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   141篇
  免费   7篇
  148篇
  2023年   2篇
  2022年   2篇
  2021年   2篇
  2020年   1篇
  2019年   1篇
  2018年   3篇
  2017年   4篇
  2016年   8篇
  2015年   6篇
  2014年   7篇
  2013年   9篇
  2012年   9篇
  2011年   8篇
  2010年   11篇
  2009年   8篇
  2008年   3篇
  2007年   12篇
  2006年   8篇
  2005年   7篇
  2004年   2篇
  2003年   4篇
  2002年   3篇
  2001年   1篇
  2000年   2篇
  1995年   1篇
  1993年   2篇
  1992年   2篇
  1987年   1篇
  1986年   1篇
  1981年   1篇
  1979年   1篇
  1977年   2篇
  1975年   1篇
  1974年   1篇
  1973年   3篇
  1972年   2篇
  1970年   1篇
  1969年   1篇
  1961年   1篇
  1957年   1篇
  1944年   1篇
  1928年   2篇
排序方式: 共有148条查询结果,搜索用时 12 毫秒
41.
A simple and highly sensitive high-performance liquid chromatographic (HPLC) method for the simultaneous determination of cis(Z)-clopenthixol and trans(E)-clopenthixol in human plasma has been developed. The chromatographic analysis was carried out isocratically on a reversed-phase column (C(8) 150 x 4.6 mm I.D., 5 microm) using a mixture of 25 mM phosphate buffer and acetonitrile (65:35 v/v, pH* 3.0) as the mobile phase, and ultraviolet detection at 230 nm. Plasma sample pretreatment was accomplished by means of an original solid-phase extraction (SPE) procedure carried out on cyanopropyl cartridges, with a high extraction yield and good selectivity. Under the optimum conditions, calibration graphs of spiked human plasma samples were obtained over the concentration ranges 1-300 ng ml(-1) for cis(Z)-clopenthixol and 1-200 ng ml(-1) for trans(E)-clopenthixol. The limit of detection (LOD) was 0.3 ng ml(-1) for both cis(Z)- and trans(E)-isomers of clopenthixol. The method was successfully applied to the determination of cis(Z)-clopenthixol and trans(E)-clopenthixol in plasma samples of schizophrenic patients undergoing therapy with zuclopenthixol.  相似文献   
42.
A rapid and feasible method was developed for the analysis of carbamazepine and its five metabolites (10,11-dihydro-10,11-epoxycarbamazepine, 10,11-dihydro-10,11-dihydroxycarbamazepine, 10,11-dihydro-10-hydroxycarbamazepine, 2-hydroxycarbamazepine and 3-hydroxycarbamazepine) in human plasma. Separation of the analytes is based on micellar electrokinetic chromatography, in untreated fused-silica capillary (48.5/40.0 cm length, 50 microm I.D.) with phosphate buffer (30 mM, pH 8.00) as background electrolyte, containing 50 mM sodium dodecylsulfate, and methanol (15%, v/v) as organic modifier. Clean up of human plasma samples was carried out by means of a solid-phase extraction procedure, which gave a high extraction yield for all six carbamazepines (>88%). The overall precision of the method gives a mean RSD of about 1.8%. The limit of quantitation for all analytes is < or = 0.30 microg ml(-1), the limit of detection < or = 0.12 microg ml(-1).  相似文献   
43.
Solid-media agar-plate tests were investigated for their potential to evaluate the biodegradation of polymeric particles by certain strains of microorganisms. A reliable test was developed. Stable suspensions of 1-m globules of two poly(3-hydroxyalkanoates) were prepared. These were mixed with liquified agar-agar to form turbid plates. Inoculation was carried out by plate puncturing. Seven strains isolated from sewage sludge formed clear zones. These zones were distinct, circular, and reproducible in outlook and growth. The maximal duration of single experiments was 192 h. A growth rate was defined. It allowed the classification of the polymers, and of the microorganisms used. Poly(3-hydroxybutyrate-co-valerate) was degraded faster by all the strains used than was the homopolyester poly(3-hydroxybutyrate). Strains achieving high zone growth rates generally did so on both of the polymers. This led to the assumption of similar enzymatic processes being active. Zone formation was concluded to be dependent on the amount of enzyme production plus the enzyme activity. The technique provides a means of either distinguishing the degradation abilities of a microorganisms, or the degradability of materials. Correspondence to: R.-J. Müller  相似文献   
44.
