全文获取类型
收费全文 | 917篇 |
免费 | 61篇 |
国内免费 | 32篇 |
出版年
2023年 | 8篇 |
2022年 | 6篇 |
2021年 | 15篇 |
2020年 | 20篇 |
2019年 | 27篇 |
2018年 | 24篇 |
2017年 | 23篇 |
2016年 | 18篇 |
2015年 | 24篇 |
2014年 | 33篇 |
2013年 | 56篇 |
2012年 | 35篇 |
2011年 | 49篇 |
2010年 | 36篇 |
2009年 | 31篇 |
2008年 | 38篇 |
2007年 | 33篇 |
2006年 | 42篇 |
2005年 | 29篇 |
2004年 | 17篇 |
2003年 | 33篇 |
2002年 | 25篇 |
2001年 | 19篇 |
2000年 | 20篇 |
1999年 | 12篇 |
1998年 | 16篇 |
1997年 | 15篇 |
1996年 | 18篇 |
1995年 | 23篇 |
1994年 | 22篇 |
1993年 | 23篇 |
1992年 | 19篇 |
1991年 | 19篇 |
1990年 | 18篇 |
1989年 | 17篇 |
1988年 | 12篇 |
1987年 | 15篇 |
1986年 | 15篇 |
1985年 | 13篇 |
1984年 | 14篇 |
1983年 | 10篇 |
1982年 | 8篇 |
1981年 | 12篇 |
1980年 | 16篇 |
1979年 | 11篇 |
1978年 | 7篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1972年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有1010条查询结果,搜索用时 140 毫秒
91.
Mosaic cycles in agricultural landscapes of Northwest Europe 总被引:2,自引:0,他引:2
Michael Kleyer Robert Biedermann Klaus Henle Elisabeth Obermaier Hans-Joachim Poethke Peter Poschlod Boris Schrder Josef Settele Doris Vetterlein 《Basic and Applied Ecology》2007,8(4):295-309
Mosaic cycles were originally understood as cyclical regeneration phases in forests. In this review, we shall examine how far the concept can be extended towards cyclical mosaics of habitat quality in patterned landscapes as a special case of ‘dynamic landscapes’. We will concentrate on habitats and plants in European temperate agricultural landscapes and grasslands in particular. Mosaic cycles of habitat quality are characterised by spatiotemporal shifts between disturbance and secondary succession. We found evidence for mosaic cycles in traditional agricultural systems, modern crop farming, and in recent conservation management. The relevant disturbance parameters to describe land-use drivers of mosaic cycles are spatial extent, frequency, and magnitude (biomass loss). Land-use-related drivers are usually regular and deterministic in space and time, with the exception of year-round grazing by free-ranging large herbivores. Fluctuating soil resources such as water and nutrients in interaction with climate variability add a stochastic component to these (land-use-related) drivers. The proportion of deterministic and stochastic components and their autocorrelation in time and space divides purely deterministic mosaic cycles from purely stochastic dynamic landscapes. In a second part, we briefly review plant life-history traits that may facilitate survival of plants in mosaic cycles of habitat quality. Theoretical studies emphasise (i) dispersal functions for extinction and recolonisation processes of metapopulations, (ii) storage effects as a component of buffered population growth in response to temporal fluctuations of habitat quality, and (iii) competitive ability in metacommunities. We propose a simple scheme relating these functions to the temporal and spatial correlation of patterned landscapes. There are only a very limited number of field studies available that give some support for the proposed scheme. We provide perspectives for further research in this field. 相似文献
92.
We examined the impact of ploughing on soil seed banks of plant communities living in temporary marshes located in agricultural
fields. The quantity, quality and vertical distribution of seeds were quantified under ploughed or unploughed treatment at
community level. We also focussed on a typical semi-aquatic species, Damasonium alisma, to investigate the impact of ploughing at population level. We used two complementary techniques to study seed banks: hand
sorting and seedling emergence. We found that species richness of seeds, seed abundance and germination ability were strongly
affected by ploughing at community level. Concerning D. alisma, most of the seeds (56%) were stored in the two deepest soil layers among the four considered in ploughed pools. Moreover,
the germination rate was higher for buried seeds (84%) than for seeds collected at the surface (33.6%). These patterns were
almost inverted in unploughed pools. Our results agree with the temporal storage effect generally suggested to describe the
seed bank property of plant communities. But in addition, we showed that ploughing induces a spatial storage effect in accumulating
species and individuals in the seed banks that favourably influence community dynamics. We conclude that, in contrast to what
is usually thought, ploughing disturbance can be of benefit for such ephemeral wetland vegetation. 相似文献
93.
