全文获取类型
收费全文 | 1281566篇 |
免费 | 114454篇 |
国内免费 | 1474篇 |
专业分类
1397494篇 |
出版年
2021年 | 17723篇 |
2020年 | 12553篇 |
2019年 | 16106篇 |
2018年 | 17021篇 |
2017年 | 15800篇 |
2016年 | 27565篇 |
2015年 | 42073篇 |
2014年 | 50096篇 |
2013年 | 76347篇 |
2012年 | 34477篇 |
2011年 | 22121篇 |
2010年 | 42428篇 |
2009年 | 44112篇 |
2008年 | 21581篇 |
2007年 | 19139篇 |
2006年 | 25148篇 |
2005年 | 26284篇 |
2004年 | 25591篇 |
2003年 | 23387篇 |
2002年 | 21406篇 |
2001年 | 28409篇 |
2000年 | 25596篇 |
1999年 | 27180篇 |
1998年 | 24849篇 |
1997年 | 24691篇 |
1996年 | 24441篇 |
1995年 | 22492篇 |
1994年 | 22364篇 |
1993年 | 21436篇 |
1992年 | 24374篇 |
1991年 | 22804篇 |
1990年 | 21416篇 |
1989年 | 22470篇 |
1988年 | 20669篇 |
1987年 | 19876篇 |
1986年 | 18868篇 |
1985年 | 20848篇 |
1984年 | 21164篇 |
1983年 | 18880篇 |
1982年 | 19928篇 |
1981年 | 19315篇 |
1980年 | 17988篇 |
1979年 | 17702篇 |
1978年 | 16925篇 |
1977年 | 16294篇 |
1976年 | 15674篇 |
1975年 | 14970篇 |
1974年 | 15513篇 |
1973年 | 15788篇 |
1972年 | 13435篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
LAWRENCE C. WATSON 《American anthropologist》1972,74(5):1189-1207
The results of this study indicate that urbanization has the effect of significantly altering the Guajiro's sense of ideal and feared identity. It was hypothesized that an attempt would be made to minimize dissonance between the ideal and the real or "true" self which the individual privately acknowledges. However, there is found to be considerable dissonance between ideal and real identities, with the latter actually including many components of feared identity. Defective socialization in the city is suggested as a possible reason for failure to achieve ideal standards. 相似文献
982.
983.
984.
985.
Feasible mechanisms for algal digestion in the king angelfish 总被引:1,自引:0,他引:1
To determine the ability of the king angelfish Holacanthus passer to digest algae, three algal species were immersed in acidic conditions similar to that found in the stomach of fish. Only one of them was not susceptible to acidic lysis; two were affected after 40 and 60 min at pH 2·0. King angelfish have an expanded region of the intestine called here the hindgut chamber (HC) containing populations of micro-organisms. Some of these micro-organisms have the capacity to grow in cellulose, agar, and alginic acid; the main components of algal cell walls. Micro-organisms grew in carboxymethylcellulose cultures under aerobic and micro-aerobic conditions. The HC is highly vascularized, which could increase absorptive efficiency of material digested in it. 相似文献
986.
Heat treatment results in a loss of transgene-encoded activities in several tobacco lines. 总被引:1,自引:0,他引:1 下载免费PDF全文
Heat treatment (37 degrees C) of transgenic tobacco (Nicotiana tabacum) plants led to a reversible reduction or complete loss of transgene-encoded activities in about 40% of 10 independent transformants carrying the luciferase-coding region fused to the 355 cauliflower mosaic virus or the soybean small subunit promoter and the nopaline synthase promoter driving the neomycin phosphotransferase gene, whereas the other lines had temperature-tolerant activities. Temperature sensitivity or tolerance of transgene-encoded activities was heritable. In some of the lines, temperature sensitivity of the transgene-encoded activities depended on the stage of development, occurring in either seedlings (40% luciferase and 50% neomycin phosphotransferase) or adult plants (both 40%). The phenomenon did not correlate with copy numbers or the homo- or hemizygous state of the transgenes. In lines harboring a temperature-sensitive luciferase activity, reduction of bioluminescence was observed after 2 to 3 h at 37 degrees C. Activity was regained after 2 h of subsequent cultivation at 25 degrees C. Irrespective of the reaction to the heat treatment, the level of luciferase RNA was slightly increased at 37 degrees C. Only in lines showing temperature sensitivity of transgene-encoded activities was the amount of luciferase and neomycin phosphotransferase strongly reduced. In sterile culture, heat treatment for 15 d did not cause visible damage or changes in plant morphology. In all plants tested a slight induction of the heat-shock response was observed at 37 degrees C. 相似文献
987.
988.
Externally attached radio transmitters do not affect the parental care behaviour of rock bass 总被引:2,自引:0,他引:2
S. J. Cooke 《Journal of fish biology》2003,62(4):965-970
Turning, pectoral fin and caudal fin rates and time spent on the nest of male rock bass Ambloplites rupestris , engaged in parental care, were not affected after the attachment of external radio transmitters. Reproductive success was similar between treatment and control fish. Micro external radio transmitters can be used on small fishes for studying parental care duration and post-care movement without altering their behaviour. 相似文献
989.
990.