
  1997年   290篇
  1996年   240篇
  1995年   200篇
  1994年   256篇
  1993年   216篇
  1992年   198篇
  1991年   144篇
  1990年   145篇
  1989年   104篇
  1988年   119篇
  1987年   71篇
  1986年   57篇
  1985年   70篇
  1984年   58篇
  1983年   51篇
  1982年   55篇
  1981年   52篇
  1980年   32篇
  1979年   32篇
  1978年   21篇
  1977年   19篇
  1975年   27篇
  1974年   25篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
4.
Predation by introduced mammals is decimating New Zealand's indigenous fauna. Understanding factors that influence this process allows resources for predator control to be applied with maximum effect. This study examines how predation of a secondary prey species (a relatively common but declining native plover, the banded dotterel Charadrius bicinctus ) varied with reductions in abundance of a major prey source (rabbits), kill-trapping of predators, nest density and habitat complexity. Banded dotterels mostly nest in open braided riverbeds alongside a number of endemic threatened species. We measured the fate of 753 dotterel clutches exposed to predation by cats, ferrets and hedgehogs. We found key times and places of high predation risk. Immediately after widespread reduction in rabbit populations by rabbit haemorrhagic disease (RHD), clutch predation rates were almost as high (mean, 50%) as those recorded during past rabbit poisoning programmes (mean, 57%). Both rates were significantly higher than the mean predation rate of 22% without rabbit control, suggesting a shift in predator diet immediately after rabbit population declines. Unlike after rabbit poisoning, clutch predation rate remained high in the years after RHD. Other patterns observed included higher clutch predation rate where nest density was lower, suggesting that predation can potentially cause local extinction. Clutch predation was also higher along riverbed margins where vegetation was dense. There was equivocal evidence for an effect of predator kill-trapping on clutch predation rate. Management strategies that could potentially reduce clutch predation risks include focusing predator mitigation measures during periods of rabbit decline, maintaining them for more than one breeding season if the rabbit declines are widespread (e.g. RHD epidemics), and applying greater effort at sites with relatively low nest density and along riverbed margins where predator use is more frequent.  相似文献   
5.
Crops producing insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) are widely planted to manage insect pests. Bt crops can provide an effective tool for pest management; however, the evolution of Bt resistance can diminish this benefit. The European corn borer, Ostrinia nubilalis Hübner, is a significant pest of maize and is widely managed with Bt maize in the Midwest of the United States. When Bt crops are grown in conjunction with non‐Bt refuges, fitness costs of Bt resistance can delay the evolution of resistance. Importantly, fitness costs often vary with ecological factors, including host‐plant genotype and diapause. In this study, we examined fitness costs associated with Cry1F resistance in O. nubilalis when insects were reared on three maize lines. Fitness costs were tested in two experiments. One experiment assessed the fitness costs when Cry1F‐resistant and Cry1F‐susceptible insects were reared on plants as larvae and experienced diapause. The second experiment tested resistant, susceptible and F1 heterozygotes that were reared on plants but did not experience diapause. Despite some evidence of greater adult longevity for Cry1F‐resistant insects, these insects produced fewer fertile eggs than Cry1F‐susceptible insects, and this occurred independent of diapause. Reduced fecundity was not detected among heterozygous individuals, which indicated that this fitness cost was recessive. Additionally, maize lines did not affect the magnitude of this fitness cost. The lower fitness of Cry1F‐resistant O. nubilalis may contribute to the maintenance of Cry1F susceptibility in field populations more than a decade after Cry1F maize was commercialized.  相似文献   
6.
Ebola virus infection can cause Ebola virus disease (EVD). Patients usually show severe symptoms, and the fatality rate can reach up to 90%. No licensed medicine is available. In this review, development of therapeutics for treatment of Ebola virus infection and EVD will be discussed.  相似文献   
7.
The present study is to determine the prevalence and implication of coeliac disease (CD) among adult Saudis and compared to those with diagnosed irritable bowel syndrome. This prospective study was conducted among 980 adults. Out of that, 482 subjects (staff and students of Riyadh Health Science College) were designated as control cohorts for undetected coeliac disease. Furthermore, another contingent of 498 subjects diagnosed with irritable bowel syndrome (IBS) at Prince Salman Hospital and Al-Iman General Hospital also constituted a segment of the overall initial 1020 subjects. Both cases and control were tested for serological markers of coeliac disease (tissues transglutaminase (tTGAs) and endomysial autoantibody (EMAs) and were confirmed by histopathology test. All the positive for cases of coeliac disease were screened for iron deficiency anaemia, Vitamin D deficiency, and osteoporosis and weight assessment. The percentage of coeliac disease in control subjects and patients diagnosed with irritable bowel syndrome (IBS) were found to be 1.9% and 9.6% respectively, about 38% of the total coeliac disease patients are among females of middle age (20–39-years) and 16% of the males in the same age range. Whereas, 20% and 25% of all coeliac disease cases with ages of 40–59 were remarked as females and males respectively. The identical nature and overlap of symptoms of the two conditions could possibly result in misdiagnosis of coeliac diseases or over-diagnosis of irritable bowel syndrome. The findings of the study might also give considerable implications of the disease in the nutritional level which is noticeable.  相似文献   
8.
Normal and limited vision gait was investigated in individuals with Parkinson disease (PD), healthy older and healthy young individuals. Participants walked a GAITRite mat with normal vision or vision of lower limbs occluded. Results indicate individuals with PD walked more slowly, with shorter and wider steps, and spent more time in double support with limited vision as compared to full vision. Healthy young and old individuals took shorter steps but were otherwise unchanged between conditions.  相似文献   
9.
10.
In 1988, 985 patients presenting with breast disease, most with a palpable abnormality, were investigated by the triple approach (clinical examination, imaging and fine needle aspiration cytology [FNAC]). Using FNAC, 28% of patients were diagnosed as having carcinoma, 45% benign disease, 4% had suspicious cytology and 3% equivocal cytology. The remaining 20% had inadequate aspirates. Two false positive diagnoses of carcinoma were made (a false positive rate of 0.7%); one was a case of high grade non-Hodgkin's lymphoma and the other a papillary lesion with epithelial atypia. The false negative rate was 6.4%. Of these 49 patients, six had carcinoma-in-situ and 19 had low grade tumours. The absolute and complete sensitivities for the diagnosis of carcinoma in this series were 84.7% and 91.9% respectively and the absolute and complete specificities 99.7% and 98.3%, respectively. These figures compare favourably with those from other centres and confirm the efficacy of FNAC as part of the triple approach to the diagnosis of breast disease. The use of FNAC has resulted in a reduction in the number of Trucut and frozen section biopsies performed. Eighty three per cent of the patients with benign disease diagnosed by the triple approach have avoided excision biopsy, none of whom have subsequently been found to have carcinoma. Eighty patients with advanced breast carcinoma were spared operative intervention.  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