全文获取类型
收费全文 | 7486篇 |
免费 | 618篇 |
国内免费 | 887篇 |
出版年
2023年 | 111篇 |
2022年 | 115篇 |
2021年 | 140篇 |
2020年 | 166篇 |
2019年 | 218篇 |
2018年 | 246篇 |
2017年 | 253篇 |
2016年 | 291篇 |
2015年 | 251篇 |
2014年 | 347篇 |
2013年 | 455篇 |
2012年 | 198篇 |
2011年 | 334篇 |
2010年 | 262篇 |
2009年 | 376篇 |
2008年 | 430篇 |
2007年 | 416篇 |
2006年 | 366篇 |
2005年 | 346篇 |
2004年 | 316篇 |
2003年 | 280篇 |
2002年 | 233篇 |
2001年 | 167篇 |
2000年 | 166篇 |
1999年 | 171篇 |
1998年 | 140篇 |
1997年 | 168篇 |
1996年 | 132篇 |
1995年 | 144篇 |
1994年 | 151篇 |
1993年 | 133篇 |
1992年 | 125篇 |
1991年 | 129篇 |
1990年 | 117篇 |
1989年 | 101篇 |
1988年 | 102篇 |
1987年 | 75篇 |
1986年 | 83篇 |
1985年 | 98篇 |
1984年 | 130篇 |
1983年 | 98篇 |
1982年 | 100篇 |
1981年 | 83篇 |
1980年 | 80篇 |
1979年 | 60篇 |
1978年 | 23篇 |
1977年 | 15篇 |
1976年 | 11篇 |
1974年 | 8篇 |
1973年 | 10篇 |
排序方式: 共有8991条查询结果,搜索用时 15 毫秒
61.
Frank H. GLEASON David J. MIDGLEY Peter M. LETCHER Peter A. McGEE 《Mycological Research》2006,110(7):869-875
Twenty isolates from soil in the orders Spizellomycetales, Blastocladiales and Chytridiales (Chytridiomycota) grew on complex solid media supplemented with 10 g l−1 sodium chloride. In a synthetic liquid medium, 4.4 g l−1 sodium chloride strongly inhibited growth in three of the five isolates, possibly because of the effect of the ions or osmolarity of the solution. The maximum concentration for growth in synthetic liquid medium with different osmotic potentials using polyethylene glycol (PEG) varied considerably amongst the isolates. Three patterns of growth with increasing concentrations of PEG were evident among isolates within the genus Rhizophydium. Up to the concentration where growth ceased, the dry weight of each isolate either decreased, remained constant, or in one case, increased. Most of the fungi survived when incubated at room temperature for 7 d in complex liquid media supplemented with 35 g l−1 sodium chloride or 300 g l−1 PEG. These data indicate that soil Chytridiomycota can survive various osmotic potentials that may occur during the wetting and drying phases in soils. 相似文献
62.
Respiratory complex I couples the transfer of electrons from NADH to ubiquinone and the translocation of protons across the mitochondrial membrane. A detailed understanding of the midpoint reduction potentials (Em) of each redox center and the factors which influence those potentials are critical in the elucidation of the mechanism of electron transfer in this enzyme. We present accurate electrostatic interaction energies for the iron-sulfur (FeS) clusters of complex I to facilitate the development of models and the interpretation of experiments in connection to electron transfer (ET) in this enzyme. To calculate redox titration curves for the FeS clusters it is necessary to include interactions between clusters, which in turn can be used to refine Em values and validate spectroscopic assignments of each cluster. Calculated titration curves for clusters N4, N5, and N6a are discussed. Furthermore, we present some initial findings on the electrostatics of the redox centers of complex I under the influence of externally applied membrane potentials. A means of determining the location of the FeS cofactors within the holo-complex based on electrostatic arguments is proposed. A simple electrostatic model of the protein/membrane system is examined to illustrate the viability of our hypothesis. 相似文献
63.
l-Glutamic acid actively loaded into resealed brain synaptic membrane vesicles was rapidly released into the incubation medium following the introduction of KCl and CaCl2, or nigericin, or veratridine into the external medium. The KCl-induced release was enhanced by the presence of low (0.1 mM), extravesicular [Ca2+]. Neither the KCl-induced nor the veratridine-stimulated l-glutamate efflux were carrier-mediated processes. Finally, the KCl-stimulated l-glutamate efflux was dependent on the ratio of intra- to extravesicular [K+]. The observations described in this study were indicative of depolarization-induced l-glutamate release from isolated synaptic plasma membrane vesicles. 相似文献
64.
