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   976篇
  免费   33篇
  国内免费   136篇
  1145篇
  2022年   9篇
  2020年   11篇
  2019年   16篇
  2018年   10篇
  2016年   10篇
  2015年   21篇
  2014年   21篇
  2013年   27篇
  2012年   37篇
  2011年   51篇
  2010年   49篇
  2009年   62篇
  2008年   45篇
  2007年   69篇
  2006年   53篇
  2005年   44篇
  2004年   31篇
  2003年   36篇
  2002年   33篇
  2001年   21篇
  1998年   14篇
  1997年   15篇
  1996年   16篇
  1995年   17篇
  1993年   15篇
  1992年   16篇
  1991年   11篇
  1990年   14篇
  1989年   14篇
  1988年   12篇
  1987年   11篇
  1986年   8篇
  1985年   11篇
  1980年   9篇
  1978年   10篇
  1977年   8篇
  1976年   10篇
  1975年   7篇
  1974年   15篇
  1972年   7篇
  1971年   11篇
  1959年   9篇
  1958年   18篇
  1957年   18篇
  1956年   10篇
  1955年   16篇
  1954年   17篇
  1953年   11篇
  1952年   8篇
  1951年   9篇
排序方式: 共有1145条查询结果,搜索用时 0 毫秒
81.
Adult mosquito surveillance was conducted using black light traps in August of 2006 and 2007 at Wangging county and Yanji city, Jilin Province, China to identify the distribution of anopheline mosquitoes in northern China. A total of 2459 female mosquitoes comprising three genera and eight species including Anopheles (Anopheles) lesteri, An. (Ano.) kleini, An. (Ano.) pullus, Culex inatomii, Cx. orientalis, Cx. pipiens, Cx. bitaeniorhynchus and Aedes vexans nipponii were collected. The most commonly collected species was An. kleini which had not been previously reported from China. Anopheles sinensis sensu stricto is commonly collected throughout China, but was not collected from these areas.  相似文献   
82.
The phylogeographical patterns of the endemic minnow, Candidia barbatus, were examined using mitochondrial (mt) cytochrome b sequences, allozyme electrophoresis, and morphology. The 23 mtDNA haplotypes recovered were grouped into six lineages (A–F). According to a molecular clock of 1.7% per million years, we estimated that the colonization of C. barbatus to Taiwan took place three million years ago with several subsequent vicariance events. In the allozyme analysis, the Bayesian approach partitioned different populations into three clusters, with clusters I, II, and III mainly found in northern, central, and southern Taiwan, respectively. Morphologically, fish from north and south of the Tropic of Cancer had distinct numbers of lateral line scales and were separated into two morphs. The cohabitation of different mtDNA lineages with high allozymic and morphological similarities suggests that interbreeding amongst them is possible. Therefore, in spite of highly differentiated mtDNA, samples from different collecting sites should be considered a single species. Based on the current population distribution and levels of divergence, we propose that both orogenic activities and marine regression during glacial epoch were the two main forces shaping population differentiation of this species. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 161 , 613–632.  相似文献   
83.
84.
Culdoscopy     
LEE ST 《California medicine》1952,76(4):294-296
Cul-de-sac puncture for introduction of a culdoscope is easily made with the patient in the knee-chest position. With the use of the instrument, the pelvic organs can be viewed clearly. Culdoscopic examination of 45 patients was carried out. In all cases in which laparotomy was done after the examination, the culdoscopic observations and diagnosis were confirmed. In no case in which tubal pregnancy was present, was the diagnosis missed in culdoscopic examination. Patients were only slightly uncomfortable after the examination. There was no evidence of pelvic peritonitis in any patient, and no pregnant patient aborted as a result of the procedure.  相似文献   
85.
Auxin and abscisic acid (ABA) modulate numerous aspects of plant development together, mostly in opposite directions, suggesting that extensive crosstalk occurs between the signalling pathways of the two hormones. However, little is known about the nature of this crosstalk. We demonstrate that ROP‐interactive CRIB motif‐containing protein 1 (RIC1) is involved in the interaction between auxin‐ and ABA‐regulated root growth and lateral root formation. RIC1 expression is highly induced by both hormones, and expressed in the roots of young seedlings. Whereas auxin‐responsive gene induction and the effect of auxin on root growth and lateral root formation were suppressed in the ric1 knockout, ABA‐responsive gene induction and the effect of ABA on seed germination, root growth and lateral root formation were potentiated. Thus, RIC1 positively regulates auxin responses, but negatively regulates ABA responses. Together, our results suggest that RIC1 is a component of the intricate signalling network that underlies auxin and ABA crosstalk.  相似文献   
86.
