全文获取类型
收费全文 | 321篇 |
免费 | 10篇 |
出版年
2013年 | 12篇 |
2012年 | 5篇 |
2011年 | 4篇 |
2010年 | 4篇 |
2009年 | 6篇 |
2008年 | 5篇 |
2007年 | 5篇 |
2006年 | 4篇 |
2005年 | 7篇 |
2004年 | 4篇 |
2000年 | 3篇 |
1999年 | 6篇 |
1998年 | 10篇 |
1997年 | 5篇 |
1996年 | 7篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1993年 | 7篇 |
1992年 | 5篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1989年 | 8篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1985年 | 5篇 |
1984年 | 10篇 |
1983年 | 4篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 9篇 |
1975年 | 11篇 |
1974年 | 3篇 |
1973年 | 7篇 |
1972年 | 12篇 |
1971年 | 10篇 |
1970年 | 4篇 |
1969年 | 3篇 |
1968年 | 5篇 |
1965年 | 4篇 |
1958年 | 4篇 |
1957年 | 9篇 |
1956年 | 3篇 |
1955年 | 4篇 |
1953年 | 6篇 |
1952年 | 3篇 |
1951年 | 3篇 |
排序方式: 共有331条查询结果,搜索用时 937 毫秒
121.
122.
RONALD G. WILSON STEPHEN K. TAHIR RAPHAEL MECHOULAM SELMA ZIMMERMAN ARTHUR M. ZIMMERMAN 《Cell biology international》1996,20(2):147-157
The negative and positive enantiomers of 7-hydroxy- Δ6-tetrahydrocannabinol-dimethylheptyl (designated HU-210 and HU-211 respectively) differentially affect undifferentiated and differentiating cultured pheochromocytoma cells (PC-12 cells). In general, cell viability and cell proliferation were suppressed to a much greater extent with HU-210 than with HU-211 in differentiating cells. The effects of these synthetic cannabinoids on the cytoskeleton of PC-12 cells were examined by epifluorescence and confocal microscopy. In both undifferentiated and differentiating PC-12 cells, HU-211 has little effect on the cytoarchitecture whereas HU-210 disrupts the distribution of microtubules and microfilaments. Vacuoles (2–4 μm) were evident in the cytoplasm of HU-210-treated cells but not in the cytoplasm of HU-211-treated cells or in vehicle controls. Tubulin and actin mRNA levels were reduced to 5 and 40 %, respectively (relative to untreated controls) in 10 μmHU-210-treated cells whereas the same concentration of HU-211 reduced tubulin and actin mRNA levels to 90 and 95 %, respectively. A comparison of the effects of the paired enantiomers and Δ1-THC on the cellular parameters studied reveals that in differentiating cells the action of Δ1-THC is intermediate between that of HU-210 and HU-211. This study demonstrates that compared to HU-210 and Δ1-THC the positive enantiomer HU-211 has little cellular activity. 相似文献
123.
Ocean acidification is an important consequence of rising levels of atmospheric CO2. The chemistry of acidification is, however, general and may disturb pH in terrestrial systems. The present study examines the effects of rising CO2 on insect eggs, which may be vulnerable to acidification because they are small, have (at least initially) poorly developed physiological systems, and support important developmental events. Newly‐laid eggs of the moth Manduca sexta are exposed to levels of CO2 between 0 and 2200 p.p.m., in air, and effects on yolk pH, total developmental time, and survival are measured. Altered CO2 has no effect, over several hours, on the pH of egg yolk, suggesting that yolk fluids are well buffered. By contrast, there is a large developmental change in yolk pH. Eggs exposed to eight different levels of CO2 for the duration of development show a small but significant parabolic response in development time. Eggs develop fastest at intermediate levels of CO2, between 400 and 1200 p.p.m., and slower at 0, 1600 and 2000 p.p.m. These results suggest that future rises in CO2 may not have strong direct effects on insect development. 相似文献
124.
125.
126.
ELKE BRANDES NAOMI KODAMA KATHERINE WHITTAKER† CHRISTOPHER WESTON† HEINZ RENNENBERG CLAUDIA KEITEL‡ MARK A. ADAMS§ ARTHUR GESSLER†‡ 《Global Change Biology》2006,12(10):1922-1939
We aimed to quantify the separate effects of photosynthetic and postphotosynthetic carbon isotope discrimination on δ13C of the fast‐turn‐over carbon pool (water soluble organic carbon and CO2 emitted from heterotrophic tissues), including their diel variation, along the pathway of carbon transport from the foliage to the base of the stem. For that purpose, we determined δ13C in total and water‐soluble organic matter of the foliage plus δ13C and δ18O in phloem organic matter of twigs and at three heights along the stem of Pinus sylvestris over a nine‐day period, including four measurements per day. These data were related to meteorological and photosynthesis parameters and to the δ13C of stem‐emitted CO2. In the canopy (foliage and twigs), the δ13C of soluble organic matter varied diurnally with amplitudes of up to 1.9‰. The greatest 13C enrichment was recorded during the night/early morning, indicating a strong influence of starch storage and remobilization on the carbon isotope signatures of sugars exported from the leaves. 13C enrichment of soluble organic matter from the leaves to the twig phloem and further on to the phloem of the stem was supposed to be a result of carbon isotope fractionation associated with metabolic processes in the source and sink tissues. CO2 emitted from the stem was enriched by 2.3–5.2‰ compared with phloem organic matter. When day‐to‐day variation was addressed, water‐soluble leaf δ13C and twig phloem δ18O were strongly influenced by ci/ca and stomatal conductance (Gs), respectively. These results show that both photosynthetic and postphotosynthetic carbon isotope fractionation influence δ13C of organic matter over time, and over the length of the basipetal transport pathway. Clearly, these influences on the δ13C of respired CO2 must be considered when using the latter for partitioning of ecosystem CO2 fluxes or when the assessment of δ13C in organic matter is applied to estimate environmental effects in ci/ca. 相似文献
127.
128.
1. Our objective was to measure the influence of hydrological connection with anabranch channels on the availability of major carbon sources in a lowland, anabranching floodplain river landscape. 2. Results show that anabranch channels are sinks for large quantities of sediment‐associated carbon, facilitated by high rates of sediment deposition, and are sources for dissolved organic carbon (DOC), partly via inundation‐stimulated release from surface sediments and leaf litter. This dual role influences ecological pattern and process at multiple spatial and temporal scales, including within‐flow pulse phase differences in carbon availability and anticlockwise hysteresis in the DOC–discharge relationship. 3. Hydrological connection with anabranch channels in riverine landscapes appears to increase the retention, concentration and diversity of carbon sources over both space and time, changing the timing of carbon transport downstream and shortening the carbon spiral at a landscape scale. 4. In contrast to floodplains, anabranches exchange carbon sources with the river ecosystem during flow pulses below bankfull. They are relatively easy to target for management because they have definable commence‐to‐flow levels and require relatively small amounts of water for connection. However, the type and amount of carbon exchanged between the anabranches and the river channel will vary depending on the frequency, magnitude and duration of flow pulses. 5. Managed, periodic connection of anabranch channels via environmental flows should be considered as an option between large flood events. 相似文献
129.
130.