全文获取类型
收费全文 | 49964篇 |
免费 | 116篇 |
国内免费 | 206篇 |
专业分类
系统科学 | 324篇 |
丛书文集 | 1078篇 |
教育与普及 | 107篇 |
理论与方法论 | 281篇 |
现状及发展 | 22785篇 |
研究方法 | 1902篇 |
综合类 | 23106篇 |
自然研究 | 703篇 |
出版年
2013年 | 387篇 |
2012年 | 657篇 |
2011年 | 1399篇 |
2010年 | 287篇 |
2008年 | 861篇 |
2007年 | 920篇 |
2006年 | 944篇 |
2005年 | 927篇 |
2004年 | 877篇 |
2003年 | 868篇 |
2002年 | 887篇 |
2001年 | 1450篇 |
2000年 | 1340篇 |
1999年 | 918篇 |
1992年 | 890篇 |
1991年 | 712篇 |
1990年 | 768篇 |
1989年 | 784篇 |
1988年 | 755篇 |
1987年 | 807篇 |
1986年 | 776篇 |
1985年 | 944篇 |
1984年 | 744篇 |
1983年 | 637篇 |
1982年 | 568篇 |
1981年 | 580篇 |
1980年 | 744篇 |
1979年 | 1538篇 |
1978年 | 1334篇 |
1977年 | 1314篇 |
1976年 | 997篇 |
1975年 | 1066篇 |
1974年 | 1483篇 |
1973年 | 1292篇 |
1972年 | 1343篇 |
1971年 | 1579篇 |
1970年 | 2050篇 |
1969年 | 1546篇 |
1968年 | 1519篇 |
1967年 | 1545篇 |
1966年 | 1313篇 |
1965年 | 943篇 |
1964年 | 275篇 |
1959年 | 535篇 |
1958年 | 870篇 |
1957年 | 656篇 |
1956年 | 561篇 |
1955年 | 488篇 |
1954年 | 545篇 |
1948年 | 326篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
1.
2.
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
3.
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
4.
5.
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
6.
7.
Photon emission of phagocytes in relation to stress and disease. 总被引:1,自引:0,他引:1
Phagocytes, the first-line cells of the body's defence mechanisms against invading pathogens, kill microorganisms by means of lysosomal degradative enzymes and highly toxic reactive oxygen intermediates. The reactive oxygen compounds are produced, in a process called the 'respiratory burst', by the NADPH oxidase complex in plasma membranes, and by myeloperoxidase in phagolysosomes after degranulation. These processes generate electronically excited states which, on relaxation, emit photons, giving rise to phagocyte chemiluminescence (CL). This paper describes the conditions for the measurement of CL, and reviews the activity of phagocytes from individuals undergoing stress or disease. The capability of phagocytes to emit photons reflects remarkably well the pathophysiological state of the host. In many cases even the magnitude of the stress, the presence of a pathogen in the body, or the activity of the disease can be estimated. Physiological changes, e.g. in the reproductive cycle, can also be predicted. 相似文献
8.
9.
10.