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   28140篇
  免费   15660篇
  国内免费   6篇
  43806篇
  2023年   20篇
  2022年   113篇
  2021年   426篇
  2020年   2214篇
  2019年   3735篇
  2018年   3870篇
  2017年   4128篇
  2016年   4129篇
  2015年   4089篇
  2014年   3725篇
  2013年   4187篇
  2012年   1883篇
  2011年   1607篇
  2010年   3132篇
  2009年   1870篇
  2008年   786篇
  2007年   398篇
  2006年   378篇
  2005年   412篇
  2004年   399篇
  2003年   381篇
  2002年   351篇
  2001年   274篇
  2000年   212篇
  1999年   159篇
  1998年   43篇
  1997年   24篇
  1996年   30篇
  1995年   43篇
  1994年   28篇
  1993年   31篇
  1992年   18篇
  1991年   21篇
  1990年   19篇
  1988年   17篇
  1987年   19篇
  1986年   11篇
  1985年   17篇
  1983年   19篇
  1982年   26篇
  1981年   14篇
  1980年   18篇
  1979年   21篇
  1978年   14篇
  1977年   14篇
  1976年   14篇
  1975年   11篇
  1971年   12篇
  1962年   11篇
  1912年   18篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
On the basis of taxonomic and nomenclatural revisions of various plant groups 37 new nomenclatural combinations are proposed mostly for European phanerogamous taxa (21 for species, 14 for subspecies, 2 for hybrids.)Medicula Medicus,Pseudofumaria Medicus andThlasidia Rafin. are accepted as taxonomically justified genera and correct generic names.Dactylorhiza longebracteata (F. W. Schmidt)Holub is proposed instead ofD. fuchsii (Druce) Soó, andHylotelephium triphyllum (Haworth) Holub isstead ofH. purpureum (L.)Holub.  相似文献   
32.
A list including 114 selected subspecies names from the bookSchübler andMartens, Flora von Würtemberg, 1834 is given. Twenty of them are very important for the present taxonomic classification.  相似文献   
33.
Four earlier proposed and invalidly published generic names ofLycopodiaceae are validated:Lateristachys (type:Lycopodium laterale R. Br.),Lycopodiastrum (type:Lycopodium casuarinoides Spring),Pseudodiphasium (type:Lycopodium volubile G. Forster) andPseudolycopodium (type:Pseudolycopodium densum (Rothm.) Holub). A new genusPseudolycopodiella (type:Lycopodium carolinianum L.) is described. New nomenclatural combinations are proposed for 17 species of these genera and ofLycopodiella.  相似文献   
34.
35.
36.
Ultrathin sectioning of submerged mycelium of Claviceps purpurea Tul. producing clavine alkaloids revealed yeast-like budding resulting in asexual sporesblastospores. These deciduous spores were born by extended hyphal cells and retained the same ultrastructure of cell organelles. Both the extended hyphae and the blastospores resembled the cells of ergot sclerotial tissue. A surface culture of C. purpurea Tul. producing no alkaloids was used as a reference.  相似文献   
37.
In many animals, mate choice is important for the maintenance of reproductive isolation between species. Traits important for mate choice and behavioral isolation are predicted to be under strong stabilizing selection within species; however, such traits can also exhibit variation at the population level driven by neutral and adaptive evolutionary processes. Here, we describe patterns of divergence among androconial and genital chemical profiles at inter‐ and intraspecific levels in mimetic Heliconius butterflies. Most variation in chemical bouquets was found between species, but there were also quantitative differences at the population level. We found a strong correlation between interspecific chemical and genetic divergence, but this correlation varied in intraspecific comparisons. We identified “indicator” compounds characteristic of particular species that included compounds already known to elicit a behavioral response, suggesting an approach for identification of candidate compounds for future behavioral studies in novel systems. Overall, the strong signal of species identity suggests a role for these compounds in species recognition, but with additional potentially neutral variation at the population level.  相似文献   
38.
39.
Many empirical studies motivated by an interest in stable coexistence have quantified negative density dependence, negative frequency dependence, or negative plant–soil feedback, but the links between these empirical results and ecological theory are not straightforward. Here, we relate these analyses to theoretical conditions for stabilisation and stable coexistence in classical competition models. By stabilisation, we mean an excess of intraspecific competition relative to interspecific competition that inherently slows or even prevents competitive exclusion. We show that most, though not all, tests demonstrating negative density dependence, negative frequency dependence, and negative plant–soil feedback constitute sufficient conditions for stabilisation of two‐species interactions if applied to data for per capita population growth rates of pairs of species, but none are necessary or sufficient conditions for stable coexistence of two species. Potential inferences are even more limited when communities involve more than two species, and when performance is measured at a single life stage or vital rate. We then discuss two approaches that enable stronger tests for stable coexistence‐invasibility experiments and model parameterisation. The model parameterisation approach can be applied to typical density‐dependence, frequency‐dependence, and plant–soil feedback data sets, and generally enables better links with mechanisms and greater insights, as demonstrated by recent studies.  相似文献   
40.
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