全文获取类型
收费全文 | 4103篇 |
免费 | 307篇 |
国内免费 | 2篇 |
出版年
2023年 | 12篇 |
2022年 | 35篇 |
2021年 | 58篇 |
2020年 | 47篇 |
2019年 | 51篇 |
2018年 | 53篇 |
2017年 | 73篇 |
2016年 | 92篇 |
2015年 | 187篇 |
2014年 | 184篇 |
2013年 | 258篇 |
2012年 | 331篇 |
2011年 | 352篇 |
2010年 | 225篇 |
2009年 | 203篇 |
2008年 | 297篇 |
2007年 | 242篇 |
2006年 | 251篇 |
2005年 | 232篇 |
2004年 | 225篇 |
2003年 | 237篇 |
2002年 | 204篇 |
2001年 | 48篇 |
2000年 | 34篇 |
1999年 | 52篇 |
1998年 | 52篇 |
1997年 | 36篇 |
1996年 | 45篇 |
1995年 | 41篇 |
1994年 | 31篇 |
1993年 | 20篇 |
1992年 | 36篇 |
1991年 | 13篇 |
1990年 | 20篇 |
1989年 | 16篇 |
1988年 | 15篇 |
1987年 | 13篇 |
1986年 | 7篇 |
1985年 | 14篇 |
1984年 | 13篇 |
1983年 | 13篇 |
1982年 | 6篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1979年 | 4篇 |
1977年 | 7篇 |
1976年 | 4篇 |
1975年 | 5篇 |
1971年 | 3篇 |
1915年 | 1篇 |
排序方式: 共有4412条查询结果,搜索用时 15 毫秒
41.
The family Draconematidae is reviewed. Diagnoses of all taxa are updated or emended, basedupon an evaluation of diagnostic features. Aphylogenetic analysis at the genus level based onparsimony suggested that Tenuidraconema belongsto the Draconematinae. A new genus Bathychaetosoma is erected to accomodate B.uchidai (Kito, 1983). It is characterized by acephalic region with a smooth, non-thickened cuticleand numerous cephalic adhesion tubes located posteriorto the head region and extending over more than twohead diameters along the cervical region. 相似文献
42.
Modeling of adaptations to physical training by using a recursive least squares algorithm 总被引:3,自引:0,他引:3
Busso Thierry; Denis Christian; Bonnefoy Regis; Geyssant Andre; Lacour Jean-Rene 《Journal of applied physiology》1997,82(5):1685-1693
Busso, Thierry, Christian Denis, Régis Bonnefoy,André Geyssant, and Jean-René Lacour. Modeling ofadaptations to physical training by using a recursive least squaresalgorithm. J. Appl. Physiol. 82(5):1685-1693, 1997.The present study assesses the usefulnessof a systems model with time-varying parameters for describing theresponses of physical performance to training. Data for two subjectswho undertook a 14-wk training on a cycle ergometer were used to testthe proposed model, and the results were compared with a model withtime-invariant parameters. Two 4-wk periods of intensive training wereseparated by a 2-wk period of reduced training and followed by a 4-wkperiod of reduced training. The systems input ascribed to the trainingdoses was made up of interval exercises and computed in arbitraryunits. The systems output was evaluated one to five times per week byusing the endurance time at a constant workload. The time-invariantparameters were fitted from actual performances by using the leastsquares method. The time-varying parameters were fitted by using arecursive least squares algorithm. The coefficients of determinationr2 were 0.875 and0.879 for the two subjects using the time-varying model, higher thanthe values of 0.682 and 0.666, respectively, obtained with thetime-invariant model. The variations over time in the model parametersresulting from the expected reduction in the residuals appearedgenerally to account for changes in responses to training. Such a modelwould be useful for investigating the underlying mechanisms ofadaptation and fatigue. 相似文献
43.
Ghislaine Le Prince Pia Delaere Christiane Fages Thierry Lefrançois Monique Touret Marcelle Salanon Marcienne Tardy 《Neurochemical research》1995,20(7):859-862
Glutamine synthetase (GS), a metabolic marker of the mature astrocyte, was investigated in the temporal neocortex of postmortem brain samples of 8 cases, either not demented or affected by senile dementia of the Alzheimer type. A negative correlation between the GS protein level and the density of both classical A4 deposits and senile plaques was evidenced. Such a correlation for GS underlies a dysfunction of the astroglial metabolism and particularly of the glutamate and ammonia neutralization. Since GS is sensitive to oxidative lesioning, the changes in GS level that were observed, occurring at the posttranslational stage, might reflect oxidative damage and have severe consequences on the pathological cascade of events. 相似文献
44.
