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   1781篇
  免费   151篇
  1932篇
  2023年   7篇
  2022年   16篇
  2021年   40篇
  2020年   24篇
  2019年   33篇
  2018年   20篇
  2017年   41篇
  2016年   72篇
  2015年   94篇
  2014年   108篇
  2013年   126篇
  2012年   128篇
  2011年   111篇
  2010年   70篇
  2009年   73篇
  2008年   81篇
  2007年   88篇
  2006年   85篇
  2005年   72篇
  2004年   77篇
  2003年   66篇
  2002年   66篇
  2001年   31篇
  2000年   19篇
  1999年   28篇
  1998年   26篇
  1997年   18篇
  1996年   12篇
  1995年   16篇
  1994年   20篇
  1993年   15篇
  1992年   17篇
  1991年   10篇
  1990年   15篇
  1989年   5篇
  1988年   10篇
  1987年   13篇
  1986年   11篇
  1985年   13篇
  1984年   10篇
  1983年   21篇
  1982年   12篇
  1981年   10篇
  1980年   11篇
  1979年   17篇
  1978年   11篇
  1977年   7篇
  1970年   5篇
  1964年   4篇
  1963年   4篇
排序方式: 共有1932条查询结果,搜索用时 0 毫秒
51.
Grey mullets are remarkably characterized by their overall uniform external morphology. Identifying species as well as positioning the Mugiliformes in a phylogenetic context is rather difficult. Most recently they were placed in the newly erected Ovalentaria, but more detailed relationships to potential sister taxa were not resolved. Studying the internal morphology, especially the osteology, might provide new insights into the evolution of the Mugiliformes as well as help clarify their phylogenetic position within the Ovalentaria. A detailed osteology of the golden grey mullet Liza aurata is presented. The use of cleared and stained specimens allowed for a complete examination of bony and cartilaginous structures, and a 3D reconstruction from a μCT data set provided additional information on the positional relationships of the bones. Following this, the data obtained were compared with different mugilid species, particularly with the flathead grey mullet Mugil cephalus. Several differences between these species could be identified, such as the position of the basisphenoid, the shape of the hyomandibular and the composition of the branchial arches. These characters might help in understanding the evolutionary changes happening within the mugiliforms and will provide the basis to study this taxon in detail, finally allowing the reconstruction of the body plan of grey mullets.  相似文献   
52.
Schwantes, H.O.: Biologie der Pilze. Eine Einführung in die angewandte Mykologie. 478 S., 60 Abb., 29 Tab. Verlag Eugen Ulmer, Stuttgart, 1996, UNI‐Taschenbücher 1871, 42,80 DM. ISBN 3–8252–1871–6

Westhoff, P.; Jeske, H.; Jürgens, G.; Kloppstock, K.; Link, G.: Molekulare Entwicklungsbiologie. Vom Gen zur Pflanze. Georg Thieme Verlag Stuttgart, New York; 1996; 112 Abb., 19 Tab. ISBN 3–13–102021–0

McNamara, K.J. (Herausgeber): Evolutionary Change and Heterochrony. John Wiley &; Sons; Chichester, New York, Brisbane, Toronto, Singapur (1995), ISBN 0–471–95837–9

Sobral, B.W.S. (Herausgeber): The Impact of Plant Molecular Genetics. Birkhäuser Verlag AG Basel (1996), ISBN 3–7643–3802–4

McIntosh, R.A.; Wellings, C.R.; Park, R.F.: Wheat rusts ‐ An atlas of resistance genes. 200 S., 119 Bildseiten, Kluwer Academic Publishers, Dordrecht, Boston, London, 1995, 125,00 Dfl., ISBN 0–7923–3430–2

Friedrich, G.; Rode, H. (Hsg.): Pflanzenschutz im integrierten Obstbau. Verlag Eugen Ulmer Stuttgart. 1996. 494 S., 128 Farbfotos, 94 Zeichnungen, 41 Tab., 3. völlig neubearbeitete Auflage. Preis 88,‐DM; ISBN 3–8001–5541–9

