全文获取类型
收费全文 | 10048篇 |
免费 | 728篇 |
国内免费 | 7篇 |
专业分类
10783篇 |
出版年
2022年 | 59篇 |
2021年 | 115篇 |
2020年 | 73篇 |
2019年 | 86篇 |
2018年 | 119篇 |
2017年 | 95篇 |
2016年 | 169篇 |
2015年 | 341篇 |
2014年 | 375篇 |
2013年 | 537篇 |
2012年 | 637篇 |
2011年 | 632篇 |
2010年 | 462篇 |
2009年 | 380篇 |
2008年 | 569篇 |
2007年 | 548篇 |
2006年 | 514篇 |
2005年 | 524篇 |
2004年 | 518篇 |
2003年 | 514篇 |
2002年 | 448篇 |
2001年 | 112篇 |
2000年 | 102篇 |
1999年 | 129篇 |
1998年 | 167篇 |
1997年 | 98篇 |
1996年 | 124篇 |
1995年 | 110篇 |
1994年 | 93篇 |
1993年 | 111篇 |
1992年 | 106篇 |
1991年 | 86篇 |
1990年 | 82篇 |
1989年 | 71篇 |
1988年 | 93篇 |
1987年 | 74篇 |
1986年 | 55篇 |
1985年 | 81篇 |
1984年 | 113篇 |
1983年 | 94篇 |
1982年 | 108篇 |
1981年 | 94篇 |
1980年 | 94篇 |
1979年 | 74篇 |
1978年 | 82篇 |
1977年 | 84篇 |
1976年 | 71篇 |
1975年 | 63篇 |
1974年 | 74篇 |
1973年 | 64篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
101.
Daniele Dendi Gabriel H. Segniagbeto Roger Meek Luca Luiselli 《African Journal of Ecology》2023,61(1):226-227
Rainbow lizards (Agama agama) are common in suburban areas throughout Africa, and have an opportunistic foraging strategy, with arthropods being the main prey source. In a coastal resort in southern Togo, West Africa, several individuals in a population were observed while feeding regularly upon non-natural human-made food (pizza) and showing a clear preference for a given type of food versus others that were offered (‘four cheeses’ being the preferred one). The fact that all monitored individuals fed upon a same type of pizza suggests that they may have some chemical cues attracting them. 相似文献
102.
J. Roger Jacobs 《Developmental neurobiology》1993,24(5):611-626
The longitudinal glia (LG), progeny of a single glioblast, form a scaffold that presages the formation of longitudinal tracts in the ventral nerve cord (VNC) of the Drosophila embryo. The LG are used as a substrate during the extension of the first axons of the longitudinal tract. I have examined the differentiation of the LG in six mutations in which the longitudinal tracts were absent, displaced, or interrupted to determine whether the axon tract malformations may be attributable to disruptions in the LG scaffold. Embryos mutant for the gene prospero had no longitudinal tracts, and glial differentiation remained arrested at a preaxonogenic state. Two mutants of the Polycomb group also lacked longitudinal tracts; here the glia failed to form an oriented scaffold, but cytological differentiation of the LG was unperturbed. The longitudinal tracts in embryos mutant for slit fused at the VNC midline and scaffold formation was normal, except that it was medially displaced. Longitudinaltracts had intersegmental interruptions in embryos mutant for hindsight and midline. In hindsight, there were intersegmental gaps in the glial scaffold. In midline, the glial scaffold retracted after initial extension. LG morphogenesis during axonogenesis was abnormal in midline. Commitment to glial identity and glial differentiation also occurred before scaffold formation. In all mutants examined, the early distribution of the glycoprotein neuroglian was perturbed. This was indicative of early alterations in VNC pattern present before LG scaffold formation began. Therefore, some changes in scaffold formation may have reflected changes in the placement and differentiation of other cells of the VNC. In all mutants, alterations in scaffold formation preceded longitudinal axon tract formation. © 1993 John Wiley & Sons, Inc. 相似文献
103.
104.
Are the spatial patterns of weeds scale-invariant? 总被引:1,自引:0,他引:1
In previous empirical and modelling studies of rare species and weeds, evidence of fractal behaviour has been found. We propose that weeds in modern agricultural systems may be managed close to critical population dynamic thresholds, below which their rates of increase will be negative and where scale-invariance may be expected as a consequence. We collected detailed spatial data on five contrasting species over a period of three years in a primarily arable field. Counts in 20×20 cm contiguous quadrats, 225,000 in 1998 and 84,375 thereafter, could be re-structured into a wide range of larger quadrat sizes. These were analysed using three methods based on correlation sum, incidence and conditional incidence. We found non-trivial scale invariance for species occurring at low mean densities and where they were strongly aggregated. The fact that the scale-invariance was not found for widespread species occurring at higher densities suggests that the scaling in agricultural weed populations may, indeed, be related to critical phenomena. 相似文献
105.
