全文获取类型
收费全文 | 1006篇 |
免费 | 81篇 |
出版年
2023年 | 8篇 |
2022年 | 10篇 |
2021年 | 29篇 |
2020年 | 19篇 |
2019年 | 22篇 |
2018年 | 35篇 |
2017年 | 40篇 |
2016年 | 45篇 |
2015年 | 59篇 |
2014年 | 51篇 |
2013年 | 57篇 |
2012年 | 84篇 |
2011年 | 78篇 |
2010年 | 48篇 |
2009年 | 50篇 |
2008年 | 41篇 |
2007年 | 44篇 |
2006年 | 51篇 |
2005年 | 45篇 |
2004年 | 35篇 |
2003年 | 38篇 |
2002年 | 39篇 |
2001年 | 26篇 |
2000年 | 24篇 |
1999年 | 15篇 |
1998年 | 12篇 |
1997年 | 4篇 |
1996年 | 3篇 |
1994年 | 5篇 |
1993年 | 2篇 |
1992年 | 6篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1989年 | 8篇 |
1988年 | 13篇 |
1987年 | 6篇 |
1983年 | 2篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1970年 | 2篇 |
1969年 | 1篇 |
1968年 | 2篇 |
1967年 | 1篇 |
排序方式: 共有1087条查询结果,搜索用时 221 毫秒
61.
Melanins are the most prevalent pigments in animals and are involved in visual communication by producing colored traits that often evolve as intraspecific signals of quality. Identifying and quantifying melanins are therefore essential to understand the function and evolution of melanin‐based signals. However, the analysis of melanins is difficult due to their insolubility and the lack of simple methods that allow the identification of their chemical forms. We recently proposed the use of Raman spectroscopy as a simple, noninvasive technique that can be used to identify and quantify melanins in feathers and hairs. Contrarily, other authors later stated that melanins are characterized by a lack of defined Raman signals. Here, we use confocal Raman microscopy to confirm previous analyses showing that the two main chemical forms of melanins (eumelanin and pheomelanin) exhibit distinct Raman signal and compare different excitation wavelengths to analyze synthetic pheomelanin and natural melanins in feathers of different species of birds. Our analyses indicate that only laser excitation wavelengths below 1064 nm are useful for the analysis of melanins by Raman spectroscopy, and only 780‐nm laser in the case of melanins in feathers. These findings show that the capacity of Raman spectroscopy to distinguish different chemical forms of melanins depends on laser power and integration time. As a consequence, Raman spectroscopy should be applied after preliminar analyses using a range of these parameters, especially in fragile biological tissues such as feathers. 相似文献
62.
Anders Pape Møller Juan J. Soler Jan Tøttrup Nielsen Ismael Galván 《Ecology and evolution》2015,5(8):1676-1685
Pathogenic bacteria constitute a serious threat to viability of many organisms. Because growth of most bacteria is favored by humid and warm environmental conditions, earlier reproducers in seasonal environments should suffer less from the negative consequences of pathogenic bacteria. These relationships, and the effects on reproductive success, should be particularly prominent in predators because they are frequently exposed to pathogenic microorganisms from sick prey. Here, we presented and tested this hypothesis by sampling bacteria on adult and nestling goshawks Accipiter gentilis. We predicted that early breeders and their offspring should have fewer bacteria than those reproducing later during the breeding season. Adult goshawks with a high abundance of Staphylococcus on their beak and claws were easier to capture and their laying date was delayed. Moreover, goshawks that laid their eggs later had offspring with more Staphylococcus on their beaks and claws. The strength of the association between laying date and bacterial density of nestlings was stronger during the warm spring of 2013, when nestlings suffered from a higher abundance of pathogenic bacteria. Hatching failure and fledging failure were more common in nests with a higher abundance of Staphylococcus independently of the number of years occupied, laying date, and age of the female nest owner. These findings imply that timing of reproduction may be under the influence of pathogenic bacteria. Because early breeding goshawks produce more recruits than later breeders, our results suggest a role for pathogenic bacteria in the optimal timing of reproduction. 相似文献
63.