1. The niche variation hypothesis predicts that among‐individual variation in niche use will increase in the presence of intraspecific competition and decrease in the presence of interspecific competition. We sought to determine whether the local isotopic niche breadth of fish inhabiting a wetland was best explained by competition for resources and the niche variation hypothesis, by dispersal of individuals from locations with different prey resources or by a combination of the two. We analysed stable isotopes of carbon and nitrogen as indices of feeding niche and compared metrics of within‐site spread to characterise site‐level isotopic niche breadth. We then evaluated the explanatory power of competing models of the direct and indirect effects of several environmental variables spanning gradients of disturbance, competition strength and food availability on among‐individual variation of the eastern mosquitofish (Gambusia holbrooki). 2. The Dispersal model posits that only the direct effect of disturbance (i.e. changes in water level known to induce fish movement) influences among‐individual variation in isotopic niche. The Partitioning model allows for only direct effects of local food availability on among‐individual variation. The Combined model allows for both hypotheses by including the direct effects of disturbance and food availability. 3. A linear regression of the Combined model described more variance than models limited to the variables of either the Dispersal or Partitioning models. Of the independent variables considered, the food availability variable (per cent edible periphyton) explained the most variation in isotopic niche breadth, followed closely by the disturbance variable (days since last drying event). 4. Structural equation modelling provided further evidence that the Combined model was best supported by the data, with the Partitioning and the Dispersal models only modestly less informative. Again, the per cent edible periphyton was the variable with the largest direct effect on niche variability, with other food availability variables and the disturbance variable only slightly less important. Indirect effects of heterospecific and conspecific competitor densities were also important, through their effects on prey density. 5. Our results support the Combined hypotheses, although partitioning mechanisms appear to explain the most diet variation among individuals in the eastern mosquitofish. The results also support some predictions of the niche variation hypothesis, although both conspecific and interspecific competition appeared to increase isotopic niche breadth in contrast to predictions that interspecific competition would decrease it. We think this resulted from high diet overlap of co‐occurring species, most of which consume similar macroinvertebrates.  相似文献   
87.
βγ-晶状体蛋白是主要分布于脊椎动物眼睛晶状体内的水溶性结构蛋白。非晶状体βγ-晶状体蛋白从微生物到高等哺乳动物都有过报道。微生物中发现的非晶状体βγ-晶状体蛋白主要作为环境胁迫应急蛋白,在恶劣环境中发挥对细菌的自我保护作用,如ProteinS。在哺乳动物中,非晶状体βγ-晶状体蛋白被推测能够参与胚胎的皮肤发育与分化调节,具有肿瘤抑制等重要的生理功能,如黑色素瘤缺失蛋白。但是,对于它们的生化特性、生理功能和作用机制知之甚少。本文从蛋白结构、基因起源和生物学功能等方面简要介绍目前非晶状体βγ-晶状体蛋白的相关研究进展。  相似文献   
88.
89.
Trophic Dynamics and Niches of Salt Marsh Foraminifera   总被引:1,自引:0,他引:1  
Energetic considerations of the growth of three species of littoralbenthic foraminifera, Allogromia laticollaris, Rosalina leei,and Spiroloculina hyalina, have been made on laboratory-grownpopulations. Under optimum laboratory conditions A. laticollarishas the greatest intrinsic rate of increase (r = 2.533 org/day);S. hyalina (r = 1.472 org/day), and R. leei (r = 0.272 org/day)being less fecund. The respiration rates of the three specieswere similar (0.5–4.5 µ1/mg body wt/hr) within thetemperature range (15–35 C) tested. The species studiedare selective feeders. Only 4-5 of 28 species of algae testedwere consumed in significant quantities (40-150 x 108 g/foram/day).Although great numbers of bacteria were eaten, their biomasswas negligible when compared to the algae. The ecological growthefficiency (Ee) of the three species tested is highest in freshcultures (5-20%) and declines rapidly. Evidence suggests thatthe species studied are well adapted for the rapid changes inthe microbial community structure which take place throughoutthe summer, and that community stability and high rate of productivityare achieved through diversity.  相似文献   
90.
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