WU Zhishan Keith R. Hopper Paul J. Ode Roger W. Fuester CHEN Jia-hua George E. Heimpel 《Insect Science》2003,10(2):81-93
Abstract In haplodiploid Hymenoptera, unfertilized eggs produce haploid males while fertilized eggs lead to diploid females under most circumstances. Diploid males can also be produced from fertilization under a system of sex determination known as complementary sex determination (CSD). Under single-locus CSD, sex is determined by multiple alleles at a single sex locus. Individuals heterozygous at the sex locus are female while hemizygous and homozygous individuals develop as haploid and diploid males, respectively. In multiple-locus CSD, two or more loci, each with two or more alleles, determine sex. Diploid individuals are female if one or more sex loci are heterozygous, while a diploid is male only if homozygous at all sex loci. Diploid males are known to occur in 43 hymenopteran species and single-locus CSD has been demonstrated in 22 of these species. Diploid males are either developmentally inviable or sterile, so their production constitutes a genetic load. Because diploid male production is more likely under inbreeding, CSD is a form of inbreeding depression. It is crucial to preserve the diversity of sex alleles and reduce the loss of genetic variation in biological control. In the parasitoid species with single-locus CSD, certain precautionary procedures can prevent negative effects of single-locus CSD on biological control. 相似文献
106.
Enoka, Roger M. Eccentric contractions require uniqueactivation strategies by the nervous system. J. Appl.Physiol. 81(6): 2339-2346, 1996.Eccentriccontractions occur when activated muscles are forcibly lengthened. Thismode of muscle function occurs frequently in the activities of dailyliving and in athletic competition. This review examines theexperimental evidence that provides the foundation for our currentunderstanding of the benefits, consequences, and control of eccentriccontractions. Over the past several decades, numerous studies haveestablished that eccentric contractions can maximize the force exertedand the work performed by muscle; that they are associated with agreater mechanical efficiency; that they can attenuate the mechanicaleffects of impact forces; and that they enhance the tissue damageassociated with exercise. More recent evidence adds a new feature tothis repertoire by suggesting a new hypothesis: that the neuralcommands controlling eccentric contractions are unique. Examination of this hypothesis is critical because the existence of such a control scheme would increase substantially the complexity of the strategies that the nervous system must use to control movement. 相似文献
107.
Response of compressed skinned skeletal muscle fibers to conditions that simulate fatigue 总被引:1,自引:0,他引:1
Myburgh, Kathryn H., and Roger Cooke. Response ofcompressed skinned skeletal muscle fibers to conditions that simulate fatigue. J. Appl. Physiol. 82(4):1297-1304, 1997.During fatigue, muscles become weaker, slower,and more economical at producing tension. Studies of skinned musclefibers can explain some but not all of these effects, and, inparticular, they are less economical in conditions that simulatefatigue. We investigated three factors that may contribute to thedifferent behavior of skinned fibers. 1) Skinned fibers have increasedmyofilament lattice spacing, which is reversible by osmoticcompression. 2) A myosin subunit becomes phosphorylated during fatigue.3) Inosine 5-monophosphate (IMP) accumulates during fatigue. We tested the response ofphosphorylated and unphosphorylated single skinned fibers (isometrictension, contraction velocity, and adenosinetriphosphatase activity) to changes in lattice spacing (0-5% dextran) and IMP (0-5 mM)in the presence of altered concentrations ofPi (3-25 mM),H+ (pH 7-6.2), and ADP(0-5 mM). The response of maximally activated skinned fibers tothe direct metabolites of ATP hydrolysis is not altered by osmoticcompression, phosphorylating myosin subunits, or increasing IMPconcentration. These factors, therefore, do not explain the discrepancybetween intact and skinned fibers during fatigue. 相似文献
108.
Effects of anatomic variability on blood flow and pressure gradients in the pulmonary capillaries 总被引:1,自引:0,他引:1
Dhadwal Amit; Wiggs Barry; Doerschuk Claire M.; Kamm Roger D. 《Journal of applied physiology》1997,83(5):1711-1720
Dhadwal, Amit, Barry Wiggs, Claire M. Doerschuk, and RogerD. Kamm. Effects of anatomic variability on blood flow and pressure gradients in the pulmonary capillaries. J. Appl. Physiol. 83(5): 1711-1720, 1997.Atheoretical model is developed to simulate the flow of blood throughthe capillary network in a single alveolar septum. The objective is tostudy the influence of random variability in capillary dimension andcompliance on flow patterns and pressures within the network. Thecapillary bed is represented as an interconnected rectangular grid ofcapillary segments and junctions; blood flow is produced by applying apressure gradient across the network. Preferred flow channels are shownto be a natural consequence of random anatomic variability, the effectof which is accentuated at low transcapillary pressures. Thedistribution of pressure drops across single capillary segments widenswith increasing network variability and decreasing capillary transmuralpressure. Blockage of one capillary segment causes the pressure dropacross that segment to increase by 60%, but the increase falls to<10% at a distance of three segments. The factors that causenonuniform capillary blood flow through the capillary network arediscussed. 相似文献
109.
110.