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   290913篇
  免费   23320篇
  国内免费   156篇
  314389篇
  2018年   2423篇
  2016年   3370篇
  2015年   4653篇
  2014年   5235篇
  2013年   7849篇
  2012年   8466篇
  2011年   8680篇
  2010年   5786篇
  2009年   5263篇
  2008年   7826篇
  2007年   7923篇
  2006年   7558篇
  2005年   7291篇
  2004年   7154篇
  2003年   6933篇
  2002年   6677篇
  2001年   11826篇
  2000年   11869篇
  1999年   9399篇
  1998年   3439篇
  1997年   3673篇
  1996年   3582篇
  1995年   3168篇
  1994年   3146篇
  1993年   3140篇
  1992年   7889篇
  1991年   7870篇
  1990年   7538篇
  1989年   7490篇
  1988年   6873篇
  1987年   6612篇
  1986年   6044篇
  1985年   6218篇
  1984年   5162篇
  1983年   4350篇
  1982年   3383篇
  1981年   3153篇
  1980年   2951篇
  1979年   4962篇
  1978年   3824篇
  1977年   3730篇
  1976年   3436篇
  1975年   3791篇
  1974年   4185篇
  1973年   4101篇
  1972年   3667篇
  1971年   3437篇
  1970年   3092篇
  1969年   3002篇
  1968年   2753篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
931.
932.
Batrachochytrium dendrobatidis (Bd), the cause of a fatal fungal skin disease of amphibians that has led to massive die-offs, global declines and extinctions, has spread internationally as a pandemic clone with low genetic diversity. A need exists to develop highly polymorphic markers to determine centers of origin and patterns of spread to assist in the development of management strategies. Comparison of paralogous sequences, obtained from the 2 sequenced Bd genomes, indicates useful levels of inter-strain polymorphism in repetitive fragments. We assessed 6 repetitive loci for variation within and among Australian isolates using standard fragment analysis and capillary electrophoresis-single strand conformation polymorphism (CE-SSCP) analysis. Confirmation of inter-isolate polymorphism was achieved for 2 marker systems, highlighting the potential of repetitive loci for the development of polymorphic markers in Bd. In addition, we found that repetitive loci in Bd include possible orthologs of virulence-related genes from pathogenic fungi.  相似文献   
933.
  总被引:2,自引:0,他引:2  
Cryotherapy of shoot tips is a new method for pathogen eradication based on cryopreservation techniques. Cryopreservation refers to the storage of biological samples at ultra-low temperature, usually that of liquid nitrogen (−196°C), and is considered as an ideal means for long-term storage of plant germplasm. In cryotherapy, plant pathogens such as viruses, phytoplasmas and bacteria are eradicated from shoot tips by exposing them briefly to liquid nitrogen. Uneven distribution of viruses and obligate vasculature-limited microbes in shoot tips allows elimination of the infected cells by injuring them with the cryo-treatment and regeneration of healthy shoots from the surviving pathogen-free meristematic cells. Thermotherapy followed by cryotherapy of shoot tips can be used to enhance virus eradication. Cryotherapy of shoot tips is easy to implement. It allows treatment of large numbers of samples and results in a high frequency of pathogen-free regenerants. Difficulties related to excision and regeneration of small meristems are largely circumvented. To date, severe pathogens in banana ( Musa spp.), Citrus spp., grapevine ( Vitis vinifera ), Prunus spp., raspberry ( Rubus idaeus ), potato ( Solanum tuberosum ) and sweet potato ( Ipomoea batatas ) have been eradicated using cryotherapy. These pathogens include nine viruses (banana streak virus, cucumber mosaic virus, grapevine virus A, plum pox virus, potato leaf roll virus, potato virus Y, raspberry bushy dwarf virus, sweet potato feathery mottle virus and sweet potato chlorotic stunt virus), sweet potato little leaf phytoplasma and Huanglongbing bacterium causing 'citrus greening'. Cryopreservation protocols have been developed for a wide variety of plant species, including agricultural and horticultural crops and ornamental plants, and can be used as such or adjusted for the purpose of cryotherapy.  相似文献   
934.
BACKGROUND AND AIMS: The mid-Cretaceous is a period of sudden turnover from gymnosperm to angiosperm-dominated floras. The aim was to investigate the fossil plant ecology in order to follow the spread of angiosperm taxa. METHODS: Floristic lists and localities from the latest Albian-Cenomanian of Europe are analysed with Wagner's Parsimony Method, a clustering method currently used in phylogeny (cladistics). KEY RESULTS: Wagner's Parsimony Method points out that (a) gymnosperms dominated brackish water-related environments while angiosperms dominated freshwater-related environments (e.g. swamps, floodplains, levees, channels), (b) angiosperms showed the highest diversity in stable, freshwater-related environments, (c) a single angiosperm, 'Diospyros' cretacea, is restricted to brackish water-related environments and (d) the families Lauraceae and Platanaceae were exclusive to disturbed, braided river environments, implying a opportunist strategy for early tree angiosperms. CONCLUSIONS: During the Mid-Cretaceous, European floras were characterized by (a) coastal gymnosperms, (b) highly diversified fluvial angiosperms and (c) the first European brackish water-related angiosperm.  相似文献   
935.