Résumé Une polyédrose nucléaire de la tordeuse de la pelure,Pandemis heparana Denis & Schiffermuller (Lep.: Tortricidae) due à un baculovirus isolé dans un verger de pommiers en France est étudiée. Le baculovirus, uniquement actif sur les larves nouvelles nées, induit un arrêt de développement des larves au stade L3 et une mortalité ultérieue des larves aux stades L5 et L6. L'étude en microscopie optique et en microscopie électronique des larves L5 et L6 présentant des sympt?mes très prononcés de la viroise (début de paralysie et couleur blanchatre des segments abdominaux) montre que c'est essentiellement le tissu adipeux et accessoirement certains hémocytes et quelques cellules de l'hypoderme qui sont le siège de la multiplication virale. La morphogénèse virale dans les cellules du tissu adipeux est décrite en détail. Certains aspects de l'étude ultrastructurale liés à la forme en massue des virions, au processus de cristallisation des polyèdres et à l'accumulation de masses fibrillaires viro-induites sont discutés.
Summary A nuclear polyhedrosis virus infecting the torticidPandemis heparana Denis & Schiffermuller, collected in an apple-orchard in France was isolated and studied. The baculovirus is active only in the newly hatched larvae. It induces a delayed arrest of development in the L3 larvae and subsequently the death of L5 and L6 larvae. Study by light and electron microscopy of L5 and L6 larvae exhibiting very marked symptoms of the virus disease (early stages of paralysis and whitish colour of abdominal segments) shows that the site of multiplication of the virus is mainly located in the fat body cells and occasionally in some hemocytes or hypoderm cells. Virus morphogenesis within the fat body cells is described in detail. Some aspects of the ultrastructure such as clubbed virions, process of crystallization of polyhedral bodies and clumps of fibrous material induced by the virus, are discussed.
  相似文献   
45.
The largest anuran diversity belongs to the Neobatrachia, which harbor more than five thousand extant species. Here, we propose a new hypothesis for the historical aspects of the neobatrachian evolution with a formal biogeographical analysis. We selected 12 genes for 144 neobatrachian genera and four archaeobatrachian outgroups and performed a phylogenetic analysis using a maximum likelihood algorithm with the rapid bootstrap test. We also estimated divergence times for major lineages using a relaxed uncorrelated clock method. According to our time scale, the diversification of crown Neobatrachia began around the end of the Early Cretaceous. Our phylogenetic tree suggests that the first split of Neobatrachia is related to the geological events in the Atlantic and Indian Oceans. Hence, we propose names for these clades that indicate this connection, i.e., Atlanticanura and Indianura. The Atlanticanura is composed of three major neobatrachian lineages: Heleophrynidae, Australobatrachia and Nobleobatrachia. On the other hand, the Indianura consists of two major lineages: Sooglossoidea and Ranoides. The biogeographical analysis indicates that many neobatrachian splits occurred as a result of geological events such as the separation between South America and Africa, between India and the Seychelles, and between Australia and South America.  相似文献   
46.
Scattered trees support high levels of farmland biodiversity and ecosystem services in agricultural landscapes, but they are threatened by agricultural intensification, urbanization, and land abandonment. This study aimed to map and quantify the decline of orchard meadows (scattered fruit trees of high nature conservation value) for a region in Southwestern Germany for the 1968 2009 period and to identify the driving forces of this decline. We derived orchard meadow loss from 1968 and 2009 aerial images and used a boosted regression trees modelling framework to assess the relative importance of 18 environmental, demographic, and socio-economic variables to test five alternative hypothesis explaining orchard meadow loss. We found that orchard meadow loss occurred in flatter areas, in areas where smaller plot sizes and fragmented orchard meadows prevailed, and in areas near settlements and infrastructure. The analysis did not confirm that orchard meadow loss was higher in areas where agricultural intensification was stronger and in areas of lower implementation levels of conservation policies. Our results demonstrated that the influential drivers of orchard meadow loss were those that reduce economic profitability and increase opportunity costs for orchards, providing incentives for converting orchard meadows to other, more profitable land uses. These insights could be taken up by local- and regional-level conservation policies to identify the sites of persistent orchard meadows in agricultural landscapes that would be prioritized in conservation efforts.  相似文献   
47.

