全文获取类型
收费全文 | 387篇 |
免费 | 24篇 |
国内免费 | 3篇 |
专业分类
414篇 |
出版年
2023年 | 5篇 |
2022年 | 6篇 |
2021年 | 10篇 |
2020年 | 4篇 |
2019年 | 6篇 |
2018年 | 7篇 |
2017年 | 5篇 |
2016年 | 17篇 |
2015年 | 25篇 |
2014年 | 18篇 |
2013年 | 20篇 |
2012年 | 29篇 |
2011年 | 33篇 |
2010年 | 19篇 |
2009年 | 18篇 |
2008年 | 25篇 |
2007年 | 33篇 |
2006年 | 12篇 |
2005年 | 13篇 |
2004年 | 9篇 |
2003年 | 16篇 |
2002年 | 5篇 |
2001年 | 9篇 |
2000年 | 3篇 |
1999年 | 7篇 |
1998年 | 9篇 |
1997年 | 5篇 |
1996年 | 7篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1990年 | 1篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1972年 | 1篇 |
1968年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有414条查询结果,搜索用时 15 毫秒
31.
The changes in germination, peroxidase activity and isoperoxidase spectrum have been studied in apple embryos at 5°C (stratification) and at 20°C in the presence or absence of seed coats. The embryo dormancy is progressively released at 5°C, but not at 20°C. The peroxidase activity in embryos covered with seed coats is very low at 5°C as well as at 20°C which corresponds to a restricted number of isoenzymes. In isolated embryos the peroxidase activity increases significantly. This is due to an increase in both the number and the activity of the isoperoxidases and it is more pronounced at 20°C than at 5°C. The obtained results suggest that the soluble peroxidases are not involved in the process of the release of embryo dormancy. The variations observed are attributed to the growth process following germination, which can occur even at low temperature. 相似文献
32.
In an attempt to recognize the possible ecological causes of the decline of a population of Hoopoes Upupa epops in the Swiss Alps, we collected data on resource exploitation. The prey provisioned to nestlings by parents was investigated at four breeding sites using photographs (n = 4353, 80% of which enabled prey identification). Molecrickets Gryllotalpa gryllotalpa and Lepidoptera (larvae and pupae) were dominant in nestling diet (93% frequency; 97% biomass). Although Molecrickets were provisioned less frequently (26%) than Lepidoptera (67%), they represented 68% of the total biomass (vs 29% for Lepidoptera). There was an overall negative relationship between the proportion of Molecricket biomass in the diet and the parents' feeding rate, whereas a comparison between broods showed that a higher provisioning activity did not lead to an increase in the biomass supplied to the chicks. A diet based on Molecrickets therefore appears to be energetically advantageous. As Molecrickets are a traditional prey of Hoopoes in central Europe, this might be relevant to other populations. In the study area, Molecrickets occur only on the intensively cultivated plain, whereas the majority of Hoopoe pairs nest at various altitudes on the foothills adjacent to the plain as the latter provides at present almost no suitable nesting sites. Hoopoes breeding higher up on the foothills seem to experience greater provisioning costs and have, on average, lower breeding success. Providing nest sites on the plain is the main conservation measure proposed for the local Hoopoe population. Further attention should also be paid to Molecrickets as these may be crucial for Hoopoes. 相似文献
33.
Jifeng Tang Samantha J Baldwin Jeanne ME Jacobs C Gerard van der Linden Roeland E Voorrips Jack AM Leunissen Herman van Eck Ben Vosman 《BMC bioinformatics》2008,9(1):374
Background
Simple Sequence Repeat (SSR) or microsatellite markers are valuable for genetic research. Experimental methods to develop SSR markers are laborious, time consuming and expensive. In silico approaches have become a practicable and relatively inexpensive alternative during the last decade, although testing putative SSR markers still is time consuming and expensive. In many species only a relatively small percentage of SSR markers turn out to be polymorphic. This is particularly true for markers derived from expressed sequence tags (ESTs). In EST databases a large redundancy of sequences is present, which may contain information on length-polymorphisms in the SSR they contain, and whether they have been derived from heterozygotes or from different genotypes. Up to now, although a number of programs have been developed to identify SSRs in EST sequences, no software can detect putatively polymorphic SSRs. 相似文献34.
Miguel A. Bedoya-Pérez Daniel D. Issa Peter B. Banks Clare McArthur 《Oecologia》2014,175(4):1167-1177
While trying to achieve their nutritional requirements, foraging herbivores face the costs of plant defenses, such as toxins. Teasing apart the costs and benefits of various chemical constituents in plants is difficult because their chemical defenses and nutrient concentrations often co-vary. We used an approach derived from predator–prey studies to quantitatively compare the foraging response of a free-ranging mammalian herbivore, the swamp wallaby (Wallabia bicolor), through three feeding trials with artificial diets that differed in their concentrations of (1) the terpene 1,8-cineole, (2) primary constituents (including nitrogen and fiber), and (3) both the terpene and the primary constituents. Applying the giving-up density (GUD) framework, we demonstrated that the foraging cost of food patches increases with higher dietary cineole concentration and decreases with higher dietary nutrient concentration. The effect of combined differences in nutrients and cineole concentrations on GUD was interactive, and high nutrient food required more cineole to achieve the same patch value as low nutrient food. Our results indicate that swamp wallabies equate low nutrient, poorly defended food with high nutrient, highly defended food, providing two contrasting diets with similar cost–benefit outcomes. This behavior suggests that equal concentrations of chemical defenses provide nutrient-poor plants with relatively greater protection as nutrient-rich plants. Nutrient-rich plants may therefore face the exacerbated problem of being preferred by herbivores and therefore need to produce more defense compounds to achieve the same level of defense as nutrient-poor plants. Our findings help explain the difference in anti-herbivore strategy of nutrient-poor and rich plants, i.e., tolerance versus defense. 相似文献
35.
