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   152篇
  免费   3篇
  国内免费   1篇
  156篇
  2016年   2篇
  2014年   2篇
  2013年   2篇
  2012年   5篇
  2011年   12篇
  2010年   14篇
  2009年   10篇
  2008年   6篇
  2007年   6篇
  2006年   5篇
  2005年   2篇
  2004年   1篇
  2003年   1篇
  2002年   1篇
  2001年   2篇
  2000年   4篇
  1999年   3篇
  1998年   2篇
  1997年   3篇
  1996年   5篇
  1995年   4篇
  1994年   4篇
  1993年   2篇
  1992年   2篇
  1991年   4篇
  1990年   2篇
  1989年   5篇
  1988年   4篇
  1987年   1篇
  1986年   3篇
  1985年   2篇
  1984年   1篇
  1983年   4篇
  1982年   2篇
  1981年   9篇
  1980年   2篇
  1978年   1篇
  1977年   1篇
  1976年   1篇
  1974年   2篇
  1973年   1篇
  1971年   2篇
  1970年   1篇
  1969年   1篇
  1968年   1篇
  1966年   1篇
  1962年   1篇
  1959年   1篇
  1956年   1篇
  1953年   1篇
排序方式: 共有156条查询结果,搜索用时 0 毫秒
151.
The ongoing changes in the global climate expose the world's ecosystems not only to increasing CO2 concentrations and temperatures but also to altered precipitation (P) regimes. Using four well-established process-based ecosystem models (LPJ, DayCent, ORCHIDEE, TECO), we explored effects of potential P changes on water limitation and net primary production (NPP) in seven terrestrial ecosystems with distinctive vegetation types in different hydroclimatic zones. We found that NPP responses to P changes differed not only among sites but also within a year at a given site. The magnitudes of NPP change were basically determined by the degree of ecosystem water limitation, which was quantified here using the ratio between atmospheric transpirational demand and soil water supply. Humid sites and/or periods were least responsive to any change in P as compared with moderately humid or dry sites/periods. We also found that NPP responded more strongly to doubling or halving of P amount and a seasonal shift in P occurrence than that to altered P frequency and intensity at constant annual amounts. The findings were highly robust across the four models especially in terms of the direction of changes and largely consistent with earlier P manipulation experiments and modelling results. Overall, this study underscores the widespread importance of P as a driver of change in ecosystems, although the ultimate response of a particular site will depend on the detailed nature and seasonal timing of P change.  相似文献   
152.
The variation in copulatory activity in three strains of Culex pipiens is investigated and the results are discussed in relation to genetic control of mosquito populations.  相似文献   
153.
SYNOPSIS. AS theory rules method, the methodological proceduresapplied to morphological explanation have to be derived fromthe law-like properties of the objects under investigation.The explanation of organismic constructions has to be basedon the hydraulic principle which describes organisms as systemscomposed of fluid contained within flexiblemembranes. This insightestablishes a supra-molecular causal principle which, in itsgenerality for morphological explanation, parallels the biochemicalprinciples of molecular biology on the molecular level. Everyform and architectural arrangement has to be conceived as theresult of the form-enforcing influence of mechanical elementsthat operatewithin an integrated mechanically coherent system.An adequate explanation of morphological configuration has toelaborate the organization of the constructional whole and explainits properties as the result of a gradual transformation processthat is constrained by internal mechanical principles. The theoriesdeveloped by such a procedure are open to criticism and canbe tested and corroborated by reference to experiments conductedby nature.  相似文献   
154.
155.
ABSTRACT. The locomotor activity patterns of wildtype Drosophila melanogaster and the mutants so (sine oculis) and sol (small optic lobes) were investigated. In all strains the proportions showing circadian rhythmicity, arrhythmicity and more complex patterns which could not be thus classified were similar. The occurrence of abnormal activity patterns is thus not a property of the mutation as previously claimed (Mack & Engelmann, 1981). In flies with a distinct circadian rhythmicity, the period lengths (τ) varied between strains, τ for wildtype Italy and the mutant so was longer than for wildtype Berlin and the mutant sol. As different τ's have been reported by others, τ does not seem to be closely determined for Drosophila melanogaster. Many flies exhibited two rhythms simultaneously, one with τ shorter and one with τ longer than 24 h, apparently implying two-oscillator control of the locomotor activity. The eyeless so mutants were entrained by LD cycles, so the compound eyes are not necessary, and so must possess the relevant photoreceptor(s) elsewhere. This may therefore also be true for the wildtype. Histology of the so adults revealed no correlation between the degree of reduction in the medulla and the occurrence of abnormal activity patterns. Since the only structures common to the medulla of so and sol are known to be large tangential cells, it is concluded that either they are of importance for the rhythmic system, or the oscillator(s) controlling locomotor activity is (are) not located in the medulla.  相似文献   
156.
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