全文获取类型
收费全文 | 147篇 |
免费 | 10篇 |
专业分类
157篇 |
出版年
2011年 | 4篇 |
2010年 | 5篇 |
2009年 | 3篇 |
2008年 | 6篇 |
2007年 | 4篇 |
2002年 | 3篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1995年 | 3篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 5篇 |
1973年 | 5篇 |
1972年 | 2篇 |
1971年 | 1篇 |
1969年 | 2篇 |
1968年 | 1篇 |
1967年 | 1篇 |
1966年 | 2篇 |
1965年 | 1篇 |
1962年 | 1篇 |
1959年 | 3篇 |
1958年 | 6篇 |
1957年 | 7篇 |
1956年 | 6篇 |
1955年 | 5篇 |
1954年 | 3篇 |
1953年 | 4篇 |
1952年 | 3篇 |
1951年 | 2篇 |
1950年 | 3篇 |
1949年 | 8篇 |
1948年 | 3篇 |
排序方式: 共有157条查询结果,搜索用时 0 毫秒
1.
2.
The xylem vessels of excised adventitious roots of onion, Alliumcepa, were perfused with unbuffered nutrient solution adjustedinitially to either pH 9·3 or 3·9; the pH of thesolution after passage through the xylem, at rates not lessthan 2 xylem volume changes min1, was close to pH 6·5in both instances. The flux of H+ across the xylem/symplastboundary into mildly alkaline, phosphate-buffered solutionsperfusing the vessels could be increased greatly with increasingbuffer strength, up to a maximum value between 0·51·0pmol H+ mm2 s1. The apparent neutralization ofacidic malic acid buffers had a slightly lower maximum capacity,equivalent to 0·3 to 0·5 pmol H+mm2 s1. The addition of 5·0 pmol m3fusicoccin (FC) to the xylem perfusion solution stimulated theentry of H+ into the xylem; in unbuffered perfusion solutionsthe pH fell to pH 3·6 after a lag of 2535 min.FC additions to phosphate-buffered solutions also stimulatedthe H+ flux to an extent similar to that in unbuffered solution,viz. 0·20·4 pmol mm2 s1. The release of K+ (36Rb-labelled) into xylem sap transientlyincreased as the [K+] in weakly buffered perfusion solutionswas raised stepwise; a very marked increase being seen whenthe concentration was raised to 100 mol m3 from 40 molm3. The addition of 5·0 mmol m3 FC to theperfusing solution containing 100 mol m3 K+ rapidly decreasedthe K+ flux to the xylem as the H+ flux increased. Fusicoccinalso inhibited the flux of K+ into unbuffered perfusion solutionsbut the effect appeared reversible. Addition of 10 mmol m3abscisic acid (ABA) to the perfusion solution quickly producedtransient increases in both K+ and H+ fluxes into the xylem.In this and other experiments using weakly phosphate-bufferedperfusing solutions, H+ fluxes were comparable in size to thoseof K+ The results are consistent with the idea that the stele of onionroots contains a proton trarislocating ATPase whose activityresponds to the pH of the xylem sap. It is evident that theactivity of the proton secreting and proton neutralizing mechanismsin the xylem parenchyma control the movement of other ions acrossthe xylem/symplast boundary. Key words: Xylem perfusion, fusicoccin, abscisic acid, pH gradient 相似文献
3.
SUMMARY.
- 1 We tracked calcite saturation and seston composition during 1987 and 1988 in a shallow, hardwater prairie lake, 1 year before and 1 year after a lake-wide fish removal. We also measured the contribution of calcite to turbidity during 1988.
- 2 In both years calcite saturation increased rapidly after all ice had thawed and peaked during mid-late summer with the mineral saturation index of calcite (SI) sometimes exceeding 30.
- 3 Removal of calcite from lake seston by gentle acidification of unfiltered water samples showed suspended calcite to be an important source of lurbidity in this lake.