Nultsch, W.: Allgemeine Botanik. 10. neubearbeitete erweiterte Auflage. Georg Thieme Verlag Stuttgart New York. 1996, 602 Seiten, 234 Abb. in 525 Einzeldarstellungen, 19 Boxen. Glossarium mit 752 Stichworten. Preis 44,‐DM; ISBN 3–13–383310–3

Berndt, J.: Umweltbiochemie. Uni‐Taschenbücher 1838. Gustav Fischer Verlag Stuttgart Jena. 1995; 278 S., 101 Abb., 33 Tab., Preis 34.80 DM. UTB‐ISBN 3–8252–1838–4

Kalusche, D.: Ökologie in Zahlen. Eine Datensammlung in Tabellen mit über 10.000 Einzelwerten. Gustav Fischer Verlag. Stuttgart, Jena, New York. 1996. 415 S. Preis 54, ‐DM; ISBN 3–437–20521–8  相似文献   
53.
54.
The season of birth has been suggested to influence the development of some diseases, but its role in lung fibrosis seems to not have been studied previously. The aim of this study was to investigate the relation between the season of birth and fibrotic abnormalities as detected radiologically in high‐resolution computed tomography (HRCT) among workers exposed to asbestos. The HRCT examination was performed on 528 study subjects. Multiple ordinal regression analysis adjusting for covariates was used to study the relations between birth month or season and radiological fibrosis signs. Subjects born in autumn or winter had more extensive fibrotic changes than those born in spring or summer. This applied to all fibrotic changes, apart from subpleural nodules, but only the overall fibrosis score, septal lines, and honeycombing showed statistically significantly higher values in comparison to spring births. The highest scores were detected among those born in autumn and winter months (September–February). These results suggest that there are differences in fibrotic radiological abnormalities according to the season of birth in adults exposed to asbestos. Several hypotheses could explain the observed findings, including the effects of early respiratory infections, cold temperature, and differences in air pollution levels, as well as some metabolic and hormonal effects.  相似文献   
55.
Root growth in biopores—evaluation with in situ endoscopy   总被引:1,自引:0,他引:1  

Background and aims

The significance of biopores for nutrient acquisition from the subsoil depends on root-soil contact, which in turn is influenced by root architecture. The aim of this study was to detect differences regarding the architecture and root-soil contact of homorhizous barley roots (Hordeum vulgare L.) and allorhizous oilseed rape roots (Brassica napus L.) growing in biopores.

Methods

In situ endoscopy was used as a technique that allows non-destructive display of pore wall characteristics and root morphology inside large biopores under field conditions.

Results

For both crops, about 85 % of all roots did establish contact to the pore wall. However, according to their different root architecture, the two crops varied in their strategy of resource acquisition: While barley was characterized by thin vertical or ingrowing roots, most of them in direct contact to the pore wall, oilseed rape established contact to the pore wall predominantly via lateral roots.

Conclusions

Root morphological and pore wall assessment with in situ endoscopy in combination with detailed studies of soil biochemical and soil physical parameters of the pore wall is considered an essential prerequisite for more precise future modelling of nutrient acquisition and uptake.  相似文献   
56.
57.
Leuconostoc carnosum 4010 is a protective culture for meat products. It kills the foodborne pathogen Listeria monocytogenes by producing two class IIa (pediocin-like) bacteriocins, leucocin A and leucocin C. The genes for leucocin A production have previously been characterised from Leuconostoc gelidum UAL 187, whereas no genetic studies about leucocin C has been published. Here, we characterised the genes for the production of leucocins A and C in L. carnosum 4010. In this strain, leucocin A and leucocin C operons were localised in different plasmids. Unlike in L. gelidum, leucocin A operon in L. carnosum 4010 only contained the structural and the immunity genes lcaAB without transporter genes lcaECD. On the contrary, leucocin C cluster included two intact operons. Novel genes lecCI encode the leucocin C precursor and the 97-aa immunity protein LecI, respectively. LecI shares 48 % homology with the immunity proteins of sakacin P and listeriocin. Another leucocin C operon lecXTS, encoding an ABC transporter and an accessory protein, was 97 % identical with the leucocin A transporter operon lcaECD of L. gelidum. For heterologous expression of leucocin C in Lactococcus lactis, the mature part of the lecC gene was fused with the signal sequence of usp45 in the secretion vector pLEB690. L. lactis secreted leucocin C efficiently, as shown by large halos on lawns of L. monocytogenes and Leuconostoc mesenteroides indicators. The function of LecI was then demonstrated by expressing the gene lecI in L. monocytogenes. LecI-producing Listeria was less sensitive to leucocin C than the vector strain, thus corroborating the immunity function of LecI.  相似文献   
58.

