全文获取类型
收费全文 | 289篇 |
免费 | 17篇 |
专业分类
306篇 |
出版年
2018年 | 3篇 |
2017年 | 6篇 |
2016年 | 5篇 |
2015年 | 12篇 |
2014年 | 8篇 |
2013年 | 5篇 |
2012年 | 7篇 |
2011年 | 12篇 |
2010年 | 9篇 |
2009年 | 7篇 |
2008年 | 6篇 |
2007年 | 2篇 |
2006年 | 8篇 |
2005年 | 9篇 |
2004年 | 11篇 |
2003年 | 3篇 |
2002年 | 6篇 |
2001年 | 10篇 |
2000年 | 2篇 |
1999年 | 10篇 |
1998年 | 12篇 |
1997年 | 7篇 |
1996年 | 10篇 |
1995年 | 14篇 |
1994年 | 3篇 |
1993年 | 6篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1990年 | 7篇 |
1989年 | 3篇 |
1988年 | 6篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1985年 | 12篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 9篇 |
1976年 | 8篇 |
1975年 | 6篇 |
1974年 | 8篇 |
1973年 | 2篇 |
1972年 | 5篇 |
1950年 | 1篇 |
1946年 | 1篇 |
1941年 | 1篇 |
1940年 | 1篇 |
排序方式: 共有306条查询结果,搜索用时 0 毫秒
1.
Summary The water balance of three different sized coexisting species of heteromyid rodents (Dipodomys merriami ca. 39 g;Perognathus fallax ca. 23 g;Perognathus longimembris ca. 9 g) was assessed while consuming two different diets (either wheat or hulled sunflower) at ambient temperatures of 15–30°C.
The metabolism of wheat as the sole food source was calculated to provide a greater metabolic water production (MWP) than
the consumption of sunflower seed because of their different composition. The state of water balance was assessed by measuring
urine concentrations and body weight maintenance on each diet at each temperature. Both measures showed that (i) all species
were able to maintain a more positive water on the higher MWP seed, (ii) for all species there was an ambient temperature
above which water balance could no longer be maintained, (iii) that this temperature was higher with the higher MWP food source
and (iv) water regulatory efficiency was negatively correlated with body mass.Dipodomys showed a reduced digestive efficiency compared toPerognathus. When presented with both seedsDipodomys showed no preference for either seed irrespective of the state of water balance whilst thePerognathus species showed a tendency for an increased preference for the high MWP food source at the higher ambient temperatures. The
ecological implications of these findings are discussed. 相似文献
2.
Methods for computing the standard errors of branching points in an evolutionary tree and their application to molecular data from humans and apes 总被引:23,自引:2,他引:21
Statistical methods for computing the standard errors of the branching
points of an evolutionary tree are developed. These methods are for the
unweighted pair-group method-determined (UPGMA) trees reconstructed from
molecular data such as amino acid sequences, nucleotide sequences,
restriction-sites data, and electrophoretic distances. They were applied to
data for the human, chimpanzee, gorilla, orangutan, and gibbon species.
Among the four different sets of data used, DNA sequences for an
895-nucleotide segment of mitochondrial DNA (Brown et al. 1982) gave the
most reliable tree, whereas electrophoretic data (Bruce and Ayala 1979)
gave the least reliable one. The DNA sequence data suggested that the
chimpanzee is the closest and that the gorilla is the next closest to the
human species. The orangutan and gibbon are more distantly related to man
than is the gorilla. This topology of the tree is in agreement with that
for the tree obtained from chromosomal studies and DNA-hybridization
experiments. However, the difference between the branching point for the
human and the chimpanzee species and that for the gorilla species and the
human-chimpanzee group is not statistically significant. In addition to
this analysis, various factors that affect the accuracy of an estimated
tree are discussed.
相似文献
3.
Native unploughed tallgrass prairie from Konza Prairie, Kansas, USA is described with respect to plant species compositional changes over a five year period in response to fire and topography. The principal gradient of variation in the vegetation is related to time since burning. Species show an individualistic response in terms of relative abundance to this gradient. Both the percentage of and cover of C4 species and all grasses decrease as the prairie remains unburnt. Forb and woody plant species numbers and abundance increase along this gradient. A secondary gradient of variation reflects topography (i.e. upland versus lowland soils). Upland soils support a higher species richness and diversity. Upland and lowland plant assemblages are distinct except on annually burnt prairie. The interaction between burning regime, topography and year-to-year climatic variation affects the relative abundance of the plant species differentially. The most dominant species overall, Andropogon gerardii, was affected only by year-to-year variation (i.e. climate). Its position at the top of the species abundance hierarchy was unaffected by burning regime or soil type. The other dominant species showed a suite of varying responses to these factors.Deceased May, 1986. 相似文献
4.
