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   4687篇
  免费   930篇
  国内免费   265篇
  2024年   19篇
  2023年   109篇
  2022年   139篇
  2021年   167篇
  2020年   263篇
  2019年   332篇
  2018年   308篇
  2017年   259篇
  2016年   243篇
  2015年   264篇
  2014年   270篇
  2013年   370篇
  2012年   214篇
  2011年   255篇
  2010年   194篇
  2009年   203篇
  2008年   203篇
  2007年   174篇
  2006年   203篇
  2005年   214篇
  2004年   211篇
  2003年   183篇
  2002年   146篇
  2001年   102篇
  2000年   65篇
  1999年   60篇
  1998年   55篇
  1997年   55篇
  1996年   43篇
  1995年   34篇
  1994年   44篇
  1993年   32篇
  1992年   28篇
  1991年   26篇
  1990年   36篇
  1989年   24篇
  1988年   13篇
  1987年   24篇
  1986年   17篇
  1985年   37篇
  1984年   31篇
  1983年   24篇
  1982年   21篇
  1981年   27篇
  1980年   20篇
  1979年   20篇
  1978年   29篇
  1977年   25篇
  1976年   15篇
  1975年   13篇
排序方式: 共有5882条查询结果,搜索用时 15 毫秒
91.
A new alkaloid, 3-methoxy-4,6-dihydroxymorphinandien-7-one, and norsinoacutine have been isolated from extracts of Croton bonplandianum.  相似文献   
92.
A mass spectral fragmentation pattern of permethyl 6-C-glycosylflavones is proposed from the MS data of permethyl derivatives mono-O-deuteriomethylated in the 2″-, 3″-, 4″- or 6″-positions. The synthesis of these compounds via O″-glycosyl-6-C-glucosylflavones is described.  相似文献   
93.
The triterpenoid saponins identified in plants during the period 1973 to 1978 inclusive are reviewed. Their natural occurrence, chemistry and biological activities are discussed. A compilation of all saponins, the structures of which have been fully elucidated, is included.  相似文献   
94.
Sitosterol and the following terpenic compounds have been isolated from the bark of Podocarpus lambertius: 3β-hydroxytotarol, 4β-carboxynortotarol, and macrophyllic and lambertic acids. The leaves yielded sitosterol, stigmastan-3β,5α-diol-6-one, isopimaric acid, phyllocladene, isophyllocladene, 8,9-abieten-15-ol and 17-isophyllocladenol.  相似文献   
95.
Two enantiomeric cuparene-type sesquiterpenoids, (R)-(−)-cuparene (1) and (R)-(−)-δ-cuparenol (2), have been isolated from the liverwort, Bazzania pompeana. The structures and absolute configurations of the two compounds have been determined.  相似文献   
96.
SYNOPSIS. Light and electron microscope observations on Dunaliella primolecta Butcher from logarithmic and stationary phases of batch cultures are correlated. Except for the lack of a cell wall the fine structure has typical volvocid features. The transition from logarithmic to stationary phase is marked by changes in content and size of cytoplasmic vacuoles, accumulation of cytoplasmic lipid, accumulation of starch in the plastid matrix, and by the formation of autophagosome-like bodies. The organelles in stationary-phase flagellates are closely packed together because of the cytoplasmic lipid and starch-distended chloroplast. Organisms from logarithmic phase have an abundant ribosome-packed groundplasm supporting the organelles. In the cytochemical demonstration of acid phosphatase activity, Golgi cisternae and smooth and coated Golgi vesicles contain Gomori reaction product. The possible roles of the Golgi apparatus in this flagellate are discussed.  相似文献   
97.
The new clerodane diterpene 1 was isolated from the aerial parts of Baccharis tucumanensis (Compositae). Its structure has been determined from spectral data combined with chemical transformations.  相似文献   
98.
《Cell》2022,185(23):4409-4427.e18
  1. Download : Download high-res image (172KB)
  2. Download : Download full-size image
  相似文献   
99.
Coloration plays an important role in sexual and social communication, and in many avian species both males and females maintain elaborate colours. Recent research has provided strong support for the hypothesis that elaborate female traits can be maintained by sexual or social selection; however, most research on female ornamentation has focused on pigment‐based colours, and less is known about how structural colours are maintained. Both sexes of the turquoise‐browed motmot (Eumomota superciliosa) have a blue‐green racket‐tipped tail, and it remains unknown if tail coloration serves as a sexual or social signal in one or both sexes. Here, we describe sexual dichromatism in the blue‐green portion of the tail racket, and we test for a relationship between coloration and condition, as indicated by growth bars. Tail colour of both sexes has a similar spectral shape, and there is significant, although moderate, sexual dichromatism: males are brighter than females, and males have marginally greater blue‐green saturation than females. The length of feather grown per day is positively related to overall feather brightness, but this relationship is only present in males. The relationship between male coloration and condition suggests that tail colour has the potential to convey information about individual quality during mate choice or contest competition. The lack of a similar relationship in females suggests that female tail colour does not convey the same condition‐dependent information that we suggest may be reflected by male colour. Female tail colour may therefore reflect other aspects of condition, be involved in other (non‐condition‐dependent) forms of communication, or be expressed as a non‐functional byproduct of genetic correlation between the sexes. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 673–681.  相似文献   
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