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   47119篇
  免费   3520篇
  国内免费   6343篇
  2024年   208篇
  2023年   953篇
  2022年   1284篇
  2021年   1524篇
  2020年   1508篇
  2019年   2099篇
  2018年   1694篇
  2017年   1564篇
  2016年   1598篇
  2015年   1484篇
  2014年   2268篇
  2013年   3220篇
  2012年   1926篇
  2011年   2139篇
  2010年   1687篇
  2009年   2276篇
  2008年   2319篇
  2007年   2452篇
  2006年   2208篇
  2005年   1995篇
  2004年   1725篇
  2003年   1640篇
  2002年   1472篇
  2001年   1176篇
  2000年   1059篇
  1999年   1027篇
  1998年   937篇
  1997年   799篇
  1996年   721篇
  1995年   687篇
  1994年   691篇
  1993年   634篇
  1992年   591篇
  1991年   601篇
  1990年   471篇
  1989年   413篇
  1988年   435篇
  1987年   349篇
  1986年   379篇
  1985年   571篇
  1984年   665篇
  1983年   384篇
  1982年   517篇
  1981年   498篇
  1980年   435篇
  1979年   323篇
  1978年   236篇
  1977年   247篇
  1976年   233篇
  1973年   183篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
A bacterial cDNA clone was identified carrying one third of the nucleotides coding for elongation factor EF-1 alpha from the brine shrimp Artemia. The sequence of codons corresponds with the known sequence of amino acids of EF-1 alpha in the region involved.  相似文献   
73.
74.
An examination of four species of Cirsium disclosed the presence of two new flavonoids in C. lineare. The structure of one was 5,4′-dihydroxy-6,7,3′-trimethoxyflavone (cirsilineol) 4′-monoglucoside and the other 5,3′,4′-trihydroxy-6,7-dimethoxyflavone (cirsiliol) 4′-monoglucoside. Luteolin 7-glucoside was found in C. suffultum, and pectolinarin and linarin in C. kamtschaticum and C. pectinellum.  相似文献   
75.
76.
In this paper, we study a two-species competitive system where both the species produce toxin against each other at some cost to their growth rates. A much wider set of outcomes is possible for our system. These outcomes are important contrasts to competitive exclusion or bistable attractors that are often the outcomes for competitive systems. We show that toxin helps to gain an advantage in competition for toxic species whenever the cost of toxin production remains within some moderate value; otherwise it may result in the extinction of the species itself.  相似文献   
77.
Carcinoma tissue consists of not only tumor cells but also fibroblasts, endothelial cells or vascular structures, and inflammatory cells forming the supportive tumor stroma. Therefore, the spatial distribution of proteins that promote growth and proliferation in these complex functional units is of high interest. Matrix-assisted laser desorption/ionization imaging mass spectrometry is a newly developed technique that generates spatially resolved profiles of protein signals directly from thin tissue sections. Surface-enhanced laser desorption/ionization mass spectrometry (MS)combined with tissue microdissection allows analysis of defined parts of the tissue with a higher sensitivity and a broader mass range. Nevertheless, both MS-based techniques have a limited spatial resolution. IHC is a technique that allows a resolution down to the subcellular level. However, the detection and measurement of a specific protein expression level is possible only by semiquantitative methods. Moreover, prior knowledge about the identity of the proteins of interest is necessary. In this study, we combined all three techniques to gain highest spatial resolution, sensitivity, and quantitative information. We used frozen tissue from head and neck tumors and chose two exemplary proteins (HNP1–3 and S100A8) to highlight the advantages and disadvantages of each technique. It could be shown that the combination of these three techniques results in congruent but also synergetic data. (J Histochem Cytochem 58:929–937, 2010)  相似文献   
78.
Abstract .The effect of the introduction of the entomophagous copepod Mesocyclops longisetus (Acuacultura F.C.B. strain) on the survival of Aedes aegypti immature stages in car tyres was evaluated under semi-natural conditions in the municipality of Merida, Yucatan, Mexico. Life tables were constructed for the immature stages of the mosquito in the presence and absence of M. longisetus , and the survival data were compared using log–linear models. The data set was adjusted using the GLIM statistical package and the quality of adjustment was evaluated with a chi-squared test . Survivorship curves were constructed for each treatment.
In the absence of M. longisetus , the survivorship of Ae. aegypti immature stages averaged 9%. The highest mortality rate was observed during the fourth larval instar (54%) and the resulting survival pattern corresponded to a type II survivorship curve. The mortality rate of Ae. aegypti first-instar larvae (fifty per tyre) increased more than 200-fold in the presence of M. longisetus (twenty per tyre) and the highest mortality was during the first two larval instars, where it reached 98.9%, with a resulting survivorship of 0.2%. Overall mortality was sixfold greater in the presence of the copepod than in its absence. The survival pattern of immature stages of Ae. aegypti in the presence of the copepod corresponded to a type III survivorship curve. As M. longisetus was so effective against Ae. aegypti immature stages in tyres under semi-natural conditions, its long-term effectiveness should be evaluated under socially and ecologically realistic field conditions in Mexico.  相似文献   
79.
An avian piroplasm, Nuttallia balearicae sp. n., is described from captive Balearica p. pavonina and B. p. gibbericeps. Other avian piroplasms and their validity and taxonomic status are discussed. The possible route of transmission of these parasites is also considered.  相似文献   
80.
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