全文获取类型
收费全文 | 6760篇 |
免费 | 749篇 |
国内免费 | 3篇 |
出版年
2021年 | 87篇 |
2020年 | 69篇 |
2019年 | 86篇 |
2018年 | 102篇 |
2017年 | 114篇 |
2016年 | 151篇 |
2015年 | 256篇 |
2014年 | 270篇 |
2013年 | 343篇 |
2012年 | 476篇 |
2011年 | 506篇 |
2010年 | 304篇 |
2009年 | 288篇 |
2008年 | 423篇 |
2007年 | 406篇 |
2006年 | 371篇 |
2005年 | 363篇 |
2004年 | 307篇 |
2003年 | 312篇 |
2002年 | 323篇 |
2001年 | 96篇 |
2000年 | 83篇 |
1999年 | 100篇 |
1998年 | 119篇 |
1997年 | 63篇 |
1996年 | 59篇 |
1995年 | 61篇 |
1994年 | 50篇 |
1993年 | 50篇 |
1992年 | 57篇 |
1991年 | 48篇 |
1990年 | 43篇 |
1989年 | 51篇 |
1988年 | 49篇 |
1987年 | 51篇 |
1986年 | 38篇 |
1985年 | 50篇 |
1984年 | 42篇 |
1983年 | 43篇 |
1982年 | 40篇 |
1981年 | 43篇 |
1980年 | 43篇 |
1978年 | 38篇 |
1977年 | 28篇 |
1976年 | 29篇 |
1975年 | 31篇 |
1974年 | 41篇 |
1973年 | 35篇 |
1972年 | 29篇 |
1971年 | 27篇 |
排序方式: 共有7512条查询结果,搜索用时 15 毫秒
101.
James Richardson Alexander C. Lees Eliot T. Miller Stuart J. Marsden 《Ecology letters》2023,26(8):1301-1313
Understanding the composition of urban wildlife communities is crucial to promote biodiversity, ecosystem function and links between nature and people. Using crowdsourced data from over five million eBird checklists, we examined the influence of urban characteristics on avian richness and function at 8443 sites within and across 137 global cities. Under half of the species from regional pools were recorded in cities, and we found a significant phylogenetic signal for urban tolerance. Site-level avian richness was positively influenced by the extent of open forest, cultivation and wetlands and avian functional diversity by wetlands. Functional diversity co-declined with richness, but groups including granivores and aquatic birds occurred even at species-poor sites. Cities in arid areas held a higher percentage of regional species richness. Our results indicate commonalities in the influence of habitat on richness and function, as well as lower niche availability, and phylogenetic diversity across the world's cities. 相似文献
102.
103.
Helen E. O'connor David R. Stevens Stuart V. Ruffle Jonathan H. A. Nugent Saul Purton 《Plant Molecular Biology Reporter》1993,11(3):207-211
The isolation of chloroplast DNA fromChlamydomonas reinhardtii requires the efficient separation of this AT-rich genome from the GC-rich nuclear genome by density-gradient centrifugation.
We describe a simple and efficient method for separating these DNA fractions by using a sodium iodide gradient in combination
with the DNA-binding dye, bisbenzimide. The yield of chloroplast DNA is close to the theoretical maximum and the DNA is suitable
for restriction enzyme analysis and cloning. This method is applicable to the isolation of AT-rich plastid genomes from other
organisms and may be appropriate as a general method for separating species of DNA that differ in their AT/GC ratios.
An erratum to this article is available at . 相似文献
104.
105.
Richard I. Samuels Stuart E. Reynolds 《Archives of insect biochemistry and physiology》1993,24(1):33-44
Developmental profiles for a number of molting fluid (MF) enzyme activities were established and related to the progress of pupal cuticle degradation during the four days that precede the eclosion of adult tobacco hornworms. Cuticle degrading activity, molting fluid protease 1 (MFP-1), and molting fluid protease 2 (MFP-2) all increased in activity at the time that loss of material from the old cuticle occurred. In contrast, chitinase and β-acetylglucosaminidase activities did not parallel weight loss from the old cuticle. These results are consistent with the hypothesis that proteolytic activity is a prerequisite for the action of chitinase on cuticle chitin. © 1993 Wiley-Liss, Inc. 相似文献
106.
