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   2572篇
  免费   82篇
  国内免费   13篇
  2015年   17篇
  2014年   19篇
  2013年   63篇
  2012年   72篇
  2011年   57篇
  2010年   73篇
  2009年   117篇
  2008年   86篇
  2007年   69篇
  2006年   63篇
  2005年   56篇
  2004年   56篇
  2003年   33篇
  2002年   29篇
  2001年   17篇
  2000年   21篇
  1999年   31篇
  1998年   37篇
  1997年   42篇
  1996年   36篇
  1995年   27篇
  1994年   23篇
  1993年   18篇
  1992年   35篇
  1991年   13篇
  1990年   23篇
  1989年   23篇
  1988年   18篇
  1987年   21篇
  1986年   19篇
  1985年   19篇
  1984年   14篇
  1983年   18篇
  1981年   21篇
  1980年   14篇
  1979年   20篇
  1977年   15篇
  1971年   16篇
  1959年   49篇
  1958年   123篇
  1957年   137篇
  1956年   134篇
  1955年   149篇
  1954年   150篇
  1953年   119篇
  1952年   104篇
  1951年   98篇
  1950年   59篇
  1949年   14篇
  1948年   23篇
排序方式: 共有2667条查询结果,搜索用时 187 毫秒
951.
The botanical career of Nathaniel Wallich is followed; his collecting activities in Asia enumerated, and the final disposition of his huge herbarium, largely the property of the Honourable East India Company, is given. His relationship with numerous eminent contemporary botanists, and particularly with the de Candolles of Geneva, is described, together with the role that each played in assisting Wallich to work up his material.
A full list of the works used in the composition of this paper is given in an Appendix rather than following the usual citation convention. Copies of the correspondence between Wallich and the de Candolles of Geneva are available on microfiche.  相似文献   
952.
Nine-day chicken embryo neuroretinal cells transdifferentiate into both lens and pigment cells after 3–4 weeks when cultured in MEM medium containing 10% foetal calf serum at pH 7.4. At pH 6.8. the appearance of lens crystallins is retarded and cholineacetyltransferase (CAT) activity persists for longer, whereas at pH 8.0 crystallins appear earlier and CAT activity declines more rapidly. Cell survival and culture growth are about 10% lower at pH 6.8 than at pH 8.0. If the concentration of foetal calf serum (FCS) is increased from 10% to 25% (at pH 7.4), cell survival and growth are both promoted, crystallins appear slightly earlier and CAT activity declines more rapidly. Converse effects are observed with 5 % serum, accumulation of crystallins being greatly inhibited and CAT activity prolonged. Crystallin production in cultures with 10% or 25% chicken serum (CS) is much less extensive than in similar FCS cultures, but in cultures with 5 % CS, crystallins appear more rapidly, reaching higher levels than in 5 % FCS cultures. However, the pattern of CAT activity in response to different serum levels is similar for both CS and FCS. This might imply the presence of some factor(s) able to stimulate transdifferentiation in FCS, whereas CS can apparently inhibit this process.  相似文献   
953.
The marine silverside Atherinops affinis has a short, simple gut and no obvious stomach or physical mechanisms for breaking down algal cells even though its diet in many estuarine habitats is primarily green macroalgae. Silverside from the Upper Newport Bay estuary in southern California had: (1) assimilation efficiencies from a green algal diet of 54% (total organic material), 74% (carbon), 84% (nitrogen), 89% (protein) and 81% (energy), values typical for marine herbivorous fishes; (2) an alkaline gut and no pepsin activity, thus confirming that the fish lacks a stomach; (3) relatively high amylase activity and low trypsin and lipase activities, a digestive enzyme profile expected in a fish consuming a high-carbohydrate, low-protein and low-lipid diet. The fish appears to be capable of functional herbivory and can be classified tentatively as a type III herbivore based on its possible use of pharyngeal jaws for lysis of algal cells.  相似文献   
954.
Contrary to the conventional wisdom 'bigger is better', evolution at high temperature invariably leads to small individuals in Drosophila melanogaster . Natural selection is known to be responsible, meaning that genotypes that are small because of adaptation to high temperature must have some temperature dependent fitness advantage. In this study we consider both preadult and adult fitness components, and show that large adults from a cold adapted population significantly outperform small adults from a warm adapted population only when tested at low temperature and low larval density. In all other conditions 'bigger is not necessarily better', meaning that environmental influences are capable of altering the association between size and fitness. Yet, 'smaller wasn't better either' under any condition, when considering the overall measure of fitness. Examination of individual fitness components revealed population by temperature interaction in preadult survival; this interaction is potentially capable of explaining the temperature specific advantage of small adult body size. At high temperature, the warm adapted population exhibits superior preadult survival while producing small adults. Geographical variation in adult body size seems to be the result of selection on larval growth and competitive strategies, resulting in alterations in the association between fitness components.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 80 , 717–725.  相似文献   
955.
A new species Exostyles godoyensis Soares-Silva & Mansano (Leguminosae, Papilionoideae, Swartzieae), endemic to Paraná state in southern Brazil, is described and illustrated. The new species is distinct from all the other species of Exostyles in having a leaf with up to five leaflets, while the other species have at least seven leaflets per leaf. The new species is distinct from Exostyles amazonica Yakovlev in its longer stamen filaments and ovary stipe and from E. glabra Vogel and E. venusta Schott by its shorter anthers. In addition to these morphological characters E. godoyensis has a disjunct geographical distribution.  © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society , 2004, 146 , 103–106.  相似文献   
956.
Tragopogon cazorlanum (Asteraceae: Lactuceae) is proposed as a new Spanish species with distribution restricted to the Baetic mountains (south-eastern Spain). Its morphological, palynological, chromosomal, biogeographical and ecological features are discussed, as well as its main relationships and differences with reference to other Spanish species of the genus.  © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society , 2004, 146 , 505–511.  相似文献   
957.
A review of the genus Semele (Ruscaceae) systematics in Madeira   总被引:2,自引:0,他引:2  
The present study attempts to review the systematics of Semele (Ruscaceae) in Madeira, based on phenotypic diversity. The variation in some vegetative (climbing shoot, second-order branches or 'phylloclades') and sexual (inflorescence and flowers) characters was analysed in 115 plant specimens from 30 field populations, herbaria of the Costa collection and Madeira Botanical Garden (MADJ) and certain gardens. Thirty-one quantitative and qualitative characters have been utilized in the analysis. Kaiser–Meyer–Olkin (KMO) analysis indicates adequate sampling. Principal component analysis (PCA) reveals that the spatial distribution of individuals has a discontinuous behaviour. Principal coordinate analysis (PCO) utilizing the Gower coefficient on standardized data revealed a significantly discontinuous distribution of individuals, such that two different clusters can be defined. The Student's t -test and Tukey test on separate characters, when individuals were classified according to the Costa classification, confirms the significant differences between grouping accessions. This leads to the recognition of two species within the genus in Madeira. Literature and herbarium studies show that these two taxa are conspecific with Semele androgyna (L.) Kunth sensu stricto (s.s.) and Semele menezesi Costa sensu lato (s.l.) . A separated statistical analysis of the S. androgyna cluster shows the possible existence of additional subgroups. Based on field population distribution, ecological behaviour and variation in features, we propose the recognition of two species, S. androgyna (L.) Kunth and S. menezesi (Costa) Pinheiro de Carvalho, and two subspecies S. androgyna (L.) Kunth androgyna Pinheiro de Carvalho and S. androgyna (L.) Kunth pterygophora Pinheiro de Carvalho.  © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society , 2004, 146 , 483–497.  相似文献   
958.
959.
960.
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