全文获取类型
收费全文 | 1500392篇 |
免费 | 139429篇 |
国内免费 | 1468篇 |
出版年
2021年 | 18084篇 |
2019年 | 16226篇 |
2018年 | 18397篇 |
2017年 | 17181篇 |
2016年 | 28657篇 |
2015年 | 42988篇 |
2014年 | 51084篇 |
2013年 | 77413篇 |
2012年 | 41298篇 |
2011年 | 30593篇 |
2010年 | 45279篇 |
2009年 | 46035篇 |
2008年 | 28501篇 |
2007年 | 27175篇 |
2006年 | 31341篇 |
2005年 | 32347篇 |
2004年 | 31541篇 |
2003年 | 29058篇 |
2002年 | 27419篇 |
2001年 | 44502篇 |
2000年 | 42244篇 |
1999年 | 39273篇 |
1998年 | 26507篇 |
1997年 | 26410篇 |
1996年 | 25835篇 |
1995年 | 23990篇 |
1994年 | 23796篇 |
1993年 | 22851篇 |
1992年 | 33945篇 |
1991年 | 32028篇 |
1990年 | 30403篇 |
1989年 | 30958篇 |
1988年 | 28550篇 |
1987年 | 26863篇 |
1986年 | 25259篇 |
1985年 | 27006篇 |
1984年 | 25778篇 |
1983年 | 22849篇 |
1982年 | 22188篇 |
1981年 | 21054篇 |
1980年 | 19700篇 |
1979年 | 22222篇 |
1978年 | 19971篇 |
1977年 | 19011篇 |
1976年 | 17947篇 |
1975年 | 17900篇 |
1974年 | 18481篇 |
1973年 | 18826篇 |
1972年 | 16334篇 |
1971年 | 14757篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Four myeloid cell lines (M1, WEHI-3B D+, FDC-P1, and 32D) were screened for the presence of J11d antigen. One of these cell lines, the myeloid leukemia M1, was found to express a high level of J11d antigen on the cell surface. Recombinant mouse leukemic inhibitory factor (rm-LIF), recombinant human LIF (rh-LIF), and steroids (hydrocortisone and dexamethasone) could induce M1 cells to undergo monocytic differentiation. The level of J11d antigen was greatly reduced after treatment of the cells with LIF or steroids. Western blotting revealed that the apparent molecular weight of the J11d antigen on M1 cells was 45-48 kDa. Furthermore, the level of J11d mRNA was also reduced during LIF-induced differentiation of M1 cells. 相似文献
102.
103.
104.
A. R. E. SINCLAIR 《African Journal of Ecology》1973,11(1):93-107
Censuses of buffalo and wildebeest in the Serengeti-Mara region have been carried out by various workers at intervals since 1958. The methods of these censuses, which normally employ a total count, are described and the errors inherent in such techniques are analysed in detail. After the appropriate corrections had been made the results showed that both populations have increased considerably over the years 1961–1971. 相似文献
105.
106.
T. B. Lapirova V. R. Mikryakova A. S. Mavrin G. A. Yinogradova 《Journal of Evolutionary Biochemistry and Physiology》2000,36(1):47-51
The paper presents data on a change in lysozyme content in tissues of spleen, liver and heart in fry of the Lena River sturgeon
exposed to the presence of sublethal concentrations of Hg2+, Cd2+, and Cu2+ under conditions of chronic experiment. It has been shown that the lysozyme content in fish tissues varies and has a phasic
character. The amplitude of fluctuations of this parameter depends on the moment of sampling, nature of the toxicant, and
structural-functional organization of the studied organs. 相似文献
107.
To maximize fitness, many animals must trade off their need to forage efficiently against their need to avoid predators. We studied such a trade-off in four species of tits (Paridae) in a forest near Oxford, UK. During winter, tits form flocks which increase feeding efficiency and reduce predation risk. These flocks feed extensively on beech (Fagus sylvatica) seeds, the abundance of which may be critical for winter survival. Because these seeds drop to the ground, where birds are exposed to sparrowhawk (Accipiter nisus) attack, tits need to trade off their need to find seeds against the proximity to protective cover, provided by dense clusters of hawthorn (Crataegus spp.). The quality of the beech crop differs markedly between trees and years. During a year of abundant beechmast, most tits searched for seeds close to protective cover. This 'safety-first' strategy precluded visits to superabundant food patches if they were too far from protective cover. Among beech trees near to cover, tits tended to prefer those with high seed density. Tits benefited from foraging under trees with high seed density because this correlated significantly with seed mass per square metre and because mean search times decreased with increasing seed density. Finally, we show experimentally that great tits, Parus major, can discriminate between edible (viable) and inedible (empty) seeds. 相似文献
108.
109.
(Z)- and (E)-4-amino-2-(trifluoromethyl)-2-butenoic acid (4 and 5, respectively) were synthesized and investigated as potential mechanism-based inactivators of gamma-aminobutyric acid aminotransferase (GABA-AT) in a continuing effort to map the active site of this enzyme. The core alpha-trifluoromethyl-alpha,beta-unsaturated ester moiety was prepared via a Reformatsky/reductive elimination coupling of the key intermediates tert-butyl 2,2-dichloro-3,3,3-trifluoropropionate and N,N-bis(tert-butoxy-carbonyl)glycinal. Both 4 and 5 inhibited GABA-AT in a time-dependent manner, but displayed non-pseudo-first-order inactivation kinetics; initially, the inactivation rate increased with time. Further investigation demonstrated that the actual inactivator is generated enzymatically from 4 or 5. This inactivating species is released from the active site prior to inactivation, and as a result, 4 and 5 cannot be defined as mechanism-based inactivators. Furthermore, 4 and 5 are alternate substrates for GABA-AT, transaminated by the enzyme with Km values of 0.74 and 20.5 mM, respectively. Transamination occurs approximately 276 and 305 times per inactivation event for 4 and 5, respectively. The enzyme also catalyzes the elimination of the fluoride ion from 4 and 5. A mechanism to account for these observations is proposed. 相似文献
110.