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   6757篇
  免费   387篇
  国内免费   504篇
  2024年   24篇
  2023年   69篇
  2022年   86篇
  2021年   160篇
  2020年   201篇
  2019年   215篇
  2018年   190篇
  2017年   172篇
  2016年   274篇
  2015年   277篇
  2014年   395篇
  2013年   721篇
  2012年   350篇
  2011年   285篇
  2010年   192篇
  2009年   241篇
  2008年   336篇
  2007年   253篇
  2006年   204篇
  2005年   208篇
  2004年   183篇
  2003年   189篇
  2002年   167篇
  2001年   171篇
  2000年   170篇
  1999年   104篇
  1998年   128篇
  1997年   130篇
  1996年   118篇
  1995年   102篇
  1994年   114篇
  1993年   117篇
  1992年   112篇
  1991年   91篇
  1990年   97篇
  1989年   84篇
  1988年   76篇
  1987年   73篇
  1986年   47篇
  1985年   62篇
  1984年   79篇
  1983年   40篇
  1982年   41篇
  1981年   50篇
  1980年   36篇
  1979年   50篇
  1978年   30篇
  1977年   27篇
  1975年   24篇
  1973年   24篇
排序方式: 共有7648条查询结果,搜索用时 109 毫秒
1.
Four new species ofCayaponia are described and illustrated: three from Brazil (C. cogniauxiana, C. nitida andC. rugosa) and one from Brazil and Bolivia (C. ferruginea).  相似文献   
2.
Ten taxa of myxomycetes growing mainly withEucalyptus, oak and pine are described. They were found in Upper Galilee, Mt. Carmel and Central parts of the country and all are new to Israel.  相似文献   
3.
4.
To effectively integrate DNA sequence analysis and classical nematode taxonomy, we must be able to obtain DNA sequences from formalin-fixed specimens. Microdissected sections of nematodes were removed from specimens fixed in formalin, using standard protocols and without destroying morphological features. The fixed sections provided sufficient template for multiple polymerase chain reaction-based DNA sequence analyses.  相似文献   
5.
Two major subgroups of erycine snakes, designated Charina and Eryx , are delimited with a cladistic analysis of 75 morphological characters. The hypotheses of species relationships within the two clades are ( reinhardtii ( bottae, trivirgata )) and ( colubrinus, conicus, elegans, jayakari, muelleri, somalicus ( miliaris ( tataricus ( jaculus, johnii )))), respectively. This pattern of grouping obtains without assuming multistate character additivity. At least 16 synapomorphies indicate that reinhardtii is an erycine and that it is the sister lineage of the ( bottae, trivirgata ) clade. Calabaria and Lichanura are synonymized with Charina for reasons of taxonomic efficiency, and to emphasize the New-Old World geographic distribution of the three species in that assemblage. Further resolution of Eryx species relationships is required before Gongylophis (type species conicus ) can be recognized.  相似文献   
6.
The gorgoniid Eugorgia is exclusively an eastern Pacific genus. It has a wide geographic and bathymetric range of distribution, found from California to Perú and extends down to 65 m deep. Two new species are herein described. The morphological characters were analyzed and illustrated by light and scanning electron microscopy. Eugorgia beebei sp. n. can be distinguished by its white, ascending, sparse colony growth. Eugorgia mutabilis sp. n. can be distinguished by its white colony that changes color after collection, and the conspicuous sharp-crested disc sclerites. From a morphological point of view the new species are related to the daniana-group, the rubens-group and the siedenburgae-group of Eugorgia; their affiliations, and the proposal of a new group are discussed. These new species increases the number of species in the genus to 15, and contribute to the knowledge of the eastern Pacific octocoral biodiversity.  相似文献   
7.
Rickettsia parkeri, a member of the spotted fever group rickettsias, was first described in 1939 and was thought to be non‐pathogenic until recently, when it was found to cause a spotted fever‐like illness in humans and areas of necrosis (eschars) at the sites of tick bites. Accordingly, there is currently much interest in this emerging pathogen. In this study, all published articles concerning R. parkeri were reviewed and analyzed for evidence of relatedness among this agent and other spotted fever group (SFG) rickettsiae which also produce similar clinical syndromes and/or eschars, including R. conorii, R. africae, and R. sibirica. A synthesis of the historical (antigenic) and recent (molecular) data supporting a phylogenetic sub‐grouping of these SFG organisms is presented and comments are offered about the taxonomy of rickettsial organisms in general, and R. parkeri in particular.  相似文献   
8.
9.
Forty-five populations of Pentanema corresponding to seven species included in the Pentanema conyzae clade have been studied using AFLP fingerprinting. The results show that allopolyploidization could have been involved in the diversification of this group, specifically in species P. langeanum and P. maletii. Molecular data confirm the presence of P. britannicum in the Iberian Peninsula and key steps are provided to identify the species that are morphologically the most challenging.  相似文献   
10.
Some taxonomic characters may escape observation when only a small part of the vegetative body of the plant is examined. For this reason, these characters are likely to be overlooked. Phyllotaxy varies in most cases from the trunk to branches. Serial buds, when they exist, may be lacking in the axil of some leaves. Syllepsis is easier to see on the trunk than on twigs. Architectural models are established after appropriate observation of the whole plant or, better, with different individuals of the same species. Nevertheless, many of these characters can be readily observed and frequently help to identify a woody plant without its flowers or fruits. Some interpretations concerning systematics and evolutionary biology are also given.  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