Ismael Ferrusquía-Villafranca José E. Ruiz-González Enrique Martínez-Hernández José Ramón Torres-Hernández Guillermo Woolrich-Piña 《Geobios》2014
Mexico's Late Neogene mammal faunas are largely known from localities in the Trans-Mexican Volcanic Belt; those from other morphotectonic provinces are few and far apart. Thus, the discovery of Late Miocene vertebrates in western Sierra Madre Oriental at San Luis Potosí, the Paso del Águila local fauna, significantly adds to this meager record. The assemblage was collected from the floodplain facies of the San Nicolás Formation, a ∼1100-m thick, dominantly fluviolacustrine and calcilithitic, 15°–20° NE dipping sequence preserved in the Peotillos-Tolentino Graben, between 22°11’–22°19’ N and 100°30’–100°39° W. It includes remains of cf. Trachemys, a small to medium-sized emydid chelonian, a large camelid, a small cervid and a new species of the equini Pliohippus s.s., comparable in size, cranial morphology and odontographic characters to the Clarendonian-Early Hemphillian horses of the Pliohippus clade. Ar-Ar dates from ash-fall tuffs seemingly above and below the fossiliferous strata, bracket the age between 12.33 and 7.41 Ma (i.e., late Middle to Late Miocene), that is, within the Late Clarendonian-Early Hemphillian NALMA interval, making this fauna the first in Mexico from this age. The Paso del Águila local fauna is at least partly correlative with the Hemphillian local faunas from the TMVB and adjacent areas (e.g., Rancho El Ocote, Guanajuato and Tecolotlán, Jalisco), the Central Plateau (e.g., Arroyo Los Fragmentos, Zacatecas), and the Sierra Madre Occidental (e.g., Yepómera). Elsewhere, it is broadly correlative with the Late Clarendonian-Early Hemphillian faunas from the California Coast Ranges (e.g., North Tejon Hills, Ricardo and Dove Springs in the Mohave Desert), and the Gulf Coast Plain, Florida (McGehee Farm and Mixon). The Paso del Águila local fauna was part of a subtropical savannah and pine-oak forest (with a well-developed understory) biome that thrived on a climate regime much more humid than today. 相似文献
64.
65.
Marcelino Herrera Ismael Hachero-Cruzado Alicia Naranjo Juan Miguel Mancera 《Reviews in Fish Biology and Fisheries》2010,20(4):489-497
In this work, several features during the wedge sole larval development have been described. The newly hatched larva presented
an acidophilic yolk with some oil drops. The digestive tract began to differentiate at 1 DAH, with a loop being discernible.
The pancreas and liver were completely formed at 2 DAH, the former showing its typical basophilic acinar structure and acidophilic
zymogen granules. The first supranuclear vesicles in enterocytes were seen at 3 DAH. At 4 DAH, yolk reserves were completely
exhausted, the number of oesophagus and intestine mucous cells increased, and the heart was differentiated into four chambers:
the venous sinus, atrium, ventricle, and arterious bulb. The development was fast and almost all organs were differentiated
at 2 DAH. It is important to emphasize that gastric glands were not detected, a factor that should be considered when deciding
diet formulation and feeding strategies for the rearing of this species. 相似文献
66.
Ismael Bustos-Jaimes Rodrigo Mora-Lugo Mario L. Calcagno Amelia Farrés 《Biochimica et Biophysica Acta - Proteins and Proteomics》2010,1804(12):2222-2227
Lipases are useful catalysts for a wide variety of industrial purposes. Herein we report the stability and thermal dependence of the activity of wild-type Bacillus pumilus lipase (BplA) and four site-directed mutants designed to improve its thermal stability. The Gly28:Ser mutation produces a dramatic four-fold increase in its kcat and a remarkable increase in its stability. While the increase in kcat is temperature-independent, the increase in stability shows that the resultant interactions of this mutation have a strong enthalpic component. Thermal dependence of stability, kcat, KM and kcat/KM were analysed to gain insight on the structural effects of mutations on BplA. Our results are consistent with a gain in enzyme mobility for those mutants displaying enhanced catalytic properties; the analysis of thermal dependence of kinetic parameters indicates that the mutations did not change either the catalytic mechanism or the rate-limiting step of catalysis. 相似文献
67.
68.
