全文获取类型
收费全文 | 4808篇 |
免费 | 585篇 |
国内免费 | 6篇 |
专业分类
5399篇 |
出版年
2021年 | 53篇 |
2019年 | 60篇 |
2018年 | 54篇 |
2017年 | 59篇 |
2016年 | 94篇 |
2015年 | 151篇 |
2014年 | 159篇 |
2013年 | 197篇 |
2012年 | 226篇 |
2011年 | 250篇 |
2010年 | 159篇 |
2009年 | 115篇 |
2008年 | 222篇 |
2007年 | 227篇 |
2006年 | 198篇 |
2005年 | 212篇 |
2004年 | 189篇 |
2003年 | 175篇 |
2002年 | 197篇 |
2001年 | 94篇 |
2000年 | 102篇 |
1999年 | 86篇 |
1998年 | 71篇 |
1997年 | 64篇 |
1996年 | 48篇 |
1995年 | 45篇 |
1994年 | 46篇 |
1993年 | 47篇 |
1992年 | 90篇 |
1991年 | 70篇 |
1990年 | 62篇 |
1989年 | 76篇 |
1988年 | 76篇 |
1987年 | 60篇 |
1986年 | 51篇 |
1985年 | 59篇 |
1984年 | 64篇 |
1983年 | 43篇 |
1982年 | 46篇 |
1981年 | 44篇 |
1980年 | 44篇 |
1979年 | 74篇 |
1978年 | 61篇 |
1977年 | 57篇 |
1976年 | 44篇 |
1975年 | 44篇 |
1974年 | 87篇 |
1973年 | 58篇 |
1971年 | 48篇 |
1970年 | 44篇 |
排序方式: 共有5399条查询结果,搜索用时 0 毫秒
231.
232.
THE ORIGINS OF ANGIOSPERMS AND THEIR BIOLOGICAL CONSEQUENCES by E. M. Friis, W. G. Chaloner and P. R. Crane (Eds). Cambridge University Press. Paperback Version 1989. ISBN 0 521 31173X. Price £15. ARGUMENTS ON EVOLUTION. A PALEONTOLOGIST'S PERSPECTIVE by Antoni Hoffman. Oxford University Press. 1989. 274 pages. HETEROCHRONY IN EVOLUTION: A MULTDDISCIPLINARY APPROACH. Editon M.L. McKinney. Plenum Press, New York. 1988. 348 pp. ISBN 0–306–42947–0. $95.40 £79.50 相似文献
233.
Anita Tragler Lakshmi Srinivasan Olga Shenderova Melvin McClauren Donald W. Brenner 《Molecular simulation》2013,39(1-2):121-130
Abstract A novel simulation interface is being developed as an educational tool to help students better understand fundamentals of materials science. This interface makes use of virtual reality (VR) technology consisting of PC-based graphics and a force-feedback haptic device. Visualization of atomistic processes with simultaneous tactile sensation via the haptic provides a powerful method for understanding complex phenomena that are otherwise difficult to comprehend. Modules are described that allow students to interactively explore interatomic bonding and single-atom diffusion through materials. 相似文献
234.
Chris Morris Greg Atkinson Barry Drust Kelly Marrin Warren Gregson 《Chronobiology international》2013,30(3):560-575
Chronobiological investigations into core temperature during and after exercise can involve ambulatory measurements of intestinal temperature during actual competitions, esophageal temperature measurements in laboratory simulations, or rectal temperature, which can be measured in both the field and laboratory. These sites have yet to be compared during both morning and afternoon exercise and subsequent recovery. At 08∶00 and 17∶00 h, seven recreationally active males exercised at 70% peak oxygen uptake for 30 min and then recovered passively for 30 min. During the experiment, esophageal, rectal, intestinal, and skin temperatures, plus sweat loss, heart rate, and ratings of perceived exertion (RPE), were monitored. We found that the diurnal variation in intestinal temperature responses (0.45±0.32°C; mean±SD) was significantly larger compared with rectal (0.33±0.24°C) and, particularly, esophageal temperature responses (0.21±0.20°C; p= 0.019). This reflected a greater difference of 0.25–0.40°C between the esophagus and the other two sites in the afternoon, compared to inter‐site differences of only 0.13–0.16°C in the morning. Diurnal variation was small for skin temperature, heart rate, sweat loss, and RPE responses during exercise (p>0.05). Our data suggest that the relative differences between intestinal, rectal, and esophageal temperature during exercise and subsequent recovery depend on time of day to the extent that inferences from studies on experimental and applied chronobiology will be affected. 相似文献
235.