- 4 The lake-wide fish removal produced detectable changes in the annual cycle of calcite saturation and precipitation. Increased grazing by Daphnia galeata and Daphnia pulex apparently reduced calcite saturation during early summer by suppressing the phytoplankton, and lowering the demand for CO2.
- 5 Lower calcite precipitation, as well as direct removal of calcite by Daphnia grazing, probably contributed to the improvement in water transparency observed after the fish kill.
4.
STOREY R.; GORHAM J.; PITMAN M. G.; HANSON A. D.; GAGE D. 《Journal of experimental botany》1993,44(10):1551-1560
Melanthera biflora (Asteraceae) is a moderately salt-tolerantplant from the Indo-Pacific region. In laboratory studies itsgrowth was inhibited by salt above 50 mol m3, but itwas able to survive salinities approaching that of seawater,namely 400 mol m3. Shoot potassium concentrations weremaintained over a range of salinities up to 400 mol m3,while sodium and chloride accumulation followed closely theincrease in external osmotic pressure. In contrast, the increasein osmotic pressure of the leaf sap of Melanthera biflora, subjectedto water stress, was due mainly to a decrease in the ratio offresh weight/dry weight. 3-dimethylsulphoniopropionate (3-DMSP)and glycinebetaine were identified by fast atom bombardmentmass and 1H -NMR spectroscopy, with 3-DMSP being the main oniumcompound and glycinebetaine absent in some accessions. Onium(quaternary ammonium and/or tertiary sulphonium) compounds andproline increased during salt and water stress due mainly toa decrease in the fresh weight/dry weight ratio of tissue, althoughpart of the increase in salt-stressed tissue was due to an increasein the accumulation of the onium compound. This salt-inducedincrease in 3-DMSP was inhibited in conditions of low sulphursupply and there was no compensatory increase in proline. Key words: Melanthera biflora, Asteraceae, salinity, glycinebetaine, 3-dimethylsulphonioproprionate 相似文献
5.
The effect of current velocity and sediment on the drift of the mayfly Ephemerella subvaria Mcdunnough 总被引:4,自引:0,他引:4
SUMMARY. Experiments conducted in an artificial stream showed that significantly more nymphs drifted from an inorganic substrate at a mean current velocity of 28.5 cm s−1 than at 18.5 cm s−1 . Drift density, however, was not affected. Disproportionately large numbers of nymphs drifted while current velocities were being increased from 18.5 to 28.5 cm s−1 .
Both drift numbers and drift density were greater in turbid water, after the addition of large amounts of inorganic sediment, than under clear-flowing conditions during dark periods but not in the light. The interaction of increasing current velocity and sediment levels resulted in a significantly greater number of drifting nymphs under lighted conditions.
Minor spates which do not seriously disturb the stream bed may initiate significant increases in macroinvertebrate drift. 相似文献
Both drift numbers and drift density were greater in turbid water, after the addition of large amounts of inorganic sediment, than under clear-flowing conditions during dark periods but not in the light. The interaction of increasing current velocity and sediment levels resulted in a significantly greater number of drifting nymphs under lighted conditions.
Minor spates which do not seriously disturb the stream bed may initiate significant increases in macroinvertebrate drift. 相似文献
6.
