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   1144111篇
  免费   121436篇
  国内免费   531篇
  2018年   10992篇
  2017年   10486篇
  2016年   14866篇
  2015年   19308篇
  2014年   22955篇
  2013年   33174篇
  2012年   36982篇
  2011年   37799篇
  2010年   25906篇
  2009年   24047篇
  2008年   33672篇
  2007年   35035篇
  2006年   32693篇
  2005年   31536篇
  2004年   31157篇
  2003年   29993篇
  2002年   29142篇
  2001年   51536篇
  2000年   51118篇
  1999年   40514篇
  1998年   14088篇
  1997年   14590篇
  1996年   13720篇
  1995年   13154篇
  1994年   12679篇
  1993年   12723篇
  1992年   33279篇
  1991年   32317篇
  1990年   31386篇
  1989年   30686篇
  1988年   28140篇
  1987年   26811篇
  1986年   24956篇
  1985年   24931篇
  1984年   20565篇
  1983年   17699篇
  1982年   13398篇
  1981年   12224篇
  1980年   11407篇
  1979年   19279篇
  1978年   15122篇
  1977年   13698篇
  1976年   12890篇
  1975年   14305篇
  1974年   15436篇
  1973年   15144篇
  1972年   13845篇
  1971年   12498篇
  1970年   10928篇
  1969年   10579篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
A survey of the freshwater algal flora of the Primorsky Region, Russian Far East, is presented. During more than 70 years of investigations 1659 species (including varieties and forms 2276 taxa) belonging to 346 genera of different groups of algae have been recorded from this territory. The Primorsky Region is rich in habitats attractive to algae-rivers, lakes, reservoirs, and ponds. The details of the distribution of algae in different types of waterbodies have been examined. The algal flora of the region is comparable to the areas, and its taxonomical structure is typical for Eurasia. From this territory some new species and varieties have been described — Achnanthes chankensis, Cymbella ventricosa var. hankensis, Surirella alisoviana, and others. The flora includes some rare species — Fragilaria magocsyi, Perone dimorpha, and Heterothrix monochloron, and some endemics of Asia — Navicula scutelloides var. baicalensis, N. globulifera var. nipponica, Gomphonema sphaerophorum, and Surirella tientsinensis.  相似文献   
82.
83.
84.
85.
86.
Conservation genetics: beyond the maintenance of marker diversity   总被引:8,自引:0,他引:8  
One of the major problems faced by conservation biologists is the allocation of scarce resources to an overwhelmingly large number of species in need of preservation efforts. Both demographic and genetic information have been brought to bear on this problem; however, the role of information obtained from genetic markers has largely been limited to the characterization of gene frequencies and patterns of diversity. While the genetic consequences of rarity may be a contributing factor to endangerment, it is widely recognized that demographic factors often may be more important. Because patterns of genetic marker variation are influenced by the same demographic factors of interest to the conservation biologist, it is possible to extract useful demographic information from genetic marker data. Such an approach may be productive for determining plant mating systems, inbreeding depression, effective population size, and metapopulation structure. In many cases, however, data consisting only of marker frequencies are inadequate for these purposes. Development of genealogical based analytical methods coupled with studies of DNA sequence variation within and among populations is likely to yield the most information on demographic processes from genetic marker data. Indeed, in some cases it may be the only means of obtaining information on the long-term demographic properties that may be most useful for determining the future prospects of a species of interest.  相似文献   
87.
Analysis of 'homes' occupied by juvenile Octopus vulgaris shows flexible behaviour which may indicate specialized intelligence and tool use. Octopuses occupied sheltered areas for a short time, average 10 days, but stayed longer in larger homes and in areas where preferred prey was available. They did not simply respond to perceptual characteristics of a site, but instead chose locations potentially suitable and modified them by removing rocks and sand and bringing in items partially to block the aperture.  相似文献   
88.
89.
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