全文获取类型
收费全文 | 790篇 |
免费 | 39篇 |
国内免费 | 53篇 |
出版年
2024年 | 4篇 |
2023年 | 7篇 |
2022年 | 5篇 |
2021年 | 9篇 |
2020年 | 19篇 |
2019年 | 24篇 |
2018年 | 16篇 |
2017年 | 27篇 |
2016年 | 21篇 |
2015年 | 24篇 |
2014年 | 21篇 |
2013年 | 61篇 |
2012年 | 16篇 |
2011年 | 40篇 |
2010年 | 19篇 |
2009年 | 35篇 |
2008年 | 26篇 |
2007年 | 34篇 |
2006年 | 38篇 |
2005年 | 35篇 |
2004年 | 27篇 |
2003年 | 27篇 |
2002年 | 22篇 |
2001年 | 29篇 |
2000年 | 18篇 |
1999年 | 18篇 |
1998年 | 37篇 |
1997年 | 19篇 |
1996年 | 18篇 |
1995年 | 22篇 |
1994年 | 20篇 |
1993年 | 15篇 |
1992年 | 12篇 |
1991年 | 8篇 |
1990年 | 18篇 |
1989年 | 14篇 |
1988年 | 14篇 |
1987年 | 11篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 9篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 6篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有882条查询结果,搜索用时 218 毫秒
121.
Changes in Hg fractionation in soil induced by willow 总被引:1,自引:0,他引:1
This study investigated the effect of willow (Salix viminalis × S. schwerinii) on soil characteristics, including changes in Hg fractionation in the soil solid phase, and Hg accumulation and distribution
in pot-grown plants cultivated for 32 and 76 days in aged Hg-contaminated soil (30 mg Hg kg−1DW). Changes in soil pH and organic carbon content as well as in Hg fractionation were monitored in both rhizospheric soil
and in soil without plants. Mercury fractionation was performed by a 5-step sequential soil extraction procedure. Organic
carbon content increased while pH decreased in the rhizospheric soil. Both chemically defined exchangeable Hg (0.1) and Hg
bound to humic and fulvic acids (1.1) decreased in the rhizospheric soil, whereas plant accumulation of Hg increased with
cultivation time. The sum of the decrease of these two soil Hg fractions after 76 days of cultivation was approximately equal
to the amount of Hg accumulated in plants. Furthermore, the major Hg fractions (Hg bound to residual organic matter (53),
sulphides (43), and the residual fraction (2.5)) remained stable. Neither whole plant accumulation of Hg from the soil, approximately
0.2 of total Hg in soil after 76 days cultivation, nor the fraction of total plant Hg in the shoots, which accounted for about
3 of the total plant Hg pool regardless of the cultivation time, were high. The overall results suggest that plants might
be suitable for phytostabilization of aged Hg-contaminated soil, where root systems trap bioavailable Hg and help to control
both leaching of Hg and re-entrainment of Hg-containing particulates from a contaminated site. 相似文献
122.
Laurie A. McConnico Michael S. Foster 《Journal of experimental marine biology and ecology》2005,324(1):61-75
Persistence of annual plant populations requires sufficient seeds and suitable habitat for development and growth each year. Competition with perennials may prevent within site persistence and result in “fugitive” annual populations. Comparisons have been made between the population biology of annual macroalgae and terrestrial plants, but demographic information necessary to make strong comparisons is lacking for most of these algae, and life history differences may make such comparisons questionable. We studied population dynamics of the kelp Alaria marginata to determine if it was an annual and, if so, how populations persisted. This kelp is the dominant macroalga on exposed mid to low rocky intertidal shores along the Big Sur coast of California. Experimental clearings at two sites were used to assess recruitment timing and survivorship. Sporophytes were collected monthly to determine growth and fecundity. Recruitment occurred in late winter to early spring, primarily on geniculate corallines and residual A. marginata holdfasts. Thinning was inversely related to density, and occurred during the February through July growing season as larger thalli rapidly increased in length (up to 1.4 m month− 1) and formed a thick canopy. Sorus development was positively related to size, began as early as March, peaked in late August-October, and decreased as adults were removed by winter surf. Spore release was generally highest (108-109 spores individual− 11 h− 1) between October and January and associated with high water motion. Survivorship of sporophytes beyond one year was < 1%, showing the populations were annual.Field observations and experiments on effects of canopy clearing, season of clearing, and influence of substrate type on recruitment were done to assess how these annual populations persist. Massive spore production at the onset of fall storms, survival of microscopic stages for 3-4 months facilitated by microhabitat refuges, rapid growth, large size and rapid maturation of sporophytes contributed to persistence. Furthermore, the dense stands with thick canopies may suppress potential competitors via shading and abrasion. Rather than being a fugitive, this combination of growth and life history features enables A. marginata and perhaps other large, annual kelps to maintain perennial populations. 相似文献
123.
