全文获取类型
收费全文 | 288篇 |
免费 | 10篇 |
专业分类
林业 | 14篇 |
农学 | 10篇 |
71篇 | |
综合类 | 65篇 |
农作物 | 31篇 |
水产渔业 | 12篇 |
畜牧兽医 | 71篇 |
园艺 | 7篇 |
植物保护 | 17篇 |
出版年
2023年 | 1篇 |
2022年 | 4篇 |
2021年 | 15篇 |
2020年 | 9篇 |
2019年 | 14篇 |
2018年 | 12篇 |
2017年 | 11篇 |
2016年 | 9篇 |
2015年 | 7篇 |
2014年 | 10篇 |
2013年 | 14篇 |
2012年 | 24篇 |
2011年 | 31篇 |
2010年 | 8篇 |
2009年 | 3篇 |
2008年 | 26篇 |
2007年 | 15篇 |
2006年 | 11篇 |
2005年 | 10篇 |
2004年 | 17篇 |
2003年 | 9篇 |
2002年 | 12篇 |
2001年 | 7篇 |
2000年 | 2篇 |
1999年 | 1篇 |
1998年 | 1篇 |
1997年 | 5篇 |
1996年 | 5篇 |
1979年 | 1篇 |
1960年 | 1篇 |
1944年 | 1篇 |
1942年 | 1篇 |
1939年 | 1篇 |
排序方式: 共有298条查询结果,搜索用时 0 毫秒
41.
Vladimir A. Basiuk Marco Salerno Alejandro Heredia Elena V. Basiuk 《Fibers and Polymers》2018,19(8):1632-1639
Bolas spiders of the genus Mastophora are very uncommon orb-weaver spiders, famous for their unusual predatory behavior. Being strictly nocturnal and very cryptic, they are extremely difficult to reveal, which explains their rarity in world arachnological collections. After a recent finding of an adult female of Mastophora corpulenta Banks (one of especially rare representatives of the genus), a few egg cases of this species became available, which seem to be very different from those of more common spiders due to their unusual hardness and coloration. We characterized some spectral, structural, and mechanical properties of this unusual silk material, by using Fourier-transform infrared spectroscopy, scanning electron microscopy (SEM), 3D profilometer imaging, and atomic force microscopy (AFM). For comparison, similar studies were performed for egg cases of the Southern black widow Latrodectus mactans Fabricius, since spiders of the genus Latrodectus are known to produce silk fibers of comparable toughness. Some similarities were observed between the silks produced by Mastophora corpulenta and Latrodectus mactans in terms of the size and chemical composition of egg cases. At the same time, SEM imaging and AFM force characterization revealed striking differences between them, due to the fact that Mastophora egg cases, unlike those produced by more common Araneae species, includes a solid (fused) wall, lacking evident porous structure, which results in their outstanding stiffness and strength. 相似文献
42.
Wei Xiao Liu Yuqiao Zhan Qiang Zhang Pengbo Zhao Dandan Xu Bo Bocharnikova Elena Matichenkov Vladimir 《Paddy and Water Environment》2018,16(1):173-181
Paddy and Water Environment - Numerous greenhouse and laboratory experiments have shown that the application of Si-rich materials may affect the mobility and transport of hazardous elements in the... 相似文献
43.
The origin of jets emitted from black holes is not well understood; however, there are two possible energy sources: the accretion disk or the rotating black hole. Magnetohydrodynamic simulations show a well-defined jet that extracts energy from a black hole. If plasma near the black hole is threaded by large-scale magnetic flux, it will rotate with respect to asymptotic infinity, creating large magnetic stresses. These stresses are released as a relativistic jet at the expense of black hole rotational energy. The physics of the jet initiation in the simulations is described by the theory of black hole gravitohydromagnetics. 相似文献
44.
45.
Armbrust EV Berges JA Bowler C Green BR Martinez D Putnam NH Zhou S Allen AE Apt KE Bechner M Brzezinski MA Chaal BK Chiovitti A Davis AK Demarest MS Detter JC Glavina T Goodstein D Hadi MZ Hellsten U Hildebrand M Jenkins BD Jurka J Kapitonov VV Kröger N Lau WW Lane TW Larimer FW Lippmeier JC Lucas S Medina M Montsant A Obornik M Parker MS Palenik B Pazour GJ Richardson PM Rynearson TA Saito MA Schwartz DC Thamatrakoln K Valentin K Vardi A Wilkerson FP Rokhsar DS 《Science (New York, N.Y.)》2004,306(5693):79-86
Diatoms are unicellular algae with plastids acquired by secondary endosymbiosis. They are responsible for approximately 20% of global carbon fixation. We report the 34 million-base pair draft nuclear genome of the marine diatom Thalassiosira pseudonana and its 129 thousand-base pair plastid and 44 thousand-base pair mitochondrial genomes. Sequence and optical restriction mapping revealed 24 diploid nuclear chromosomes. We identified novel genes for silicic acid transport and formation of silica-based cell walls, high-affinity iron uptake, biosynthetic enzymes for several types of polyunsaturated fatty acids, use of a range of nitrogenous compounds, and a complete urea cycle, all attributes that allow diatoms to prosper in aquatic environments. 相似文献
46.
