全文获取类型
收费全文 | 741篇 |
免费 | 92篇 |
专业分类
833篇 |
出版年
2021年 | 11篇 |
2020年 | 6篇 |
2019年 | 10篇 |
2018年 | 9篇 |
2017年 | 7篇 |
2016年 | 10篇 |
2015年 | 24篇 |
2014年 | 27篇 |
2013年 | 47篇 |
2012年 | 53篇 |
2011年 | 46篇 |
2010年 | 31篇 |
2009年 | 22篇 |
2008年 | 40篇 |
2007年 | 41篇 |
2006年 | 32篇 |
2005年 | 32篇 |
2004年 | 26篇 |
2003年 | 34篇 |
2002年 | 25篇 |
2001年 | 12篇 |
2000年 | 15篇 |
1999年 | 10篇 |
1998年 | 5篇 |
1997年 | 9篇 |
1995年 | 9篇 |
1993年 | 5篇 |
1992年 | 18篇 |
1991年 | 20篇 |
1990年 | 11篇 |
1989年 | 19篇 |
1988年 | 20篇 |
1987年 | 9篇 |
1986年 | 17篇 |
1985年 | 10篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 7篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1973年 | 4篇 |
1972年 | 5篇 |
1971年 | 5篇 |
1970年 | 5篇 |
1969年 | 4篇 |
排序方式: 共有833条查询结果,搜索用时 0 毫秒
831.
A new triterpenoid, calaminthadiol, 3,4-seco-4(23),12-ursadien-3,28-diol, was isolated from the leaves of Satureia calamintha and Satureia graeca. Its structure was elucidated by spectroscopic methods and partial synthesis. The natural occurrence of this compound can be conclusive for the chemotaxonomic characterization of the genus because of the rarity of the 3,4-fission in the α- and β-amyrin skeletons. A biogenetic mechanism for its formation is proposed. Sitosterol, stigmasterol and campesterol were also isolated. 相似文献
832.
Animals show several behavioral strategies to reduce predation risks. Presumably, moonlight avoidance is a strategy used by some nocturnal species to reduce the risk of predation. In bats, some research indicates that foraging activity is negatively correlated with moonlight intensity, a phenomenon better known as lunar phobia. However, the currently available evidence is contradictory because some bat species reduce their activity during nights with more moonlight while the opposite occurs in other species. We quantitatively evaluated the strength and direction of the relationship between moonlight intensity and bat activity using a meta-analysis. We also looked at some ecological correlates of lunar phobia in bats. Specifically, we examined foraging habitat and latitude as potential moderators of the size of the lunar phobia effect. Our results show that, regardless of the method used to evaluate bat activity, the overall relationship between moonlight intensity and bat activity is significant and negative (r = ?0.22). Species foraging on the surface of the water (piscivores and insectivores; r = ?0.83) and forest canopy species (i.e., big frugivores; r = ?0.30) are more affected by moonlight than those with different foraging habitats (understory, subcanopy, open air). Latitude was positively correlated with lunar phobia (r = 0.023). The stronger lunar phobia for bats foraging on the water surface and in the forest canopy may suggest that the risk of predation is greater where moonlight penetrates more easily. The significant effect of latitude as a moderator of lunar phobia suggests that there is a weak geographic pattern, with this phobia slightly more common in tropical bats than in temperate species. 相似文献
833.
Romeo V. Yordanov 《Aquatic Ecology》1992,26(2-4):581-585
A series of laboratory experiments onGlyptotendipes salinus were carried out in order to assess cytogenetic effects of different doses of gamma-radiation on polytene chromosomes, isolated from salivary glands. Chrinonomid larvae (III–IV larval stage) were irradiated with doses varying from 0.05 to 1.00 Gy (5–100 rad) and were bred under laboratory conditions until the fourth larval stage. Cytogenetic slides were analyzed for an estimation of occurrence of changes in the organization of the polytene chromosomes caused by gamma-radiation. A specific heterochromatin effect was found in certain chromosomes of the investigated species after 1.00 Gy irradiation. Decondensation of the centromeric heterochromatin and increased functional activity of Balbiany ring 2 were observed in the fourth (G) chromosome. Regression of the nucleolus of the first (AB) chromosome was detected. 相似文献