全文获取类型
收费全文 | 387篇 |
免费 | 2篇 |
出版年
2017年 | 3篇 |
2015年 | 2篇 |
2014年 | 4篇 |
2013年 | 10篇 |
2012年 | 15篇 |
2011年 | 20篇 |
2010年 | 17篇 |
2009年 | 30篇 |
2008年 | 22篇 |
2007年 | 20篇 |
2006年 | 14篇 |
2005年 | 21篇 |
2004年 | 3篇 |
2003年 | 8篇 |
2002年 | 5篇 |
2001年 | 10篇 |
2000年 | 4篇 |
1999年 | 2篇 |
1998年 | 8篇 |
1997年 | 16篇 |
1996年 | 11篇 |
1995年 | 6篇 |
1994年 | 5篇 |
1993年 | 8篇 |
1992年 | 7篇 |
1991年 | 8篇 |
1990年 | 3篇 |
1989年 | 7篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 9篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1972年 | 9篇 |
1971年 | 2篇 |
1970年 | 5篇 |
1964年 | 2篇 |
1961年 | 2篇 |
1958年 | 2篇 |
1954年 | 2篇 |
1946年 | 1篇 |
排序方式: 共有389条查询结果,搜索用时 15 毫秒
11.
12.
- 1 A simple periphyton sampler, which is cheap and easy to construct, is described. The key component is a disposable scouring disc which also serves to adsorb detached material.
- 2 Field trials show that the scouring sampler is significantly more efficient at removing and retaining periphyton from stones in streams for biomass estimation than other commonly used techniques.
- 3 The scouring sampler requires only one operator. Its small size and speed of use facilitate replication of samples. Samples can be collected from stones as small as 30mm diameter.
13.
Boron in Relation to Membrane Function in Higher Plants 总被引:18,自引:0,他引:18
POLLARD ANNE S.; PARR ADRIAN J.; LOUGHMAN BRIAN C. 《Journal of experimental botany》1977,28(4):831-841
The capacity for absorption of phosphate was shown to be reducedin Zea mays and Vicia faba suffering from boron deficiency;addition of 105 M boron rapidly restored this capacity.Root tips of normal plants also responded to the addition ofboron during a short pretreatment period prior to the determinationof phosphate absorption. Comparable effects of boron on theabsorption of chloride and rubidium ions were also demonstratedin Zea mays. Specific inhibition of ion uptake by auxins maybe relevant to the mechanism of the impaired ion transport seenduring boron deficiency. The KCl-stimulated adenosine triphosphatase (ATPase) of theroots of Zea mays was also studied, with a view to examiningthe effect of boron deficiency on the activity of the enzyme.Boron-deficient roots had a reduced ATPase activity which couldbe rapidly restored by the addition of 104 M H3BO3 1h before extraction of the enzyme. The results suggest that the activity of specific membrane componentscan be directly influenced by boron. Possible mechanisms wherebythis control is exercised include direct interaction of boronwith polyhydroxy components of the membrane and the elevationof endogenous levels of auxins. The evidence presented stronglysupports the view that boron plays an essential role in theregulation of the functions of higher plant membranes and thatthe ATPase is a possible component of the transplant process. 相似文献
14.
ROSS KONING ANNE TKACZYK PETER B. KAUFMAN RICHARD P. PHARIS WILLIAM MORF 《Physiologia plantarum》1977,40(2):119-124
It has been found in recent studies that the inflorescence and nodes (node-pulvini) are the primary sources for native gibberellins in the Avena shoot, and that GA3 is the predominant gibberellin in the inflorescence. In the present work, linear growth of next-to-last internode is drastically reduced by removal of the inflorescence and last leaf. This growth is completely abolished when the nodes are also excised. It is restored fully by the addition of GA3 when the nodes are present, and restored only partially when the nodes are deleted. Internodal growth in Avena stem segments with basal node present is also restored by native GA3-like substances extracted from Avena inflorescences and partially purified by silica gel partition column chromatography. Evidence from these studies, taken in toto, indicates that the inflorescence, nodes, and leaves supply gibberellins, leaves supply substrate, and nodes modulate the gibberellin growth response in next-to-last Avena internodes. 相似文献
15.
16.
Using a model system of 3mM NaCl and purified water, predictions were made for subjects’ability to discriminate the odd sample in a set of nine triadic tasting sequences associated with the triangle test. With such individual triads rather than triangular testing protocols which involve more than one triad, it was found necessary to modify the regular Sequential Sensitivity Analysis. The modification required a consideration of sensory adaptation effects caused by repeated tasting of the same stimulus in some of the sequences. Further confirmation was also obtained for the order of magnitude of signal strengths of water and NaCl stimuli tasted after water or NaCl prerinses. 相似文献
17.
