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   657816篇
  免费   61910篇
  国内免费   503篇
  2018年   10345篇
  2017年   10004篇
  2016年   10490篇
  2015年   9179篇
  2014年   10855篇
  2013年   16035篇
  2012年   19156篇
  2011年   23949篇
  2010年   16999篇
  2009年   15728篇
  2008年   20763篇
  2007年   22837篇
  2006年   16066篇
  2005年   16042篇
  2004年   15792篇
  2003年   15622篇
  2002年   15346篇
  2001年   26766篇
  2000年   26921篇
  1999年   21155篇
  1998年   6935篇
  1997年   7314篇
  1996年   6956篇
  1995年   6718篇
  1994年   6626篇
  1993年   6527篇
  1992年   17887篇
  1991年   17552篇
  1990年   17570篇
  1989年   17171篇
  1988年   16128篇
  1987年   15289篇
  1986年   14019篇
  1985年   14652篇
  1984年   11946篇
  1983年   10329篇
  1982年   7778篇
  1981年   7012篇
  1980年   6537篇
  1979年   11411篇
  1978年   8826篇
  1977年   8303篇
  1976年   7944篇
  1975年   8814篇
  1974年   9550篇
  1973年   9431篇
  1972年   9052篇
  1971年   8308篇
  1970年   6912篇
  1969年   6685篇
排序方式: 共有10000条查询结果,搜索用时 14 毫秒
1.
2.
3.
4.
A conceptual model is proposed, describing potential Zostera marina habitats in the Wadden Sea, based on reported data from laboratory, mesocosm and field studies. Controlling factors in the model are dynamics, degree of desiccation, turbidity, nutrients and salinity. A distinction has been made between a higher and a lower zone of potential habitats, each suitable for different morphotypes of Z. marina. The model relates the decline of Z. marina in the Wadden Sea to increased sediment and water dynamics, turbidity, drainage of sediments (resulting in increased degree of desiccation) and total nutrient loads during the twentieth century. The upper and lower delineation of both the higher and the lower zone of potential Z. marina habitats appear to be determined by one or a combination of several of these factors. Environmental changes in one of these factors will therefore influence the borderlines of the zones. The lower zone of Z. marina will be mainly affected by increased turbidity, sediment dynamics, degree of desiccation during low tide and nutrient load. The higher zone will be affected by increases in water and sediment dynamics, desiccation rates and nutrient loads. Potential Z. marina habitats are located above approx. –0.80 m mean sea level (when turbidity remains at the same level as in the early 1990s) in sheltered, undisturbed locations, and preferably where some freshwater influence is present. At locations with a high, near-marine, salinity, the nutrient load has to be low to allow the growth of Z. marina. The sediment should retain enough water during low tide to keep the plants moist. Our results suggest that the return of Z. marina beds within a reasonable time-scale will require not only suitable habitat conditions, but also revegetation measures, as the changes in the environment resulting from the disappearance of Z. marina may impede its recovery, and the natural import of propagules will be unlikely. Furthermore, the lower zone of Z. marina may require a genotype that is no longer found in the Wadden Sea. Received: 26 April 1999 / Received in revised form: 15 October 1999 / Accepted: 16 October 1999  相似文献   
5.
6.
7.
8.
N Bains  J Hoey 《CMAJ》1998,159(2):163
  相似文献   
9.
Random amplified polymorphic DNA (RAPD) analysis was used tostudy variation among and within selectedIxora (Rubiaceae) populationsand mutants. Six populations of I. congesta yielded identicalbanding patterns suggesting genetic uniformity of this species.However, six populations of I. coccinea varieties (three red-flowered,two yellow-flowered and one red-flowered wild-type) exhibitedinfraspecific differences in RAPD profiles. Small and largeleaves of an atavistic mutant cultivar of I. coccinea were alsosubjected to RAPD analysis. An extra band was amplified in thelarge leaves that was absent in small leaves, suggesting thatthe phenotypic alteration in this taxon is due to genetic mutationrather than epigenetic changes. Similarly, an extra band wasdetected in the white sectors of I. Variegated compared to thegreen sectors, suggesting that the shoot apical meristems ofthis cultivar exist as a genetic chimera. DNA gel blot hybridizationwas performed to confirm the specificities of selected bands.Our study indicates that differences among individuals of variouspopulations and mutants may be detected using RAPD markers.Copyright 1999 Annals of Botany Company Ixora L., variegated variety, RAPD fingerprinting, DNA gel blot, intraspecific genetic similarity, atavistic mutant.  相似文献   
10.
Recent studies have shown that Salmonella shedding status affects sows’ microbiota during gestation and that these modifications are reflected in the faecal microbiota of their piglets at weaning. The aims of this study were: (a) to evaluate the persistence, up to the fattening period, of the previously measured link between the microbiota of piglets and their mothers’ Salmonella shedding status; and (b) measure the impact of the measured microbiota variations on their Salmonella excretion at this stage. To achieve this, 76 piglets born from 19 sows for which the faecal microbiota was previously documented, were selected in a multisite production system. The faecal matter of these swine was sampled after 4 weeks, at the fattening stage. The Salmonella shedding status and faecal microbiota of these animals were described using bacteriological and 16S rRNA gene amplicon sequencing respectively. The piglet digestive microbiota association with the Salmonella shedding status of their sows did not persist after weaning and did not affect the risk of Salmonella excretion during fattening, while the birth mother still affected the microbiota of the swine at fattening. This supports the interest in sows as a target for potentially transferrable microbiota modifications.  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