Background and aims
Through recruitment, plants establish in novel environments. Recruitment also is the stage where plants undergo the highest mortality. We investigate the recruitment niche for Microstegium vimineum, an annual grass from East Asia spreading throughout eastern North American forests.Methods
Current observational and greenhouse research indicates that M. vimineum recruitment may be inhibited by leaf litter and promoted by soil moisture; we use field studies to experimentally test how these factors influence M. vimineum germination, seedling survival and reproduction. Specifically, we introduce M. vimineum seeds into forest microhabitats with experimentally varied levels of soil moisture and leaf litter.Results
Soil moisture increases M. vimineum germination regardless of leaf litter thickness and ameliorates seedling mortality in deep leaf litter. Seed production per m2 increases with watering, reflecting higher germination and survival, whereas per capita seed production increases with leaf litter thickness, reflecting density-dependent limits on seed production.Conclusions
The interactive effects of varied levels of soil moisture and leaf litter thickness on key M. vimineum life history stages highlight the need to consider multiple drivers, such as rainfall and local forest disturbance, when assessing how soil properties influence the establishment of invasive plants. 相似文献236.
Lesley A. Warren Patricia A. Maurice Nagina Parmar F. Grant Ferris 《Geomicrobiology journal》2013,30(1):93-115
Microbial degradation of urea was investigated as a potential geochemical catalyst for Ca carbonate precipitation and associated solid phase capture of common groundwater contaminants (Sr, UO2, Cu) in laboratory batch experiments. Bacterial degradation of urea increased pH and promoted Ca carbonate precipitation in both bacterial control and contaminant treatments. Associated solid phase capture of Sr was highly effective, capturing 95% of the 1 mM Sr added within 24 h. The results for Sr are consistent with solid solution formation rather than discrete Sr carbonate phase precipitation. In contrast, UO2 capture was not as effective, reaching only 30% of the initial 1 mM UO2 added, and also reversible, dropping to 7% by 24 h. These results likely reflect differing sites of incorporation of these two elements-Ca lattice sites for Sr versus crystal defect sites for UO2. Cu sequestration was poor, resulting from toxicity of the metal to the bacteria, which arrested urea degradation and concomitant Ca carbonate precipitation. Scanning electron microscopy (SEM) indicated a variety of morphologies reminiscent of those observed in the marine stromatolite literature. In bacterial control treatments, X-ray diffraction (XRD) analyses indicated only calcite; while in the presence of either Sr or UO2, both calcite and vaterite, a metastable polymorph of Ca carbonate, were identified. Tapping mode atomic force microscopy (AFM) indicated differences in surface microtopography among abiotic, bacterial control, and bacterial contaminant systems. These results indicate that Ca carbonate precipitation induced by passive biomineralization processes is highly effective and may provide a useful bioremediation strategy for Ca carbonate-rich aquifers where Sr contamination issues exist. 相似文献
237.
Aung Than Melvin Khee-Shing Leow Peng Chen 《The Journal of biological chemistry》2013,288(22):15520-15531
Angiotensin II (AngII), a peptide hormone released by adipocytes, can be catabolized by adipose angiotensin-converting enzyme 2 (ACE2) to form Ang(1–7). Co-expression of AngII receptors (AT1 and AT2) and Ang(1–7) receptors (Mas) in adipocytes implies the autocrine regulation of the local angiotensin system upon adipocyte functions, through yet unknown interactive mechanisms. In the present study, we reveal the adipogenic effects of Ang(1–7) through activation of Mas receptor and its subtle interplays with the antiadipogenic AngII-AT1 signaling pathways. Specifically, in human and 3T3-L1 preadipocytes, Ang(1–7)-Mas signaling promotes adipogenesis via activation of PI3K/Akt and inhibition of MAPK kinase/ERK pathways, and Ang(1–7)-Mas antagonizes the antiadipogenic effect of AngII-AT1 by inhibiting the AngII-AT1-triggered MAPK kinase/ERK pathway. The autocrine regulation of the AngII/AT1-ACE2-Ang(1–7)/Mas axis upon adipogenesis has also been revealed. This study suggests the importance of the local regulation of the delicately balanced angiotensin system upon adipogenesis and its potential as a novel therapeutic target for obesity and related metabolic disorders. 相似文献
238.
