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   394621篇
  免费   45494篇
  国内免费   360篇
  2016年   3571篇
  2015年   5348篇
  2014年   6243篇
  2013年   8978篇
  2012年   9950篇
  2011年   10017篇
  2010年   6669篇
  2009年   6294篇
  2008年   8910篇
  2007年   9401篇
  2006年   8935篇
  2005年   8693篇
  2004年   8618篇
  2003年   8219篇
  2002年   8207篇
  2001年   18201篇
  2000年   18753篇
  1999年   14895篇
  1998年   4899篇
  1997年   5159篇
  1996年   4809篇
  1995年   4616篇
  1994年   4547篇
  1993年   4552篇
  1992年   12057篇
  1991年   11645篇
  1990年   11303篇
  1989年   10962篇
  1988年   10344篇
  1987年   9866篇
  1986年   9373篇
  1985年   9460篇
  1984年   7813篇
  1983年   6720篇
  1982年   5348篇
  1981年   4980篇
  1980年   4523篇
  1979年   7627篇
  1978年   6156篇
  1977年   5654篇
  1976年   5340篇
  1975年   5996篇
  1974年   6627篇
  1973年   6570篇
  1972年   6086篇
  1971年   5510篇
  1970年   4757篇
  1969年   4718篇
  1968年   4300篇
  1967年   3616篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
971.
The seasonal growth and decline of a population of Elodea canadensis Mich. growing in an irrigation drain are described, together with some characteristics of the aquatic environment (turbidity, photosynthetically-available radiation, temperature and pH). Overwintering buds (up to 5000 m?2) in the form of swollen dormant stem apices are produced in autumn with the onset of cold weather, remain in the mud, and grow out in the spring. In late summer vegetative reproduction also occurs when large numbers of the leafless stem portions which are capable of growing into independent plants are swept downstream from established populations. Results suggest that control measures should be applied in early summer when populations are approaching nuisance size, and again in late summer before fragmentation occurs and overwintering propagules are initiated. In irrigation channels in Australia, where draining and drying are not feasible, biomass in successfully reduced by widespread use of herbicides.  相似文献   
972.
973.
974.
975.
976.
Succinate dehydrogenase activity in mitochondria, which were isolated by centrifuging partially purified mitochondria through 1. 315 M sucrose, was completely suppressed when [14C]succinate uptake was abolished by prior incubation of the mitochondria with carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and valinomycin. The conclusion that these mitochondria were intact was confirmed by the fact that, when these mitochondria were broken by a freeze-thaw cycle followed by sonication, such inhibition was totally abolished. The yield of mitochondria, microsomes, and peroxisomes from the initial homogenate was 17.8, <0.1, and 0%, respectively, indicating that the mitochondria were not only intact but also essentially free of contamination from microsomes and peroxisomes. The overt form of carnitine palmitoyltransferase (CPT I) in these intact and pure mitochondria was totally inhibited by malonyl CoA, indicating that previous reports of incomplete inhibition in mitochondrial preparations resulted from interference from CPT activity in the inner mitochondrial membrane (CPT II), microsomes, or peroxisomes.  相似文献   
977.
978.
979.
Gross and microscopic anatomical evidence indicates that pain fibers involved in causalgia are those distributed to blood vessels-possibly to the arterioles-and that, for the greater part, these fibers constitute part of the general visceral afferent system. Several investigators have reported evidence that injury to a peripheral nerve of such a type as to cause damage to the vasomotor control of any area produces the initial pain in an extremity, and it is predicated that the arteriolar constriction causing the pain is then prolonged by the sensitization of arteriolar smooth muscle to the amount of epinephrine normally in the blood. If the condition is not treated, tissue anoxia occurs to such an extent that irreversible changes take place in the affected area.Treatment of causalgia in the lower extremities is directed toward interruption of either the vasomotor or afferent supply of blood vessels by blocking or excision of the second to fourth lumbar ganglia inclusive with the intervening chains. For the upper extremities, the blocking or disconnection of the second and third thoracic ganglia with interruption of the sympathetic chain between the third and fourth ganglia is considered a feasible method of treatment which does not produce the concomitant disability of Horner's syndrome.  相似文献   
980.
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