全文获取类型
收费全文 | 122884篇 |
免费 | 2745篇 |
国内免费 | 1645篇 |
出版年
2024年 | 40篇 |
2023年 | 295篇 |
2022年 | 547篇 |
2021年 | 974篇 |
2020年 | 715篇 |
2019年 | 830篇 |
2018年 | 12523篇 |
2017年 | 11269篇 |
2016年 | 8496篇 |
2015年 | 2406篇 |
2014年 | 2248篇 |
2013年 | 2594篇 |
2012年 | 6935篇 |
2011年 | 15106篇 |
2010年 | 13449篇 |
2009年 | 9639篇 |
2008年 | 11511篇 |
2007年 | 12926篇 |
2006年 | 1910篇 |
2005年 | 1961篇 |
2004年 | 2324篇 |
2003年 | 2210篇 |
2002年 | 1880篇 |
2001年 | 554篇 |
2000年 | 344篇 |
1999年 | 280篇 |
1998年 | 299篇 |
1997年 | 269篇 |
1996年 | 199篇 |
1995年 | 173篇 |
1994年 | 168篇 |
1993年 | 170篇 |
1992年 | 138篇 |
1991年 | 155篇 |
1990年 | 119篇 |
1989年 | 78篇 |
1988年 | 87篇 |
1987年 | 72篇 |
1986年 | 44篇 |
1985年 | 87篇 |
1984年 | 101篇 |
1983年 | 62篇 |
1982年 | 55篇 |
1981年 | 58篇 |
1980年 | 55篇 |
1978年 | 35篇 |
1977年 | 41篇 |
1976年 | 35篇 |
1972年 | 256篇 |
1971年 | 286篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Juan Viruel Pilar Catalán José Gabriel Segarra‐Moragues 《Journal of Biogeography》2012,39(6):1073-1085
Aim The Chilean endemic Dioscorea biloba (Dioscoreaceae) is a dioecious geophyte that shows a remarkable 600 km north–south disjunction in the peripheral arid area of the Atacama Desert. Its restricted present‐day distribution and probable Neogene origin indicate that its populations have a history linked to that of the Atacama Desert, making this an ideal model species with which to investigate the biogeography of the region. Location Chile, Atacama Desert and peripheral arid area. Methods Two hundred and seventy‐five individuals from nine populations were genotyped for seven nuclear microsatellite loci, and plastid trnL–F and trnT–L sequences were obtained for a representative subset of these. Analyses included the estimation of genetic diversity and population structure through clustering, Bayesian and analysis of molecular variance analyses, and statistical parsimony networks of chloroplast haplotypes. Isolation by distance was tested against alternative dispersal hypotheses. Results Microsatellite markers revealed moderate to high levels of genetic diversity within populations, with those from the southern Limarí Valley showing the highest values and northern populations showing less exclusive alleles. Bayesian analysis of microsatellite data identified three genetic groups that corresponded to geographical ranges. Chloroplast phylogeography revealed no haplotypes shared between northern and southern ranges, and little haplotype sharing between the two neighbouring southern valleys. Dispersal models suggested the presence of extinct hypothetical populations between the southern and northern ranges. Main conclusions Our results are consistent with prolonged isolation of the northern and southern groups, mediated by the life‐history traits of the species. Significant isolation was revealed at both large and moderate distances as gene flow was not evident even between neighbouring valleys. Bayesian analyses of microsatellite and chloroplast haplotype diversity identified the southern area of Limarí as the probable area of origin of the species. Our data do not support recent dispersal of D. biloba from the southern range into Antofagasta, but indicate the fragmentation of an earlier wider range, concomitant with the Pliocene–Pleistocene climatic oscillations, with subsequent extinctions of the Atacama Desert populations and the divergence of the peripheral ones as a consequence of genetic drift. 相似文献
2.
The influence of fruit ingestion by the bat, Sturnira lilium, on germination of the seeds of the tree Solanum riparium was studied in a secondary rain forest in northwestern Argentina. Bat frequencies in disturbed areas were analyzed by mist net captures. Germination rates were determined for seeds collected from trees and bat feces. S. lilium was the most abundant fruit bat in the study area. Fruit digestion and the passage of seeds through the intestine did not significantly affect germination in S. riparium. In this case the fruit bats, therefore, probably provide only seed dispersal. 相似文献
3.
Identifying common prognostic factors in genomic cancer studies: A novel index for censored outcomes
Background
With the growing number of public repositories for high-throughput genomic data, it is of great interest to combine the results produced by independent research groups. Such a combination allows the identification of common genomic factors across multiple cancer types and provides new insights into the disease process. In the framework of the proportional hazards model, classical procedures, which consist of ranking genes according to the estimated hazard ratio or the p-value obtained from a test statistic of no association between survival and gene expression level, are not suitable for gene selection across multiple genomic datasets with different sample sizes. We propose a novel index for identifying genes with a common effect across heterogeneous genomic studies designed to remain stable whatever the sample size and which has a straightforward interpretation in terms of the percentage of separability between patients according to their survival times and gene expression measurements. 相似文献4.
