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   103437篇
  免费   7609篇
  国内免费   258篇
  111304篇
  2012年   12383篇
  2011年   13759篇
  2010年   2072篇
  2009年   950篇
  2008年   10259篇
  2007年   10554篇
  2006年   9688篇
  2005年   9085篇
  2004年   8528篇
  2003年   7808篇
  2002年   6608篇
  2001年   5148篇
  2000年   6579篇
  1999年   2605篇
  1998年   302篇
  1997年   194篇
  1996年   138篇
  1995年   145篇
  1994年   139篇
  1993年   137篇
  1992年   121篇
  1991年   104篇
  1990年   74篇
  1989年   104篇
  1988年   111篇
  1987年   90篇
  1986年   82篇
  1985年   61篇
  1984年   68篇
  1983年   75篇
  1982年   62篇
  1981年   49篇
  1979年   34篇
  1978年   32篇
  1974年   38篇
  1972年   42篇
  1971年   36篇
  1970年   45篇
  1959年   135篇
  1958年   315篇
  1957年   270篇
  1956年   220篇
  1955年   247篇
  1954年   260篇
  1953年   222篇
  1952年   186篇
  1951年   209篇
  1950年   180篇
  1949年   80篇
  1948年   80篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
141.
142.
Discovery of two ill horses and three dogs naturally infected with Trypanosoma evansi near an experimental station in the Eastern Plains of Colombia led to a search for reservoir hosts of the parasite. Infection was detected in 8/33 healthy capybaras (Hydrochoerus hydrochaeris), none of the remaining 14 horses, and none of 32 Zebu cattle (Bos indicus), 18 paca (Cuniculus paca) and 20 spiny rats (Proechimys sp.). Contrary to common opinion, the results indicated a carrier state in the capybara. Diagnosis was based on morphology, behaviour in albino rats, and pathogenicity and host range in domestic animals.  相似文献   
143.
A gastric ulcer was diagnosed in a South American Sea Lion, Otario byronia, captured along the northern coast of Chile. Species of Anisakis and Phocanema were found to be associated with the lesions. A discussion of the histopathologic damage observed is presented, along with probable etiology of the ulcer.  相似文献   
144.
145.
The objectives of this research were to determine the variation of chemical composition across botanical fractions of cornstover, and to probe the potential of Fourier transform near-infrared (FT-NIR) techniques in qualitatively classifying separated cornstover fractions and in quantitatively analyzing chemical compositions of cornstover by developing calibration models to predict chemical compositions of cornstover based on FT-NIR spectra. Large variations of cornstover chemical composition for wide calibration ranges, which is required by a reliable calibration model, were achieved by manually separating the cornstover samples into six botanical fractions, and their chemical compositions were determined by conventional wet chemical analyses, which proved that chemical composition varies significantly among different botanical fractions of cornstover. Different botanic fractions, having total saccharide content in descending order, are husk, sheath, pith, rind, leaf, and node. Based on FT-NIR spectra acquired on the biomass, classification by Soft Independent Modeling of Class Analogy (SIMCA) was employed to conduct qualitative classification of cornstover fractions, and partial least square (PLS) regression was used for quantitative chemical composition analysis. SIMCA was successfully demonstrated in classifying botanical fractions of cornstover. The developed PLS model yielded root mean square error of prediction (RMSEP %w/w) of 0.92, 1.03, 0.17, 0.27, 0.21, 1.12, and 0.57 for glucan, xylan, galactan, arabinan, mannan, lignin, and ash, respectively. The results showed the potential of FT-NIR techniques in combination with multivariate analysis to be utilized by biomass feedstock suppliers, bioethanol manufacturers, and bio-power producers in order to better manage bioenergy feedstocks and enhance bioconversion.  相似文献   
146.
147.
The importance of DNA methylation in mammalian and plant systems is well established. In recent years there has been renewed interest in DNA methylation in insects. Accumulating evidence, both from mammals and insects, points towards an emerging role for DNA methylation in the regulation of phenotypic plasticity. The migratory locust (Locusta migratoria) is a model organism for the study of phenotypic plasticity. Despite this, there is little information available about the degree to which the genome is methylated in this species and genes encoding methylation machinery have not been previously identified. We therefore undertook an initial investigation to establish the presence of a functional DNA methylation system in L. migratoria. We found that the migratory locust possesses genes that putatively encode methylation machinery (DNA methyltransferases and a methyl-binding domain protein) and exhibits genomic methylation, some of which appears to be localised to repetitive regions of the genome. We have also identified a distinct group of genes within the L. migratoria genome that appear to have been historically methylated and show some possible functional differentiation. These results will facilitate more detailed research into the functional significance of DNA methylation in locusts.  相似文献   
148.
The postural and locomotor activity and its relation to substrates was observed in 3 Saguinus oedipus, and 3 Saimiri sciureus for comparison, during a period of 10 h for each individual. The animals moved freely in cages of 3.40 m x 3.40 m x 2.40 m (height) on rather diverse substrates. Observations were made according to the focal-animal-method, with combined instantaneous and continuous sampling. They were protocoled in schematic form and video-recorded. In addition, 3 further Saguinus oedipus were subjected to an X-ray cinematographic study on a modified treadmill to unveil metric parameters of the locomotor pattern preferred on slender and compliant ("arboreal") substrates, the walk. Independent from the substrates, the postures of the two species differed in details, as do the preferred substrates. Horizontal, comfortable substrates are favored most. Walking ranked top in frequency, followed by jumping and galloping (in a strict sense). All other locomotor modes described for primates played a minor role or lacked entirely, like the trot. Average distance of leaps was only 0.60 m, landings were mainly on the same level as take-offs. In Saguinus, the movements of both limbs, including the shoulder blade, followed the pattern common to small mammals in general: At the end of the stance phase, humerus and tibia are nearly parallel to the substrate, while just before touchdown ulna and femur are in this position. The walk in both species was surprisingly fast (1 m/s), reaching the speed of much larger cursorial animals, like humans.  相似文献   
149.
150.
Tan M  Jing T  Lan KH  Neal CL  Li P  Lee S  Fang D  Nagata Y  Liu J  Arlinghaus R  Hung MC  Yu D 《Molecular cell》2002,9(5):993-1004
ErbB2 overexpression confers resistance to taxol-induced apoptosis by inhibiting p34(Cdc2) activation. One mechanism is via ErbB2-mediated upregulation of p21(Cip1), which inhibits Cdc2. Here, we report that the inhibitory phosphorylation on Cdc2 tyrosine (Y)15 (Cdc2-Y15-p) is elevated in ErbB2-overexpressing breast cancer cells and primary tumors. ErbB2 binds to and colocalizes with cyclin B-Cdc2 complexes and phosphorylates Cdc2-Y15. The ErbB2 kinase domain is sufficient to directly phosphorylate Cdc2-Y15. Increased Cdc2-Y15-p in ErbB2-overexpressing cells corresponds with delayed M phase entry. Expressing a nonphosphorylatable mutant of Cdc2 renders cells more sensitive to taxol-induced apoptosis. Thus, ErbB2 membrane RTK can confer resistance to taxol-induced apoptosis by directly phosphorylating Cdc2.  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