全文获取类型
收费全文 | 390篇 |
免费 | 28篇 |
出版年
2021年 | 3篇 |
2018年 | 7篇 |
2017年 | 3篇 |
2016年 | 9篇 |
2015年 | 20篇 |
2014年 | 16篇 |
2013年 | 27篇 |
2012年 | 14篇 |
2011年 | 16篇 |
2010年 | 25篇 |
2009年 | 27篇 |
2008年 | 15篇 |
2007年 | 13篇 |
2006年 | 12篇 |
2005年 | 15篇 |
2004年 | 13篇 |
2003年 | 5篇 |
2002年 | 5篇 |
2001年 | 7篇 |
2000年 | 5篇 |
1999年 | 4篇 |
1998年 | 10篇 |
1997年 | 10篇 |
1996年 | 5篇 |
1995年 | 6篇 |
1994年 | 3篇 |
1993年 | 7篇 |
1991年 | 5篇 |
1989年 | 3篇 |
1988年 | 7篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1982年 | 18篇 |
1981年 | 5篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1972年 | 3篇 |
1971年 | 2篇 |
1970年 | 2篇 |
1968年 | 3篇 |
1967年 | 2篇 |
1965年 | 3篇 |
1963年 | 2篇 |
排序方式: 共有418条查询结果,搜索用时 15 毫秒
1.
Improved chemistry for oligodeoxyribonucleotide synthesis substantially improves restriction enzyme cleavage of a synthetic 35mer. 总被引:1,自引:1,他引:0
Two DNA duplexes of identical sequence and 35 nt in length were synthesized by an original and a highly improved version of phosphoramidite chemistry. By base composition analysis, DNA synthesized by improved chemistry (termed DMTS-imp) contained no detectable modified bases while DNA synthesized by the original chemistry (termed DMTS-std) had a large number of modifications. Under optimal reaction conditions, HhaI and RsaI cleaved the DMTS-std duplex to 76-77% completion and the DMTS-imp duplex to 96-99% completion. Restriction analysis and piperidine treatment yielded estimates of approximately 3.0% modified nucleotides in DMTS-std and approximately 1.0% in DMTS-imp. Overall, the improvements in chemistry increased the restriction efficiency of synthetic DNA up to 10-fold. 相似文献
2.
3.
Summary Hainsworth and Wolf (1976) reported that under certain conditions hummingbirds made food choices which did not maximize their net rate of energy intake while foraging. They concluded that the birds were not foraging optimally. We show here that their birds probably maximized a different utility function, the net energy per unit volume consumed (NEVC), which appears to be an optimal choice on a time scale longer than that of a foraging bout. Our own experiments with Archilochus colubris support the conclusion that hummingbirds make foraging decisions that maximize NEVC. A simulation model shows that, in nature, NEVC maximization would require fewer foraging trips and visits to fewer flowers per day to balance daily energy budgets. For territorial birds this can lead to smaller territory sizes and reduced costs of territorial defense. Plants that evolutionarily increase corolla length to enhance pollinator specificity need only increase nectar concentration slightly to maintain the same net energy per unit volume consumed (NEVC) by a given hummingbird pollinator. 相似文献
4.
2-Aminoethylphosphonic acid concentrations in some rumen ciliate protozoa. 总被引:2,自引:2,他引:0
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The concentration of 2-aminoethylphosphonic acid has been measured in seven genera of rumen ciliate protozoa. Expressed as milligrams per gram of total nitrogen, 2-aminoethylphosphonic acid concentrations ranged from 17.2 in Ophryoscolex spp. to 72.4 in Eremoplastron spp. 相似文献
5.
6.
7.
The phylogeny of Greya Busck (Lepidoptera: Prodoxidae) was inferred from
nucleotide sequence variation across a 765-bp region in the cytochrome
oxidase I and II genes of the mitochondrial genome. Most parsimonious
relationships of 25 haplotypes from 16 Greya species and two outgroup
genera (Tetragma and Prodoxus) showed substantial congruence with the
species relationships indicated by morphological variation. Differences
between mitochondrial and morphological trees were found primarily in the
positions of two species, G. variabilis and G. pectinifera, and in the
branching order of the three major species groups in the genus. Conflicts
between the data sets were examined by comparing levels of homoplasy in
characters supporting alternative hypotheses. The phylogeny of Greya
species suggests that host-plant association at the family level and larval
feeding mode are conservative characters. Transition/transversion ratios
estimated by reconstruction of nucleotide substitutions on the phylogeny
had a range of 2.0-9.3, when different subsets of the phylogeny were used.
The decline of this ratio with the increase in maximum sequence divergence
among taxa indicates that transitions are masked by transversions along
deeper internodes or long branches of the phylogeny. Among transitions,
substitutions of A-->G and T-->C outnumbered their reciprocal
substitutions by 2-6 times, presumably because of the approximately 4:1
(77%) A+T-bias in nucleotide base composition. Of all transversions,
73%-80% were A<-->T substitutions, 85% of which occurred at third
positions of codons; these estimates did not decrease with an increase in
maximum sequence divergence of taxa included in the analysis. The high
frequency of A<-->T substitutions is either a reflection or an
explanation of the 92% A+T bias at third codon positions.
相似文献
8.
We tested whether predation on duck nests ( Anas spp.) was density dependent at three spatial scales using artificial and natural nests in the Suisun Marsh, California, USA. At the largest spatial scale, we used 5 years (1998–2002) of data to examine the natural variation in duck nest success and nest densities among 8–16 fields per year, each 5–33 ha in size (n=62 fields). At an intermediate spatial scale, we deployed artificial nests (2000, n=280) within 1-ha plots at three experimental densities (5, 10, and 20 nests ha−1 ) in a complete randomized block design and examined differences in nest predation. At the smallest spatial scale, we examined nest success in relation to nearest-neighbor fates and distances for artificial (2000, n=280) and natural nests (2000, n=507). We detected no relationship between nest success and the density of natural nests among fields in any year, nor when we pooled data for all years after controlling for year effects. The proportion of artificial nests that survived also did not depend on experimental nest densities within 1-ha plots. Overall, 15.0±12.4%, 15.0±11.0%, and 6.2±4.3% of artificial nests survived the 32-day exposure period in the low, intermediate, and high nest densities, respectively. Additionally, we detected no consistent effect of nearest-neighbor fate or distance on the success of artificial or natural nests. Thus, our results provide no evidence of density-dependent predation on duck nests at any scale of analysis, in contrast to a number of previous studies. Variation among geographical locations in the degree to which predation is density-dependent may reflect the composition of the predator community and the availability of alternate prey. 相似文献
9.
10.