Köping-Höggård M Issa MM Köhler T Tronde A Vårum KM Artursson P 《The journal of gene medicine》2005,7(9):1215-1222
BACKGROUND: The available methods for administration of gene delivery systems to the lungs of small animals via nebulization have several drawbacks. These include lack of control over the delivered dose and a negative impact on the stability of the formulation. This paper describes a new nebulization catheter device for the administration of plasmid-based gene delivery systems (polyplexes) as aerosols to the mouse lung in vivo. METHODS: The physical stability of naked pDNA and polyplexes formulated with chitosan oligomers and PEI was examined following nebulization with the catheter device. We also examined the in vitro transfection efficiency of the polyplexes recovered after nebulization. Lung distribution and gene expression after administration of the selected gene delivery systems to the mouse lung were also investigated. RESULTS: In contrast to previously described nebulization methods, the structural integrity of the unprotected naked pDNA was maintained following nebulization by the catheter device, which indicates relatively mild nebulization conditions. In addition, the nebulization procedure did not affect the physical stability of the formulated polyplexes. Small volumes of the pDNA aerosol (10-20 microl) were delivered in a highly controlled and reproducible manner. The aerosol droplet size varied with the molecular weight of the polycations. Aerosol delivery via this method resulted in improved lung distribution of pDNA polyplexes and a six-fold increase in the efficiency of gene delivery in vivo over that seen with the commonly used intratracheal instillation method. CONCLUSION: The use of the nebulization catheter device provides a promising alternative for aerosol gene delivery to the mouse lung. 相似文献
36.
A new series of CU(III) complexes of the type [Cu(SB)]ClO4, where SB is a dibasic tetradentate Schiff's base, were synthesized and characterized by elemental analysis, IR and electronic spectra. The prepared complexes are compared with Cu(II) complexes of the same ligands. The room temperature magnetic susceptibility and the x-band EPR spectra of the two types of complexes showed different behaviour. Cu(II) complexes were non conducting whereas Cu(III) complexes gave a conductance corresponding to 1:1 electrolyte. In fact this is the first report on the Cu(III) complexes with the tetradentate Schiff base ligands. 相似文献
37.
Villard V Agak GW Frank G Jafarshad A Servis C Nébié I Sirima SB Felger I Arevalo-Herrera M Herrera S Heitz F Bäcker V Druilhe P Kajava AV Corradin G 《PloS one》2007,2(7):e645
To identify malaria antigens for vaccine development, we selected alpha-helical coiled coil domains of proteins predicted to be present in the parasite erythrocytic stage. The corresponding synthetic peptides are expected to mimic structurally "native" epitopes. Indeed the 95 chemically synthesized peptides were all specifically recognized by human immune sera, though at various prevalence. Peptide specific antibodies were obtained both by affinity-purification from malaria immune sera and by immunization of mice. These antibodies did not show significant cross reactions, i.e., they were specific for the original peptide, reacted with native parasite proteins in infected erythrocytes and several were active in inhibiting in vitro parasite growth. Circular dichroism studies indicated that the selected peptides assumed partial or high alpha-helical content. Thus, we demonstrate that the bioinformatics/chemical synthesis approach described here can lead to the rapid identification of molecules which target biologically active antibodies, thus identifying suitable vaccine candidates. This strategy can be, in principle, extended to vaccine discovery in a wide range of other pathogens. 相似文献
38.
Deforestation usually results in significant losses of soil organic carbon (SOC). The rate and factors determining the recovery of this C pool with afforestation are still poorly understood. This paper provides a review of the influence of afforestation on SOC stocks based on a meta-analysis of 33 recent publications (totaling 120 sites and 189 observations), with the aim of determining the factors responsible for the restoration of SOC following afforestation. Based on a mixed linear model, the meta-analysis indicates that the main factors that contribute to restoring SOC stocks after afforestation are: previous land use, tree species planted, soil clay content, preplanting disturbance and, to a lesser extent, climatic zone. Specifically, this meta-analysis (1) indicates that the positive impact of afforestation on SOC stocks is more pronounced in cropland soils than in pastures or natural grasslands; (2) suggests that broadleaf tree species have a greater capacity to accumulate SOC than coniferous species; (3) underscores that afforestation using pine species does not result in a net loss of the whole soil-profile carbon stocks compared with initial values (agricultural soil) when the surface organic layer is included in the accounting; (4) demonstrates that clay-rich soils (> 33%) have a greater capacity to accumulate SOC than soils with a lower clay content (< 33%); (5) indicates that minimizing preplanting disturbances may increase the rate at which SOC stocks are replenished; and (6) suggests that afforestation carried out in the boreal climate zone results in small SOC losses compared with other climate zones, probably because trees grow more slowly under these conditions, although this does not rule out gains over time after the conversion. This study also highlights the importance of the methodological approach used when developing the sampling design, especially the inclusion of the organic layer in the accounting. 相似文献
39.
40.
Gisele M. S. Ouedraogo Güler Demirbas-Uzel Jean-Baptiste Rayaisse Geoffrey Gimonneau Astan C. Traore Antonios Avgoustinos Andrew G. Parker Issa Sidibe Anicet G. Ouedraogo Amadou Traore Bale Bayala Marc J. B. Vreysen Kostas Bourtzis Adly m. M. Abd-Alla 《BMC microbiology》2018,18(1):153