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   3573篇
  免费   258篇
  国内免费   5篇
  3836篇
  2023年   22篇
  2022年   44篇
  2021年   56篇
  2020年   52篇
  2019年   52篇
  2018年   59篇
  2017年   53篇
  2016年   91篇
  2015年   165篇
  2014年   203篇
  2013年   268篇
  2012年   280篇
  2011年   260篇
  2010年   186篇
  2009年   150篇
  2008年   238篇
  2007年   231篇
  2006年   174篇
  2005年   179篇
  2004年   192篇
  2003年   181篇
  2002年   132篇
  2001年   39篇
  2000年   17篇
  1999年   36篇
  1998年   42篇
  1997年   29篇
  1996年   29篇
  1995年   29篇
  1994年   28篇
  1993年   18篇
  1992年   23篇
  1991年   22篇
  1990年   20篇
  1989年   25篇
  1988年   16篇
  1987年   20篇
  1986年   14篇
  1985年   12篇
  1984年   18篇
  1983年   11篇
  1982年   10篇
  1981年   9篇
  1980年   13篇
  1979年   6篇
  1978年   10篇
  1976年   9篇
  1975年   9篇
  1974年   10篇
  1973年   6篇
排序方式: 共有3836条查询结果,搜索用时 15 毫秒
11.
12.
When they were subjected to a range of physical and chemical treatments, spores of Pasteuria penetrans showed properties similar to those of other endospore-forming bacteria. The spores did not take up some stains, were resistant to desiccation and sonication and showed extrusion of spore contents ('spore popping') on prolonged exposure to 0.1% KMnO4 in 0.3 n HNO3. Calcium and dipicolinic acid (DPA) were present at concentrations of 0.28% and 0.96% of the spore dry weight respectively, giving a Ca: DPA molar ratio of 1.2. The infectivity of P. penetrans spores was reduced to a low level after heating at 100°C for 5 min, but spore attachment was not markedly affected by heating at 100°C for 15 min. Evidence for the presence of catalase in P. penetrans spores was equivocal because the low levels of catalase activity observed in spore suspensions may have been due to contamination from catalase-positive nematode tissue. When P. penetrans spores were exposed to a range of substances known to act as germinants for spores of Bacillus spp., germination or loss of refractility was not observed by phase microscopy. In vitro culture of P. penetrans was attempted by inoculating either spores or vegetative mycelial bodies onto a diverse range of simple and complex media and incubating them in aerobic, reduced oxygen, anaerobic and increased CO2 environments. Signs of spore germination or growth of vegetative stages were never observed.  相似文献   
13.
14.
Monoclonal Antibodies for Detection and Serotyping of Cucumber Mosaic Virus   总被引:3,自引:0,他引:3  
Two monoclonal antibodies (MAbs) to cucumber mosaic virus (CMV) were selected from a panel of MAbs for use in the direct DAS (double antibody sandwich)-ELISA. Two different test procedures were developed: an ELISA with polyclonal and monoclonal antibodies (mixed ELISA) for the routine detection of CMV and a MAb-ELISA with two MAbs directed against different epitopes for the specific detection of the N serotype which is prevalent in GDR. The conventional two-step incubation of plates precoated with IgG was compared with simultaneous incubation of test sample and labelled antibody (one-step incubation). The mixed ELISA proved to be more sensitive than the direct DAS-ELISA with polyclonal antisera in detecting CMV in crude sap of infected plants. On the other hand, the MAb-ELISA could be used for serotyping of CMV isolates which is important in epidemiological investigations and in resistance breeding. Both the two-step and the one-step procedures gave similar results with some advantages of the latter procedure. One-step incubation is not only time-saving but seems to be also more sensitive with regard to the detection limit. However, care must be taken to circumvent the hook-effect occurring at high virus concentrations.  相似文献   
15.
Summary We have recently demonstrated tight linkage of the Duffy blood group marker to the -spectrin gene in an extended pedigree with Charcot-Marie-Tooth neuropathy. To determine a more precise location of the Duffy blood group locus on the chromosome 1 map we have tested several more chromosome 1 genes for linkage with this marker. We found suggestive linkage with the antithrombin III and apolipoprotein A2 genes and conclusive linkage with the gene coding for -nerve growth factor.  相似文献   
16.
Summary The regulatory mutation bronze mutable 4 Derivative 6856 (bz-m4 D6856) contains a complex 6.7 kb Dissociation (Ds) element tagged with a duplication of low copy bz 3 flanking sequences (Klein et al. 1988). This creates a unique opportunity to study the transposition of a single member of the repetitive family of Ds elements. Eighteen full purple revertants (Bz alleles) of bz-m4 were characterized enzymatically and by genomic mapping. For 17 of the Bz alleles, reversion to a wild-type phenotype was caused by excision of the 6.7 kb Ds transposon. Nine of these Bz alleles retained the transposon somewhere in their genome. In this study we show that like Ac (Schwartz 1989; Dooner and Belachew 1989), the 6.7 kb Ds element can transpose within a short physical distance, both proximal and distal to its original position. Additional bz sequences have been mapped immediately distal to the mutant locus in bz-m4 D6856; genetic evidence suggests these are flanked by two additional Ds elements. The remaining Bz revertant, Bz :107, arose from excision of a more complex 13 kb Ds element.  相似文献   
17.
