全文获取类型
收费全文 | 114750篇 |
免费 | 1544篇 |
国内免费 | 2540篇 |
专业分类
系统科学 | 3000篇 |
丛书文集 | 2634篇 |
教育与普及 | 2316篇 |
理论与方法论 | 779篇 |
现状及发展 | 35439篇 |
研究方法 | 1459篇 |
综合类 | 71011篇 |
自然研究 | 2196篇 |
出版年
2021年 | 1089篇 |
2015年 | 1378篇 |
2014年 | 1917篇 |
2013年 | 2175篇 |
2012年 | 2290篇 |
2011年 | 4660篇 |
2010年 | 2186篇 |
2009年 | 2716篇 |
2008年 | 3389篇 |
2007年 | 3310篇 |
2006年 | 2917篇 |
2005年 | 3064篇 |
2004年 | 3430篇 |
2003年 | 2942篇 |
2002年 | 2723篇 |
2001年 | 2958篇 |
2000年 | 2640篇 |
1999年 | 3749篇 |
1998年 | 3062篇 |
1997年 | 3278篇 |
1996年 | 2873篇 |
1995年 | 2446篇 |
1994年 | 2589篇 |
1993年 | 2343篇 |
1992年 | 2482篇 |
1991年 | 2142篇 |
1990年 | 2088篇 |
1989年 | 1802篇 |
1988年 | 1602篇 |
1987年 | 1326篇 |
1986年 | 1164篇 |
1985年 | 1151篇 |
1984年 | 941篇 |
1981年 | 894篇 |
1980年 | 1038篇 |
1979年 | 1738篇 |
1978年 | 1586篇 |
1977年 | 1563篇 |
1976年 | 1262篇 |
1975年 | 1300篇 |
1974年 | 1393篇 |
1973年 | 1496篇 |
1972年 | 1560篇 |
1971年 | 1730篇 |
1970年 | 2017篇 |
1969年 | 1676篇 |
1968年 | 1596篇 |
1967年 | 1543篇 |
1966年 | 1295篇 |
1965年 | 1015篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Archive for History of Exact Sciences - We show that Dedekind, in his proof of the principle of definition by mathematical recursion, used implicitly both the concept of an inductive cone from an... 相似文献
22.
Russell L. Burke Beverly Clendening Alexandra Kanonik 《Journal of Natural History》2018,52(25-26):1723-1732
Clutch size (CS) is a well-studied life history characteristic, and optimal egg size theory predicts that CS should correlate positively with reproductive investment. Turtles are good subjects for studies of reproductive strategies because few turtle species exhibit parental care; therefore quantifying their short-term reproductive investment is mostly limited to measuring egg size and number. Clutch size is usually reported as an average value for each turtle population, and where CS variation is noted, it is usually attributable to variation in adult body size. In two long-term studies of ecologically dissimilar species we detected a dramatic increase in CS in a common snapping turtle (Chelydra serpentina) population and in a diamondback terrapin (Malaclemys terrapin) population. It is unknown whether these changes are due to either variation in adult body size or resource availability; but the temporal patterns we observed have apparently never been reported previously. These trends remain unexplained and should be explored further. 相似文献
23.
24.
25.
26.
Reverberation is significant in shallow water and produces obvious notches in OBC spec-tra. It also degrades the quality of sections and increases the difficulty of processing and interpretation. This ... 相似文献
27.
Danielle Kamato Muhamad Ashraf Rostam Rebekah Bernard Terrence J. Piva Nitin Mantri Daniel Guidone Wenhua Zheng Narin Osman Peter J. Little 《Cellular and molecular life sciences : CMLS》2015,72(4):799-808
G protein-coupled receptor (GPCR) signalling is mediated through transactivation-independent signalling pathways or the transactivation of protein tyrosine kinase receptors and the recently reported activation of the serine/threonine kinase receptors, most notably the transforming growth factor-β receptor family. Since the original observation of GPCR transactivation of protein tyrosine kinase receptors, there has been considerable work on the mechanism of transactivation and several pathways are prominent. These pathways include the “triple membrane bypass” pathway and the generation of reactive oxygen species. The recent recognition of GPCR transactivation of serine/threonine kinase receptors enormously broadens the GPCR signalling paradigm. It may be predicted that the transactivation of serine/threonine kinase receptors would have mechanistic similarities with transactivation of tyrosine kinase pathways; however, initial studies suggest that these two transactivation pathways are mechanistically distinct. Important questions are the relative importance of tyrosine and serine/threonine transactivation pathways, the contribution of transactivation to overall GPCR signalling, mechanisms of transactivation and the range of cell types in which this phenomenon occurs. The ultimate significance of transactivation-dependent signalling remains to be defined but it appears to be prominent and if so will represent a new cell signalling frontier. 相似文献
28.
29.
To evaluate the biocide effect of quaternary ammonium chloride (QAC), a whitewater sample was taken from a fine papermachine headbox. By plate spreading method, 51 strains of facultative anaerobe were isolated morphologically. Then the strains were separately transferred to basal medium and were incubated before the beginning of log phase. To identify strains with different QAC resistance, 30-120 ppm of N-Alkyl-benzyl-dimethyl ammonium chloride were added to basal medium.Biocide effect was investigated by comparison of bacterial growth, which can be monitored by 600 nm light absorbance of basal medium suspension. Among 51 strains, only 2 strains can Survive for QAC concentration up to 120 ppm. API20E aud 16S rRNA gene sequencing technique were applied to identify two strains with highest (120 ppm)QAC resistance. One strain (HB22)was identified as Morganella morganii. HB22 can resist QAC concentration up to 150 ppm. HB22 is Gram-negative rod, motile with flagella, catalase positive, oxidase negative, Indole positive, H2S production negative, facultative anaerobe. HB22 has optimum growth condition of 35℃ and pH 7.0.HB22 can catabolize only glucose and D (+)-Mannose.The other (HB45) was identified with high similarity among Pseudomonas cf. monteili or Pseudomonas mosselii or Pseudomonas putida. HB45 can resist QAC concentration up to 200 ppm.HB45 is Gram-negative rod, motile with flagella,catalase positive, oxidase negative, Indole negative,H2S production negative, aerobe. HB45 has optimum growth condition of 30℃ and pH 7.4. HB45 can catabolize L-Arabinose, L(+)-Rhanmose, D (+)-Mannose, Glucose, Glycerol, Lactose, Maltose,Raffmose, Xylose, Cellulose and Xylan. The implication of this work to paper industry is also discussed. 相似文献
30.