  总被引:3,自引:0,他引:3  
Electrical penetration graphs (EPGs) of aphids on plants demonstrate distinct periods of lowered potential level: the potential drop (pd). Such pds are produced frequently during the stylet pathway to the phloem. Experimental evidence supports the hypothesis that the pd corresponds with a stylet puncture of a (epidermal or mesophyll) cell membrane. The intracellular potentials of-100 to-180 mV are thought to be measured thereby with the stylets acting as microelectrodes. The short pd of 5–15 s can be described as a sequence of three distinct phases. Besides the short pd, occurring during pattern B and C, long pds are produced during the complete pattern D+E. Pattern D+E may occur on the potential level of the pd (abbr.: D+E (pd)) and on the potential level of the preceding pattern C (abbr.: D+E(c)). Pattern D+E(pd) seems to be related to sieve element penetration, at least in a number of cases. Both pds, short and long, are produced on host and non-host plants, on susceptible and resistant cultivars, using several aphid species.
Potentiels membraneux comme indication pour des pénétrations intracellulaires des plantes par les stylets des aphides
Résumé L'enrégistrement électrique de la pénétration du stylet des aphides dans les tissus de la plante se caracterise par des périodes distinctes de chute de potentiel (c.p.). Ces c.p. existent fréquemment au cours du trajet du stylet vers le phloème. Une évidence expérimentale soutient l'hypothèse de la correspondance entre la c.p. et la piqûre de la membrane des cellules épidermales et parenchymateuses. Les potentiels intracellulaires de-100 à-180 mV pourraient être mesurés par l'intermédiaire des stylets comme microélectrodes. Les. c.p. brèves, de 5 à 15 s peuvent être décrites comme des séquences composées de 3 phases distinctes. Outre les c.p. brèves pendant les ondes B et C, les pucerons produisent des c.p. durables qui se maintiennent au cours des ondes D+E. Ou bien l'onde D+E peuve apparaite au niveau de potentiel de c.p. (D+E(cp)), ou au niveau de potentiel de l'onde C précédente (D+E(c)). L'onde D+E(cp) semble être reliée à la pénétration des vaisseaux conducteurs, du moins dans certain cas. Les c.p. brêves et durables sont produits sur les plantes hôtes et non-hôtes chez plusiers espèces d'aphids.
  相似文献   
936.
Primulina tabacum Hance is an endangered perennial herb distributed in calcium-rich and nitrogen-limited soil of the karst limestone areas in southern China. The morphological, ultrastructural, and physiological traits were determined for P. tabacum populations growing in three different environment conditions: twilight zone of a cave (site TZ, extremely low light intensity), at a cave entrance (site EZ, low light intensity), and in an open area (site OA, high light intensity). At site OA, P. tabacum plants were exposed to high light (635 μmol m−2 s−1 of mean daily photosynthetically active radiation) with drought stress, and expressed traits to minimize light capture and water loss. Compared to plants at sites EZ and TZ, those at site OA had thicker leaves with higher densities of stomata and pubescence, higher palisade/spongy ratio, higher light-saturated rate of net photosynthetic rate (P max), higher biomass, higher non-photochemical quenching coefficient (NPQ), and higher light saturation point (LSP) but fewer grana per chloroplast and less thylakoid stacking per granum. In contrast, P. tabacum growing at the cave vicinities: EZ (mean daily irradiance 59 μmol m−2 s−1) and TZ (mean daily irradiance 11 μmol m−2 s−1) showed typical shade-adapted characteristics for optimum light capture. The presence of sun- and shade-adapted characteristics indicates that P. tabacum has different strategies to cope with different environments but whether these strategies reflect genetic selection or phenological plasticity is yet to be determined. Such variability in physiological and morphological traits is important for the survival of P. tabacum in heterogeneous light conditions.  相似文献   
937.
Sulfation is a major modification of many molecules in eukaryotes that is dependent on the enzymatic synthesis of an activated sulfate donor, 3'-phosphoadenosine 5'-phosphosulfate (PAPS). While sulfate activation has long been assumed to occur in the cytosol, we show in this study that human PAPS synthetase 1 (PAPSS1), a bifunctional ATP sulfurylase/adenosine 5'-phosphosulfate (APS) kinase enzyme sufficient for PAPS synthesis, accumulates in the nucleus of mammalian cells. Nuclear targeting of the enzyme is mediated by its APS kinase domain and requires a catalytically dispensable 21 amino acid sequence at the amino terminus. Human PAPSS1 and Drosophila melanogaster PAPSS localize to the nucleus in yeast and relieve the methionine auxotrophy of ATP sulfurylase- or APS kinase-deficient strains, suggesting that PAPSS1 is fully functional in vivo when targeted to the nucleus. A second PAPS synthetase gene, designated PAPSS2, has recently been described, mutations of which are responsible for abnormal skeletal development in human spondyloepimetaphyseal dysplasia and murine brachymorphism. We found that PAPSS2, which localizes to the cytoplasm when ectopically expressed in mammalian cells, is relocated to the nucleus when coexpressed with PAPSS1. Taken together, these results indicate that a sulfation pathway might exist in the nucleus of eukaryotic cells. -Besset, S., Vincourt, J.-B., Amalric, F., Girard, J.-P. Nuclear localization of PAPS synthetase 1: a sulfate activation pathway in the nucleus of eukaryotic cells.  相似文献   
938.
939.
940.
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