全文获取类型
收费全文 | 19432篇 |
免费 | 2026篇 |
国内免费 | 8篇 |
出版年
2021年 | 327篇 |
2020年 | 172篇 |
2019年 | 215篇 |
2018年 | 285篇 |
2017年 | 253篇 |
2016年 | 409篇 |
2015年 | 670篇 |
2014年 | 805篇 |
2013年 | 994篇 |
2012年 | 1257篇 |
2011年 | 1295篇 |
2010年 | 840篇 |
2009年 | 732篇 |
2008年 | 996篇 |
2007年 | 1025篇 |
2006年 | 964篇 |
2005年 | 852篇 |
2004年 | 909篇 |
2003年 | 811篇 |
2002年 | 820篇 |
2001年 | 317篇 |
2000年 | 293篇 |
1999年 | 316篇 |
1998年 | 255篇 |
1997年 | 196篇 |
1996年 | 178篇 |
1995年 | 174篇 |
1994年 | 182篇 |
1993年 | 181篇 |
1992年 | 227篇 |
1991年 | 201篇 |
1990年 | 201篇 |
1989年 | 187篇 |
1988年 | 177篇 |
1987年 | 167篇 |
1986年 | 167篇 |
1985年 | 182篇 |
1984年 | 212篇 |
1983年 | 140篇 |
1982年 | 198篇 |
1981年 | 139篇 |
1980年 | 127篇 |
1979年 | 146篇 |
1978年 | 142篇 |
1977年 | 128篇 |
1976年 | 126篇 |
1975年 | 143篇 |
1974年 | 131篇 |
1973年 | 120篇 |
1971年 | 94篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
David Jackson 《BMJ (Clinical research ed.)》1989,298(6675):753-754
53.
54.
Abstract. Although there are many studies in searching for the most useful community diversity index, the importance of choosing an appropriate parameter has been neglected. Here, we examined dominance and diversity in early post-fire chaparral communities using different variables, i.e. plant density, cover and biomass. Significant different results were produced by applying different parameters and the difference may be caused by the inconsistency in density, cover, and biomass allocated in each life form. Among the three parameters, biomass was most successful in detecting differences among communities because the apportionment of biomass among species was more variable than that of density. Although the three species variables represent different aspects of community properties and their relative performance may vary among communities, we recommend the use of biomass or productivity data as the most appropriate variable because it can best represent per capita resource use and resource partitioning among organisms in competitive situations. 相似文献
55.
David Philip Arthur Craig James W. Grice Chris A. Varnon B. Gibson Michel B. C. Sokolowski Charles I. Abramson 《PloS one》2012,7(10)
Free-flying honey bees (Apis mellifera L.) reactions were observed when presented with varying schedules of post-reinforcement delays of 0 s, 300 s, or 600 s. We measured inter-visit-interval, response length, inter-response-time, and response rate. Honey bees exposed to these post-reinforcement delay intervals exhibit one of several patterns compared to groups not encountering delays, and had longer inter-visit-intervals. We observed no group differences in inter-response time. Honey bees with higher response rates tended to not finish the experiment. The removal of the delay intervals increased response rates for those subjects that completed the trials. 相似文献
56.
57.
R H Jackson 《BMJ (Clinical research ed.)》1976,1(6003):220-221
58.
59.
60.