Indices of water deficit were determined under conditions of non-limited water supply in cucumber mosaic virus (CMV)-infected seedlings of the susceptible cv. Bet Alpha. An increase in the concentration of soluble solids, decrease of water and osmotic potentials, and increase of proline concentration were found in the CMV-infected cotyledons. In the cv. Shimshon, which is resistant to CMV, virus infection caused only a slight change in the concentration of the soluble solids and in the osmotic potential; water potential and proline content were not affected. Concomitantly, infectivity of cotyledons by CMV was much lower in the tolerant cv. Shimshon than in the susceptible cv. Bet Alpha. The possible association of water deficit with virus-induced growth retardation is discussed. 相似文献
65.
Alexander P.R. Theuvenet Willem M.H. Van De Wijngaard Josephus W. Van De Rijke George W.F.H. Borst-Pauwels 《生物化学与生物物理学报:生物膜》1984,775(2):161-168
The applicability of 9-aminoacridine as a probe of the surface potential of yeast cells is examined. Yeast cells are found to quench the fluorescence of the dye and it is shown that this quenching is caused by a decrease in the dye concentration in the bulk aqueous phase. Consistent with predictions of the Gouy-Chapman theory the dye is displaced from the surface of the yeast cells by addition of salts, the effectiveness of the salts being related to the valency of the cation: . It is shown that 9-aminoacridine is predominantly bound by the plasma membrane of the cells. Only a minor part of the binding occurs in the cell wall, in line with the finding that enzymic removal does not significantly affect the binding of the dye to the cells. A single relationship for the distribution ratio of the dye between cells and medium with the ζ potential of the cells is found, irrespective of the way the ζ potential is changed, either by varying the pH or the Ca2+ concentration. It is argued that the electrostatic potentials probed by the dye are much higher than the corresponding ζ potentials and are of the same order of magnitude of the presumed discrete charge potentials experienced by cation transporters in the plasma membrane. It is concluded that 9-aminoacridine may be applied as a convenient and almost quantitative probe of the surface potential that effects the kinetics of ion uptake by the yeast cells. 相似文献
66.
John Boreman 《Environmental Biology of Fishes》1997,48(1-4):399-405
Sturgeons and paddlefish exhibit unusual combinations of morphology, habits, and life history characteristics, which make them highly vulnerable to impacts from human activities, particularly fisheries. Five North American sturgeons (shortnose, Gulf, pallid, Alabama, and green sturgeon) are listed as endangered or threatened by management authorities. Managers have instituted fishery closures for the three other species of North American sturgeons (Atlantic, white, and shovelnose) and paddlefish because of low stock abundance at some point in this century. Reproductive potential in four species I examined (Atlantic, white, and shortnose sturgeon, and paddlefish) is more sensitive to fishing mortality than it is for three other intensively-fished coastal species in North America: striped bass, winter flounder, and bluefish. The sturgeons and paddlefish are generally longer-lived than the three other coastal species, and also have an older age at full maturity, lower maximum fecundity values, and older ages at which 50% of the lifetime egg production is realized with no fishing mortality. 相似文献
67.
