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   1894篇
  免费   236篇
  2021年   29篇
  2020年   14篇
  2019年   18篇
  2018年   19篇
  2017年   16篇
  2016年   34篇
  2015年   66篇
  2014年   60篇
  2013年   90篇
  2012年   90篇
  2011年   95篇
  2010年   52篇
  2009年   49篇
  2008年   84篇
  2007年   73篇
  2006年   75篇
  2005年   86篇
  2004年   87篇
  2003年   68篇
  2002年   52篇
  2001年   42篇
  2000年   53篇
  1999年   58篇
  1998年   30篇
  1997年   24篇
  1996年   25篇
  1995年   18篇
  1994年   24篇
  1993年   15篇
  1992年   47篇
  1991年   35篇
  1990年   39篇
  1989年   41篇
  1988年   31篇
  1987年   36篇
  1986年   20篇
  1985年   30篇
  1984年   20篇
  1983年   15篇
  1982年   14篇
  1980年   19篇
  1979年   27篇
  1978年   23篇
  1977年   22篇
  1975年   14篇
  1974年   21篇
  1972年   12篇
  1971年   13篇
  1970年   13篇
  1967年   15篇
排序方式: 共有2130条查询结果,搜索用时 375 毫秒
101.
In an experimental clinic, run by nurse specialists in family planning, a total of 768 patients were seen in the first year. Oral contraception was dispensed for 377 patients and 187 intrauterine devices (IUCDs) were inserted; a further 204 IUCD patients attended only for follow-up visits. All side effects were adequately diagnosed by the nurse specialist.  相似文献   
102.
Summary Chromosome analysing using quinacrine fluorescence was performed on 930 consecutive newborn infants. The total incidence of major chromosome aberrations including numerical changes of the sex chromosomes, and structural changes of autosomes, was 0.54%. Incidences of XYY (0.4%) and XXY (0.2%) were relatively higher as compared to other studies. About 0.75% of the newborn infants were found to have a variable bright fluorescent band located on the proximal area of the short arm (p11) rather than on the proximal long arm (q11) of chromosome No. 3. Attempts were also made to record the variable fluorescent regions on 7 autosomes and the Y chromosome.  相似文献   
103.
Various factors involved in the production of "Q-bands" have been studied. It was found that a Zeiss standard WL fluorescent microscope required a shorter exposure time for photography as compared to a Zeiss photomicroscope. The minimal exposure time was obtained when the standard WL microscope was equipped with a UV light source containing a DC powered mercury burner and a concave mirror. Further, the pH and type of water used in the staining, washing and mounting of the slide were also important factors in producing clear and well differentiated "Q-bands". It also appears that the factors involved in the production of "Q-bands" effect the enhancement or quenching of fluorescence by poly d(A-T)-poly d(A-T) and salmon sperm DNA or poly dG-poly dC respectively. This preliminary report also suggests that DNA or polynucleotides with a specific base sequence may play an important role in Q-banding patterns on chromosomes.  相似文献   
104.
105.
106.
107.
108.
109.
Monitoring successional advancement is a complex field involving a constant search for applied ecological indicators which facilitate monitoring of secondary forests for both active and passive restoration. In this study, the authors investigate the successional advancement of floristics and tree structure within Araucaria Forest (AF) fragments under passive restoration in a context where exotic tree plantations (mainly Pinus L. genus) dominate the landscape. The ecological indicators used were floristic dissimilarity (β‐diversity inferences), indicator species, ecological groups of species, basal area, and species abundance distribution (SAD) models (α‐diversity inferences). A total of 182 tree species belonging to 91 genera and 43 botanical families were identified. A high β diversity was verified for which each site has indicator species (for the locations CD—Dicksonia sellowiana; CO—Cryptocarya aschersoniana; and PG—Pinus taeda), where pioneer species contributed to much of the abundance. Different SAD models are useful for describing passive restoration sites in exotic tree plantation landscapes, namely Lognormal, Mandelbrot, and Zipf. SAD models together with basal area, taxonomic group (e.g. Myrtaceae assemblage), and tree abundance in ecological groups are strategic ecological indicators for monitoring successional advancement in AF.  相似文献   
110.
The extinction debt, delayed species extinctions following landscape degradation, is a widely discussed concept. But a consensus about the prevalence of extinctions debts is hindered by a multiplicity of methods and a lack of comparisons among habitats. We applied three contrasting species–area relationship methods to test for plant community extinction debts in three habitats which had different degradation histories over the last century: calcareous grassland, heathland and woodland. These methods differ in their data requirements, with the first two using information on past and current habitat area alongside current species richness, whilst the last method also requires data on past species richness. The most data‐intensive, and hence arguably most reliable method, identified extinction debts across all habitats for specialist species, whilst the other methods did not. All methods detected an extinction debt in calcareous grassland, which had undergone the most severe degradation. We conclude that some methods failed to detect an extinction debt, particularly in habitats that have undergone moderate degradation. Data on past species numbers are required for the most reliable method; as such data are rare, extinction debts may be under‐reported.  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