首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   787篇
  免费   55篇
  国内免费   2篇
  2022年   3篇
  2021年   4篇
  2019年   3篇
  2017年   5篇
  2016年   9篇
  2015年   20篇
  2014年   27篇
  2013年   24篇
  2012年   48篇
  2011年   39篇
  2010年   29篇
  2009年   33篇
  2008年   46篇
  2007年   38篇
  2006年   37篇
  2005年   34篇
  2004年   34篇
  2003年   42篇
  2002年   29篇
  2001年   13篇
  2000年   13篇
  1999年   10篇
  1998年   18篇
  1997年   7篇
  1996年   11篇
  1995年   4篇
  1994年   9篇
  1993年   15篇
  1992年   13篇
  1991年   13篇
  1990年   14篇
  1989年   13篇
  1988年   14篇
  1987年   16篇
  1986年   17篇
  1985年   11篇
  1984年   9篇
  1983年   10篇
  1982年   10篇
  1981年   10篇
  1980年   13篇
  1979年   10篇
  1978年   6篇
  1977年   4篇
  1976年   13篇
  1975年   6篇
  1974年   4篇
  1973年   7篇
  1968年   5篇
  1967年   5篇
排序方式: 共有844条查询结果,搜索用时 15 毫秒
841.
The investigation of the essential oil from Molopospermum peloponnesiacum roots afforded, in addition to some well known terpenoids, two aromatic aldehydes, which were found for the first time as volatile oil components. Their structures were elucidated by spectroscopic methods. The formation of these aldehydes during steam distillation from unstable precursors is discussed.  相似文献   
842.
We present measurements of the maximum diameter of the planktonic foraminifer Neogloboquadrina pachyderma sin. from six sediment cores (Ocean Drilling Program sites 643, 644, 907, 909, 985 and 987) from the Norwegian–Greenland Sea. Our data show a distinct net increase in mean shell size of N. pachyderma sin. at all sites during the last 1.3 Ma, with largest shell sizes reached after 0.4 Ma. External factors such as glacial–interglacial variability and carbonate dissolution alone cannot account for the observed variation in mean shell size of N. pachyderma sin. We consider the observed shell size increase to mirror an evolutionary trend towards better adaptation of N. pachyderma sin. to the cold water environment after 1.1–1.0 Ma. Probably, the Mid Pleistocene climate shift and the associated change of amplitude and frequency of glacial–interglacial fluctuations have triggered the evolution of this planktonic foraminifer. Oxygen and carbon stable isotope analyses of different shell size classes indicate that the observed shell size increase could not be explained by the functional concept that larger shells promote increasing sinking velocities during gametogenesis. For paleoceanographic reconstructions, the evolutionary adaptation of Neogloboquadrina pachyderma sin. to the cold water habitat has significant implications. Carbonate sedimentation in highest latitudes is highly dependent on the presence of this species. In the Norwegian–Greenland Sea, carbonate-poor intervals before 1.1 Ma are, therefore, not necessarily related to severe glacial conditions. They are probably attributed to the absence of this not yet polar-adapted species. Further, transfer function and modern analog techniques used for the reconstruction of surface water conditions in high latitudes could, therefore, contain a large range of errors if they were applied to samples older than 1.1–1.0 Myrs.  相似文献   
843.
The “expensive tissue hypothesis” states that large brains are active at high metabolic rates, which have to be financed by a significant trade-off with other organs such as the alimentary tract. Recent morphological findings on primate brains and guts support this idea also considering the importance of high-energy diets as a possible driving power of this process. However, the trade-off correlation between brain and alimentary tract, the essence of the “expensive tissue hypothesis”, has not yet been tested using molecular data to complement morpho-functional findings. We therefore hypothesize that the activity of marker proteins expressed both in brain and alimentary tract should parallel functional morphology in organs at the molecular level. Thus, in animals feeding on hard to digest diet, we would expect a high concentration per unit mass of that marker protein in the digestive tract and reversely a lower concentration in the brain. In contrast, in animals feeding on easily-digested, high-energy food we would expect the reverse pattern. Recent preliminary studies suggest that carbonic anhydrase II (CA-II) could act as a marker. The enzyme concentration was found to increase in the brain with higher cerebral activity from cattle to humans and to reversely decrease in salivary secretions. The reverse concentration of CA-II in saliva and brains of cattle and primates might be the first molecular evidence of the validity of the “expensive tissue hypothesis”.  相似文献   
844.
Alcaligenes latus strains can accumulate poly-D(-)-3-hydroxybutyrate (PHB) up to about 85% of cell dry weight. The abilities to store poly-D(-)-3-hydroxyvalerate (PHV) of three strains ofA. latus were investigated. With Na-propionate as PHV precursor, strainA. latusDSM 1122 had better PHV accumulation ability than strainsA. latusDSM 1123 and 1124. StrainA. latus DSM 1123 could store PHV when Na-valerate but not Na-propionate served as the PHV precursor. PHB and PHV accumulation byA. latus DSM 1124 rapidly increased when propionic acid and acetic acid were together added to the fermentor. This increase was not obtained in the culture shaker flask and fermentor growing the same strain when Na-propionate alone served as a PHV precursor.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号