首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4169篇
  免费   256篇
  国内免费   3篇
  4428篇
  2024年   13篇
  2023年   17篇
  2022年   42篇
  2021年   66篇
  2020年   42篇
  2019年   39篇
  2018年   55篇
  2017年   43篇
  2016年   103篇
  2015年   180篇
  2014年   202篇
  2013年   243篇
  2012年   290篇
  2011年   302篇
  2010年   192篇
  2009年   182篇
  2008年   218篇
  2007年   247篇
  2006年   227篇
  2005年   232篇
  2004年   209篇
  2003年   191篇
  2002年   189篇
  2001年   56篇
  2000年   49篇
  1999年   50篇
  1998年   57篇
  1997年   35篇
  1996年   36篇
  1995年   41篇
  1994年   22篇
  1993年   35篇
  1992年   29篇
  1991年   37篇
  1990年   32篇
  1989年   18篇
  1988年   29篇
  1987年   35篇
  1986年   32篇
  1984年   35篇
  1983年   23篇
  1982年   30篇
  1981年   24篇
  1980年   17篇
  1979年   23篇
  1978年   13篇
  1977年   13篇
  1976年   15篇
  1972年   18篇
  1971年   14篇
排序方式: 共有4428条查询结果,搜索用时 15 毫秒
1.
Indigenous peoples of western Arnhem Land, central northern Australia, have detailed knowledge of the rock kangaroos of the region, species that are little known to science. Information about the ecology of the species is required for their conservation and management. Ethnoecological studies can assist senior indigenous people with transfer of knowledge and can give respect and meaningful employment to those involved. We used semidirected interviews in the regional vernacular, Bininj Kunwok, to record indigenous knowledge of the ecology of the four rock kangaroo species (Petrogale brachyotis, P. concinna, Macropus bernardus and M. robustus). Discussions focussed on habitat preferences, diet, activity patterns, reproduction, predation, and hunting practices. The ethnoecological knowledge of the rock kangaroo species was extensive, and both complemented and extended that reported in the scientific literature. In contrast to scientific understanding of taxonomy and ecology, consultants recognized the rock kangaroos as a natural group. They also described subtle differences in the species’ comparative ecology. The methodology used proved highly successful and we recommend recording indigenous knowledge of the ecology of fauna species in the local vernacular wherever possible. This study is one of the most comprehensive ethnozoological studies of a group of species undertaken in Australia.
Wendy R. TelferEmail:
  相似文献   
2.
Abstract: The cerebral metabolic rates for O2 and for glucose were measured in conscious, fasted male Fischer-344 rats at the ages of 3, 12, and 24 months, and cerebral blood flow was determined with 14C-iodoantipyrine. The metabolic rates for oxygen and glucose were obtained by multiplying blood flow by the O2 and glucose concentration differences, respectively, between blood in the femoral artery and in the superior sagittal sinus. Mean cerebral blood flow and the metabolic rates for oxygen and glucose did not differ significantly (p > 0.05) between 3 and 12 or between 12 and 24 months. Nor did the arteriovenous differences for O2 and for glucose change significantly with age. Because the superior sagittal sinus drains blood mainly from the cerebral cortex, the results indicate that average cerebral cortical oxidative metabolism, and the coupling ratios between the cerebral metabolic rate for oxygen and cerebral blood flow and between the cerebral metabolic rate for glucose and cerebral blood flow, do not change significantly with age in the Fischer-344 rat.  相似文献   
3.
An approach of combining flow cytometry (FCM) analysis with morphological and chemical profiling was used to assess the genetic stability and bioactive compound diversity in a Scutellaria baicalensis Georgi (Huang-qin) germplasm collection that was clonally maintained in in vitro for a period of over 6 years. Based on the FCM analysis of nuclei samples from young shoots, the nuclear DNA content of S. baicalensis was calculated as 0.84 pg/2C. FCM analysis showed no significant variation in the nuclear DNA contents and ploidy levels in the long-term in vitro maintained germplasm lines. Germplasm lines, acclimatized to ex vitro conditions, exhibited distinctive plant growth and bioactive compound production capacities. The high level of genetic stability observed in in vitro maintained S. baicalensis lines opens up a variety of opportunities such as allowing long-term aseptic preservation and easy distribution of well-characterized germplasm lines of this medicinal plant species. This study represents a novel approach for continuous maintenance, monitoring, and production of medicinal plant tissues with specific chemistry.  相似文献   
4.
