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   333784篇
  免费   37218篇
  国内免费   190篇
  371192篇
  2018年   2866篇
  2017年   2716篇
  2016年   3895篇
  2015年   5345篇
  2014年   6250篇
  2013年   8794篇
  2012年   10003篇
  2011年   9970篇
  2010年   6827篇
  2009年   6185篇
  2008年   8877篇
  2007年   9326篇
  2006年   8611篇
  2005年   8306篇
  2004年   8042篇
  2003年   7794篇
  2002年   7793篇
  2001年   18176篇
  2000年   18399篇
  1999年   14133篇
  1998年   4268篇
  1997年   4534篇
  1996年   4341篇
  1995年   4039篇
  1994年   3949篇
  1993年   3801篇
  1992年   11495篇
  1991年   11140篇
  1990年   10696篇
  1989年   10252篇
  1988年   9491篇
  1987年   8853篇
  1986年   8089篇
  1985年   7974篇
  1984年   6402篇
  1983年   5569篇
  1982年   4114篇
  1981年   3603篇
  1980年   3363篇
  1979年   6114篇
  1978年   4583篇
  1977年   4134篇
  1976年   3768篇
  1975年   4333篇
  1974年   4504篇
  1973年   4396篇
  1972年   4100篇
  1971年   3499篇
  1970年   3172篇
  1969年   2976篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
The fragments of cotton Gossipium hirsutum c.v. 108-f chloroplast genome were cloned in Escherichia coli cells. The cloned psbA and rbcL genes have been selected using the heterologous probes from spinach. The preliminary attempts to clone the complete psbA gene in pUC19 vector failed, probably, due to the toxicity of its product to Escherichia coli cells, and its 5'- and 3'-ends were cloned separately. Reconstruction experiments revealed that while the complete psbA gene was unable to be stably inherited by Escherichia coli cells, its structural part lacking the promoter region could be readily cloned in the bacterial cells.  相似文献   
992.
Thirteen Arabidopsis thaliana mutants with deviating epicuticular wax layers (i.e., cer mutants) were isolated by screening 13 000 transformed lines produced by the seed transformation method. After crossing the 13 mutants to some of the previously known cer mutant lines, 12 of our mutants mapped to 6 of the 21 known complementation groups (cer1 through cer4 as well as cer6 and cer10), while the other mutant corresponded to a previously unknown locus, cer21. Mutant phenotypes of 6 of the 13 mutant lines were caused by T-DNA insertions within cer genes. We also analyzed the chemical composition of the epicuticular wax layers of the cer mutants isolated in this study relative to that of Arabidopsis wild-type plants. Our results suggest that the five genes we tagged regulate different steps in wax biosynthesis, i.e., the decarbonylation of fatty aldehydes to alkanes, the elongation of hexacosanoic acid to octacosanoic acid, the reduction of fatty aldehydes to primary alcohols and the production of free aldehydes, while an insertion in the fifth gene causes an alteration in the chain length distribution of the different classes of wax compounds.  相似文献   
993.
994.
995.
996.
The level of urine selenium in healthy adult population, 230 persons, was examined. Persons were selected regarding sex, ages, and smoking habits. No differences versus these observations have been found. For a total, group values are 16.96 Se nmol/creatinine mmol, SD=5.44. It is possible from a single-void specimen to express daily excretion of selenium.  相似文献   
997.
Administration of high-dose Antide to ovariectomized monkeys results in rapid, prolonged, and reversible inhibition of gonadotropin secretion. The present study examined whether similar long-term control would be manifested in the menstrual cycle of intact primates. Antide administration at a dose of either 3.0 or 18.0 mg/kg induced rapid suppression of bioassayable LH concentrations, precipitating a concurrent fall in serum progesterone concentrations from 7.9 +/- 3.6 and 5.8 +/- 1.0 ng/ml (mean +/- SEM) on the day of injection to 0.6 +/- 0.2 and 0.5 +/- 0.1 ng/ml by 2 days post-treatment, respectively. This Antide-induced luteolysis was accompanied by the premature onset of menses within 3 days. The next menses following Antide administration was delayed. Ultimately, folliculogenesis culminating in normal follicular-phase estradiol production, ovulation, and subsequent normal luteal-phase progesterone production did occur in all treated monkeys. Menses resumed 54 +/- 9 and 75 +/- 13 days after treatment with 3.0 and 18.0 mg/kg Antide, respectively. No allergic cutaneous or peripheral reactions were seen, even at the highest dose of Antide. Thus, the long duration of action of high-dose Antide reported earlier in ovariectomized monkeys is also demonstrated in intact primates. These findings, along with the apparent absence of histamine-release effects even at high doses, suggest that Antide is a GnRH antagonist deserving clinical evaluation.  相似文献   
998.
999.
1000.
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