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   449篇
  免费   24篇
  国内免费   9篇
  2021年   3篇
  2020年   5篇
  2018年   12篇
  2017年   10篇
  2016年   6篇
  2015年   10篇
  2014年   6篇
  2013年   13篇
  2012年   17篇
  2011年   15篇
  2010年   16篇
  2009年   19篇
  2008年   19篇
  2007年   13篇
  2006年   12篇
  2005年   6篇
  2004年   11篇
  2003年   6篇
  2002年   3篇
  2001年   9篇
  2000年   4篇
  1999年   11篇
  1998年   8篇
  1997年   10篇
  1996年   11篇
  1995年   3篇
  1994年   5篇
  1992年   3篇
  1991年   3篇
  1989年   2篇
  1983年   4篇
  1981年   2篇
  1980年   3篇
  1979年   2篇
  1978年   5篇
  1974年   3篇
  1972年   2篇
  1959年   7篇
  1958年   23篇
  1957年   26篇
  1956年   26篇
  1955年   22篇
  1954年   22篇
  1953年   13篇
  1952年   13篇
  1951年   10篇
  1950年   9篇
  1949年   2篇
  1948年   2篇
  1912年   2篇
排序方式: 共有482条查询结果,搜索用时 15 毫秒
81.
82.
We used partial sequences of mitochondrial 16S and cytochrome oxidase I genes to perform a phylogenetic study of collared frogs (Anura: Aromobatidae: Mannophryne ), a genus endemic to Venezuela and the islands of Trinidad and Tobago. We analysed 1.2 kb from 13 of the 15 described species of Mannophryne . Maximum parsimony, maximum likelihood and Bayesian analyses support the monophyly of Mannophryne . Mannophryne consists of three deeply differentiated clades that split from each other in a relatively short period of time. The diversification of Mannophryne occurred well before the glacial-interglacial periods of the Quaternary. Our data support the taxonomic validity of M. olmonae , a species endemic to Tobago Island. Mannophryne olmonae is more closely related to the continental species Mannophryne riveroi than to the Trinidad island endemic Mannophryne trinitatis . As in most tropical clades of frogs, molecular evidence indicates that species richness in Mannophryne is largely underestimated and, consequently, current priorities for conservation are inadequate.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 185–199.  相似文献   
83.
A multivariate morphometric study of Daniellia, an endemic genus of tropical and subtropical Africa, indicates that nine species may be recognized: D. alsteeniana, D. klainei, D. oblonga, D. ogea, D. oliveri, D. pilosa, D. pynaertii, D. soyauxii and D. thurifera. In our study we found that some characters, not previously studied in detail, were significant in species delimitation: petiole indumentum, petiole width, number and position of glands on the lower surface of the leaflets and presence or absence of glands at the insertion of each pair of leaflets. The rare and scattered material of D. pilosa and D. soyauxii made their classification uncertain, although some qualitative characters support their differentiation. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 159 , 268–279.  相似文献   
84.
Behçet's disease is a complex vasculitis of unknown etiology. Abundant neutrophils suggest the involvement of innate immunity. Cytokines are skewed to the T-helper-1 pattern. Few sterile organs are easily accessible for analysis in Behçet's disease. Cañete and coworkers identify inflamed joints as a feasible model and suggest the involvement of innate immunity in Behçet's disease.  相似文献   
85.
This study provides a checklist of the Caesalpinioideae (Leguminosae) present in Equatorial Guinea, comprising 52 genera and 124 taxa. Seven species are known from Annobón, 33 from Bioko and 109 from Río Muni. The best represented genus is Senna with eight species. In addition, bibliographic references for Caesalpinioideae (Leguminosae) from Equatorial Guinea have been gathered and checked. Fourteen species are included based on literature records, because their distribution ranges suggest they may occur in Equatorial Guinea, 11 introduced species could be naturalized, and 45 taxa are recorded for the first time from the country. This represents an increase of over 35% in the floristic knowledge of Caesalpinioideae from Equatorial Guinea. A statistical summary is presented at the end of the checklist.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 151 , 541–562.  相似文献   
86.
87.
