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   8037篇
  免费   755篇
  国内免费   4篇
  8796篇
  2023年   29篇
  2022年   62篇
  2021年   121篇
  2020年   78篇
  2019年   109篇
  2018年   142篇
  2017年   136篇
  2016年   224篇
  2015年   361篇
  2014年   375篇
  2013年   456篇
  2012年   629篇
  2011年   668篇
  2010年   402篇
  2009年   345篇
  2008年   451篇
  2007年   478篇
  2006年   400篇
  2005年   412篇
  2004年   392篇
  2003年   391篇
  2002年   334篇
  2001年   118篇
  2000年   160篇
  1999年   143篇
  1998年   87篇
  1997年   89篇
  1996年   66篇
  1995年   79篇
  1994年   77篇
  1993年   64篇
  1992年   76篇
  1991年   75篇
  1990年   83篇
  1989年   66篇
  1988年   43篇
  1987年   47篇
  1986年   70篇
  1985年   41篇
  1984年   25篇
  1983年   32篇
  1982年   39篇
  1981年   31篇
  1980年   23篇
  1979年   32篇
  1978年   25篇
  1977年   19篇
  1976年   27篇
  1973年   21篇
  1972年   23篇
排序方式: 共有8796条查询结果,搜索用时 15 毫秒
101.
The conserved influenza virus hemagglutinin (HA) stem domain elicits cross-reactive antibodies, but epitopes in the globular head typically elicit strain-specific responses because of the hypervariability of this region. We isolated human monoclonal antibody 5J8, which neutralized a broad spectrum of 20th century H1N1 viruses and the 2009 pandemic H1N1 virus. Fine mapping of the interaction unexpectedly revealed a novel epitope between the receptor-binding pocket and the Ca2 antigenic site on HA. This antibody exposes a new mechanism underlying broad immunity against H1N1 influenza viruses and identifies a conserved epitope that might be incorporated into engineered H1 virus vaccines.  相似文献   
102.
103.
Species distribution modeling has been widely used to address questions related to ecology, biogeography and species conservation on global and regional scales. Here, we study palms (Arecaceae) in a tropical biodiversity hotspot (Thailand) using species distribution modeling to assess range‐limiting factors and estimate distribution and diversity patterns based on a comprehensive compilation of occurrence records. We focused on palms as a model group due to their key‐stone importance for ecosystem functioning and socio‐economics. Different combinations of climatic, non‐climatic environmental and spatial predictors were used. The most accurate models as indicated by the ‘area under the receiver operating characteristic curve’ (AUC) statistic were those that combined all predictors. The four strongest single predictors of palm species distributions were, in decreasing order of importance, 1) latitude, 2) precipitation of driest quarter, 3) annual precipitation, and 4) minimum temperature of the coldest month, suggesting rainfall patterns and latitudinal spatial constraints as the main range determinants. Overlaying the predicted distributions revealed that potential palm hotspots are situated in the provinces of Satun and Yala in southern Thailand where vast areas remain relatively open to the discovery of new palm records and perhaps even new species.  相似文献   
104.
105.
Two little known species of brown algae, Litosiphon mortensenii S. Lund and L. groenlandicus S. Lund, described from East Greenland have been compared with the type species of the genera Litosiphon Harv. and Asperococcus Lamour. They are distinguished from the genus Litosiphon by their basally sheathed hairs, hecatonematoid microthalli, mediate differentiation of embryospores, and lack of sympodial branching of the macrothallus. They are considered more primitive than Asperococcus by the absence of paraphyses and by the non-localized formation of reproductive structures. The new genus Trachynema is proposed to accommodate these plants; it finds a natural position in the Punctariaceae.  相似文献   
106.
OBJECTIVE--To study the acceptability, costs, psychosocial consequences, and organisation of screening for carcinoma of the prostate. DESIGN--A randomly selected population was personally invited for digital rectal examination by a urologist and a general practitioner. Further examinations were performed if induration was felt. Each man completed a questionnaire on his response to the examination. SETTING--General practices in the area of Norrköping. PATIENTS--1494 Men aged 50-69 randomly selected from a population of 9026. MAIN OUTCOME MEASURE--Prostates having a firm nodular consistency. RESULTS--Carcinoma of the prostate was suspected in 45 of 1163 patients examined; in 10 by the general practitioners, in 10 by the urologists, and in 25 by both. Forty four men had a fine needle aspiration biopsy, and carcinomas were found in 13 cases. Of these, one had been suspected by the general practitioner, four by urologists, and eight by both. The cost for each man was 11.60 pounds, and the cost for each case of carcinoma detected and treated by potentially curative methods was 2477 pounds. Of the 13 men with carcinoma, 10 underwent radical prostatectomy and one radiotherapy. One man had advanced disease and was given endocrine treatment, another was not treated. Only 193 men felt distress during the initial examination. Of the 44 men who had an aspiration biopsy, 25 experienced anxiety. CONCLUSIONS--Screening for carcinoma of the prostate by a urologist or a general practitioner using digital rectal examination is a cost effective method of early diagnosis. Whether such screening leads to prolonged survival, however, remains doubtful.  相似文献   
107.
108.
PGF or hypertonic (20 per cent) saline followed by oxytocin was used to terminate pregnancy in 160 women between the 10th and 20th weeks of gestation.In the patients treated with PGF minor side-effects were reported in about 50 per cent of the cases, however, the method was superior to hypertonic saline with regard to the number of complications and the length of stay in hospital.  相似文献   
109.
Potentilla fruticosa L. (Rosaceae) is a complex of large morphological, karyological, geographical and ecological amplitudes. A survey of the distribution of the morphological variation is given and the underlying reasons discussed. Migration paths are also discussed. In southeast central Asia there is a complex in need of further investigation. Outside this area two species should be recognized, a diploid and hermaphrodite species, P. floribunda Pursh, in N America, S Europe and Siberia and a tetraploid and dioecious species, P. fruticosa L., in N Europe and Siberia.  相似文献   
110.
Xylose is a second‐most abounded sugar after glucose in lignocellulosic hydrolysates and should be efficiently fermented for economically viable second‐generation ethanol production. Despite significant progress in metabolic and evolutionary engineering, xylose fermentation rate of recombinant Saccharomyces cerevisiae remains lower than that for glucose. Our recent study demonstrated that peroxisomedeficient cells of yeast Ogataea polymorpha showed a decrease in ethanol production from xylose. In this work, we have studied the role of peroxisomes in xylose alcoholic fermentation in the engineered xylose‐utilizing strain of S. cerevisiae. It was shown that peroxisome‐less pex3Δ mutant possessed 1.5‐fold decrease of ethanol production from xylose. We hypothesized that peroxisomal catalase Cta1 may have importance for hydrogen peroxide, the important component of reactive oxygen species, detoxification during xylose alcoholic fermentation. It was clearly shown that CTA1 deletion impaired ethanol production from xylose. It was found that enhancing the peroxisome population by modulation the peroxisomal biogenesis by overexpression of PEX34 activates xylose alcoholic fermentation.  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