Background

Apomixis is an intriguing trait in plants that results in maternal clones through seed reproduction. Apomixis is an elusive, but potentially revolutionary, trait for plant breeding and hybrid seed production. Recent studies arguing that apomicts are not evolutionary dead ends have generated further interest in the evolution of asexual flowering plants.

Results

In the present study, we investigate karyotypic variation in a single chromosome responsible for transmitting apomixis, the Apospory-Specific Genomic Region carrier chromosome, in relation to species phylogeny in the genera Pennisetum and Cenchrus. A 1 kb region from the 3' end of the ndhF gene and a 900 bp region from trnL-F were sequenced from 12 apomictic and eight sexual species in the genus Pennisetum and allied genus Cenchrus. An 800 bp region from the Apospory-Specific Genomic Region also was sequenced from the 12 apomicts. Molecular cytological analysis was conducted in sixteen Pennisetum and two Cenchrus species. Our results indicate that the Apospory-Specific Genomic Region is shared by all apomictic species while it is absent from all sexual species or cytotypes. Contrary to our previous observations in Pennisetum squamulatum and Cenchrus ciliaris, retrotransposon sequences of the Opie-2-like family were not closely associated with the Apospory-Specific Genomic Region in all apomictic species, suggesting that they may have been accumulated after the Apospory-Specific Genomic Region originated.

Conclusions

Given that phylogenetic analysis merged Cenchrus and newly investigated Pennisetum species into a single clade containing a terminal cluster of Cenchrus apomicts, the presumed monophyletic origin of Cenchrus is supported. The Apospory-Specific Genomic Region likely preceded speciation in Cenchrus and its lateral transfer through hybridization and subsequent chromosome repatterning may have contributed to further speciation in the two genera.  相似文献   
48.
49.
The particular combination of polydeoxyribonucleotides, l-carnitine, calcium ions, proteolytic enzyme and other ingredients acts in a synergetic way in the regeneration of skin and connective tissues. This new formulation of active principles was tested in vitro as a cell and tissue culture medium and in vivo for various preparations in support of tissue regeneration. In vitro, the new blend allowed the maintenance of skin biopsies for more than 1 year in eutrophic conditions. Immunocytochemical analyses of fibroblasts isolated from these biopsies confirmed a significant increase of the epidermal and connective wound-healing markers such as collagen type I, collagen type IV, cytokeratin 1 (CK1), CK5, CK10 and CK14 versus controls. To examine the effects of the new compound in vivo, we studied impaired wound healing in genetically diabetic db/db mice. At day 18, diabetic mice treated with the new composition showed 100% closure of wounds and faster healing than mice treated with the other solutions. This complex of vital continuity factors or life-keeping factors could be used as a tissue-preserving solution or a cosmetic/drug/medical device to accelerate wound healing in the treatment of patients with deficient wound repair to promote the regeneration of cutaneous and connective tissues (injuries-wound, dermatitis) and prevent the recurrent relapses.  相似文献   
50.
The Hindak strain of a Cryptomonas species (Cryptophyceae) produces extracellular polysaccharides. Because there is no information on the structure of these compounds in the Cryptophyceae we conducted structural studies. Gas–liquid chromatographic analyses showed that the polysaccharide is composed of fucose, rhamnose, xylose, mannose, glucose, galactose, galacturonic acid, glucuronic acid, and traces of 3-O-methyl galactose. The polysaccharide was separated into two subtractions by ion-exchange chromatography. Fraction A consisted mainly of 1,3-linked galactose units and 1,4-linked galacturonic acid. Unlike fraction B, fraction A did not have xylose, 3-O-methyl galactose, or glucuronic acid. Also, its degree of branching was low compared to that of fraction B. Only traces of sulfate were present infraction A, but fraction B was 10–15% sulfated. Protein was approximately 1% in both fractions. These polysaccharides appear to be a novel type of polymer in algae.  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