全文获取类型
收费全文 | 546篇 |
免费 | 30篇 |
国内免费 | 1篇 |
出版年
2021年 | 4篇 |
2020年 | 5篇 |
2019年 | 10篇 |
2018年 | 12篇 |
2017年 | 13篇 |
2016年 | 20篇 |
2015年 | 20篇 |
2014年 | 19篇 |
2013年 | 27篇 |
2012年 | 38篇 |
2011年 | 37篇 |
2010年 | 23篇 |
2009年 | 22篇 |
2008年 | 28篇 |
2007年 | 32篇 |
2006年 | 27篇 |
2005年 | 23篇 |
2004年 | 22篇 |
2003年 | 18篇 |
2002年 | 22篇 |
2001年 | 10篇 |
2000年 | 9篇 |
1999年 | 11篇 |
1998年 | 7篇 |
1997年 | 5篇 |
1996年 | 7篇 |
1995年 | 5篇 |
1994年 | 3篇 |
1993年 | 6篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1987年 | 7篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1979年 | 6篇 |
1978年 | 2篇 |
1977年 | 10篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1969年 | 5篇 |
1968年 | 4篇 |
1967年 | 6篇 |
排序方式: 共有577条查询结果,搜索用时 31 毫秒
1.
2.
MAJOR CLADES OF THE ANGIOSPERMS 总被引:2,自引:0,他引:2
ROLF DAHLGREN KÅRE BREMER 《Cladistics : the international journal of the Willi Hennig Society》1985,1(4):349-368
Abstract— Our knowledge of fundamental angiosperm interrelationships is still very incomplete. The absence of a narrowly circumscribed gymnosperm outgroup, ideally the sister group, makes character evaluation, necessary for a cladistic analysis, difficult. According to current views the superorder Magnoliiflorae with a number of other groups, for example the monocotyledons, may represent a complex of families near the base of the angiosperms. Interrelationships of groups within the monocotyledons are much better understood than those between groups within the dicotyledons. A cladogram of monocotyledon orders based on earlier work by R. Dahlgren, H. T. Clifford, and F. N. Rasmussen is presented. A data matrix for a sample of the angiosperms with 61 characters for 49 taxa, mostly magnoliifloran and related families, is presented. The characters are polarized mainly according to the current view that the primitive angiosperm morphotype is a woody dicotyledon with strobiloid flowers. As an alternative the matrix is adjusted following W. C. Burger's conjecture that the primitive angiosperm was a herbaceous monocotyledon with trimerous flowers. Both matrices were run in a computerized parsimony analysis, resulting in numerous equally parsimonious solutions. This result is illustrative of the great homoplasy in the available character information, and also of how little actually is known about fundamental angiosperm interrelationships or phylogeny. 相似文献
3.
Assignment of orthologous relationships among mammalian alpha-globin genes by examining flanking regions reveals a rapid rate of evolution 总被引:1,自引:0,他引:1
In order to study the relationships among mammalian alpha-globin genes, we
have determined the sequence of the 3' flanking region of the human alpha 1
globin gene and have made pairwise comparisons between sequenced
alpha-globin genes. The flanking regions were examined in detail because
sequence matches in these regions could be interpreted with the least
complication from the gene duplications and conversions that have occurred
frequently in mammalian alpha-like globin gene clusters. We found good
matches between the flanking regions of human alpha 1 and rabbit alpha 1,
human psi alpha 1 and goat I alpha, human alpha 2 and goat II alpha, and
horse alpha 1 and goat II alpha. These matches were used to align the
alpha-globin genes in gene clusters from different mammals. This alignment
shows that genes at equivalent positions in the gene clusters of different
mammals can be functional or nonfunctional, depending on whether they
corrected against a functional alpha-globin gene in recent evolutionary
history. The number of alpha-globin genes (including pseudogenes) appears
to differ among species, although highly divergent pseudogenes may not have
been detected in all species examined. Although matching sequences could be
found in interspecies comparisons of the flanking regions of alpha- globin
genes, these matches are not as extensive as those found in the flanking
regions of mammalian beta-like globin genes. This observation suggests that
the noncoding sequences in the mammalian alpha-globin gene clusters are
evolving at a faster rate than those in the beta-like globin gene clusters.
The proposed faster rate of evolution fits with the poor conservation of
the genetic linkage map around alpha-globin gene clusters when compared to
that of the beta-like globin gene clusters. Analysis of the 3' flanking
regions of alpha-globin genes has revealed a conserved sequence
approximately 100-150 bp 3' to the polyadenylation site; this sequence may
be involved in the expression or regulation of alpha-globin genes.
相似文献
4.
The interaction of antigen with specific receptor sites of primed murine spleen lymphocytes has been estimated quantitatively. The antigen-specific receptors are restricted to a small segment of the entire spleen cell population, one which may be enriched by centrifugation of the cells in a discontinuous density gradient. The specificity of the receptors, their variable affinities for the specific ligand, their quantitative fluctuations following immunization, have been investigated. Attempts have also been initiated for their isolation. The possibility that these receptors may represent passively adherent cytophilic immunoglobulins was excluded. The results of these experiments suggest that the union of antigen with a portion of cell membrane receptors initiates a sequence of reaction steps involving DNA production and multiplication, leading to a small net synthesis of immunoglobulins. The continued presence of antigen during this period leads to complex formation with these immunoglobulins. The resulting immune complexes bind to both adherent and nonadherent cells, thereby amplifying the immune response. 相似文献
5.
6.
7.
8.
9.
10.
Removal of Cr(VI) from ground water by Saccharomyces cerevisiae 总被引:1,自引:0,他引:1
Chromium can be removed from ground water by the unicellular yeast, Saccharomyces cerevisiae. Local ground water maintains chromium as CrO4
2- because of bicarbonate buffering and pH and E
h
conditions (8.2 and +343 mV, respectively). In laboratory studies, we used commercially available, nonpathogenic S. cerevisiae to remove hexavalent chromium [Cr(VI)] from ground water. The influence of parameters such as temperature, pH, and glucose concentration on Cr(VI) removal by yeast were also examined. S. cerevisiae removed Cr(VI) under aerobic and anaerobic conditions, with a slightly greater rate occurring under anaerobic conditions. Our kinetic studies reveal a reaction rate (Vmax) of 0.227 mg h-1 (g dry wt biomass)-1 and a Michaelis constant (Km) of 145 mg/l in natural ground water using mature S. cerevisiae cultures. We found a rapid (within 2 minutes) initial removal of Cr(VI) with freshly hydrated cells [55–67 mg h-1 (g dry wt biomass)-1] followed by a much slower uptake [0.6–1.1 mg h-1 (g dry wt biomass)-1] that diminished with time. A materials-balance for a batch reactor over 24 hours resulted in an overall shift in redox potential from +321 to +90 mV, an increase in the bicarbonate concentration (150–3400 mg/l) and a decrease in the Cr(VI) concentration in the effluent (1.9-0 mg/l). 相似文献