Temperature response of root‐lesion nematode (Pratylenchus thornei) reproduction on wheat cultivars has implications for resistance screening and wheat production 下载免费PDF全文
The root‐lesion nematode Pratylenchus thornei is a major pathogen of wheat and other field crops, particularly in the northern grain region of sub‐tropical eastern Australia. Research was conducted into the temperature requirements of P. thornei for reproduction on wheat to increase the reliability of selection in resistance tests for wheat breeding. Final population densities (Pf) of P. thornei were determined on four wheat cultivars (Gatcher, GS50a, Potam and Suneca) at fortnightly intervals from 8 to 18 weeks at a range of six soil temperatures (15°C, 20°C, 22.5°C, 25°C, 27.5°C and 30°C) in a glasshouse experiment. Pratylenchus thornei had the highest Pf in the temperature range of 20–25°C on all wheat cultivars at all growth times after sowing, with no nematode reproduction measured at 30°C and very little at 15°C. The wheat cv. GS50a consistently produced lower Pf than cvs Gatcher, Potam and Suneca in the optimum temperature range of 20–25°C. In carrot disc cultures, P. thornei had an optimum temperature of 25°C with little reproduction at 17.5°C and none detectable at 30°C. A standard soil temperature of 23°C was chosen to maximise differences in nematode reproduction between resistant and susceptible wheat genotypes for selection in wheat breeding, and to improve reproducibility among successive experiments. The relationships derived from these experiments will be valuable for simulation of P. thornei reproduction in crop growth models. They also indicate that early sowing of wheat into cool soil (≤15°C) in farmers' fields of the northern grain region should favour wheat growth over nematode reproduction and increase grain yield. 相似文献
94.
95.
AAV‐mediated expression of BAG1 and ROCK2‐shRNA promote neuronal survival and axonal sprouting in a rat model of rubrospinal tract injury 下载免费PDF全文
Malleswari Challagundla Jan Christoph Koch Vinicius Toledo Ribas Uwe Michel Sebastian Kügler Thomas Ostendorf Frank Bradke Hans Werner Müller Mathias Bähr Paul Lingor 《Journal of neurochemistry》2015,134(2):261-275
A lesion to the rat rubrospinal tract is a model for traumatic spinal cord lesions and results in atrophy of the red nucleus neurons, axonal dieback, and locomotor deficits. In this study, we used adeno‐associated virus (AAV)‐mediated over‐expression of BAG1 and ROCK2‐shRNA in the red nucleus to trace [by co‐expression of enhanced green fluorescent protein (EGFP)] and treat the rubrospinal tract after unilateral dorsal hemisection. We investigated the effects of targeted gene therapy on neuronal survival, axonal sprouting of the rubrospinal tract, and motor recovery 12 weeks after unilateral dorsal hemisection at Th8 in rats. In addition to the evaluation of BAG1 and ROCK2 as therapeutic targets in spinal cord injury, we aimed to demonstrate the feasibility and the limits of an AAV‐mediated protein over‐expression versus AAV.shRNA‐mediated down‐regulation in this traumatic CNS lesion model. Our results demonstrate that BAG1 and ROCK2‐shRNA both promote neuronal survival of red nucleus neurons and enhance axonal sprouting proximal to the lesion.
96.
Ragna Redelstorff Shoji Hayashi Bruce M. Rothschild Anusuya Chinsamy 《Lethaia: An International Journal of Palaeontology and Stratigraphy》2015,48(1):47-55
Palaeopathologies are relatively common in the fossil record; however, bone histological analyses of these are rare as they are more commonly discovered and described using radiological scans. Here, we describe the bone microstructure of infectious abscesses (osteomyelitis) in the long bones of two Stegosaurus individuals from the Como Bluff area of the Late Jurassic Morrison Formation. Multiple lytic, geographical lesions filled with reactive lamellar bone occur in a femur of a juvenile specimen. A single lesion was found in an adult tibia. This lesion is characterized by the presence of spiculated periosteal reactive growth perpendicular to the bone surface and the formation of new trabeculae within the lesion. 相似文献
97.