THERESE F. MOE ANNE K. BRYSTING TOM ANDERSEN SUSANNE C. SCHNEIDER ØYVIND KASTE DAG O. HESSEN 《Freshwater Biology》2013,58(1):114-127
1. Invasive species can transform aquatic ecosystems, and the nuisance growth of the freshwater macrophyte Juncus bulbosus has become a problem in many lakes and rivers in northern Europe. It affects biodiversity strongly and conflicts with human uses, not least compromising the generation of hydroelectricity. The causes of the proliferation of these massive stands of J. bulbosus are not finally resolved, however. 2. In this study, a wide range of catchment, lake and sediment parameters (n = 34) were assessed for 139 lakes in Southern Norway, with the aim of explaining the presence or absence of J. bulbosus and to assess potential drivers behind its prolific growth. 3. Juncus bulbosus was more often present in lakes with lower pH and phosphate concentrations, and a higher element ratio of dissolved inorganic nitrogen (DIN) to total phosphorus (DIN : TotP). 4. Despite the many parameters measured across substantial environmental gradients, none explained nuisance growth. Genetic screening (amplified fragment length polymorphism fingerprinting) of plants from a subset of lakes and additional river sites also showed no genetic differences between the various growth forms. A macrophyte trophic index, however, suggested that the most problematic growth occurred in the most oligotrophic lakes. 5. The lack of consistent patterns may reflect either factors not assessed in our survey, or that the current extension of stands represents a gradual cumulative response over time, not characterised effectively in our snapshot survey. Nevertheless, we can now exclude some putative causes of nuisance growth, including in particular genetics and N‐deposition. 相似文献
18.
Different values have resulted in conflicts between anglers and conservation lobbies in the management of trout in South Africa. Key to the conflict is the demarcation of boundaries to areas in which brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss currently occur, or are likely to establish following stocking for angling. To provide a longer-term perspective on these areas, we developed models to link salmonid biological thermal thresholds to elevation. These, when applied spatially using a digital elevation model with a probability of occurrence model, provided the basis for estimating potentially available thermal habitat for these two cold water species. Here, we acknowledge that other variables (stocking history; river connectivity) also play a role in understanding trout distributions. Using a simple scenario of an increase in mean daily water temperatures of 2 °C, we demonstrated that both brown and rainbow trout are likely to exhibit considerable range reductions in the future. Because it is possible that these range restrictions will result in an increasing desire to introduce trout into areas above their current distribution limits for the maintenance of angling opportunities, conservation managers should prioritise these areas, with management interventions seeking to understand what will help to limit introductions. 相似文献
19.
PHILIP W. ATKINSON ALLAN J. BAKER KAREN A. BENNETT NIGEL A. CLARK JACQUIE A. CLARK KIMBERLY B. COLE ANNE DEKINGA AMANDA DEY SIMON GILLINGS PATRICIA M. GONZÁLEZ KEVIN KALASZ CLIVE D. T. MINTON JASON NEWTON LAWRENCE J. NILES THEUNIS PIERSMA ROBERT A. ROBINSON HUMPHREY P. SITTERS 《Journal of Applied Ecology》2007,44(4):885-895
20.
Unstable snow cover and more frequent freeze–thaw events have been predicted for montane areas in southern Norway, where stable winters are common today. These systems are important contributors to the flux of carbon (C) and nitrogen (N) to air and water. Here we quantify and compare the effects of freeze–thaw on C and N release from soils collected below Calluna, Molinia or Sphagnum. Intact organic soil cores were subjected to four different freeze–thaw regimes for four consecutive 2‐week periods: (1) slow cycling (SC) with one long freezing event during each 2‐week period, (2) fast cycling (FC) with four short freezing events during each 2‐week period, (3) permanent frost (PF) and (4) permanent thaw (PT). The freezing temperature was −5 °C and the thawing temperature was 5 °C. Before start of treatment, at the end of each 2‐week period, and during postincubation periods, carbon dioxide (CO2) emission as well as leachable dissolved organic C (DOC), dissolved organic N (DON), ammonium (NH4), nitrate (NO3) and absorbance at 254 nm were measured. In soils from all three vegetations, PF increased the release of CO2, DOC, DON and NH4 compared with PT. SC caused some scattered effects whereas FC only resulted in some increase in NO3 release below Molinia. Generally, the emission of CO2 and leaching of DOC, DON and NH4 increased in the following order: Sphagnum < Calluna < Molinia. The release of NO3 was greatest below Calluna. Our data suggest that vegetation cover and composition seem at least as important as increased soil frost for future winter fluxes of CO2, DOC, DON and dissolved inorganic N (DIN) from the soil to air and water. The freezing period needs to be sufficiently long to give significant effects. 相似文献