全文获取类型
收费全文 | 3777篇 |
免费 | 281篇 |
国内免费 | 340篇 |
出版年
2023年 | 51篇 |
2022年 | 49篇 |
2021年 | 75篇 |
2020年 | 75篇 |
2019年 | 100篇 |
2018年 | 97篇 |
2017年 | 94篇 |
2016年 | 101篇 |
2015年 | 97篇 |
2014年 | 163篇 |
2013年 | 290篇 |
2012年 | 128篇 |
2011年 | 155篇 |
2010年 | 136篇 |
2009年 | 185篇 |
2008年 | 199篇 |
2007年 | 204篇 |
2006年 | 165篇 |
2005年 | 127篇 |
2004年 | 134篇 |
2003年 | 132篇 |
2002年 | 111篇 |
2001年 | 88篇 |
2000年 | 86篇 |
1999年 | 95篇 |
1998年 | 83篇 |
1997年 | 83篇 |
1996年 | 72篇 |
1995年 | 44篇 |
1994年 | 64篇 |
1993年 | 60篇 |
1992年 | 62篇 |
1991年 | 49篇 |
1990年 | 54篇 |
1989年 | 58篇 |
1988年 | 42篇 |
1987年 | 45篇 |
1986年 | 49篇 |
1985年 | 47篇 |
1984年 | 60篇 |
1983年 | 40篇 |
1982年 | 42篇 |
1981年 | 49篇 |
1980年 | 51篇 |
1979年 | 38篇 |
1978年 | 23篇 |
1977年 | 19篇 |
1976年 | 21篇 |
1973年 | 48篇 |
1972年 | 20篇 |
排序方式: 共有4398条查询结果,搜索用时 328 毫秒
1.
Henry C. Stevens Elaine M. Metz Percy Saboya Del Castillo Juan Díaz Alvn Mark T. Bowler 《Journal of Field Ornithology》2019,90(1):70-79
White‐sand forests are patchily distributed ecosystems covering just 5% of Amazonia that host many specialist species of birds not found elsewhere, and these forests are threatened due to their small size and human exploitation of sand for construction projects. As a result, many species of birds that are white‐sand specialists are at risk of extinction, and immediate conservation action is paramount for their survival. Our objective was to evaluate current survey methods and determine the relative effect of the size of patches of these forests on the presence or absence of white‐sand specialists. Using point counts and autonomous recorders, we surveyed avian assemblages occupying patches of white‐sand forest in the Peruvian Amazon in April 2018. Overall, we detected 126 species, including 21 white‐sand forest specialists. We detected significantly more species of birds per survey point with autonomous recorders than point counts. We also found a negative relationship between avian species richness and distance from the edge of patches of white‐sand forest, but a significant, positive relationship when only counting white‐sand specialists. Although we detected more species with autonomous recorders, point counts were more effective for detecting canopy‐dwelling passerines. Therefore, we recommend that investigators conducting surveys for rare and patchily distributed species in the tropics use a mixed‐method approach that incorporates both autonomous recorders and visual observation. Finally, our results suggest that conserving large, continuous patches of white‐sand forest may increase the likelihood of survival of species of birds that are white‐sand specialists. 相似文献
2.
3.
Hiroyoshi Kubo 《Mycoscience》2012,53(2):147-151
Pilobolus crystallinus has three putative glyceraldehyde-3-phosphate dehydrogenase (gapdh) genes (pcgapdh1, pcgapdh2 and pcgapdh3). The results of this study demonstrate that expression of pcgapdh2 was increased by irradiation and that this increased expression was correlated with the formation of asexual reproductive
organs (trophocysts). Interestingly, expression of pcgapdh2 was restricted to trophocysts. The formation of trophocysts was likely promoted by light, and the expression of pcgapdh2 was increased as a result of trophocyst formation. This is the first report that shows the regulation of a gapdh gene in an organ-specific manner in fungi. 相似文献
4.
5.
6.
7.
M. Masuyama 《Biometrical journal. Biometrische Zeitschrift》1991,33(6):725-733
A mathematical model of the linearized growth curve and its physiological interpretation by the introduction of proper age, which is proportional to the chronological age, are presented here. In the second phase, but not in the first phase, this constant of proportionality is highly correlated with the age at menarche. 相似文献
8.
9.
I Gallien F Caurant M Bordes P Bustamante P Miramand B Fernandez N Quellard P Babin 《Comparative biochemistry and physiology. Toxicology & pharmacology : CBP》2001,130(3):389-395
Top predators from the northern sub-polar and polar areas exhibit high cadmium concentrations in their tissues. In the aim to reveal possible adverse effects, samples of five Atlantic white-sided dolphins Lagenorhyncusacutus have been collected on the occasion of the drive fishery in the Faroe Islands, for ultrastructural investigations and energy dispersive X-ray microanalyses. Cadmium concentrations were less than the limit of detection in both immature individuals and ranged from 22.7 to 31.1 μg g?1 wet weight in the mature individuals. Two individuals with the highest cadmium concentrations exhibited electron dense mineral concretions in the basal membranes of the proximal tubules. They are spherocrystals made up of numerous strata mineral deposit of calcium and phosphorus together with cadmium. Cadmium has been detected with a molar ratio of Ca:Cd of 10:1 in the middle of these concretions. To our knowledge, this is the first report of such granules in a wild vertebrate. The role of these granules in the detoxification of the metal and the possible pathological effects are considered. 相似文献
10.
Cultures of the unicellular green alga Chlamydomonas reinhardii can be synchronized by light/dark cycling not only under photoautotrophic but also under mixotrophic growth conditions. We observed that cultures synchronized in the presence of acetate continue to divide synchronously for one cell-cycle period when transferred to heterotrophic growth conditions. This finding enabled us to investigate the differential effects of light on cell growth and cell division. When cells were exposed to continuous light at the beginning of the growth period they entered the division phase earlier than dark-grown cells as a consequence of an increased growth rate. Illumination at the end of the growth period, however, caused a considerable delay in cell division and an extended growth period. The light-induced delay in cell division was also observed in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of photosystem II. This finding demonstrates that cell division is directly influenced by a light/dard-responsive cell-cycle switch rather than by light/dark-dependent changes in energy metabolism. The importance of this light/dark control to the regulation of the Chlamydomonas cell cycle was investigated in comparison with other control mechanisms (size control, time control). We found that the light/dard-responsive cell-cycle switch regulates the transition from G1-to S-phase. This control mechanism is effective in cells which have attained the commitment to at least one round of DNA replication and division but have not attained the maximal cell mass which initiates cell division in the light.Abbreviations dCTP
deoxycytidine 5-triphosphate
- DCMU
3-(3,4-dichlorophenyl)-1,1-dimethylurea 相似文献