Background

The pro-inflammatory status of the elderly triggers most of the age-related diseases such as cancer and atherosclerosis. Atherosclerosis, the leading cause world wide of morbidity and death, is an inflammatory disease influenced by life-style and genetic host factors. Stimuli such as oxLDL or microbial ligands have been proposed to trigger inflammation leading to atherosclerosis. It has recently been shown that oxLDL activates immune cells via the Toll-like receptor (TLR) 4/6 complex. Several common single nucleotide polymorphisms (SNPs) of the TLR system have been associated with atherosclerosis. To investigate the role of TLR-6 we analyzed the association of the TLR-6 SNP Pro249Ser with atherogenesis.

Results

Genotyping of two independent groups with CAD, as well as of healthy controls revealed a significant association of the homozygous genotype with a reduced risk for atherosclerosis (odds ratio: 0.69, 95% CI 0.51-0.95, P?=?0.02). In addition, we found a trend towards an association with the risk of restenosis after transluminal coronary angioplasty (odds ratio: 0.53, 95% CI 0.24-1.16, P?=?0.12). In addition, first evidence is presented that the frequency of this protective genotype increases in a healthy population with age. Taken together, our results define a role for TLR-6 and its genetic variations in modulating the inflammatory response leading to atherosclerosis.

Conclusions

These results may lead to a better risk stratification, and potentially to an improved prophylactic treatment of high-risk populations. Furthermore, the protective effect of this polymorphism may lead to an increase of this genotype in the healthy elderly and may therefore be a novel genetic marker for the well-being during aging.
  相似文献   
59.

Background

Recently, Marcus et al. (Bioinformatics 30:3476–83, 2014) proposed to use a compressed de Bruijn graph to describe the relationship between the genomes of many individuals/strains of the same or closely related species. They devised an \(O(n\log g)\) time algorithm called splitMEM that constructs this graph directly (i.e., without using the uncompressed de Bruijn graph) based on a suffix tree, where n is the total length of the genomes and g is the length of the longest genome. Baier et al. (Bioinformatics 32:497–504, 2016) improved their result.

Results

In this paper, we propose a new space-efficient representation of the compressed de Bruijn graph that adds the possibility to search for a pattern (e.g. an allele—a variant form of a gene) within the pan-genome. The ability to search within the pan-genome graph is of utmost importance and is a design goal of pan-genome data structures.
  相似文献   
60.
Fish are known for their high phenotypic plasticity in life‐history traits in relation to environmental variability, and this is particularly pronounced among salmonids in the Northern Hemisphere. Resource limitation leads to trade‐offs in phenotypic plasticity between life‐history traits related to the reproduction, growth, and survival of individual fish, which have consequences for the age and size distributions of populations, as well as their dynamics and productivity. We studied the effect of plasticity in growth and fecundity of vendace females on their reproductive traits using a series of long‐term incubation experiments. The wild parental fish originated from four separate populations with markedly different densities, and hence naturally induced differences in their growth and fecundity. The energy allocation to somatic tissues and eggs prior to spawning served as a proxy for total resource availability to individual females, and its effects on offspring survival and growth were analyzed. Vendace females allocated a rather constant proportion of available energy to eggs (per body mass) despite different growth patterns depending on the total resources in the different lakes; investment into eggs thus dictated the share remaining for growth. The energy allocation to eggs per mass was higher in young than in old spawners and the egg size and the relative fecundity differed between them: Young females produced more and smaller eggs and larvae than old spawners. In contrast to earlier observations of salmonids, a shortage of maternal food resources did not increase offspring size and survival. Vendace females in sparse populations with ample resources and high growth produced larger eggs and larvae. Vendace accommodate strong population fluctuations by their high plasticity in growth and fecundity, which affect their offspring size and consequently their recruitment and productivity, and account for their persistence and resilience in the face of high fishing mortality.  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