Background The cost–benefit model for the evolution of botanical carnivory provides a conceptual framework for interpreting a wide range of comparative and experimental studies on carnivorous plants. This model assumes that the modified leaves called traps represent a significant cost for the plant, and this cost is outweighed by the benefits from increased nutrient uptake from prey, in terms of enhancing the rate of photosynthesis per unit leaf mass or area (AN) in the microsites inhabited by carnivorous plants.Scope This review summarizes results from the classical interpretation of the cost–benefit model for evolution of botanical carnivory and highlights the costs and benefits of active trapping mechanisms, including water pumping, electrical signalling and accumulation of jasmonates. Novel alternative sequestration strategies (utilization of leaf litter and faeces) in carnivorous plants are also discussed in the context of the cost–benefit model.Conclusions Traps of carnivorous plants have lower AN than leaves, and the leaves have higher AN after feeding. Prey digestion, water pumping and electrical signalling represent a significant carbon cost (as an increased rate of respiration, RD) for carnivorous plants. On the other hand, jasmonate accumulation during the digestive period and reprogramming of gene expression from growth and photosynthesis to prey digestion optimizes enzyme production in comparison with constitutive secretion. This inducibility may have evolved as a cost-saving strategy beneficial for carnivorous plants. The similarities between plant defence mechanisms and botanical carnivory are highlighted. 相似文献
68.
69.
Two experiments examined simultaneous changes in leaf area (AL), root length (Lr), stomatal conductance (gs), leaf water potential (ΨL), transpiration and hydraulic plant conductance per unit leaf area (G) during the first three shoot cycles of northern red oak (Quercus rubra L.) grown under favourable and controlled conditions. Each shoot cycle consisted of bud swell, stem elongation, leaf expansion and rest; roots grew almost continuously. The gs of all leaves decreased substantially while leaves of the newest flush were expanding and increased modestly when seedling leaf area remained constant. Overall, gs decreased. The ΨL of mature leaves decreased during leaf expansion and increased by an equivalent amount during intervening periods. Possible explanations for the paired changes in gs and ΨL are considered. Changes in G closely paralleled those of canopy gs. These parallel changes during polycyclic seedling growth should act to keep seedling ΨL relatively constant as plant size increases and thereby help prevent ΨL from dropping to levels that would cause runaway embolism. 相似文献
70.
An objective, quantifiable index of structural biodiversity that could be rapidly obtained with reduced or no field effort is essential for the use of structure as universal ecological indicator for ecosystem management. Active remote sensing provides a rapid assessment tool to potentially guide land managers in highly dynamic and spatially complex landscapes. These landscapes are often dependent on frequent disturbance regimes and characterized by high endemism.We propose a modified Shannon–Wiener Index and modified Evenness Index as stand structural complexity indices for surrogates of ecosystem health. These structural indices are validated at Tall Timbers Research Station the site of one of the longest running fire ecology studies in southeastern U.S. This site is dominated by highly dynamic pine-grassland woodlands maintained with frequent fire. Once the dominant ecosystem in the Southeast, this woodland complex has been cleared for agriculture or converted to other cover types, and depends on a frequent (1- to 3-year fire return interval) low- to moderate-intensity fire regime to prevent succession to mixed hardwood forests and maintain understory species diversity. Structural evaluation of the impact of multiple disturbance regimes included height profiles and derived metrics for five different fire interval treatments; 1-year, 2-year, 3-year, mixed fire frequency (a combination of 2- and 4-year fire returns), and fire exclusion. The 3-dimensional spatial arrangement of structural elements was used to assess hardwood encroachment and changes in structural complexity. In agreement with other research, 3-year fire return interval was considered to be the best fire interval treatment for maintaining the pine-grassland woodlands, because canopy cover and vertical diversity indices were shown to be statistically higher in fire excluded and less frequently burned plots than in 1- and 2-year fire interval treatments. We developed a LiDAR-derived structural diversity index, LHDI, and propose that an ecosystem-specific threshold target for management intervention can be developed, based on significant shifts in structure and composition using this new index.Structural diversity indices can be valuable surrogates of ecosystem biodiversity, and ecosystem-specific target values can be developed as objective quantifiable goals for conservation and ecosystem integrity, particularly in remote areas. 相似文献