Genetic variation is described at 15 loci in 2 neotenic and 12 nonneotenic populations of red-spotted newts. Though high levels of genetic similarity (I=0.990) were found among all populations, allele frequencies at six of the eight most polymorphic loci show significant heterogeneity across populations. Change in allele frequencies at two of these loci (Pep-2 and Ldh-1) is significantly correlated with latitude. Interspecific homologies are established for newt peptidases based on substrate specificities and lactate dehydrogenases based on tissue distribution, thermal stability, and kinetic properties. Nonneotenic populations are highly variable (H=0.157) and neotenic populations are only slightly, but significantly, less variable (H=0.120). The high levels of heterozygosity detected in nonneotenic populations may result from large effective population size and/or environmental heterogeneity. The unexpectedly high heterozygosity values obtained for the neotenic populations may indicate adult dispersal or the presence of some previously undetected red efts at these localities. In any case, a major change in life history has apparently had little effect on the genetic structure of these populations.This research was supported by grants from the Blakeslee Fund of Smith College.  相似文献   
5.
6.
  总被引:2,自引:0,他引:2  
General cognitive ability ( g ), which refers to what cognitive abilities have in common, is an important target for molecular genetic research because multivariate quantitative genetic analyses have shown that the same set of genes affects diverse cognitive abilities as well as learning disabilities. In this first autosomal genome-wide association scan of g , we used a two-stage quantitative trait locus (QTL) design with pooled DNA to screen more than 500 000 single nucleotide polymorphisms (SNPs) on microarrays, selecting from a sample of 7000 7-year-old children. In stage 1, we screened for allele frequency differences between groups pooled for low and high g . In stage 2, 47 SNPs nominated in stage 1 were tested by individually genotyping an independent sample of 3195 individuals, representative of the entire distribution of g scores in the full 7000 7-year-old children. Six SNPs yielded significant associations across the normal distribution of g , although only one SNP remained significant after a false discovery rate of 0.05 was imposed. However, none of these SNPs accounted for more than 0.4% of the variance of g , despite 95% power to detect associations of that size. It is likely that QTL effect sizes, even for highly heritable traits such as cognitive abilities and disabilities, are much smaller than previously assumed. Nonetheless, an aggregated 'SNP set' of the six SNPs correlated 0.11 ( P  < 0.00000003) with g . This shows that future SNP sets that will incorporate many more SNPs could be useful for predicting genetic risk and for investigating functional systems of effects from genes to brain to behavior.  相似文献   
7.
8.
We are responding to a Letter to the Editor addressing the Method section of our paper “Different measures of ‘genome-wide’ DNA methylation exhibit unique properties in placental and somatic tissues.” The letter raised concerns that the protocol for Epigentek’s MethylFlash kit was followed incorrectly based on the wording of an online publication of our article. We admittedly made an error in the language used to describe the MethylFlash protocol in our initial submission and thus this was corrected as soon as it was brought to our attention. However, the error was only in language and not procedure. We are confident that the protocol was followed as stated in the insert provided with the MethylFlashTM Methylated DNA Quantification kit (Colorimetric).We are responding to a Letter to the Editor addressing the Method section of our paper “Different measures of ‘genome-wide’ DNA methylation exhibit unique properties in placental and somatic tissues” (Price ME, Cotton AM, PeÒaherrera MS, McFadden DE, Kobor MS, Robinson WP. Different measures of “genome-wide” DNA methylation exhibit unique properties in placental and somatic tissues. Epigenetics 2012; 7: 652–63; PMID: 22531475; 10.4161/epi.20221). The letter raised concerns that the protocol for Epigentek’s MethylFlash kit was followed incorrectly based on the wording of an online publication of our article. We admittedly made an error in the language used to describe the MethylFlash protocol in our initial submission and thus this was corrected as soon as it was brought to our attention. However, the error was only in language and not procedure. We are confident that the protocol was followed as stated in the insert provided with the MethylFlashTM Methylated DNA Quantification kit (Colorimetric).  相似文献   
9.
A bending technique was used to infer the spatial distributions of rheological properties within the growth zone of the root of corn, Zea mays. “Bending modulus” (ratio of stress to strain, calculated from engineering theory of bending) falls from 20 MPa near the root tip (3 mm from the tip) to 6 MPa at the location 6 mm from the tip and then remains uniform through the basal region of the growth zone. Where growth stops, at 11–12 mm, there is a sharp rise in bending modulus. The profile of bending moduli is not changed by root incubation temperature during the growth period prior to bending, but it is shifted to the left in roots growing more slowly than the average at either of two temperatures (19 and 29 C). The spatial distribution of “compliance” (reciprocal of bending modulus and a measure of tissue extensibility) resembles the distribution of swelling in response to osmotic perturbation. The distribution of compliance does not parallel that of growth rate. Attempts to explain the discrepancy between compliance and growth rate lead us to examine the theoretical basis for the calculations and to suggest that the dependence of compliance on rate of stretching is physiologically important.  相似文献   
10.
There is an urgent need for the development of novel antimicrobial agents that offer effective treatment against MRSA. Using a new class of dipeptide antibiotic TAN-1057A/B as lead, we designed, synthesized and evaluated analogs of TAN-1057A/B. Several novel dihydropyrimidinone antibiotics demonstrating comparable antibiotic efficacy while possessing favorable selectivity were identified.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号