Luisa F. Gómez-Correa Camila Jaramillo-Hernández Alvaro Duque Raquel Fernandes Araujo Gabriel Arellano Stuart J. Davies Nicolas Castaño Jörg Haarpaintner Fabian Enßle Daniel Zuleta 《Biotropica》2023,55(6):1136-1147
Understanding what drives changes in tree mortality as well as the covariates influencing trees' response is a research priority to predict forest responses to global change. Here, we combined drone photogrammetry and ground-based data to assess the influence of crown exposure to light (relative to total crown area), growth deviations (relative to conspecifics), tree size, and species' wood density (as a surrogate for light-demanding and shade-tolerant life-history strategies) on the mortality of 984 canopy trees in an Amazon terra firme forest. Trees with lower wood density were less prone to die when their proportion of crown was more exposed to sunlight, but this relationship with relative crown exposure weakened and slightly reversed as wood density increased. Trees growing less than their species average had higher mortality, especially when the species' wood density decreased. The role of wood density in determining the survival of canopy trees under varying light conditions indicates differential responses of light-demanding versus shade-tolerant species. Our results highlight the importance of accounting for life-history strategies, via plant functional types, in vegetation dynamic models aiming to predict forest demography under a rapidly changing climate. Abstract in Spanish is available with online material. 相似文献
107.
108.
Stuart P. Hardegree 《American journal of botany》1989,76(3):356-360
The pressure chamber and the thermocouple psychrometer often provide different values when used to estimate plant water potential. One hypothesis to explain the discrepancy between instruments is that water movement between the xylem and symplast occurs during pressurization in the pressure chamber. Pressure chamber and thermocouple psychrometer measurements of Pinus ponderosa (Laws.) seedling shoots and mature Quercus agrifolia (Nee) shoots showed that the discrepancy is greater for Quercus. It was hypothesized that the xylem water content-water potential relationship of these species would explain the magnitude of the discrepancy between instruments. The xylem water holding capacity alone, however, does not explain the difference between species. The larger discrepancy in Quercus is likely due to a greater volume of water held in the xylem relative to the volume held in the symplast. 相似文献
109.
110.
Yu-Sen Wang Anne F. Frederick Mary M. Senior Barbara A. Lyons Stuart Black Paul Kirschmeier Louise M. Perkins Oswald Wilson 《Journal of biomolecular NMR》1996,7(2):89-98
Summary The growth factor receptor-bound protein-2 (Grb2) is an adaptor protein that mediates signal transduction pathways. Chemical shift assignments were obtained for the SH2 domain of Grb2 by heteronuclear NMR spectroscopy, employing the uniformly 13C-/15N-enriched protein as well as the protein containing selectively 15N-enriched amino acids. Using the Chemical Shift Index (CSI) method, the chemical shift indices of four nuclei, 1H, 13C, 13C and 13CO, were used to derive the secondary structure of the protein. Nuclear Overhauser enhancements (NOEs) were then employed to confirm the secondary structure. The CSI results were compared to the secondary structural elements predicted for the Grb2 SH2 domain from a sequence alignment [Lee et al. (1994) Structure, 2, 423–438]. The core structure of the SH2 domain contains an antiparallel -sheet and two -helices. In general, the secondary structural elements determined from the CSI method agree well with those predicted from the sequence alignment.Abbreviations crk
viral p47gag-crk
- EGF
epidermal growth factor
- GAP
GTPase-activating protein
- PI3K
phosphatidylinositol-3-kinase
- PLC-
phospholipase-C-, shc, src homologous and collagen
- src
sarcoma family of nonreceptor tyrosine kinase 相似文献