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   15070篇
  免费   1240篇
  国内免费   12篇
  16322篇
  2023年   75篇
  2022年   133篇
  2021年   275篇
  2020年   186篇
  2019年   193篇
  2018年   266篇
  2017年   224篇
  2016年   412篇
  2015年   645篇
  2014年   744篇
  2013年   934篇
  2012年   1180篇
  2011年   1129篇
  2010年   745篇
  2009年   673篇
  2008年   931篇
  2007年   957篇
  2006年   886篇
  2005年   831篇
  2004年   781篇
  2003年   735篇
  2002年   741篇
  2001年   184篇
  2000年   130篇
  1999年   178篇
  1998年   227篇
  1997年   151篇
  1996年   160篇
  1995年   151篇
  1994年   132篇
  1993年   128篇
  1992年   133篇
  1991年   82篇
  1990年   77篇
  1989年   59篇
  1988年   67篇
  1987年   55篇
  1986年   52篇
  1985年   69篇
  1984年   54篇
  1983年   51篇
  1982年   66篇
  1981年   50篇
  1980年   50篇
  1979年   37篇
  1978年   27篇
  1977年   43篇
  1976年   39篇
  1974年   24篇
  1973年   32篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
33.
Anin situ method, derived from anin vivo method, was used to determine nitrate reductase activity (NRA) in:i) excised barley and corn shoots and excised soybean leaves during a N-depletion experiment and; ii) roots and shoots of N-depleted barley and corn seedlings during induction of nitrate, reductase (NR). Nitrate reduction, calculated from thesein situ RNA measurements, was compared with estimates of each organ's nitrate reduction in light aerobic conditions from NO 3 consumption and a15N model (Gojonet al., 1986b). Thein situ RNA of roots strongly underestimated their15NO 3 reduction. In contrast, in barley and corn shoots and in the first trifoliolate leaves from 26-day-old, soybean, thein situ NRA assay gave a fair approximation of the true NO 3 reduction rate (relative differences ranging from −14 to +32%). In young soybean leaves (from 20-day-old plants), however, thein situ NRA strongly underestimated the actual NO 3 reduction. The physiological significance of thein situ NRA assay in shoots and roots, and its value for field studies are discussed from these results.  相似文献   
34.
Summary The inhibitory effect of various alkanols, benzyl alcohol and phenethyl alcohol on the growth rate of Clostridium acetobutylicum ATCC 824 was investigated. Inhibition of cell growth was studied by treating cultures with varied concentrations of alcohols. There was a threshold concentration above which growth inhibition occurred. The degree of inhibition was a linear function of the alcohol concentration used. The natural logarithm of the inhibition constant was shown to be: (1) a linear function of the chain length of the alkanols, (2) a linear function of the natural logarithm of the octanol/water partition coefficient for both aliphatic and cyclic alcohols.  相似文献   
35.
Four samples of the musselMytilus edulis were taken between 1984 and 1987 from Stony Brook, New York, and used to study the glucose-6-phosphate isomerase (GPI) polymorphism in this species.In vitro specific activity andin vivo flux measured in the same animals were found to be significantly correlated. A significant effect of GPI genotype on flux was observed in one of the samples; overall, significant evidence of effect of genotype on enzyme activity was also obtained. GPI activities of common genotypes tend to deviate less from the population mean than those of rare (frequency less than 5%) genotypes. This suggests the possibility that rare GPI genotypes are rare as a consequence of having biochemical properties that deviate from an optimum level and, therefore, having a lower fitness. In support of this hypothesis, we found in one of our samples that shell length is a concave function of GPI activity with an intermediate optimum activity level. The financial support provided to P.J.N.S. by the Luso-American Educational Commission (Fulbright Program), the Instituto Nacional de Investigacao Científica (Portugal), and the Faculdade de Ciências da Universidade de Lisboa during several stages of this research is gratefully acknowledged. Financial support from the Ministerio de Educatión y Ciencia (Spain) in the form of a postdoctoral Fulbright/MEC fellowship to M.S. is also gratefully acknowledged. Research was supported by National Science Foundation Grant BSR-8415060 to R.K.K. This is contribution No. 736 from the Program in Ecology and Evolution, State University of New York at Stony Brook. On leave from Departamento de Biologia Vegetal, Faculdade de Ciências, Universidade de Lisboa, Campo Grande C2, Lisboa, Portugal.  相似文献   
36.
37.
The sexual behavior and female reproductive cycles of a group of island-dwelling stumptail macaques (Macaca arctoides)were monitored over a 6-month period, yielding 530 observation hr and 268 copulations. Compared to nondominant males, the dominant male copulated at a relatively high rate throughout the cycle, but largely with one high-ranking female. The non-dominant males copulated most frequently at midcycle. Female presenting was highest at midcycle, but only to the dominant male. Cross-study discrepancies may be due to different observation methods and restricted environmental conditions that mask female-initiated sexual behavior. The more naturalistic setting of this study allowed for a fuller expression of proceptivity. Contrary to some previous conclusions, present findings suggest that both hormonal and socioenvironmental factors influence the patterns of sexual behavior found in stumptail macaque colonies.  相似文献   
38.
39.
We have developed a new algorithm ‘Complete sentencescompatibility’ (CSC) which uses single and double digestionfragments to rapidly determine restriction maps of circularDNA. From possible combinations of fragments of each simpledigestion, which we call ‘sentences of decomposition’,we construct a restriction map which combines the sentenceswhile taking into account compatibility rules. The algorithmcan also deal with experimental errors of fragment weight andcan suggest solutions that account for non-readable bands (fragmentsof zero length or multiple bands) on the gel. Because experimentsusing pairs of restrictive enzymes often result in multiplesolutions, a complementary algorithm tries to reduce the numberof proposed solutions by establishing consensus maps. The restrictionmap construction algorithm was tested on real cases, some containingmore than fifteen fragments. Execution times range from 1 –10 s on an IBM PC compatible microcomputer. Received on July 21, 1987; accepted on December 31, 1987  相似文献   
40.
InChlamydomonas eugametos gametes, phosphatidylinositol 4-phosphate (PtdInsP) and phosphatidylinositol 4,5-bisphosphate (PtdInsP2) comprised 0.4 and 0.3% of the whole-cell phospholipids. They were concentrated in the plasma membrane around the cell body and were present in low concentrations in the flagellar membrane. When gametes were fed32PO 4 - , the label was rapidly incorporated into PtdInsP and PtdInsP2 and only slowly incorporated into structural lipids such as phosphatidylethanolamine and phosphatidylglycerol. Similarly, when a pulse of32PO 4 - was chased with PO 4 - , the label was rapidly lost from the polyphosphoinositol lipids but not from the structural lipids. The major fatty acids in the polyphosphoinositides were C-22 carbon polyenoic acids (70%). The significance of these results in relationship to intracellular signalling via inositol phosphates and Ca2+ is discussed.Abbreviations InsP3 inositol 1,4,5-trisphosphate - mt/mt+ mating-type plus or minus - PtdA phosphatidic acid - PtdEtn phosphatidylethanolamine - PtdGro phosphatidylglycerol - PtdIns phosphatidylinositol - PtdInsP phosphatidylinositol 4-phosphate; - PtdInsP2 phosphatidylinositol 4,5-bisphosphate - TCA trichloroacetic acid We thank Frank Schuring for Fig. 5A and Susan Kenter, Hans Kruisselbrink, Saskia Bijvank and Nelleke Corbett for their enthousiastic assistance.  相似文献   
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