The reaction of purine nucleobases (adenine, 3-methyladenine and 9-methylguanine) with a metallic salt in the presence of potassium oxalate yields three compounds with formulae {[Cd(μ-ox)(H2O)(Hade)]·H2O}n (1), {[Cu(μ-ox)(H2O)(3Meade)]·H2O}n (2) and [Cu(ox)(H2O)2(9Megua)]·2.5H2O (3). Crystal structures of compounds 1-2 consist of 1D zig-zag chains in which cis-[M(H2O)(nucleobase)]2+ fragments are linked by bis-bidentate oxalato ligands. In compound 1, the nucleobase is coordinated through the minor groove N3 atom, and the resulting non-canonical 7H-adenine tautomer is stabilized by non-covalent interactions involving more basic N9 and N7 sites. In compound 2, the mutagenic 3-methyladenine is attached to the metal atoms by means of the imidazole N7 atom. The dissimilar binding pattern of the nucleobases produces significant differences in the supramolecular architectures of compounds 1 and 2 which are essentially governed by an extensive network of non-covalent interactions such as hydrogen bonded adenine-adenine base pairs, hydration of the nucleobases, carboxylato-nucleobase associations, and face-to-face π-π stacking. The model 9-methylguanine nucleobase of compound 3 exhibits its usual coordination mode through the major groove N7 atom to form two monomeric [(Cu(ox)(H2O)2(9Megua)] units which are held together by means of Watson-Crick like hydrogen bonds between the guanine moieties and the inorganic frameworks generating almost planar tetrameric metal-organic aggregates. The 3D packing of the complex entities affords an open structure containing voids which are filled by decameric (H2O)10 clusters. Variable-temperature magnetic susceptibility measurements of compound 2 show the occurrence of antiferromagnetic intrachain interactions in good agreement with the structural features of its 1D metal-oxalato framework.  相似文献   
88.
Laparoscopic artificial insemination has an important role in felid conservation but it is costly and includes surgical risk. Therefore, radiographic contrast medium combined with non-surgical transcervical AI to verify intrauterine gamete placement could be a viable alternative. Gamete-rescued fresh and frozen-thawed sperm were extended with one of two commercial contrast media (nonionic and ionic), with osmolarity adjusted to 320-330 mOsm, or feline optimized culture medium (control). Percent motility, forward progression status, and acrosomal integrity were recorded every 30 min for 4 h. Sperm penetration abilities were assessed by coincubating treated sperm with conspecific in vitro matured oocytes for 18 to 20 h, and presumptive zygotes and embryos were fixed and stained to determine sperm penetration and fertilization rate. There was reduced motility and acrosomal integrity in frozen-thawed versus fresh sperm (P < 0.05). Neither radiographic contrast medium induced adverse effects on fresh sperm motility relative to control medium (P > 0.05), but motility of frozen-thawed sperm decreased when treated with nonionic radiographic contrast medium compared to control medium (P < 0.05). There were no differences in acrosomal integrity between radiographic contrast and control media in fresh (P > 0.05) or frozen sperm (P > 0.05). Neither radiographic contrast media decreased the numbers of morphologically normal sperm (P > 0.05) or reduced the ability of domestic cat sperm to penetrate (P > 0.05) or fertilize (P > 0.05) conspecific oocytes. Ionic radiographic contrast medium can be added to fresh or frozen-thawed domestic cat sperm with no adverse effect on motility, morphology, acrosomal integrity or oocyte penetration rates, and thus may be used to facilitate further development of transcervical AI procedures.  相似文献   
89.
90.
本研究旨在探讨谷糠结合态多酚(bound phenol of inner shell,BPIS)发挥抗乳腺癌细胞活性的作用机制。首先采用细胞计数法检测BPIS对乳腺癌细胞以及正常乳腺细胞活性的影响;然后综合运用SEA、SIB以及GeneCards等数据库获得BPIS和乳腺癌的相关靶点,并分析活性成分与作用靶点的互作网络以及通路。本研究筛选得到BPIS抗乳腺癌相关靶点39个,主要涉及糖脂代谢和细胞自噬等生物过程以及MAPK、PI3K/AKT、FoxO等多条信号通,表明BPIS抗乳腺癌是多成分、多靶点、多通路协同作用的过程,而与细胞死亡相关的细胞自噬很可能在BPIS抑制乳腺癌过程中发挥主要作用。  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