K. S. Hong T. E. Richter J. L. Bennetzen S. H. Hulbert 《Molecular & general genetics : MGG》1993,239(1-2):115-121
Rp1 is a disease resistance complex and is the terminal morphological marker on the short arm of maize chromosome 10. Several restriction fragment length polymorphisms (RFLPs), which map within 5 map units of Rp1, were examined to determine if they are also complex in structure. Two RFLP loci, which mapped distally to Rp1, BNL3.04 and PIO200075, existed in a single copy in all maize lines examined. These two loci cosegregated perfectly in 130 test cross progeny. Two RFLP loci that map proximally to Rp1 had unusual structures, which have not yet been reported for maize RFLPs; the loci were complex, with variable numbers of copies in different maize lines. One of the loci, NPI285, occasionally recombined in meiosis to yield changes in the number of copies of sequences homologous to the probe. The other proximal locus, detected by the probes NPI422, KSU3, and KSU4, was relatively stable in meiosis and no changes in the number of restriction fragments were observed. The similarity in map position between Rp1 and the complex RFLP loci indicate there may be genomic areas where variable numbers of repeated sequences are common. The structure of these complex loci may provide insight into the structure and evolution of Rp1. 相似文献
5.
6.
7.
Phospholipids were extracted from tissues (heart, skeletal muscle, kidney cortex, liver and brain) of mammals representing a 9,000-fold range in body mass (mouse, rat, rabbit, sheep and cattle) and their fatty acid composition was determined. In heart, skeletal muscle and kidney cortex, there were significant allometric decreases in the Unsaturation Index (UI; average number of double bonds per 100 fatty acid molecules) with increasing body mass. There were significant inverse allometric relationships between body mass and the proportion of docosahexaenoic acid (2263) in heart and skeletal muscle. In heart, skeletal muscle and kidney cortex, larger mammals also had shorter fatty acid chains in their phospholipids and a higher proportion of monounsaturates. In liver, smaller mammals had a higher UI than larger mammals (except the rabbit, which had the lowest UI and very low proportions of 3 fatty acids). The brain of all mammals maintained a high UI with similar levels of polyunsaturated fatty acids, especially 2263. Our results suggest that in heart, skeletal muscle and kidney cortex the activity of the elongases and desaturases are reduced in large mammals compared to small mammals. The allometric trends in membrane composition may be involved in modifying membrane permeability. It is proposed that the elevated degree of polyunsaturation in the membranes of several tissues from small mammals is related to their higher metabolic activity.This work was supported by an Australian Commonwealth Postgraduate Research Scholarship from the University of Wollongong to P. Couture and by a grant from the Australian Research Council to A.J. Hulbert. We wish to thank Voytek Mantaj for technical assistance. 相似文献
8.
New Rust Resistance Specificities Associated with Recombination in the Rp1 Complex in Maize 总被引:14,自引:1,他引:13
下载免费PDF全文
![点击此处可从《Genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We address the question of whether genetic reassortment events, including unequal crossing over and gene conversion, at the Rp1 complex are capable of generating novel resistance specificities that were not present in the parents. Some 176 events involving genetic reassortment within the Rp1 complex were screened for novel resistance specificities with a set of 11 different rust biotypes. Most (150/176) of the events were susceptible to all tested rust biotypes, providing no evidence for new specificities. Eleven events selected as double-resistant recombinants, when screened with the 11 test biotypes, showed the combined resistance of the two parental types consistent with a simple recombination and pyramiding of the parental resistances. Nine events selected either as having partial resistance or complete susceptibility to a single biotype possessed resistance to a subset of the biotypes that the parents were resistant to, suggesting segregation of resistance genes present in the parental Rp1 complex. Four events gave rise to novel specificities being resistant to at least one rust biotype to which both parents were susceptible. All four had flanking marker exchange, demonstrating that crossing over within the Rp1 complex is associated with the appearance of new rust resistance specificities. 相似文献
9.
A highly conserved nuclear gene for low-level phylogenetics: elongation factor-1 alpha recovers morphology-based tree for heliothine moths 总被引:8,自引:2,他引:6
Cho S; Mitchell A; Regier JC; Mitter C; Poole RW; Friedlander TP; Zhao S 《Molecular biology and evolution》1995,12(4):650-656
Molecular systematists need increased access to nuclear genes. Highly
conserved, low copy number protein-encoding nuclear genes have attractive
features for phylogenetic inference but have heretofore been applied mostly
to very ancient divergences. By virtue of their synonymous substitutions,
such genes should contain a wealth of information about lower-level
taxonomic relationships as well, with the advantage that amino acid
conservatism makes both alignment and primer definition straightforward. We
tested this postulate for the elongation factor-1 alpha (EF-1 alpha) gene
in the noctuid moth subfamily Heliothinae, which has probably diversified
since the middle Tertiary. We sequenced 1,240 bp in 18 taxa representing
heliothine groupings strongly supported by previous morphological and
allozyme studies. The single most parsimonious gene tree and the
neighbor-joining tree for all nucleotides show almost complete concordance
with the morphological tree. Homoplasy and pairwise divergence levels are
low, transition/transversion ratios are high, and phylogenetic information
is spread evenly across gene regions. The EF-1 alpha gene and presumably
other highly conserved genes hold much promise for phylogenetics of
Tertiary age eukaryote groups.
相似文献
10.
Identification of the signalling molecules involved in mesoderm formation in amphibian embryos still presents problems. None of the original candidates, such as activin, have been definitively ruled out, and the new factors, such as the nodal-related genes, have come on to the scene. Of the original candidates, activin has been definitively shown to act as a morphogen, whereas bone morphogenetic protein (BMP)-4 has emerged as a ventral inducer and an inhibitor of neural differentiation. The effects of BMP-4 are antagonized by chordin, a molecule related to the product of the Drosophila gene short gastrulation. 相似文献