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   3849篇
  免费   329篇
  国内免费   728篇
  2024年   30篇
  2023年   120篇
  2022年   162篇
  2021年   152篇
  2020年   185篇
  2019年   165篇
  2018年   154篇
  2017年   165篇
  2016年   174篇
  2015年   170篇
  2014年   236篇
  2013年   333篇
  2012年   255篇
  2011年   247篇
  2010年   235篇
  2009年   235篇
  2008年   217篇
  2007年   209篇
  2006年   189篇
  2005年   172篇
  2004年   133篇
  2003年   125篇
  2002年   112篇
  2001年   86篇
  2000年   70篇
  1999年   64篇
  1998年   50篇
  1997年   45篇
  1996年   49篇
  1995年   46篇
  1994年   33篇
  1993年   38篇
  1992年   29篇
  1991年   20篇
  1990年   24篇
  1989年   14篇
  1988年   17篇
  1987年   17篇
  1986年   19篇
  1985年   18篇
  1984年   10篇
  1983年   10篇
  1982年   12篇
  1981年   7篇
  1980年   7篇
  1979年   6篇
  1978年   13篇
  1977年   6篇
  1975年   6篇
  1973年   6篇
排序方式: 共有4906条查询结果,搜索用时 352 毫秒
991.
The occurrence of Cassava tuber rot in regions of Kolli hills, Kollam, and Kottayam of South India, causes major economic loss up to 70% in Cassava production. The disease tuber is characterised by brown watery lesions with foul smell, making it unfit for further use. The sporangia of the pathogen were oval and ellipsoid with a short pedicle. Identification of the isolate from these regions was also confirmed by ribosomal internal transcribed spacer (ITS) of rDNA region. The pathogen was highly aggressive when pathogenicity was tested. Based on morphological, pathogenicity and ITS sequences, the pathogen was identified as Phytophthora palmivora. Development of integrated disease management practices is essential to combat the disease. This is the first report recording the spread of Cassava tuber rot disease in regions of Kolli hills of Tamil Nadu and Kollam and Kottayam, of Kerala.  相似文献   
992.
High-throughput RNA-Seq affords a cost and time effective means of obtaining large numbers of genetic markers for aquatic genomics. Here, we present thousands of novel microsatellite loci developed for the pearl oyster, Pinctada martensii from the Illumina HiSeq™ 2000 library of the pearl sac. Free user-friendly bioinformatics tools were employed to screen for microsatellite loci and design appropriate primers in 102,762 unigenes with 7216 microsatellite loci identified in total, 4862 of which had flanking sequences suitable for polymerase chain reaction primer design. The 50 randomly chosen primer pairs were tested in two populations of pearl oyster (base population (POP1) and selected population (POP2), with 30 individuals of each population). All the primer pairs were amplified successfully in two populations. All loci were polymorphic in POP1, while there were 3 loci showing monomorphism in POP2. In POP1 and POP2, observed heterozygosity from 0.033 to 1.000 and 0.000 to 1.000, 19 and 16 microsatellite loci deviated significantly from Hardy–Weinberg expectations including a Bonferroni correction (P < 0.001). Thirteen loci were highly informative content (PIC ≥ 0.5) in both populations. These identified loci will be useful for potential application for evolutionary, population genetic and chromosome linkage mapping research on pearl oyster.  相似文献   
993.
England  K. W. 《Hydrobiologia》1991,(1):691-697
A brief historical review of nematocyst terminology is given and three nomenclatural problems are discussed. It is proposed to combine the terms initiated by Weill (1934) with those of Schmidt (1969). A new mesobasic grade, intermediate between microbasic and macrobasic is proposed for amastigophores and p-mastigophores possessing a short Faltstück. A more liberal interpretation of Weill's (1934) terminology for nematocysts than that proposed by Cutress (1955) is suggested in respect of microbasic amastigophores and p-mastigophores. Basitrichs and b-mastigophores continue to be recognized as separate categories.deceased  相似文献   
994.
995.
