全文获取类型
收费全文 | 364篇 |
免费 | 32篇 |
国内免费 | 1篇 |
出版年
2022年 | 3篇 |
2021年 | 3篇 |
2019年 | 6篇 |
2018年 | 9篇 |
2017年 | 10篇 |
2016年 | 12篇 |
2015年 | 13篇 |
2014年 | 17篇 |
2013年 | 24篇 |
2012年 | 19篇 |
2011年 | 20篇 |
2010年 | 14篇 |
2009年 | 12篇 |
2008年 | 16篇 |
2007年 | 16篇 |
2006年 | 20篇 |
2005年 | 18篇 |
2004年 | 12篇 |
2003年 | 6篇 |
2002年 | 10篇 |
2001年 | 12篇 |
2000年 | 14篇 |
1999年 | 7篇 |
1998年 | 10篇 |
1997年 | 4篇 |
1996年 | 8篇 |
1995年 | 2篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 4篇 |
1986年 | 7篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1981年 | 4篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1975年 | 1篇 |
1974年 | 4篇 |
1973年 | 2篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1969年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有397条查询结果,搜索用时 125 毫秒
51.
Plant diversity patterns in the Aljibe Mountains (S. Spain): a comprehensive account 总被引:6,自引:0,他引:6
The Aljibe Mountains are located in the southern tip of the Iberian Peninsula and have a remarkable biogeographical interest. The complete plant species list (trees, climbers, shrubs, perennial and annual herbs, ferns, lichens, bryophytes and macroscopic algae) was recorded in four 0.1 ha plots from each of the most representative community types (Quercus suber woodland, Q. canariensis forest, open heathland and Q. coccifera shrubland). Up to 119 plant species were found in total in the Q. suber woodland plot. The diversity of woody plants was analysed from 44 samples of cover (100 m line), and the herbaceous layer was explored in 200 quadrats (of 0.5 × 0.5 m). Three biodiversity components (species richness, endemism, and taxonomic singularity) were evaluated in both shrub and herbaceous layers. Open heathlands showed the highest richness of endemic species, both woody and herbaceous. The highest number of woody species was found in the evergreen Q. suber woodland, and of herbaceous species in the semi-deciduous Q. canariensis woodland. Taxonomic singularity was higher in Q. canariensis woodlands and Q. coccifera shrublands for woody species, but there were no significant differences in the herbaceous layer. Local species diversity of heathlands in this region resembles that of South African heathlands (fynbos), despite the obvious geographic and floristic distance, and contrasts with the low diversity of biogeographically closer, European temperate heathlands. The Aljibe Mountains show high diversity values for different life forms (from trees to mosses) and spatial scales (from community to region), and are rich in endemic species. Thus, this area should be recognised as a relevant unit within the Mediterranean plant diversity hot spots. 相似文献
52.
Origin, diversification, and historical biogeography of the genus Trachurus (Perciformes: Carangidae) 总被引:3,自引:0,他引:3
Cárdenas L Hernández CE Poulin E Magoulas A Kornfield I Ojeda FP 《Molecular phylogenetics and evolution》2005,35(2):496-507
We addressed phylogenetic relationships in the genus Trachurus using cytochrome b gene and D-loop sequences. The trees showed five groups: (1) the Southwest Pacific species (T. japonicus, T. novaezelandiae, and T. declivis); (2) The Mediterranean Sea and Eastern Atlantic species (T. mediterraneus); (3) The Atlantic Ocean species (T. lathami and T. trecae); (4) Eastern Atlantic species (T. trachurus and T. capensis); and (5) a group of highly mobile pelagic species, two from the Eastern Pacific (T. symmetricus and T. murphyi) and one from the Eastern Atlantic (T. picturatus). The phylogeny based on Cyt b, supports the molecular clock hypothesis and our results agree with the reported fossil indicating that the origin of this genus occur when the Thetys Sea closed (around 18.4 MYA). In addition, a very slow neutral substitution rate is reported identified only two periods of maximum diversification: the first occurring between 18.4 and 15.0 MYA and the second between 8.4 MYA and present day. 相似文献
53.
54.
55.
Seed availability may limit the size of granivorous bird populations, particularly in desert environments. Seasonal and annual fluctuations in the abundance of granivorous birds were studied at three sites in the Northern Monte Desert, Argentina. The way in which these changes were related to seed supply was also studied. Granivore abundance and biomass, as well as seed abundance, were greater in winter. Changes in total granivore abundance were mostly because of variations in numbers of the most mobile bird species, all of which were found to have similar patterns of variation in their abundance. Evidence suggests that during the non‐breeding season, the most mobile species are able to track patches of high seed availability over long distances, whereas the abundance of less vagile species largely depends on the local ecological conditions. Seed supply is a proximate factor that limits the abundance of granivorous birds in the Monte Desert during the winter, but does not limit species richness. Granivore abundance was not correlated with seed supply in spring. 相似文献
56.
The small-subunit ribosomal RNA gene sequences from the hypotrichous ciliates Oxytricha nova and Stylonychia pustulata 总被引:17,自引:0,他引:17
We have determined the complete nucleotide sequence of the small- subunit
ribosomal RNA genes for the ciliate protozoans Stylonychia pustulata and
Oxytricha nova. The sequences are homologous and sufficiently similar that
these organisms must be closely related. In a phylogeny inferred from
comparisons of several eukaryotic small-subunit ribosomal RNAs, the
divergence of the ciliates from the eukaryotic line of descent is seen to
coincide with the radiation of the plants, the animals, and the fungi. This
radiation is preceded by the divergence of the slime mold, Dictyostelium
discoideum.