PurposeTo investigate whether a newly-developed single-energy metal artifact reduction (SEMAR) algorithm applied to images acquired on a 320-MDCT volume scanner reduces image artifacts from dental metal.MethodsWe inserted the lower right teeth covered with a dental metal alloy and crown in a skull phantom and performed single-volume scanning on a second-generation 320-MDCT scanner. A 12-mm diameter spherical lesion was placed either close to or far from the dental metal. The tube voltage and current were 120 kVp and 80 or 155 mA, respectively. Images were reconstructed with filtered back projection (FBP) or iterative reconstruction (IR), with or without SEMAR. We calculated the signal-to-artifact ratios (SAR) to quantify the visibility of the lesion. Two radiologists inspected 96 images (48 with lesion and 48 without) for the presence or absence of the lesion using a 5-point ordinal scale (1 = definitely absent to 5 = definitely present).ResultsOn images reconstructed with FPB and IR with SEMAR, streak artifacts from the dental metal were reduced substantially compared to images without SEMAR. At 155 mA with the lesion near the dental metal, the SARs were better on FBP and IR images (FBP: 1.7 and 0.5 with and without SEMAR, respectively; IR: 1.6 and 0.9 with and without SEMAR, respectively). The observer visual scores improved with SEMAR (FBP: 4.2 and 3.2 with and without SEMAR, respectively; IR: 4.2 and 3.0).ConclusionThe SEMAR algorithm reduces dental metal artifacts and improves lesion detectability and image quality in patients with oral cavity lesions. 相似文献
98.
Manosalva PM Bruce M Leach JE 《The Plant journal : for cell and molecular biology》2011,68(5):777-787
Plant 14-3-3 proteins regulate important cellular processes, including plant immune responses, through protein-protein interactions with a wide range of target proteins. In rice (Oryza sativa), the GF14e gene, which encodes a 14-3-3 protein, is induced during effector-triggered immunity (ETI) associated with pathogens such as Xanthomonas oryzae pv. oryzae (Xoo). To determine whether the GF14e gene plays a direct role in resistance to disease in rice, we suppressed its expression by RNAi silencing. GF14e suppression was correlated with the appearance of a lesion-mimic (LM) phenotype in the transgenic plants at 3 weeks after sowing. This indicates inappropriate regulation of cell death, a phenotype that is frequently associated with enhanced resistance to pathogens. GF14e-silenced rice plants showed high levels of resistance to a virulent strain of Xoo compared with plants that were not silenced. Enhanced resistance was correlated with GF14e silencing prior to and after development of the LM phenotype, higher basal expression of a defense response peroxidase gene (POX22.3), and accumulation of reactive oxygen species (ROS). In addition, GF14e-silenced plants also exhibit enhanced resistance to the necrotrophic fungal pathogen Rhizoctonia solani. Together, our findings suggest that GF14e negatively affects the induction of plant defense response genes, cell death and broad-spectrum resistance in rice. 相似文献
99.
The methane yield of digestate: effect of organic loading rate, hydraulic retention time, and plant feeding 总被引:4,自引:0,他引:4
In biogas plants, huge volumes of digestate are produced daily and stored in uncovered tanks, which leak methane into the atmosphere and cause negative environmental impacts. To better understand the effect that different operating parameters of anaerobic digestion plants have on digestate residual methane yield, four digestate samples collected from plants with very different operations were analysed in batch reactors. Their methane yields were very heterogeneous and varied between 2.88 and 37.63 NL/kgVS. The methane yield was shown to be highly influenced by the A.D. plant Organic Loading Rate and by feedstock quality; hydraulic retention time had only limited effects. 相似文献
100.
Changes of Genotype,Sensitivity and Aggressiveness in Phytophthora infestans Isolates Collected in European Countries in 1997, 2006 and 2007 总被引:1,自引:0,他引:1
Ulrich Gisi Florian Walder Zipora Resheat‐Eini Dominique Edel Helge Sierotzki 《Journal of Phytopathology》2011,159(4):223-232
A total of 241 isolates of Phytophthora infestans were collected in 1997, 2006 and 2007 in eight European countries and characterized with molecular markers (simple sequence repeats, SSR genotypes) and phenotypic traits such as sensitivity to fungicides, mating type and aggressiveness. The mating type distribution changed from mainly A1 in 1997 to a majority of A2 in 2007. No resistant isolates were detected for fluazinam and mandipropamid, whereas the proportion of isolates resistant to mefenoxam (MFX) was high and increased over the years. There was no genetic link between mating type and MFX resistance. Aggressiveness (product between lesion expansion and sporulation capacity) was slightly higher for MFX‐resistant compared to sensitive isolates and for isolates collected later compared to earlier in the same season. It was about equally high for A1 and A2 types, and for French isolates in 1997 and British isolates in 2007, but lower for French isolates in 2007. Six different SSR genotype families were distinguished. In 1997, populations were dominated by genotype families I and III/IV, which significantly declined in 2007 being largely displaced by genotype families II (‘blue 13’ type) and V, which are by coincidence mainly A2 MFX resistant and A1 MFX sensitive, respectively. However, mating type and MFX resistance were genetically not linked to SSR genotypes. 相似文献