In this communication, we used electrochemical sensor for recording the electrochemical profiles of eleven species of Yulania spp. from leaf extract. Two solvents and two buffer conditions were used for electrochemical fingerprints collection. Their electrochemical fingerprints can be converted to different patterns and consequently for species recognition. The results indicate the pattern recognition is much convenient than that of the recognition of species directly using voltammetric signal. The current information in electrochemical fingerprinting represents the type and amount of electrochemically active molecules, which linked to the genetic differences among the plants. Therefore, the electrochemical fingerprints were applied for further phylogenetic study. The phylogenetic tree deduced from voltametric curves is divided into three main groups. The first clade contains Y. denudate, Liriodendron chinense, Y. cylindrica, Y. biondii, Y. sprengeri. The second clade contains Y. zenii, Y. liliiflora, Y. kobus, and Y. amoena. The third clade contains Y. × soulangeana, Manglietia fordiana and Y. sinostellata. In addition, Y. salicifolia is not in these main clades. The results demonstrate that electrochemical fingerprinting can be used as a com-plementary tool in the study of phylogenetics.  相似文献   
996.
瓦山锥植物富含植物多酚类成分且资源丰富,目前尚无该植物化学成分及生物活性方面的报道。为了明确瓦山锥的物质基础,为该植物资源的合理开发与可持续利用提供科学依据,该研究采用Sephadex LH-20、Diaion HP20SS、Toyopearl HW-40F等多种柱层析方法对瓦山锥树叶乙醇提取物进行分离纯化,从中得到11个单体化合物,它们的结构经波谱数据分析及文献对照鉴定为没食子酸(1)、咖啡酸(2)、1-(3',4'-二羟基肉桂酰)-环戊-2,3-二酚(3)、绿原酸(4)、绿原酸甲酯(5)、kaempferol 3-O-β-D-glucuronopyranoside(6)、kaempferol 3-O-{β-D-xylopyranosyl-(1→2)-[α-L-rhamnopyranosyl-(1→6)]-β-D-glucopyranoside}(7)、quercetin 3-O-β-D-glucuronopyranoside(8)、quercetin 3-O-β-glucuronide-6″-methyl ester(9)、芦丁(10)、quercetin5-O-[α-L-rhamnopyranosyl-(1→6)]-β-D-glucopyranoside(11)。所有化合物均为首次从瓦山锥中分离得到,其中化合物3,7-11为首次从锥属植物中分离得到。  相似文献   
997.
998.
999.
A chemical investigation of Musineon divaricatum has resulted in the isolation of 19 coumarins, five of which are novel compounds; 18 are khellactone derivatives. The coumarins were isolated from the crude extracts by a combination of adsorption chromatography, gel permeation and HPLC; the more successful HPLC separations utilized a nitrile bonded phase column. The structures were determined by 1H NMR and mass spectral studies and by comparisons with literature data. The relative configuration for the entire series was secured from 1H NMR data, while the absolute configuration could be assigned with any certainty only in the cases of two of the compounds.  相似文献   
1000.
The taxonomy of ceratioid fishes is challenging and frequently based on a small number of female specimens described for each species. Twenty ceratioid specimens caught on Flemish Cap and Grand Bank (western North Atlantic), representing 12 species and six families: Ceratias holboelli and Cryptopsaras couesii (Ceratiidae); Himantolophus albinares (Himantolophidae); Melanocetus johnsonii (Melanocetidae); Lophodolos acanthognathus, Oneirodes eschrichtii, Dolopichthys karsteni, and Leptacanthichthys gracilispinis (Oneirodidae); Caulophryne polynema (Caulophrynidae); and Haplophryne mollis, Linophryne brevibarbata, and L. bicornis (Linophrynidae) were identified by examination of morphological characters. DNA barcode sequences, from the 5′ end of the COI mitochondrial gene, were developed for 18 specimens and compared with all ceratioid barcode sequences available in public repositories. The analyses extended the ranges of some quantitative traits for certain species, highlighted the possible existence of cryptic species in Ccouesii with distinct ranges in the Atlantic and Pacific oceans, and indicated a close relationship between Bertella and Dolopichthys meriting further attention. The authoritative identification of the 18 voucher specimens made possible detection of erroneous identifications of some sequences extracted from the repositories and highlighting of taxonomic conflicts that should be the subject of future studies.  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