Davis S Begon M De Bruyn L Ageyev VS Klassovskiy NL Pole SB Viljugrein H Stenseth NC Leirs H 《Science (New York, N.Y.)》2004,304(5671):736-738
In Kazakhstan and elsewhere in central Asia, the bacterium Yersinia pestis circulates in natural populations of gerbils, which are the source of human cases of bubonic plague. Our analysis of field data collected between 1955 and 1996 shows that plague invades, fades out, and reinvades in response to fluctuations in the abundance of its main reservoir host, the great gerbil (Rhombomys opimus). This is a rare empirical example of the two types of abundance thresholds for infectious disease-invasion and persistence- operating in a single wildlife population. We parameterized predictive models that should reduce the costs of plague surveillance in central Asia and thereby encourage its continuance. 相似文献
47.
Gaschen B Taylor J Yusim K Foley B Gao F Lang D Novitsky V Haynes B Hahn BH Bhattacharya T Korber B 《Science (New York, N.Y.)》2002,296(5577):2354-2360
Globally, human immunodeficiency virus-type 1 (HIV-1) is extraordinarily variable, and this diversity poses a major obstacle to AIDS vaccine development. Currently, candidate vaccines are derived from isolates, with the hope that they will be sufficiently cross-reactive to protect against circulating viruses. This may be overly optimistic, however, given that HIV-1 envelope proteins can differ in more than 30% of their amino acids. To contend with the diversity, country-specific vaccines are being considered, but evolutionary relationships may be more useful than regional considerations. Consensus or ancestor sequences could be used in vaccine design to minimize the genetic differences between vaccine strains and contemporary isolates, effectively reducing the extent of diversity by half. 相似文献
48.
Effects of oral supplementation of probiotic strains of Lactobacillus rhamnosus and Enterococcus faecium on diarrhoea events of foals in their first weeks of life 下载免费PDF全文
Christina Ströbel Elena Günther Vladimir Urubschurov Annette Zeyner 《Journal of animal physiology and animal nutrition》2018,102(5):1357-1365
Foal first diarrhoea is one of the most prominent problems in the early life of horses. Probiotics might have the potency to prevent or at least diminish neonatal diarrhoea. We hypothesised that the treatment of foals with probiotic strains of Lactobacillus rhamnosus and Enterococcus faecium starting early after birth and then daily over 2 weeks would prevent or mitigate foal heat diarrhoea. The influence of this probiotic treatment on diarrhoea incidence and growth and health performance of young foals was investigated. Thirty‐four foals were randomly allocated to two groups. From day 1 to 14 of life, the foals received either placebo (PG, n = 16) or the probiotic treatment (TG, n = 18). Clinical examination was performed, and the faeces consistency score (FCS, 1–5; with diarrhoea defined by ≤3) was recorded once per day in weeks 1 and 2 and once weekly in weeks 3–8 of life (WL). The body height was measured at birth and after two and eight WL. Diarrhoea occurred in the 1st WL in 19% and 61% of PG and TG foals respectively. In the 1st WL, diarrhoea lasted 0.3 ± 0.8 and 1.6 ± 1.4 days in PG and TG foals respectively. In the 2nd WL, diarrhoea occurred in 94% and 84% of PG and TG foals, respectively, and lasted for 3.0 ± 1.5 and 3.7 ± 1.6 days respectively. At least two periods of diarrhoea developed in 33% and 65% of PG and TG foals respectively. The TG foals grew slightly slower than the PG foals. The results indicated that the probiotic treatment of neonatal foals as performed in this study was not suitable to reduce diarrhoea within the first two WL, because contrary to the hypothesis, the TG foals suffered more frequently and for longer periods from diarrhoea than the PG foals. 相似文献
49.
Commercial intensive fry production of Atlantic cod will be dependent on production of viable eggs independent of season. This can only be done by manipulation of maturation by photoperiod, but little is known about potential effects on egg characteristics and larval viability. In two cod broodstocks, maturation was successfully advanced or delayed 6 months compared with normal spawning season (March–April) by manipulation of photoperiod. The advanced broodstock spawned both in spring and autumn the same year. In two of the spawning tanks during autumn, ambient temperature was reduced after reaching 13.7°C during the first half of the spawning period. Egg quality and viability were monitored, and several egg batches were incubated, hatched and start‐fed for examination of growth and survival. Temperatures above 9.6°C resulted in significant reductions in fertilization and normal egg development. Concurrently, fractions of dead and unfertilized eggs increased with elevated temperature. Actual relative fecundity was not affected by temperature. Egg characteristics improved when temperature was controlled and lowered below 9.6°C. Occurrence of irregular spawners suggests that handling of broodstock fish should be avoided during maturation and spawning. Cod larvae originating from eggs of the advanced or delayed broodstocks were successfully reared beyond metamorphosis. Survival was 9.0–46.6% and 29.3% in green and clear water respectively. Survival correlated with both initial and average feeding conditions, but growth rate did not correlate with either of survival and feeding conditions. Specific growth rates (8.3–13.6% day−1) is comparable with other intensive rearing trials with cod, but were lower than reported from nature‐like systems. 相似文献
50.