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   29133篇
  免费   3697篇
  国内免费   29篇
  2016年   251篇
  2015年   320篇
  2014年   406篇
  2013年   670篇
  2012年   647篇
  2011年   697篇
  2010年   490篇
  2009年   407篇
  2008年   596篇
  2007年   648篇
  2006年   615篇
  2005年   623篇
  2004年   622篇
  2003年   638篇
  2002年   647篇
  2001年   1860篇
  2000年   1832篇
  1999年   1443篇
  1998年   476篇
  1997年   474篇
  1996年   481篇
  1995年   422篇
  1994年   427篇
  1993年   420篇
  1992年   1070篇
  1991年   1084篇
  1990年   1066篇
  1989年   1029篇
  1988年   960篇
  1987年   901篇
  1986年   770篇
  1985年   747篇
  1984年   587篇
  1983年   510篇
  1982年   367篇
  1981年   343篇
  1980年   333篇
  1979年   562篇
  1978年   449篇
  1977年   413篇
  1976年   338篇
  1975年   457篇
  1974年   469篇
  1973年   415篇
  1972年   393篇
  1971年   311篇
  1970年   258篇
  1969年   240篇
  1968年   225篇
  1967年   205篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
152.
In-situ studies on sublittoral soft bottom macrofauna (depth: 14–16 m) employing the underwater laboratory (UWL) “Helgoland” were carried out. Sets of samples were compared for small-scale local and short-term changes in species richness, faunal abundance, numerical dominance, diversity, evenness, homogeneity, and similarity. It could be shown that minor differences in sediment quality can cause conspicuous heterogeneity within a small sampling area (diameter: ca. 100 m). Both spatfall and mortality of benthic invertebrates can change the faunal structure within a short period (two months). The degree of change varies between species and thus at stations harbouring different faunal assemblages as well.  相似文献   
153.
154.
155.
156.
157.
The alkaloid composition of mycelium and culture liquid filtrate of the fungus Penicillium roqueforti IBPM-F-141 was studied. The new metabolite--3,12-dihydroroquefortine, a derivative of roquefortine, the main component of the alkaloid fraction of this culture, has been isolated for the first time. The structure of 3,12-dihydroroquefortine was determined by chemical and physico-chemical methods. In addition to roquefortine and 3,12-dihydroroquefortine, representatives of a new alkaloid group, the clavine alkaloids, e. g. isofumigaclavine A, isofumigaclavine B and festuclavine, were also isolated and identified. The data on the content of these compounds in mycelium and culture medium are presented.  相似文献   
158.
159.
Duodenal goblet cells and Brunner's-gland cells obtained from two species of New World monkeys (Saimiri sciureus and Saguinus fuscicollis) were studied using conventional histochemical methods and by applying a panel of 17 labelled lectins. The secretions of both goblet and Brunner's-gland cells were found to contain neutral mucosubstances, while those of goblet cells also exhibit acid and sulphated carbohydrate components. Lectin binding studies allowed a more detailed analysis of the mucus glycoproteins. Marked differences between the two examined species were not detected. N-Acetyl-galactosamine, galactose, fucose and N-Acetyl-glucosamine were found to be the predominant sugar residues in Brunner's-glands glycoproteins, with mannose and glucose being only minor components.  相似文献   
160.
Systematics and taxonomy of hares of the genus Lepus (Lagomorpha) are under contentious debate, and phylogenetic relationships among many taxa are not well understood. Here we study genetic differentiation and evolutionary relationships among North African hares, currently considered subspecies of Lepus capensis , cape hares ( L. capensis ) from the Cape province in South Africa, and brown hares ( L. europeaus ) from Europe and Anatolia, using maternally (mtDNA) and biparentally (allozymes) inherited markers. A polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis of a c. 1.8 kb long segment of the mitochondrial control region using eight hexanucleotide-recognizing restriction endonucleases yielded 28 haplotypes, and horizontal starch gel electrophoresis of proteins encoded by 25 structural gene loci revealed 52 alleles at 18 polymorphic loci. Diverse phylogenetic analyses (neighbor joining dendrogram, median joining network, multidimensional scaling of pairwise distances, AMOVA, F -statistics, hierarchical F -statistics) of genetic variants revealed marked substructuring of mtDNA into three phylogeographic groups, namely an African, a central European, and an Anatolian, but a somewhat less pronounced overall differentiation of the nuclear genome, despite a relatively high number of population-specific (private) alleles. However, all our results are not incongruent with Petter's (1959: Mammalia 23 , 41; 1961: Z. f. Säugetierkunde 26 , 30; 1972 : Société Des Sciences Naturelles et Physiques du Maroc 52 , 122) hypothesis that North African hares generally belong to L. capensis and that brown hares should be included in this species as well.  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