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   1382435篇
  免费   128532篇
  国内免费   1456篇
  2021年   17904篇
  2019年   16172篇
  2018年   17674篇
  2017年   16262篇
  2016年   28081篇
  2015年   42597篇
  2014年   50689篇
  2013年   76918篇
  2012年   38888篇
  2011年   27554篇
  2010年   43756篇
  2009年   45126篇
  2008年   25939篇
  2007年   23932篇
  2006年   28784篇
  2005年   29813篇
  2004年   29074篇
  2003年   26541篇
  2002年   24492篇
  2001年   37833篇
  2000年   35169篇
  1999年   34020篇
  1998年   25915篇
  1997年   25773篇
  1996年   25245篇
  1995年   23339篇
  1994年   23174篇
  1993年   22317篇
  1992年   28782篇
  1991年   26897篇
  1990年   25511篇
  1989年   26285篇
  1988年   23967篇
  1987年   22778篇
  1986年   21520篇
  1985年   23169篇
  1984年   23015篇
  1983年   20346篇
  1982年   20778篇
  1981年   20001篇
  1980年   18639篇
  1979年   19137篇
  1978年   17860篇
  1977年   17183篇
  1976年   16408篇
  1975年   15707篇
  1974年   16181篇
  1973年   16503篇
  1972年   13868篇
  1971年   12829篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
261.
STACE, C. A. & JARVIS, C. E., 1985. TypiHcation of Linnaean taxa of annual Poaceae: Poeae related to Vulpia and Desmazeria. The status and typification of 15 Linnaean species of annual grasses related to Vulpia and Desmazeria are discussed. Of these 15, eight are represented by holotypes or lectotypes in LINN, two by lectotypes in Herb. A. van Royen (L), and one by a neotype in LINN. One (Festuca marina) is based on a pre-Linnaean polynomial and is represented by a lectotype in Herb. Sloane (BM); one (Cynosurus durus) has no known type specimens and we have chosen a Barrelier (1714) illustration as lectotype; one (Nardus aristatus) is an illegitimate name change for Nardus incurvus Gouan, for which we have selected a Scheuchzer (1719) illustration as lectotype; and finally Festuca incrassala appeared on a cancelled page of Species Plantarum and has no nomenclatural standing.  相似文献   
262.
Environmental unpredictability can influence strategies of maternal investment among eggs within a clutch. Models predict that breeding females should adopt a diversified bet-hedging strategy in unpredictable environments, but empirical field evidence from Asia is scarce. Here we tested this hypothesis by exploring spatial patterns in egg size along an altitudinal gradient in a frog species(Rana kukunoris) inhabiting the Tibetan Plateau. Within-clutch variability in egg size increased as the environment became variable(e.g., lower mean monthly temperature and mean monthly rainfall at higher altitudes), and populations in environments with more unpredictable rainfall produced eggs that were smaller and more variable in size. We provide support for a diversified bet-hedging strategy in high-altitude environments, which experience dynamic weather patterns and therefore are of unpredictable environmental quality. This strategy may be an adaptive response to lower environmental quality and higher unpredictable environmental variance. Such a strategy should increase the likelihood of breeding success and maximize maternal lifetime fitness by producing offspring that are adapted to current environmental conditions. We speculate that in high-altitude environments prone to physical disturbance, breeding females are unable to consistently produce the optimal egg size due to physiological constraints imposed by environmental conditions(e.g., duration of the active season, food availability). Species and populations whose breeding strategies are adapted to cope with uncertain environmental conditions by adjusting offspring size and therefore quality show a remarkable degree of ability to cope with future climatic changes.  相似文献   
263.
264.
265.
Energy storage in arthropods has important implications for survival and reproduction. The lipid content of 276 species of adult arthropods with wet mass in the range 0.2–6.13 g is determined to assess how lipid mass scales with body mass. The relative contribution of lipids to total body mass is investigated with respect to phylogeny, ontogeny and sex. The lipid content of adult insects, arachnids, and arthropods in general shows an isometric scaling relationship with respect to body mass (M) (Marthropod lipid = ?1.09 ×Mdry1.01 and Marthropod lipid = ?1.00 ×Mlean0.98). However, lipid allocation varies between arthropod taxa, as well as with sex and developmental stage within arthropod taxa. Female insects and arachnids generally have higher lipid contents than males, and larval holometabolous insects and juvenile arachnids have higher lipid contents than adults.  相似文献   
266.
267.
268.
269.
Mycotoxin production by various toxigenic fungi, growing on a bread analogue, was investigated at various water activities (aw) and pH combinations. Citrinin, ochratoxin A and sterigmatocystin could be detected at aw > 0·80, while patulin was only observed at aw= 0·95. These results show that some toxins may be produced at lower water activities than have been reported on synthetic media and suggest that, where possible, natural substrates should be used to investigate factors affect-ing mycotoxin production in foodstuffs.  相似文献   
270.
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