相似文献
57.
Barrera Enrique González Federico Velásquez Celso Ojeda María Luisa Sánchez Marciano Rentería-Tapia Víctor 《Plasmonics (Norwell, Mass.)》2018,13(6):2099-2109
Plasmonics - This paper reports the optical properties of a black silver–epoxy nanocomposite deposited on copper substrate for use as selective solar absorber. The silver nanoparticles at... 相似文献
58.
Voltage-clamp experiments were performed on single bovine adrenal fasciculata cells in short-term primary culture using either
standard (broken membrane) or perforated whole-cell patch clamp recording. The membrane current measured with the perforated
method was dominated by a very stable transient outward current. By contrast, the transient outward current recorded using
the standard method was unstable. The reversal potential of the transient outward current varied linearly with the logarithm
of [K+]
e
with a slope of 47 mV per decade. The onset of activation was sigmoidal and was fitted with a power function where n= 4. Time constants ranged from 1 to 4 msec with a maximum at −25 mV. The steady-state activation curve spanned the voltage
range −50 to +80 mV without reaching a clear maximum. During a pulse, the current decayed in a biexponential manner. Time
constants τ1 and τ2 were voltage-dependent and ranged from 50 to 200 msec respectively for a voltage step at +50 mV. The steady-state inactivation
was dependent on the conditioning pulse duration. Using short conditioning pulses (1.2 sec), the curve which spanned the voltage
range −40 to −20 mV, was 15 mV more positive than that obtained with longer conditioning pulses (60 sec). Time constants of
this ``very slow inactivation' process (τvs) determined for voltage steps at −60 and −50 mV were 15 and 10 sec respectively. A ``facilitation process' of the peak current
was observed when the duration or the amplitude of conditioning pulses were increased in the voltage range −100 to −50 mV.
Recovery from inactivation followed a biexponential time course which seemed a mixture of both inactivation processes. In
some experimental conditions, isolated cells were able to produce overshooting action potentials. These results are discussed
in relation with the membrane electrogenesis of this cell type.
Received: 14 November 1994/Revised: 24 October 1995 相似文献
59.
Ying Jun Ma Gregory A. Dissen Florence Rage Sergio R. Ojeda 《Methods (San Diego, Calif.)》1996,10(3):273-278
The RNase protection assay is a highly sensitive technique developed to detect and measure the abundance of specific mRNAs in samples of total cellular RNA. The assay utilizesin vitrotranscribed32P-labeled antisense RNA probes that are hybridized in solution to their complementary cellular mRNAs. This is followed by digestion of nonhybridizing (single-stranded) RNA species with RNases, removal of the RNases by treatment with proteinase K, phenol extraction of the cRNA:mRNA complexes, and electrophoretic isolation of the hybridizing cRNA fragments. Since one can synthesize “sense” mRNAs having the same sequence as the target cellular mRNA, appropriate standard curves can be generated and used to quantitate the changes in tissue mRNA levels. Because the assay requires perfect sequence complementarity for full protection, it not only serves as a quantitative tool but also provides conclusive evidence for the existence of a specific mRNA in a given tissue. The procedure described here is a modification of that originally described by M. Gilman [1993,inCurrent Protocols in Molecular Biology (Ausubel, F. M., Brent, R., Kingston, R. E., Moore, D. D., Seidman, J. G., Smith, J. A., and Struhl, K., Eds.), Vol. 1, pp. 4.7.1–4.7.6, Greene and Wiley–Interscience, New York]. 相似文献
60.
Molecular population genetics of ref(2)P, a locus which confers viral resistance in Drosophila 总被引:4,自引:0,他引:4
The ref(2)P locus (2-54.2) is polymorphic for two allelic forms in natural
populations of Drosophila melanogaster, ref(2)Po and ref(2)Pp. The latter
allele confers resistance to the rhabdovirus sigma infecting wild
populations. Previous work, based on a small sample of prescreened
restrictive (resistant) and permissive (susceptible) alleles, identified a
large number of amino acid replacement changes (7) relative to synonymous
changes (1). Such protein variability could be the result of
variation-enhancing selection. To further test the selection hypothesis, we
have examined the DNA sequences of ten randomly chosen lines of D.
melanogaster and one line of D. simulans. Nine of the ten lines are
permissive; D. simulans does not harbor the virus. The melanogaster alleles
contain 4 synonymous changes, 19 noncoding changes, and 13 amino acid
replacement changes, indicating a relatively high level of polymorphism.
Three sequenced restrictive alleles have nearly identical sequences,
indicating that they are relatively young. Compared to the permissive
alleles, they share only a complex deletion at codon 34, CAG-AAT to GGA,
which our analysis indicates to be the site conferring the restrictive
phenotype. Patterns of polymorphism and divergence differ from neutral
predictions by several criteria for the amino terminal region, which
contains the complex deletion (codons 1-91), but not the remainder of the
protein (codons 92-599). We find a higher rate of evolution on the D.
melanogaster lineage than on the D. simulans lineage. The relatively large
amount of both replacement and silent polymorphism in the permissive
alleles and the lack of divergence between permissive and restrictive
alleles suggests that the sigma virus and ref(2)P may be engaged in an
evolutionary race in which new restrictive alleles are continually arising
but are relatively short-lived.
相似文献