BAI YANG STEPHEN G. PALLARDY TILDEN P. MEYERS LIAN‐HONG GU PAUL J. HANSON STAN D. WULLSCHLEGER MARK HEUER KEVIN P. HOSMAN JEFFERY S. RIGGS DANIEL W. SLUSS 《Global Change Biology》2010,16(8):2252-2271
To accurately predict ecosystem responses induced by climate warming at local‐to‐global scales, models are in need of more precise knowledge of response during periods of environmental stress such as drought. In this paper, we studied environmental control of canopy‐level water use efficiency (WUE) during drought at an eddy flux site in an oak‐hickory forest in central Missouri, USA. Two consecutive severe droughts in the summers of 2006 and 2007 afforded coverage of a broad range of environmental conditions. We stratified data to obtain subranges that minimized cross‐correlations among putative WUE‐controlling factors. Our results showed that WUE was subject to control by atmospheric saturation deficit (ASD), soil water potential (SWP) and the ratio of diffuse to total photosynthetically active radiation (If/It). Generally, WUE was found to scale with 1/(ASD)0.5, consistent with predictions from stomatal optimization theory. In contrast, SWP and If/It were related to WUE in a linear fashion. ASD was better correlated with WUE than either of the other two factors. It was also observed that the relationship between WUE and any single controlling factor was subject to influence of the other two. One such example was an opposite response of WUE to SWP between low and high ASD values, suggesting a breakdown of stomatal optimality under severe environmental stresses and a shift from optimal stomatal regulation to nonstomatal regulation at leaf scale. We have demonstrated that different data handling (stratified vs. nonstratified) or selection (hourly vs. daily) could lead to different conclusions on the relationship between WUE and its controls. For this reason, we recommend modelers to be cautious when applying WUE‐response formulas at environmental conditions or at time scales different from those at which they are derived. 相似文献
7.
8.
PATRICIA A. CHAMBERS JOHN MARK HANSON † JANICE M. BURKE ELLIE E. PREPAS 《Freshwater Biology》1990,24(1):81-91
SUMMARY. 1. The impact of crayfish on the biomass, density and shoot morphology of four submersed plant species was examined under semi-natural conditions. Male or female crayfish ( Orconectes virilis ) were held for 5 weeks at biomasses of 0, 5, 10 or 18 g m−2 (live weight) in twelve plastic pools (4.67 m2 , surface area) containing Potamogeton richardsonii, Myriophyllum exalbescens, Nuphar variegatum and Sparganium eurycarpum .
2. Crayfish significantly affected biomass, density and/or shoot morphology of all four macrophyte species. Differences in the effect of crayfish on macrophyte growth were related to plant species, crayfish sex and activity, and the abundance of alternative foods.
3. The effect of female crayfish on macrophyte growth was generally stimulatory. Myriophyllum and Potamogeton biomass, Potamogeton density and Myriophyllum length increased in the presence of female crayfish, possibly due to the reduction in herbivorous snails as a result of crayfish predation. In contrast, plant growth decreased in the presence of male crayfish: Myriophyllum, Nuphar and Potamogeton biomass, Myriophyllum and Sparganium density, and Sparganium and Poiamogeton length were reduced at male crayfish biomasses between 5 and 18 g m−2 .
4. These results indicate that even relatively low densities of crayfish can greatly affect the growth of submersed aquatic plants. Because of their ability to modify aquatic macrophyte, macroinvertebrate and, ultimately, fish communities, the introduction of crayfish into lakes where they do not occur could have a major effect on the structure and composition of the littoral zone. 相似文献
2. Crayfish significantly affected biomass, density and/or shoot morphology of all four macrophyte species. Differences in the effect of crayfish on macrophyte growth were related to plant species, crayfish sex and activity, and the abundance of alternative foods.
3. The effect of female crayfish on macrophyte growth was generally stimulatory. Myriophyllum and Potamogeton biomass, Potamogeton density and Myriophyllum length increased in the presence of female crayfish, possibly due to the reduction in herbivorous snails as a result of crayfish predation. In contrast, plant growth decreased in the presence of male crayfish: Myriophyllum, Nuphar and Potamogeton biomass, Myriophyllum and Sparganium density, and Sparganium and Poiamogeton length were reduced at male crayfish biomasses between 5 and 18 g m
4. These results indicate that even relatively low densities of crayfish can greatly affect the growth of submersed aquatic plants. Because of their ability to modify aquatic macrophyte, macroinvertebrate and, ultimately, fish communities, the introduction of crayfish into lakes where they do not occur could have a major effect on the structure and composition of the littoral zone. 相似文献
9.
10.