T.-C. Francis Pan Warren W. Burggren 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》2013,183(1):123-133
The ontogeny of cardiac hypoxic responses, and how such responses may be modified by rearing environment, are poorly understood in amphibians. In this study, cardiac performance was investigated in Xenopus laevis from 2 to 25 days post-fertilization (dpf). Larvae were reared under either normoxia or moderate hypoxia (PO2 = 110 mmHg), and each population was assessed in both normoxia and acute hypoxia. Heart rate (f h ) of normoxic-reared larvae exhibited an early increase from 77 ± 1 beats min?1 at 2 dpf to 153 ± 1 beats min?1 at 4 dpf, followed by gradual decreases to 123 ± 3 beats min?1 at 25 dpf. Stroke volume (SV), 6 ± 1 nl, and cardiac output (CO), 0.8 ± 0.1 μl min?1, at 5 dpf both increased by more than 40-fold to 25 dpf with rapid larval growth (~30-fold increase in body mass). When exposed to acute hypoxia, normoxic-reared larvae increased f h and CO between 5 and 25 dpf. Increased SV in acute hypoxia, produced by increased end-diastolic volume (EDV), only occurred before 10 dpf. Hypoxic-reared larvae showed decreased acute hypoxic responses of EDV, SV and CO at 7 and 10 dpf. Over the period of 2–25 dpf, cardiac scaling with mass showed scaling coefficients of ?0.04 (f h ), 1.23 (SV) and 1.19 (CO), contrary to the cardiac scaling relationships described in birds and mammals. In addition, f h scaling in hypoxic-reared larvae was altered to a shallower slope of ?0.01. Collectively, these results indicate that acute cardiac hypoxic responses develop before 5 dpf. Chronic hypoxia at a moderate level can not only modulate this cardiac reflex, but also changes cardiac scaling relationship with mass. 相似文献
239.
Dana R. Warren Kristin E. Judd Darren L. Bade Gene E. Likens Clifford E. Kraft 《Hydrobiologia》2013,717(1):119-131
Forested headwater streams play an important role in watershed nutrient dynamics, and wood is thought to be a key factor influencing habitat structure and nitrate-nitrogen dynamics in many forested streams. Because wood in streams can promote nitrogen uptake through denitrification, we hypothesized that nitrate uptake velocities would decrease following wood removal. We measured stream characteristics and nitrate uptake velocities before and after wood manipulation experiments conducted at Hubbard Brook Experimental Forest, NH, and the Sleepers River watershed, VT. The mean size of stream substrates and the amount of riffle habitat increased following wood removal. In contrast to our expectations, summer nitrate uptake velocities increased in the wood removal treatments relative to the reference treatments, possibly because wood removal increased the availability of stable substrates for periphyton growth, therefore increasing nitrate demand in these streams. Our results highlight that effects of wood on stream ecosystems occur through multiple pathways and suggest that the relative importance of these pathways may vary seasonally. 相似文献
240.
David C. Kombo Terry A. Hauser Vladimir P. Grinevich Matthew S. Melvin Jon-Paul Strachan Serguei S. Sidach Joseph Chewning Nikolai Fedorov Kartik Tallapragada Scott R. Breining Craig H. Miller 《Bioorganic & medicinal chemistry letters》2013,23(5):1450-1455
We have carried out a pharmacological evaluation of arylmethylene quinuclidine derivatives interactions with human α3β4 nAChRs subtype, using cell-based receptor binding, calcium-influx, electrophysiological patch-clamp assays and molecular modeling techniques. We have found that the compounds bind competitively to the α3β4 receptor with micromolar affinities and some of the compounds behave as non-competitive antagonists (compounds 1, 2 and 3), displaying submicromolar IC50 values. These evidences suggest a mixed mode of action for these compounds, having interactions at the orthosteric site and more pronounced interactions at an allosteric site to block agonist effects. One of the compounds, 1-benzyl-3-(diphenylmethylene)-1-azoniabicyclo[2.2.2]octane chloride (compound 3), exhibited poorly reversible use-dependent block of α3β4 channels. We also found that removal of a phenyl group from compound 1 confers a partial agonism to the derived analog (compound 6). Introducing a hydrogen-bond acceptor into the 3-benzylidene quinuclidine derivative (compound 7) increases agonism potency at the α3β4 receptor subtype. Docking into the orthosteric binding site of a α3β4 protein structure derived by comparative modeling accurately predicted the experimentally-observed trend in binding affinity. Results supported the notion that binding requires a hydrogen bond formation between the ligand basic nitrogen and the backbone carbonyl oxygen atom of the conserved Trp-149. 相似文献