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   38407篇
  免费   65篇
  国内免费   94篇
系统科学   190篇
丛书文集   862篇
教育与普及   107篇
理论与方法论   239篇
现状及发展   17143篇
研究方法   1540篇
综合类   17998篇
自然研究   487篇
  2013年   252篇
  2012年   509篇
  2011年   1028篇
  2010年   227篇
  2008年   654篇
  2007年   673篇
  2006年   709篇
  2005年   716篇
  2004年   627篇
  2003年   673篇
  2002年   691篇
  2001年   1118篇
  2000年   1029篇
  1999年   689篇
  1992年   652篇
  1991年   542篇
  1990年   567篇
  1989年   579篇
  1988年   584篇
  1987年   586篇
  1986年   592篇
  1985年   738篇
  1984年   569篇
  1983年   486篇
  1982年   413篇
  1981年   407篇
  1980年   532篇
  1979年   1202篇
  1978年   1034篇
  1977年   1019篇
  1976年   719篇
  1975年   769篇
  1974年   1152篇
  1973年   969篇
  1972年   1014篇
  1971年   1223篇
  1970年   1608篇
  1969年   1238篇
  1968年   1213篇
  1967年   1226篇
  1966年   1028篇
  1965年   743篇
  1964年   208篇
  1959年   445篇
  1958年   685篇
  1957年   574篇
  1956年   488篇
  1955年   414篇
  1954年   458篇
  1948年   282篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
111.
Summary An ectomycorrhiza, a specialized root organ, is the result of a complex interaction leading to a finely-tuned symbiosis between a plant and a compatible ectomycorrhizal fungus. Ultrastructural observations combined with cytochemical and biochemical studies reveal that structural and metabolic changes in the symbiont cells lead to the final phenotype of the active ectomycorrhiza. In the present review these changes are interpreted as changes in gene expression and discussed within the context of ectomycorrhiza development. Recent genetic data indicate that the continued vegetative growth of the ectomycorrhizal hyphae and the root tissues, and their ability to switch to symbiotic organ formation, is basically controlled by developmentally critical genes. The activity of these symbiotic genes during the differentiation of ectomycorrhizas is associated with extensive changes in the concentration of particular polypeptides and protein biosynthesis. The present state of knowledge about the developmental biology of ectomycorrhizas allows only speculation about the events during their development.Puisant mes forces aux sources des galaxies En buvant la sève des arbres M. Jonasz  相似文献   
112.
113.
As there would be appear to be no limit to the process of successive emergence, the emergence of suprahuman autopoietic systems seems possible, if not yet necessary. Maybe there really are constraints on human behavior which would not permit such entities to emerge, or maybe the emergence of such entities entails the dissipation of more free energy than available at present. But unless such conditions are present, then, it appears to me, the emergence of suprahuman autopoietic systems is not just probable, it is inevitable.In any event, whether or not they share my concern about suprahuman autopoietic systems, all concerned with metaphor manufacture should attend to this important insight from Swenson (1989a, b, c, d) and should not be deterred by my so inadequate reflection of a new and important view of how the organizational world works.  相似文献   
114.
Summary Gigantecin (I), a novel tetrahydroxy-di-tetrahydrofuran fatty acid -lactone (acetogenin), was isolated from an ethanolic extract of the stem bark ofGoniothalamus giganteus Hook. f., Thomas (Annonaceae), by means of activity-directed fractionation (brine shrimp lethality test). This new compound is extremely cytotoxic to human tumor cells, inhibits crown gall tumors on potato discs, and is active in an assay designed to detect antimitotic agents (9 ASK).  相似文献   
115.
Summary Cyclic voltammetry of molecular oxygen in aprotic media (dimethylformamide) and in the presence of bilirubin and other bile pigments shows that superoxide anion ( ) undergoes proton-induced dismutation. Lactam hydrogens of bile pigments are sufficiently acid to induce disproportionation to O2 and H2O2. Because of its characteristic lipophilic behavior, a biological role for natural bilirubin similar to that of other non-enzymatic lipophilic scavengers of is suggested.  相似文献   
116.
Coyne JA 《Nature》1990,344(6261):30
  相似文献   
117.
In multivariate discrimination of several normal populations, the optimal classification procedure is based on quadratic discriminant functions. We compare expected error rates of the quadratic classification procedure if the covariance matrices are estimated under the following four models: (i) arbitrary covariance matrices, (ii) common principal components, (iii) proportional covariance matrices, and (iv) identical covariance matrices. Using Monte Carlo simulation to estimate expected error rates, we study the performance of the four discrimination procedures for five different parameter setups corresponding to standard situations that have been used in the literature. The procedures are examined for sample sizes ranging from 10 to 60, and for two to four groups. Our results quantify the extent to which a parsimonious method reduces error rates, and demonstrate that choosing a simple method of discrimination is often beneficial even if the underlying model assumptions are wrong.The authors wish to thank the editor and three referees for their helpful comments on the first draft of this article. M. J. Schmid supported by grants no. 2.724-0.85 and 2.038-0.86 of the Swiss National Science Foundation.  相似文献   
118.
Phytoalexins, defensive compounds produced by plants against microbial infections, were purified fromSophora exigua (Leguminosae) and their growth inhibitory effects on oral cariogenic bacteria were determined in vitro. Among three isolated compounds, 5,7,2,4-tetrahydroxy-8-lavandulylflavanone completely inhibited the growth of oral bacteria including primary cariogenic mutans streptococci, other oral streptococci, actinomycetes, and lactobacilli, at concentrations of 1.56 to 6.25 g/ml.  相似文献   
119.
A comparison of sterol utilization by 3 stored-products insects revealed very different capabilities. The fluor beetle,Tribolium castaneum dealkylates and converts dietary sitosterol to about equal amounts of cholesterol (43.7%) and 7-dehydrocholeterol (39.8%), whereas another flour beetle,Tenebrio molitor, produces considerably less 7-dehydrocholesterol (16.8%) and relatively more cholesterol (66.7%) from sitosterol. The lepidopteran,Plodia interpunctella, utilized dietary sterol very similar to plant-feeding lepidoptera, producing primarily cholesterol (86.5%) from sitosterol.  相似文献   
120.
The Indian antHarpegnathos saltator may be unique among insects in using its jumping capacity not only as an escape mechanism but also as a normal means of locomotion, and for catching its prey in flight. High-speed cinematography used to analyse the various phases of the jump suggests thatHarpegnathos employs a novel jumping mechanism to mediate these behaviours: namely the synchronous activation of its middle and hindlegs. Electrophysiological recordings from muscles or nerves in pairs of middle and hindlegs show remarkably synchronous activity during fictive jumping, supporting the synchronous activation hypothesis.Harpegnathos is not the only ant to jump, and a cladistic analysis suggests that jumping behaviour evolved independently three times during ant evolutionary history.  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