Pollution in industrial areas is a serious environmental concern, and interest in bacterial resistance to heavy metals is of practical significance. Mercury (Hg), Cadmium (Cd), and lead (Pb) are known to cause damage to living organisms, including human beings. Several marine bacteria highly resistant to mercury (BHRM) capable of growing at 25 ppm (mg L(-1)) or higher concentrations of mercury were tested during this study to evaluate their potential to detoxify Cd and Pb. Results indicate their potential of detoxification not only of Hg, but also Cd and Pb. Through biochemical and 16S rRNA gene sequence analyses, these bacteria were identified to belong to Alcaligenes faecalis (seven isolates), Bacillus pumilus (three isolates), Bacillus sp. (one isolate), Pseudomonas aeruginosa (one isolate), and Brevibacterium iodinium (one isolate). The mechanisms of heavy metal detoxification were through volatilization (for Hg), putative entrapment in the extracellular polymeric substance (for Hg, Cd and Pb) as revealed by the scanning electron microscopy and energy dispersive x-ray spectroscopy, and/or precipitation as sulfide (for Pb). These bacteria removed more than 70% of Cd and 98% of Pb within 72 and 96 h, respectively, from growth medium that had initial metal concentrations of 100 ppm. Their detoxification efficiency for Hg, Cd and Pb indicates good potential for application in bioremediation of toxic heavy metals. 相似文献
124.
Vanadium-dependent peroxidase activity in extracts of Ascophyllum nodosum growing in the intertidal region close to Roscoff/France, and algal vanadium levels, followed approximately similar seasonal variation, as deduced from a study lasting from April 2005 to March 2006. High peroxidase (PO) activity was found in extracts obtained from algae collected in between midwinter to spring [∼100-190 U per g dry mass (dm), triiodide assay] with a maximum in April. Periods of reduced PO activity lasted from summer to early winter (∼50-90 U per g dm). High vanadium levels (1.5-2.2 mg kg−1 dm) were found in algae collected from midwinter to spring, whereas reduced levels (0.6-1.4 mg kg−1 dm) were found in summer to early winter. 相似文献
125.
126.
Abstract. This study examined the influence of the fungal endophyte Neotyphodium lolii on the competitive interactions between its perennial, cool season host, Lolium perenne (perennial ryegrass), and a warm season, annual grass, Digitaria sanguinalis (large crabgrass), in densely planted stands (>1000 plant.m?2) in the glasshouse. Endophyte infection had little or no effect on L. perenne tiller production, above‐ or below‐ground biomass or root: shoot ratio in monoculture. However, endophyte infection significantly reduced L. perenne tiller production and above‐ground biomass in mixtures with D. sanguinalis. Conversely, D. sanguinalis had significantly higher above‐ground biomass and yielded more seed (g) when competing with endophyte infected L. perenne. An apparent trade‐off between allocation of resources to reproductive vs root tissues was observed in D. sanguinalis– root: shoot ratio was significantly lower when competing with endophyte infected L. perenne. Results indicate negative ecological consequences for harbouring the fungal endophyte when competing with the fast growing annual grass in newly established stands. These findings underscore the existence of a physiological cost of harbouring the fungal endophyte which is often overlooked. 相似文献
127.
The new Cu/Hg bimetallic thiocyanato-bridged coordination polymer, [Cu(2-bzpy)Hg(SCN)4]n, has been synthesized and characterized by elemental analysis, magnetic measurements, EPR, and X-ray crystallography. X-ray diffraction analysis reveals that the title complex structure creates 3D network. 相似文献
128.
