全文获取类型
收费全文 | 174篇 |
免费 | 9篇 |
出版年
2022年 | 1篇 |
2021年 | 6篇 |
2019年 | 2篇 |
2018年 | 1篇 |
2017年 | 3篇 |
2016年 | 4篇 |
2015年 | 7篇 |
2014年 | 10篇 |
2013年 | 12篇 |
2012年 | 11篇 |
2011年 | 16篇 |
2010年 | 12篇 |
2009年 | 6篇 |
2008年 | 7篇 |
2007年 | 6篇 |
2006年 | 4篇 |
2005年 | 4篇 |
2004年 | 8篇 |
2003年 | 1篇 |
2002年 | 4篇 |
2001年 | 6篇 |
2000年 | 4篇 |
1999年 | 1篇 |
1998年 | 6篇 |
1995年 | 5篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1985年 | 1篇 |
1978年 | 2篇 |
1976年 | 4篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1972年 | 2篇 |
1968年 | 2篇 |
1966年 | 1篇 |
1963年 | 1篇 |
1962年 | 1篇 |
1960年 | 1篇 |
1958年 | 1篇 |
1956年 | 1篇 |
排序方式: 共有183条查询结果,搜索用时 15 毫秒
41.
42.
The evolutionary origin of the pinnipeds (seals, sea lions, and walruses)
is still uncertain. Most authors support a hypothesis of a monophyletic
origin of the pinnipeds from a caniform carnivore. A minority view suggests
a diphyletic origin with true seals being related to the mustelids (otters
and ferrets). The phylogenetic relationships of the walrus to other
pinniped and carnivore families are also still particularly problematic.
Here we examined the relative support for mono- and diphyletic hypotheses
using DNA sequence data from the mitochondrial small subunit (12S) rRNA and
cytochrome b genes. We first analyzed a small group of taxa representing
the three pinniped families (Phocidae, Otariidae, and Odobenidae) and
caniform carnivore families thought to be related to them. We inferred
phylogenetic reconstructions from DNA sequence data using standard
parsimony and neighbor-joining algorithms for phylogenetic inference as
well as a new method called spectral analysis (Hendy and Penny) in which
phylogenetic information is displayed independently of any selected tree.
We identified and compensated for potential sources of error known to lead
to selection of incorrect phylogenetic trees. These include sampling error,
unequal evolutionary rates on lineages, unequal nucleotide composition
among lineages, unequal rates of change at different sites, and
inappropriate tree selection criteria. To correct for these errors, we
performed additional transformations of the observed substitution patterns
in the sequence data, applied more stringent structural constraints to the
analyses, and included several additional taxa to help resolve long,
unbranched lineages in the tree. We find that there is strong support for a
monophyletic origin of the pinnipeds from within the caniform carnivores,
close to the bear/raccoon/panda radiation. Evidence for a diphyletic origin
was very weak and can be partially attributed to unequal nucleotide
compositions among the taxa analyzed. Subsequently, there is slightly more
evidence for grouping the walrus with the eared seals versus the true
seals. A more conservative interpretation, however, is that the walrus is
an early, but not the first, independent divergence from the common
pinniped ancestor.
相似文献
43.
F. M. Salzano H. Gershowitz P. C. Junqueira J. P. Woodall F. L. Black W. Hierholzer 《American journal of physical anthropology》1972,36(3):417-425
Five hundred and twenty-six individuals from four populations were studied in relation to the ABO, MNSs, P, Rh, Lutheran, Kell, Lewis, Duffy, Kidd, Diego and I systems, as well as for the Wright antigen of blood groups. The H-Lea salivary secretion of 406 of them was also investigated. Considering the gene markers which show variation in South American Indians, the Cayapo frequencies are in the middle of the distribution range for genes LMs, R1 (CDe), R2 (cDE), P1, Jka, Dia and Se but present high values of Le and Fya and low ones of LMS, LNs, Ro (cDe) and/or r (cde), LNS and Rz (CDE). Unusual findings in relation to previous studies are the high prevalence of Le (a+) persons (which, however, could be expected since the frequency of gene Se is not too high) and the presence of one Lu (a+) and one PP1 (Tja) (?) individuals. Comparison of the pattern of inter-village variation in relation to these polymorphisms with those furnished by historical, demographic and morphological data gives in general reasonable concordance, but some results are difficult to interpret. New approaches and further multidisciplinary studies are needed to obtain a clearer picture of the genetic relationships present among different tribes of South American Indians as well as to understand how polymorphisms are created and maintained in human populations. 相似文献
44.
45.
P. F. Woodall 《Journal of Zoology》1991,223(1):79-90
Factor analysis of the linear dimensions of 92 species of kingfisher indicated that morphological differences were associated with four diet categories (aquatic, littoral, terrestrial and fossorial animals). Analysis of covariance confirmed the significance of these differences for culmen, tarsus and tail length but not for wing length. Habitat factors resulted in some significant differences but these were less easy to interpret. One case of intraspecific variation showed similar trends for culmen length. 相似文献
46.
