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   4623篇
  免费   552篇
  国内免费   63篇
  2022年   38篇
  2021年   56篇
  2020年   42篇
  2019年   61篇
  2018年   59篇
  2017年   52篇
  2016年   78篇
  2015年   135篇
  2014年   145篇
  2013年   179篇
  2012年   223篇
  2011年   209篇
  2010年   167篇
  2009年   122篇
  2008年   213篇
  2007年   209篇
  2006年   177篇
  2005年   182篇
  2004年   159篇
  2003年   139篇
  2002年   139篇
  2001年   132篇
  2000年   162篇
  1999年   123篇
  1998年   63篇
  1997年   64篇
  1996年   64篇
  1995年   55篇
  1994年   41篇
  1993年   45篇
  1992年   100篇
  1991年   102篇
  1990年   93篇
  1989年   84篇
  1988年   87篇
  1987年   85篇
  1986年   62篇
  1985年   71篇
  1984年   77篇
  1983年   57篇
  1982年   47篇
  1981年   47篇
  1980年   49篇
  1979年   82篇
  1978年   55篇
  1977年   37篇
  1976年   45篇
  1975年   36篇
  1974年   41篇
  1973年   42篇
排序方式: 共有5238条查询结果,搜索用时 46 毫秒
1.
2.
3.
A new species, Galearis huanglongensis Q.W.Meng & Y.B.Luo, is described and illustrated. It is similar to Galearis cyclochila (Franch. & Sav.) Soó and Galearis diantha (Schltr.) P.F.Hunt, but differs in having a short spur, two elliptical lateral stigma lobes and distinctly separated bursicles. This new species is known only from the type locality, the Huanglong Valley, Songpan County, western Sichuan, China, growing amongst mosses under alpine shrubs at an elevation of about 3000 m. Based on two years of observations of its population size, the species was categorized as critically endangered CR (B1a, B2a) according to the World Conservation Union (IUCN) Red List Categories and Criteria, Version 3.1. The micromorphology of pollinia and seeds was observed by scanning electron microscopy and compared with that of G. cyclochila and G. diantha. The results supported G. huanglongensis Q.W.Meng & Y.B.Luo as a new species. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158 , 689–695.  相似文献   
4.
The effects of three aryl acetylenes, 1-ethynylpyrene (EP), 2-ethynylnaphthalene (EN) and 3-ethynylperylene (EPE), upon the metabolism of benzo[a]pyrene (BaP) by microsomes isolated from rat liver were investigated. These aryl acetylenes all inhibited the total metabolism of BaP. Formation of BaP 7,8-dihydrodiol and BaP tetrol products by microsomal preparations from rats that had been pretreated with 3-methylcholanthrene (3MC) were preferentially inhibited. The effects of EP upon the metabolism of BaP 7,8-dihydrodiol by microsomes from rat liver were also studied. This aryl acetylene strongly inhibited the formation of BaP tetrols from BaP 7,8-dihydrodiol by liver microsomes both from untreated rats and from rats pretreated with 3MC, but enhanced the conversion of the BaP dihydrodiol into other metabolites.  相似文献   
5.
Camera trapping studies have become increasingly popular to produce population estimates of individually recognisable mammals. Yet, monitoring techniques for rare species which occur at extremely low densities are lacking. Additionally, species which have unpredictable movements may make obtaining reliable population estimates challenging due to low detectability. Our study explores the effectiveness of intensive camera trapping for estimating cheetah (Acinonyx jubatus) numbers. Using both a more traditional, systematic grid approach and pre-determined, targeted sites for camera placement, the cheetah population of the Northern Tuli Game Reserve, Botswana was sampled between December 2012 and October 2013. Placement of cameras in a regular grid pattern yielded very few (n = 9) cheetah images and these were insufficient to estimate cheetah density. However, pre-selected cheetah scent-marking posts provided 53 images of seven adult cheetahs (0.61 ± 0.18 cheetahs/100km²). While increasing the length of the camera trapping survey from 90 to 130 days increased the total number of cheetah images obtained (from 53 to 200), no new individuals were recorded and the estimated population density remained stable. Thus, our study demonstrates that targeted camera placement (irrespective of survey duration) is necessary for reliably assessing cheetah densities where populations are naturally very low or dominated by transient individuals. Significantly our approach can easily be applied to other rare predator species.  相似文献   
6.
7.
Cimetidine is widely prescribed for the treatment of peptic ulcer disease and has recently been shown to inhibit the metabolism of warfarin, antipyrine and diazepam. To further examine this phenomenon we investigated the effect of various doses of cimetidine and other related drugs on 14C-aminopyrine, 14C-phenacetin and 14C-caffeine breath tests. Cimetidine caused a dose-related inhibition of the metabolism of aminopyrine and caffeine but had no effect on the phenacetin breath test. Metiamide, H1-antihistamines, phenothiazines and local anesthetics also inhibited the aminopyrine breath test. Cyproheptadine had no effect on either phenacetin or caffeine elimination. This study demonstrates a complex drug-drug interaction which may have widespread clinical implications.  相似文献   
8.
9.
The transplantation of small pieces of tissue from the limb buds of 9 1/2 -10 day hamster embryos to the wing bud of the chick results in the induction of supernumerary wing structures. The anteroposterior polarity of these induced structures is under the control of the transplanted hamster tissue. The developing hamster limb thus has limb polarizing activity similar to that found in avian species and, as in the chick, the activity is found primarily in the posterior region of the limb bud.  相似文献   
10.
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