Deborah M. Buehler Anita Koolhaas Thomas J. Van’t Hof Ingrid Schwabl Anne Dekinga Theunis Piersma B. Irene Tieleman 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》2009,195(5):445-451
The winter immunoenhancement hypothesis associates long nights and increased exposure to melatonin with enhanced immune function
in winter when resource availability is low and the chances of becoming ill are high. Thus, increased exposure to melatonin
in the winter could be adaptive for species facing difficult winter conditions. This idea has found some support in studies
of resident mammals. In birds, the link between day length and melatonin over the annual cycle is weaker, and contributions
of melatonin to seasonal timing are unclear. Furthermore, many species, especially migrants, do not experience the most difficult
conditions of their annual cycle in winter. In this study, we tested whether the winter immunoenhancement hypothesis holds
in an avian species, the red knot Calidris canutus. We found that melatonin duration and amplitude varied significantly over the annual cycle with the highest values occurring
in winter. However, peaks did not correspond to the winter solstice or with annual variation in immune function. Our findings
do not support the winter immunoenhancement hypothesis in knots and question whether the idea that immune function should
be bolstered in winter can be generalized to systems where winter is not the most difficult time of the year. 相似文献
129.
Donald Armstrong Mark Azevedo Dallice Mills Bonnie Bailey Brian Russell Aleta Groenig Anne Halgren Gary Banowetz Kerry McPhail 《Biological Control》2009,51(1):181-190
A novel, naturally-occurring herbicide (Germination-Arrest Factor, GAF), produced by Pseudomonas fluorescens WH6 and several related isolates of rhizosphere bacteria, irreversibly arrests germination of the seeds of a wide range of graminaceous species, including a number of important grassy weed species. GAF activity has been shown previously to be associated with a hydrophilic, low molecular weight compound that contains an acid group. In the present study, thin-layer chromatography (TLC) of extracts of WH6 culture filtrate demonstrated that GAF activity migrates on TLC plates with a particular ninhydrin-reactive compound. This compound was found to be present in GAF-producing P. fluorescens isolates and absent in P. fluorescens strains that lack the ability to produce GAF. Treatments, including mutagenesis, which resulted in the loss of GAF activity in culture filtrates from P. fluorescens WH6 were shown to result in the disappearance of this ninhydrin-reactive compound from extracts of WH6 culture filtrates or in alteration of its appearance on TLC chromatograms. The ninhydrin-reactivity of GAF indicates that it probably contains an amino group, as well as the acid group previously demonstrated, and suggests that GAF may be a small peptide or amino acid analog. Biological investigations motivated by this conclusion demonstrated that the effects of GAF in inhibiting the germination of seeds of annual bluegrass (Poa annua L.) could be counteracted by treatment with alanine or glutamine and, to lesser extent, by several other amino acids, suggesting that this compound may act by interfering with some aspect of amino acid metabolism or function. 相似文献
130.
The reproductive biology of the fanray, Platyrhina sinensis, was examined in Ariake Bay, Japan, from May 2002 until December 2006. Females reached sexual maturity at a larger size than
males [total length (TL) at 50% sexual maturity: male, 393 mm; female, 421 mm]. The present data support a distinct annual
reproductive cycle for P. sinensis. The gonadosomatic index (GSI) for mature males showed a clear seasonal trend, declining from August to November. Histological
observations showed that mature sperm in the testes occurred from August to November when monthly GSI declined. Concomitantly,
pre-ovulatory ova were observed in females collected from August to November. These data indicate that mating, ovulation and
fertilization occurred from August to November. Near-term embryos, neonates and recent post-partum females also occurred from
August to November. Additionally, all post-partum females possessed large pre-ovulatory ova. Parturition occurred from August
to November followed immediately by mating, ovulation and fertilization. Mature females become pregnant every year, and the
gestation period is almost 1 year. Fertilized uterine eggs without macroscopic embryonic development were present throughout
the annual reproductive cycle, indicating that P. sinensis utilizes embryonic diapause as its reproductive strategy. Both reproductive tracts of females were functional, and fecundity
ranged from 1 to 12 with a mean of 6.0, increasing with TL. 相似文献