Plant somatic cells have the capability to switch their cell fates from differentiated to undifferentiated status under proper
culture conditions, which is designated as totipotency. As a result, plant cells can easily regenerate new tissues or organs
from a wide variety of explants. However, the mechanism by which plant cells have such remarkable regeneration ability is
still largely unknown. In this study, we used a set of meristem-specific marker genes to analyze the patterns of stem cell
differentiation in the processes of somatic embryogenesis as well as shoot or root organogenesis in vitro. Our studies furnish preliminary and important information on the patterns of the de novo stem cell differentiation during various types of in vitro organogenesis. 相似文献
5.
Claudia Raedig Carsten F. Dormann Anke Hildebrandt Sven Lautenbach 《Biodiversity and Conservation》2010,19(6):1523-1546
Monographic data rely on specimens deposited in herbaria and museums, which have been thoroughly revised by experts. However,
monographic data have been rarely used to map species richness at large scale, mainly because of the difficulties caused by
spatially heterogeneous sampling effort. In this paper we estimate patterns of species richness and narrow endemism, based
on monographic data of 4,055 Neotropical angiosperm species. We propose a geometric interpolation method to derive species
ranges at a 1° grid resolution. To this we apply an inverse distance-weighted summation scheme to derive maps of species richness
and endemism. In the latter we also adjust for heterogeneous sampling effort. Finally, we test the robustness of the interpolated
species ranges and derived species richness by applying the same method but using a leave-one-out-cross-validation (LOOCV).
The derived map shows four distinct regions of elevated species richness: (1) Central America, (2) the Northern Andes, (3)
Amazonia and (4) the Brazilian Atlantic coast (‘Mata Atlantica’). The region with the highest estimated species richness is
Amazonia, with Central America following closely behind. Centers of narrow endemism are located over the entire Neotropics,
several of them coinciding with regions of elevated species richness. Sampling effort has a minor influence on the interpolation
of overall species richness, but it substantially influences the estimation of regions of narrow endemism. Thus, in order
to improve maps of narrow endemism and resulting conservation efforts, more collection and identification activity is required. 相似文献
6.
Cristian A. Acevedo Elizabeth Y. Sanchez Juan G. Reyes Manuel E. Young 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2010,878(3-4):449-455
It is known that skin releases volatile organic compounds to the environment, and also that its emission pattern changes with aging of the skin. It could be considered, that these compounds are intermediaries in cell metabolism, since many intermediaries of metabolic pathways have a volatile potential. In this work, a simple and non-destructive method consisting of SPME sampling and GC/MS analysis was developed to identify volatile organic emanations from cell cultures. This technique, applied to skin cells culture, indicates that the cells or cell metabolism produce several skin emissions. Chemometric analysis was performed in order to explore the relationship between a volatile profile and the senescence of cell cultures. Volatile profiles were different for cell cultures in different degrees of senescence, indicating that volatile compound patterns could be used to provide information about the age of skin cells. 相似文献
7.
Hostile intercommunity relations, including attacking and killing extra-community infants of both sexes have occurred at most
wild chimpanzee sites. We describe three recent cases of intercommunity attacks on infants committed by members of the Ngogo
chimpanzee community in Kibale National Park, Uganda. Two of the attacks resulted in confirmed infanticides while a third
attack probably resulted in the infant's death. In common with previous accounts of chimpanzee infanticides, the attacks described
here occurred during boundary patrols outside the Ngogo community's usual range, adult and adolescent males were the main
participants, one infant was cannibalized after being killed, and the victims’ mothers did not accompany the attacking party
back to the Ngogo range. However, the patrol parties during each infanticide were larger than before and included females
from the Ngogo community. Our observations indirectly support both the range expansion and imbalance of power hypotheses,
which address why and under which conditions chimpanzee intercommunity encounters lead to aggression. These cases of intercommunity
infanticide add to the growing database of the phenomenon in wild chimpanzees. 相似文献
8.
Lincoln D Stein 《Genome biology》2010,11(5):207
With DNA sequencing now getting cheaper more quickly than data storage or computation, the time may have come for genome informatics
to migrate to the cloud. 相似文献
9.
Rice fields are an alternative habitat for waterbirds and provide food and shelter for many avian species, but there is a lack of information about how the use of rice fields translates into population level effects. The aim of this study was to test the relationship between the use of rice fields by European waterbirds and trends in their populations. We tested this relationship during the autumn migration season and during the breeding season. Based on counts conducted over the last 23 years in natural marshes and areas of rice fields in Doñana (SW Spain), an index of rice field use was constructed for 76 bird species, which was then compared to these species’ European population trends obtained from the literature. A positive relationship was found between waterbird population trends and the use of rice fields during autumn migration season. Our study suggests that changes in the Common Agriculture Policy in Europe leading to reductions in areas of rice cultivation may have important effects on waterbirds. The restoration of former marsh areas and the maintenance of rice cultivation would seem to be more environmentally friendly approaches than the use of these areas to grow alternative crops or solar farms. 相似文献