Summary Free-living nitrogen-fixingNostoc PCC 73102, a filamentous heterocystous cyanobacterium originally isolated from coralloid roots of the cycadMacrozamia sp., were examined for the presence of an uptake hydrogenase (H2ase) enzyme. In vivo and in vitro hydrogen uptake measurements were used to study activities and SDS-PAGE and Western immunoblots to reveal occurrence of the hydrogenase protein. Also, transmission electron microscopy and immunocytological labeling were used to study the cellular and subcellular distribution of H2ase in theNostoc cells. In vivo measurements demonstrated an active uptake of hydrogen in both light and darkness. Light stimulated in vivo hydrogen uptake with approximately 100%, and this was further doubled by increasing the pH2, from 56 to 208 M H2. An in vitro hydrogen uptake of 1.1 mol H2/ mg (protein)/h was observed when using phenazinemethosulphate as e-acceptor. Western immunoblots revealed that a polypeptide with a molecular weight of about 55 kDa was immunologically related to uptake H2ase holoenzyme purified fromAlcaligenes latus. Immunolocalization demonstrated that the H2ase protein was located both in heterocysts and vegetative cells. A higher specific labeling was associated with the cytoplasmic membranes where the vegetative cells are in contact with each other and where they actually are dividing into two vegetative cells. Using the particle analysis of an image processor, approximately equal H2ase-gold labeling per cell area was observed in the nitrogen-fixing heterocysts compared to the photosynthetic vegetative cells. This study also shows that there was no correlation between presence of phycoerythrin and uptake H2ase activity.Abbreviations H2ase hydrogenase - IgG immunoglobulin G  相似文献   
18.
Machado-Joseph disease (MJD) is an autosomal dominant spinocerebellar degeneration originally described in families of Portuguese-Azorean ancestry. The hypothesis that its present world distribution could result from the spread of an original founder mutation has been raised. To test this possibility we have conducted a linkage disequilibrium study of markers segregating with the MJD1 locus in a total of 64 unrelated families of different geographical origins. Significant association was detected between the MJD1 locus and marker alleles at loci D14S280, D14S1050 and D14S81. All affected individuals, except one Chinese family, had allele 3 (237 bp) at D14S280. This finding is consistent with a founder effect in our MJD population. However, distinct haplotypes were observed in patients originating from the two Azorean islands showing the highest disease prevalence; therefore, the possible existence of more than one founder mutation can not be excluded with the markers currently available. Received: 27 February 1996 / Revised: 4 June 1996  相似文献   
19.
20.
Over a period of 7 years the biology and phenotypic variability of Chusquea culeou were studied at 5 locations in cool temperate forests of southern Argentina. Excavated rhizomes had an average of 1.1 successful rhizome buds, and an average of 2.1 years elapsed between successive generations of rhizomes. Rhizome buds usually develop within the first four years after a rhizome forms. Height, volume and weight of a culm can be calculated from its diameter 1 m above the ground. Culm size, length of foliage leaf blades, and pattern of secondary branching differed among study sites. Dead culms were numerous and commonly remained erect for more than 7 years after dying. New culm shoots appear in spring and reach full size within a few months. Shoots can grow more than 9 cm/day. Less than half of the shoots survived a year; most were killed by moth larvae. Multiple primary branch buds emerge through the culm leaf sheaths in the second spring. The mean number of branch buds at mid-culm nodes varied between 34.8 and 81.5, and the mean number of primary branches was between 22.8 and 40.8. Number and length of branches, and number and length of foliage leaf blades at each node is related to the position of the node on a culm. Most branches grow about 3 cm and produce 1 to 3 foliage leaves annually. Foliage leaf blades generally live 2 years or more; few survive 6 years. Relative lengths of foliage leaf blades and their spacing along a branch permit recognition of annual cohorts.Both gregarious and sporadic flowering have been reported, and every year a few isolated plants flower and die. Length of the life cycle is unknown. Seedlings require up to 15 years to produce culms of mature size. Foliage branches may live more than 23 years, and culms may survive 33 years. Extensive loss of new shoots to predation suggests that gregarious flowering may be driven by a need to escape parasitism. C. culeou clumps expand slowly. Average annual rate of increase of the number of live culms in a clump was 4.6%. Methods of seed dispersal are undocumented. A dense stand of Chusquea culeou had an estimated phytomass of 179 tons/hectare (dry weight), 28% of which was underground. Net annual production was about 16 t/ha dry weight.  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