Global scale patterns of fish species richness in rivers 总被引:6,自引:0,他引:6
Explanations of spatial and temporal variation in species richness is a central theme in community ecology Until recently, most research has focused on small-scale phenomena, often emphasizing on local environmental factors and, thus, poorly reflecting large-scale processes that organize species richness In this paper, we analyze variations in species richness of indigeneous freshwater fish on a worldwide scale We show that factors related to species-area and species-energy theories statistically explain most of the variation in freshwater fish species richness across continents Historical events supposed to influence present distributions offish are of little assistence in explaining variations in fish species richness at the global scale Our model, which uses easily measured factors, should also be of practical value to aquatic conservation biology and natural resource management 相似文献
45.
46.
Conformational change in yeast tRNAAsp 总被引:1,自引:0,他引:1
The structure of yeast tRNAAsp in aqueous solutions has been analyzed in the light of results obtained from Raman spectra recorded at from 5 to 82°C and compared to those of tRNAPhe. Firm evidence is given of a reversible conformation transition for tRNAAsp at 20°C. This transition is observed for the first time in the tRNA series. The low-temperature conformation appears to have a more regular ribose–phosphate backbone and a more effective G base-stacking. This conformational change, which occurs essentially in the D loop, could be connected to the existence of two (A and B) crystal forms obtained depending on crystallization conditions. The melting temperatures, which are different for each base stacking in tRNAAsp, lie in a range of about 70°C, much higher than for tRNAPhe. This fact is interpreted by a higher ratio of G-C base pairs in tRNAAsp. 相似文献
47.
Systematic crystallization studies on brewer's yeast aspartic acid transfer RNA have yielded different crystal forms, one of them diffracting to 3 Å resolution. The high resolution crystal form is orthorhombic ( with one molecule per asymmetric unit) and is stable for over four days under X-rays. 相似文献
48.
A new way of studying RNA-protein complexes, using neutron small angle scattering in solution, is described and was applied in the case of the system, yeast valyl-tRNA synthetase, interacting with its cognate and non cognate yeast tRNAs. It was shown that, when limited amounts of tRNA (either cognate or non cognate) are added to valyl-tRNA synthetase, a complex consisting of two enzyme molecules and one tRNA molecule is first formed. It is subsequently dissociated to a one to one complex when more tRNA is present in the solution. The association curve shows a maximum for a molecular ratio, enzyme over tRNA, equal to 2. 相似文献
49.
Mohamed Saïd Hifnawy Jacqueline Vaquette Thierry Sévenet Jean-Louis Pousset André Cavé 《Phytochemistry》1977,16(7):1035-1039
Stems and leaves of Myrtopsis macrocarpa, M. myrtoidea, M. novae-caledoniae and M. sellingii yielded terpenes, sterols, coumarins, alkaloids (furoquinolines and quinolones) and amides. A new quinolone (8-methoxy flindersine) occurs in Myrtopsis macrocarpa, a new amide (N-benzoyltryptamine) in M. myrtoidea, two new coumarins (myrsellin and myrsellinol) and a new dihydrofuroquinoline (myrtopsine) in M. sellingii. Structures of the new compounds are proposed from chemical and spectroscopic evidence. 相似文献
50.
Variations in litter size and reproductive effort within and between some populations of Lacerta vivipara 总被引:1,自引:0,他引:1
We studied the relationships between litter size, litter weight, newborn weight, relative clutch mass and the female snout-vent length in some Lacerta vivipara populations over a period of three years.
Litter size and litter weight were positively correlated with female snout-vent length in all the populations for all the years, as in most other lizard species. Relative clutch mass generally increased with female size, though correlations appear not to be very tight.
Considering the two best studied populations suggests that montane females invest less in reproduction than lowland ones.
The main reproductive traits of the species appeared highly variable between as well as within the different populations hitherto studied.
We argue that current theory about lizard reproductive strategy requires, first to work out a good estimate of reproductive effort, and second to get more information about the relations between the species and their environmental, biotic and abiotic conditions. 相似文献
Litter size and litter weight were positively correlated with female snout-vent length in all the populations for all the years, as in most other lizard species. Relative clutch mass generally increased with female size, though correlations appear not to be very tight.
Considering the two best studied populations suggests that montane females invest less in reproduction than lowland ones.
The main reproductive traits of the species appeared highly variable between as well as within the different populations hitherto studied.
We argue that current theory about lizard reproductive strategy requires, first to work out a good estimate of reproductive effort, and second to get more information about the relations between the species and their environmental, biotic and abiotic conditions. 相似文献