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   808137篇
  免费   72510篇
  国内免费   322篇
  880969篇
  2016年   8650篇
  2015年   13092篇
  2014年   15095篇
  2013年   20921篇
  2012年   23873篇
  2011年   23795篇
  2010年   16148篇
  2009年   15350篇
  2008年   21820篇
  2007年   22375篇
  2006年   20945篇
  2005年   20205篇
  2004年   20017篇
  2003年   19394篇
  2002年   18819篇
  2001年   34396篇
  2000年   34818篇
  1999年   28092篇
  1998年   10774篇
  1997年   11453篇
  1996年   11022篇
  1995年   10397篇
  1994年   10239篇
  1993年   10031篇
  1992年   23595篇
  1991年   22848篇
  1990年   22479篇
  1989年   21806篇
  1988年   20081篇
  1987年   19744篇
  1986年   18109篇
  1985年   18190篇
  1984年   15117篇
  1983年   13201篇
  1982年   10551篇
  1981年   9400篇
  1980年   8952篇
  1979年   14547篇
  1978年   11691篇
  1977年   10564篇
  1976年   9962篇
  1975年   10806篇
  1974年   11629篇
  1973年   11417篇
  1972年   10224篇
  1971年   9483篇
  1970年   8111篇
  1969年   7664篇
  1968年   6843篇
  1967年   6173篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Eighty-two cultures of Fusarium species isolated in 1986 from moldy maize in Minnesota were each cultured on rice for 4 weeks and found to produce the following mycotoxins: F. graminearum isolates, deoxynivalenol (DON, 4–225 g/g), 3-acetyldeoxynivalenol (3-ADON, 2–4g/g), 15-acetyldeoxynivalenol (15-ADON, 1–35 g/g) and zearalenone (ZEA, 5–4350 g/g); F. moniliforme, fusarin C (detectable amounts to 1000 g/g); F. mòniliforme, F. oxysporum, F. proliferatum and F. subglutinans isolates, moniliformin (15–6775 g/g); F. moniliforme, F. proliferatum, and F. subglutinans isolates, fusaric acid (detectable amounts). Other mycotoxins screened for in each rice sample and not detected were T-2 toxin, HT-2 toxin, neosolaniol, T-2 tetraol, nivalenol, fusarenon-X, scirpenols, alpha and beta trans-zearalenols, wortmannin, and fusarochromanone. The rat feeding bioassay indicated that other, unidentified toxins may be present.  相似文献   
52.
An analysis of flows through primary root and first node root tissues of plants grown under conditions of salinity and nutrient deficiency induced by temperature gradients was carried out using. a mathematical model. The results obtained show that high KNO3 concentration in Knop’s nutrient solution (salinity) causes an inhibition of volume and heat flows and that the omission of KNO3 from Knop’s nutrient solution (deficiency) stimulates these flows. The causes of the inhibition lay in the fact that salinity reduced hydraulic, electric, and osmotic conductivity when compared with the control (Knop’s solution), but relative to nutrient deficiency, it increased osmotic conductivity, electrodiffusion, diffusion, and filtration of heat flow induced by the electric and heat power. The causes of the stimulation were that deficiency partially decreased conductivities, similarly as salinity when compared with the control, and also decreased osmotic abilities of the system. By contrast, it increased heat conductivity and corresponding filtrations (diffusion-thermal, thermoosmotic). In first node root tissues, it increased all conductivities with the exception of electric conductivity, then osmotic, electroosmotic, diffusion, electrodiffusion, and filtration of heat flow and current flow, that is the number of possible ways of solution transport through root tissues increased. Part II.  相似文献   
53.
Contents of volume 183, 1989  相似文献   
54.
Abstract Serial dilutions of methanogenic sludges in propionate medium gave a methanogenic non-acetoclastic enrichment degrading 1 mol of propionate to 1.6 mol of acetate and 0.17 mol of methane, with a transient accumulation of butyrate. NMR recordings showed the conversion of [2-13C]- and [3-13C]-propionate to [3-13C]- and [4-13C]-butyrate, respectively, thus demonstrating a reductive carboxylation of propionate to butyrate. The labelling found in the accumulated acetate and fermentation balances also suggested that reductive carboxylation was the major pathway involved in propionate conversion to acetate.  相似文献   
55.
Summary When 458 parents of children suffering from cystic fibrosis (CF) from all over the German Democratic Republic were interviewed to determine the number of their siblings, it was found that the maternal families had a total of 1369 children and the paternal, 1220. While the fathers of CF patients tended to originate from families with one or two children, more mothers than fathers came from families with three to twelve children (P=0.01). The average number of children in the maternal families was 2.99; in the paternal families, only 2.66. To rule out any methodological errors, sibs of mothers and fathers of various control groups were studied. We found that the number of siblings in these groups was balanced. The differences in our findings are probably due to CF heterozygosity. The underlying mechanism is unknown.  相似文献   
56.
57.
One hundred eighty sexually mature Saguinus mystax were imported from Peru in six lots over a period of 1 year. Within 1 year after arrival, the mortality was 60% and the majority of the tamarins showed signs similar to "wasting marmoset syndrome" (WMS). In an effort to improve the survival rate, an open formula diet replaced the commercial closed formula diet that had been fed since arrival of the tamarins. The open formula diet contained 26.2% crude protein, 12.3% ether extract, 43.3% nitrogen free extract and 5.9% crude fiber on a dry matter basis. The diet was evaluated on the basis of palatability, weight gain, mortality, digestibility, nitrogen balance, serum biochemical parameters and blood counts. The mean daily consumption on an as-is basis was 44.8g or 335 Kcal gross energy/Kg of body wt./day. During the 3 month open formula diet evaluation period average weight increased by 56g (p less than .05), mortality decreased demonstratively, and alopecia and chronic diarrhea were nearly eliminated. Mean daily gross energy intake for S. mystax (335 Kcal/Kg of body wt/day) was substantially greater than previously reported values for callitrichids. WMS signs observed in the S. mystax colony were controlled by providing what appears to be an adequate diet.  相似文献   
58.
This paper describes our experience with the use of high performance liquid chromatography in the analysis and preparation of several NAD-dependent dehydrogenases and oxygen-dependent oxidases. The chromatographic materials tested were from Pharmacia (Sweden), LKB (Sweden) and Lachema (Czechoslovakia), the columns were attached to the fast protein liquid chromatographic (FPLC) system from Pharmacia. The preparative use of high performance ion exchange, molecular sieve and hydrophobic interaction chromatographies as well as of chromatofocusing made it possible to prepare tens of milligrams of completely pure enzymes in several hours. In most cases a combination of two high performance methods was sufficient to yield a homogeneous enzyme. The purified enzymes were used as analytical reagents for determining the concentrations of several metabolites and activities of some enzymes. A biotechnological application of immobilized alcohol dehydrogenase for the production of reduced nicotineadenine dinucleotide from the oxidized form of the coenzyme is discussed in a greater detail.  相似文献   
59.
60.
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