全文获取类型
收费全文 | 6997篇 |
免费 | 613篇 |
国内免费 | 1篇 |
专业分类
7611篇 |
出版年
2024年 | 11篇 |
2023年 | 41篇 |
2022年 | 108篇 |
2021年 | 203篇 |
2020年 | 91篇 |
2019年 | 117篇 |
2018年 | 157篇 |
2017年 | 130篇 |
2016年 | 235篇 |
2015年 | 456篇 |
2014年 | 481篇 |
2013年 | 468篇 |
2012年 | 656篇 |
2011年 | 626篇 |
2010年 | 355篇 |
2009年 | 301篇 |
2008年 | 469篇 |
2007年 | 447篇 |
2006年 | 398篇 |
2005年 | 368篇 |
2004年 | 331篇 |
2003年 | 346篇 |
2002年 | 286篇 |
2001年 | 49篇 |
2000年 | 34篇 |
1999年 | 68篇 |
1998年 | 64篇 |
1997年 | 44篇 |
1996年 | 40篇 |
1995年 | 34篇 |
1994年 | 23篇 |
1993年 | 31篇 |
1992年 | 22篇 |
1991年 | 15篇 |
1990年 | 18篇 |
1989年 | 7篇 |
1988年 | 9篇 |
1987年 | 10篇 |
1986年 | 4篇 |
1985年 | 10篇 |
1984年 | 9篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1973年 | 2篇 |
排序方式: 共有7611条查询结果,搜索用时 8 毫秒
1.
2.
3.
Michelle van der Bank F. H. van der Bank B. -E. van Wyk 《Plant Systematics and Evolution》1999,219(1-2):27-38
We have tested the hypothesis that reseeding is a plesiomorphic character state and that sprouting is a derived state inAspalathus linearis, and that the latter is an adaptation to ensure fire-survival in a fireprone environment. Samples of five seeder and four sprouter populations of A.linearis were examined by horizontal starch gel electrophoresis to assess the amount of genetic differentiation within and between sprouting and seeding populations, and to determine the extent of gene flow between the populations. Leaf extracts were surveyed for ten enzymes and gene products revealed genetic variation at 13 (76%) of 17 protein coding loci. Allele frequency differences were found between sprouting and seeding populations and genetic distance values show that the sprouters are grouped separate from the seeders, thus providing support for the morphological data on which the above mentioned hypothesis is based. It is evident that evolution operates at the population level inA. linearis. 相似文献
4.
Rachael V. Gallagher Lesley Hughes Michelle R. Leishman Peter D. Wilson 《Biological invasions》2010,12(12):4049-4063
Potential interactions between climate change and exotic plant invasions may affect areas of high conservation value, such
as land set aside for the protection of endangered species or ecological communities. We investigated this issue in eastern
Australia using species distribution models for five exotic vines under climate regimes for 2020 and 2050. We examined how
projected changes in the distribution of climatically suitable habitat may coincide with the remaining remnants of an endangered
ecological community—littoral rainforests—in this region. The number of known infestations of each weed in tropical, subtropical
and temperate areas was used to assess the likelihood of further expansion into areas projected to provide suitable habitat
under future conditions. Littoral rainforest reserves were consistently predicted to provide bioclimatically suitable habitat
for the five vines examined under both current and future climate scenarios. We explore the consequences and potential strategies
for managing exotic plant invasions in these protected areas in the coming decades. 相似文献
5.
The HsdS and HsdM polypeptides of the type IC restriction enzyme EcoR124 have been purified independently and used in a set of gel retardation experiments to determine the minimum requirements for sequence-specific recognition of DNA by this enzyme. The HsdS polypeptide alone is able to bind to DNA in a sequence-specific manner. In addition, whilst the presence of the HsdM polypeptide gives rise to a stimulation of DNA binding by the HsdS subunit it is not clear whether, under the conditions of the experiments reported here, the HsdS subunit maintains the same interactions with the HsdM subunits observed in the absence of DNA. 相似文献
6.
7.
Cystic fibrosis typing with DNA probes and screening for ΔF508 deletion in families from Southern France 总被引:1,自引:1,他引:0
Mireille Claustres Marie Desgeorges Paule Kjellherg Hélène Bellet Jacques Demaille Michelle Ramsay 《Human genetics》1990,85(4):398-399
Summary A sample of 235 individuals from 49 French cystic fibrosis (CF) families with at least one living affected child was typed
with probes for restriction fragment length polymorphisms (RFLPs) known to be linked to the CF gene, and was screened for
the ΔF508 mutation. Using a combination of six probes, 44 out of the 49 families were sufficiently informative to enable prenatal
diagnosis or carrier determination. As in many other populations, linkage disequilibrium was found between the CF locus and
the haplotype B (XV2c: allele 1; KM19: allele 2), which accounts for about 78% of CF chromosomes in our families. The ΔF508
deletion was present in 64.3% of CF chromosomes. 相似文献
8.
Michelle Lesimple Christian Dournon Charles Houillon 《Development genes and evolution》1990,198(7):420-429
Summary In urodele amphibians, the lack of a reliable germ cell marker restricts the experimental study of the germ lineage. In the present work, we conducted genetic and histological analyses in order to demonstrate that melanin from oocytes constitutes a germ cell marker available for intraspecific experiments in Ambystoma mexicanum. Then, using this marker, we implanted germ cells from undifferentiated gonads (stage 48) into the blastocoel of host embryos and investigated their fate and determined state. Our results show that, from this stage on, the donor cells do not differentiate into other cell types; therefore, they are restricted in developmental capacity and irreversibly determined as germ cells. On the other hand, exogenous germ cells were found in an isotopic position until the young tail-bud stage, and then were found in an ectopic position; these results suggest that, from the middle tail-bud stage on, an active process contributes to migration of primordial germ cells to the gonadal territory. 相似文献
9.
Summary
Sedum wrightii is one of only a few species in the Crassulaceae for which there is evidence for a high degree of variability in the ratio of daytime to nighttime CO2 assimilation. There are both environmental and genetic components to this variability. S. wrightii grows over a wide altitudinal gradient. The purpose of this study was to compare low, intermediate, and high altitude populations with respect to the degree of CAM expression and the capability to tolerate limited water availability. We utilized clonallyreplicated genotypes of plants from each population in common environment greenhouse experiments. Genetic differences among the populations were found in long-term water use efficiency, in 24 hour CO2 exchange patterns, in biomass 13C values, in carbon allocation, and in water status and ultimately survival during prolonged drought. The differences among the populations appear to be closely related to differences in the native habitats. The low altitude, desert plants had the greatest ability to grow and survive under conditions of limited water availability and appear to have the greatest shift to nighttime CO2 uptake during periods without water, while the high altitude plants had the poorest performance under these conditions and appear to shut down net carbon uptake when severely water limited. 相似文献
10.
In the UK several quantitative methods exist for the examination of bivalve molluscan shellfish for sewage contamination. These methods include roll tubes, pour plates and most probable number (MPN) techniques, but there is no national standard method. A comparative study was made of the most commonly used methods for detection of Escherichia coli in bivalve shellfish. Schemes employing solid media, such as the roll tube and pour plate methods, underestimated faecal contamination in shellfish tissue compared with a liquid MPN multiple test-tube method using minerals-modified-glutamate broth (MMGB) as primary enrichment medium. The composition of MMGB apparently permits repair of sublethally injured cells of E. coli. Incorporation of resuscitation stages into the pour plate technique did not yield higher counts. A standardized MPN technique for examination of bivalve molluscan shellfish for E. coli content is proposed as a possible national reference procedure pending further collaborative assessment. 相似文献