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   5059篇
  免费   380篇
  国内免费   354篇
  2024年   11篇
  2023年   111篇
  2022年   137篇
  2021年   177篇
  2020年   187篇
  2019年   251篇
  2018年   190篇
  2017年   233篇
  2016年   204篇
  2015年   189篇
  2014年   286篇
  2013年   432篇
  2012年   191篇
  2011年   294篇
  2010年   205篇
  2009年   282篇
  2008年   263篇
  2007年   259篇
  2006年   231篇
  2005年   219篇
  2004年   196篇
  2003年   160篇
  2002年   150篇
  2001年   85篇
  2000年   85篇
  1999年   73篇
  1998年   59篇
  1997年   61篇
  1996年   66篇
  1995年   57篇
  1994年   44篇
  1993年   52篇
  1992年   65篇
  1991年   33篇
  1990年   33篇
  1989年   22篇
  1988年   24篇
  1987年   25篇
  1986年   24篇
  1985年   30篇
  1984年   27篇
  1983年   15篇
  1982年   11篇
  1981年   9篇
  1980年   11篇
  1979年   4篇
  1978年   4篇
  1977年   7篇
  1971年   2篇
  1970年   2篇
排序方式: 共有5793条查询结果,搜索用时 31 毫秒
91.
92.
ochotonidaeThomas,l897AlloptoxDaw8on,l96lAlloptoxsihongensis8p.nov.(Figs.lAi6)HolotypeArightp3,V884o.7.Paratypesldp3,V884o.l;3p3,V884o.3,5,6;1p4,V884O.9;2ml/m2,V884o.l2,16;Afragmentof1eftmandiblewithp4,V884O.l8;thefrontpartofrightman-diblewithi2,p3-p4,V884o.24;2I2,V884O.21,23;4P3,V884O-31-33,56;P4,V884o.27;AfragmentofleftupperjawwithP3-M1,V884o-57;2M2,V884o.5o,53.TypelocalityS0nglinzhuang,Sih0ngC0unty,Jiangsu.StratumtypicumandageXiaca0wanFormation,.EarIyMi0cene(Shan-wangian),eq…  相似文献   
93.
Abstract A mutant screening procedure is described which allows the identification of mutants carrying lesions in lipoprotein, membrane-derived oligosac-charides (MDO), and other compounds of the E. coli cell envelope containing glycerol derived from phospholipid metabolism. Two mutants lacking glycerol in MDO and one mutant devoid of lipoprotein demonstrate the usefulness of the procedure.  相似文献   
94.
中国下泥盆统脊椎动物化石组合层序   总被引:8,自引:4,他引:4  
本文综合了滇、黔、川、桂等地的一些早泥盆世地层剖面中的脊椎动物化石的出露情况,粗略地描绘出了早期脊椎动物演化的一般特征,归纳成三个发展阶段。并以此为依据,讨论了下泥盆统的分层与对比。  相似文献   
95.
Gingival eruption of the permanent teeth of Australopithecus, Paranthropus, and early Homo, diagnosed from enamel scratches and facets, is similar save for two sequences: eruption of the canines relative to the premolars which may be sexually dimorphic; and agenesis of M3 with delayed eruption of M2, first seen in Homo at two million years. Gingival eruption sequences are similar also for early and modern Homo, except that in some individuals today M3, M2, M1 and I2 take longer to form and emerge through the gingiva as functioning teeth. Probably, from two million years to the present in the evolutionary history of Homo dental development slowed-down. More and more of ontogeny has been taken over for eruption.  相似文献   
96.
Recent freshwater cyanophyte oncoids from Canandaigua Lake, New York, consist almost entirely of minute irregular, bush-like, micritic masses comparable with the calcareous alga Angulocellularia Vologdin from Lower Cambrian algal-arehaeocyathan bioherms and biostromes of the Siberian Platform and western Mongolia. The Recent specimens arc radially orientated within the oncoids and occasionally contain moulds of axial filaments 15 μm in diameter. Acid-residues of the oncoids yield abundant Schizothrix calcicola. The micritic bush-like masses are interpreted to have formed by the calcification of the sheaths of these cyanophytes. This analogue allows Angulocellularia to be reinterpreted as a calcified oscillatoriacean cyanophyte, rather than a rhodophyte as previously suggested. It indicates that filamentous cyanophytes can produce apparently solid calcareous fossils, not only porostromatc tubes. The occurrence of Angulocellularia has been overlooked in some previous studies of Cambrian and Ordovician bioherms. The generic diagnosis is emended.  相似文献   
97.
Summary Examples are presented to illustrate some of the effects aberrant values, in particular, measurement errors, may have on estimates of the genetic parameters related to selection studies. It is shown that aberrant values may cause observed response to selection pressure to differ considerably from predicted response. Possible dangers of indiscriminate screening are also discussed.  相似文献   
98.
Summary A vigorous fast growing mutant which ends vegetative growth sixty days earlier than the parent variety Co 527 was isolated from gamma irradiated vegetative buds. The mutant initially segregated but stabilized in the vM4 generation. Its growth rate was almost fifty per cent higher than Co 527 beginning in the early stages of growth. It produced a significantly higher early shoot population which enabled it to yield a higher number of millable canes at maturity. Economic characters like sucrose content and juice purity remained unaffected. This mutant had a chromosomal basis in that the number was 2 to 3 lower than in the parent variety.Research carried out under the FAO/IAEA Co-ordinated Research Programme on the Improvement of Vegetatively Propagated Plants and Tree Crops through Induced mutations (Research Contract No. 1680)  相似文献   
99.
100.
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