Sánchez-Ramos I Alvarez-Alfageme F Castañera P 《Experimental & applied acarology》2007,41(1-2):87-100
The developmental rate of immature stages and the reproduction of adults of Tyrophagus putrescentiae (Schrank), T. neiswanderi Johnston and Bruce and Acarus farris (Oudemans) were examined at 70, 80 and 90% r.h. and a constant temperature of 25°C. At 70% r.h., T. putrescentiae and A. farris immature stages failed to reach the protonymph stage as 100% of the larvae died, whereas T. neiswanderi was able to complete development. The developmental time of all immature stages for the three species was significantly increased
as relative humidity was reduced. The mobile stages were particularly susceptible, as the time needed to complete their development
at lower relative humidities suffered greater increases than the egg stage. At 70% r.h., T. putrescentiae and A. farris were not able to lay eggs and only 24% of T. neiswanderi pairs were fertile. The reproductive parameters of the three species at the relative humidities at which they were able to
lay eggs showed significant differences, except for the percentage of fertile mating at 80 and 90% r.h. As relative humidity
increased, preoviposition period was reduced and fecundity and daily fecundity was increased, whereas the oviposition period
showed different patterns for the three species. The intrinsic rate of increase (r
m
) of T. neiswanderi at 70% r.h. was negative indicating that, at these conditions, mite populations of this species will diminish until they
disappear. As relative humidity increased from 80 to 90% r.h. this parameter was almost twofold for both Tyrophagus species. The r
m
obtained for A. farris at 90% r.h. was similar to that of T. neiswanderi at the same humidity while at 80% r.h. it was very small so that the population doubling time was more than 84 days. The
influence of relative humidity on biology of these mites and its practical application as control measure are discussed. 相似文献
69.
Ebensen T Link C Riese P Schulze K Morr M Guzmán CA 《Journal of immunology (Baltimore, Md. : 1950)》2007,179(4):2065-2073
The glycolipid alpha-galactosylceramide (alphaGalCer) has immunomodulatory properties, which have been exploited to combat cancer, chronic inflammatory diseases, and infections. However, its poor solubility makes alphaGalCer a suboptimal compound for in vivo applications. In this study, a pegylated derivative of alphaGalCer is characterized, which exhibits improved physical and biological properties. The new compound, alphaGalCerMPEG, is water-soluble and retains the specificity for the CD1d receptor of alphaGalCer. The in vitro stimulatory properties on immune cells (e.g., dendritic cells and splenocytes) are maintained intact, even when tested at a 33-fold lower concentration of the active moiety than alphaGalCer. NK cells isolated from mice treated with alphaGalCerMPEG also had stronger cytotoxic activity on YAC-1 cells than those obtained from animals receiving either alphaGalCer or CpG. Intranasal immunization studies performed in mice showed that alphaGalCerMPEG exerts stronger adjuvant activities than the parental compound alphaGalCer when tested at 0.35 vs 11.7 nM/dose. Coadministration of beta-galactosidase with alphaGalCerMPEG resulted not only in high titers of Ag-specific Abs in serum (i.e., 1:512,000), but also in the stimulation of stronger Th2 and secretory IgA responses, both at local and remote mucosal effector sites (i.e., nose, lung, and vagina). The new synthetic derivative alphaGalCerMPEG represents a promising tool for the development of immune interventions against infectious and noninfectious diseases. 相似文献
70.
Morales-Rodríguez I Yañez-Morales M Silva-Rojas HV García-de-Los-Santos G Guzmán-de-Peña DA 《Mycopathologia》2007,163(1):31-39
Fusarium
proliferatum, F. subglutinans, and F. verticillioides are known causes of ear and kernel rot in maize worldwide. In Mexico, only F. verticillioides and F. subglutinans, have been reported previously as causal agents of this disease. However, Fusarium isolates with different morphological characteristics to the species that are known to cause this disease were obtained in
the Highland-Valley region of this country from symptomatic and symptomless ears of native and commercial maize genotypes.
Moreover, while the morphological studies were not sufficient to identify the correct taxonomic position at the species level,
analyses based in the Internal Transcribed Spacer region and the Nuclear Large Subunit Ribosomal partial sequences allowed
for the identification of F. subglutinans, F. solani, and F. verticillioides, as well as four species (F. chlamydosporum, F. napiforme, F. poae, and F. pseudonygamai) that had not previously been reported to be associated with ear rot. In addition, F. napiforme and F. solani were absent from symptomless kernels. Phylogenetic analysis showed genetic changes in F. napiforme, and F. pseudonygamai isolates because they were not true clones, and probably constitute separate sibling species. The results of this study suggest
that the biodiversity of Fusarium species involved in ear rot in Mexico is greater than that reported previously in other places in the world. This new knowledge
will permit a better understanding of the relationship between all the species involved in ear rot disease and their relationship
with maize. 相似文献