Leaf developmental patterns were characterized in four rainforest tree
species of Syzgium . Leaf optical properties, pigment
changes, expansion characteristics, stomatal development, and
photosynthetic rates were studied. In both S.
luehmannii and S. wilsonii photosynthetic
development was delayed until after full leaf expansion. Rates of O2
evolution were negative during expansion of S.
luehmannii and S. wilsonii leaves and
stomatal conductance was 10-20 mmol m-2
s-1 lower than for corresponding leaves of
S. moorei . Stomatal conductance showed that the
development of functional stomata was delayed until after full leaf
expansion in S. luehmannii and S.
wilsonii , however, low stomatal conductance was not responsible
for the lack of photosynthetic potential during leaf expansion in these
species. Leaves of S. luehmannii and S.
wilsonii required less than 10 d for full leaf expansion and
contained anthocyanin during expansion. In contrast, leaves of S.
moorei and S. corynanthum expanded slowly
(20-40 d required for full leaf expansion), exhibited positive rates of O2
evolution and did not accumulate anthocyanin. In S.
luehmannii and S. wilsonii anthocyanin was
located in the vacuole of distinct cell layers just below the upper
epidermis and the possible functions of anthocyanin accumulation are
discussed. This is the first report where such variation in leaf
development has been characterized in the one genus.Key
words : Anthocyanin, leaf expansion, photosynthetic development,
delayed leaf greening, stomatal development.
相似文献
47.
Nienhuis HL de Leeuw K Bijzet J van Doormaal JJ van Roon AM Smit AJ Graaff R Kallenberg CG Bijl M 《Arthritis research & therapy》2010,12(5):R181
Introduction
The objectives of this study were to determine small arterial elasticity (SAE) in systemic lupus erythematosus (SLE) and to investigate its relationship with intima media thickness (IMT), accumulation of advanced glycation end products (AGEs), endothelial activation and inflammation. 相似文献48.
The molecular mechanisms by which plants sense their micronutrient status, and adapt to their environment in order to ensure a sufficient micronutrient supply, are poorly understood. Zinc is an essential micronutrient for all living organisms. when facing a shortage in zinc supply, plants adapt by enhancing the zinc uptake capacity. The molecular regulators controlling this adaptation were recently identified. in this mini-review, we highlight recent progress in understanding the adaptation to zinc deficiency in plants and discuss the future challenges to fully unravel its molecular basis.Key words: adaptation, zinc deficiency, biofortification, molecular regulators, plant nutritionIn an increasingly populated world, agricultural production is an essential element of social development. Agriculture is the primary source of all nutrients required for human life, and nutrient sufficiency is the basis for good health and welfare of the human population.1 Soils with zinc deficiency are widespread in the world, affecting large areas of cultivated soils in India, Turkey, China, Brazil and Australia,2,3 making zinc the most common crop micronutrient deficiency.4 In addition, risk of inadequate zinc diet and zinc malnutrition are estimated to affect one-third of the global human population, i.e., around two billion people.5 Most affected are people living in developing countries, where diets are rich in cereal-based foods. Cereal grains are rich in phytate, which is a potent anti-nutrient, limiting micronutrient bioavailability.6 Zinc deficiency in crop production can be easily ameliorated through zinc fertilization, making agronomic biofortification an important strategy,3 however in the poorer regions, the required infrastructure to provide a reliable supply of zinc fertilizers of sufficient quality, is often not available. In those situations, biofortified crops, in which the zinc status of crops is genetically improved by selective breeding or via biotechnology, offer a rural-based intervention that will more likely reach the population.7 Different traits can be targeted to developing such improved crops, such as plant zinc deficiency tolerance, zinc use efficiency and the accumulation of zinc in edible parts. However, insufficient knowledge on the molecular mechanisms and the regulation of the zinc homeostasis network in plants is a serious bottleneck when pursuing zinc biofortification. 相似文献
49.
Vegard Eldholm Mecky Matee Sayoki GM Mfinanga Manfred Heun Ulf R Dahle 《BMC microbiology》2006,6(1):76
Background
Tanzania has a high tuberculosis incidence, and genotyping studies of Mycobacterium tuberculosis in the country are necessary in order to improve our understanding of the epidemic. Spoligotyping is a potentially powerful genotyping method due to fast generation of genotyping results, high reproducibility and low operation costs. The recently constructed SpolDB4 database and the model-based program 'spotclust' can be used to assign isolates to families, subfamilies and variants. The results of a study can thus be analyzed in a global context. 相似文献50.
Reefman E de Jong MC Kuiper H Jonkman MF Limburg PC Kallenberg CG Bijl M 《Arthritis research & therapy》2006,8(6):R156-13
Apoptotic cells are thought to play an essential role in the pathogenesis of systemic lupus erythematosus (SLE). We hypothesise that delayed or altered clearance of apoptotic cells after UV irradiation will lead to inflammation in the skin of SLE patients. Fifteen SLE patients and 13 controls were irradiated with two minimal erythemal doses (MEDs) of ultraviolet B light (UVB). Subsequently, skin biopsies were analysed (immuno)histologically, over 10 days, for numbers of apoptotic cells, T cells, macrophages, and deposition of immunoglobulin and complement. Additionally, to compare results with cutaneous lesions of SLE patients, 20 biopsies of lupus erythematosus (LE) skin lesions were analysed morphologically for apoptotic cells and infiltrate. Clearance rate of apoptotic cells after irradiation did not differ between patients and controls. Influx of macrophages in dermal and epidermal layers was significantly increased in patients compared with controls. Five out of 15 patients developed a dermal infiltrate that was associated with increased epidermal influx of T cells and macrophages but not with numbers of apoptotic cells or epidermal deposition of immunoglobulins. Macrophages were ingesting multiple apoptotic bodies. Inflammatory lesions in these patients were localised near accumulations of apoptotic keratinocytes similar as was seen in the majority of LE skin lesions. In vivo clearance rate of apoptotic cells is comparable between SLE patients and controls. However, the presence of inflammatory lesions in the vicinity of apoptotic cells, as observed both in UVB-induced and in LE skin lesions in SLE patients, suggests that these lesions result from an inflammatory clearance of apoptotic cells. 相似文献