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   4077篇
  免费   250篇
  2023年   28篇
  2022年   43篇
  2021年   116篇
  2020年   88篇
  2019年   107篇
  2018年   140篇
  2017年   124篇
  2016年   187篇
  2015年   236篇
  2014年   278篇
  2013年   321篇
  2012年   348篇
  2011年   293篇
  2010年   202篇
  2009年   176篇
  2008年   216篇
  2007年   200篇
  2006年   206篇
  2005年   152篇
  2004年   155篇
  2003年   117篇
  2002年   115篇
  2001年   54篇
  2000年   40篇
  1999年   39篇
  1998年   18篇
  1997年   22篇
  1996年   16篇
  1995年   14篇
  1994年   17篇
  1993年   14篇
  1992年   25篇
  1991年   22篇
  1990年   22篇
  1989年   19篇
  1988年   13篇
  1987年   18篇
  1986年   10篇
  1985年   15篇
  1984年   14篇
  1983年   10篇
  1982年   7篇
  1981年   10篇
  1980年   5篇
  1979年   7篇
  1977年   6篇
  1975年   4篇
  1974年   5篇
  1973年   6篇
  1972年   4篇
排序方式: 共有4327条查询结果,搜索用时 15 毫秒
1.
Primates have long been used as indicator species for assessing overall ecosystem health. However, area‐wide census methods are time consuming, costly, and not always feasible under many field conditions. Therefore, it is important to establish whether monitoring a subset of a population accurately reflects demographic changes occurring in the population at large. Over the past 35 years, we have conducted 15 area‐wide censuses in Sector Santa Rosa, Costa Rica. These efforts have revealed important trends in population growth patterns of capuchin monkeys following the protection and subsequent regeneration of native forests. During this same period, we have also intensively studied a subset of the capuchin groups. Comparing these two datasets, we investigate whether the population structures of the closely monitored groups are reliable indicators of area‐wide demographic patterns. We compare the overall group size and the individual age/sex class compositions of study groups and nonstudy groups (i.e., those contacted during area‐wide censuses only). Our study groups contained more individuals overall with a larger proportion of infants, and there were indications that the proportion of adult and subadult males was lower. These differences can be ascribed either to sampling errors or real differences attributable to human presence and/or better habitat quality for the study groups. No other sex/age classes differed, and major demographic changes were simultaneously evident in both study and nonstudy groups. This study suggests that the Santa Rosa capuchin population is similarly impacted by large‐scale ecological patterns observable within our study groups.  相似文献   
2.
International Journal of Primatology - The selection of sleeping sites and sleeping trees in nonhuman primates is related to social and ecological factors. We investigate the role of body...  相似文献   
3.
Summary A protocol for in vitro propagation of Rollinia mucosa, an important medicinal plant, was developed. The presence of 500 mg l−1 polyvinylpyrrolidone (PVP) during explant excision was important to avoid browning. Axillary buds, adventitious buds, and shoot cluster proliferation were achieved from epicotyl and hypocotyl explants from nursery-grown seedlings. The highest direct organogenesis percentage from hypocotyl explants was obtained upon culture of explants on Murashige and Skoog medium supplemented with 2.2 μM benzyladenine (BA) plus 2.32 μM kinetin. Epicotyl explants display highest regeneration frequency on a medium containing 8.8 μM BA and 0.54 μM naphthaleneacetic acid. Gibberellic acid was necessary for shoot elongation. Root induction was observed when shoots were pretreated with activated charcoal for 7 d in the dark before culture on Woody Plant Medium supplemented with 49.21 μM indolebutyric acid for 10 d. Root development was observed when 20 g l−1 sucrose was used. Rooted plantlets were acclimatized and grown in the greenhouse.  相似文献   
4.
FTA® cards were used for long‐term storage of avian blood samples. Blood DNA was extracted by a simple method and used in PCR for sex identification of adult and nestling Great Grey Shrikes Lanius excubitor.  相似文献   
5.
6.
Changes in phytoplankton community composition potentially affect the entire marine food web. Because of seasonal cycles and inter-annual variations in species composition, long-term monitoring, covering many sequential years, is required to establish a baseline study and to reveal long-term trends. The current study describes the phytoplankton biomass variations and species composition in relation to hydrographic and meteorological conditions in the Raunefjord, western Norway, over a 6-year period from 2001 to 2006. The extent of inflow or upwelling in the fjord varied from year to year and resulted in pronounced differences in water column stability. The annual phytoplankton community succession showed some repeated seasonal patterns, but also high variability between years. Two to four diatom blooms were observed per year, and the spring blooms occurring before water column stratification in March were dominated by Skeletonema marinoi and Chaetoceros socialis, and other Chaetoceros and Thalassiosira spp. Blooms of the haptophytes Phaeocystis pouchetii and Emiliania huxleyi were irregular and in some years totally absent. Although E. huxleyi was present all year round it appeared in bloom concentrations only in 2003, when the summer was warm and the water column characterized by high surface temperatures and pronounced stratification. The annual average abundance of both diatoms and flagellates increased during the six years. Despite the high variation from year to year, our investigation provides valuable knowledge about annual phytoplankton community patterns in the region, and can be used as a reference to detect possible future changes.  相似文献   
7.
Jorge Lobo’s disease (JLD) is a chronic infection that affects the skin and subcutaneous tissues. Its etiologic agent is the fungus Lacazia loboi. Lesions are classified as localized, multifocal, or disseminated, depending on their location. Early diagnosis and the surgical removal of lesions are the best therapeutic options currently available for JLD. The few studies that evaluate the immunological response of JLD patients show a predominance of Th2 response, as well as a high frequency of TGF-β and IL-10 positive cells in the lesions; however, the overall immunological status of the lesions in terms of their T cell phenotype has yet to be determined. Therefore, the objective of this study was to evaluate the pattern of Th1, Th2, Th17 and regulatory T cell (Treg) markers mRNA in JLD patients by means of real-time PCR. Biopsies of JLD lesions (N = 102) were classified according to their clinical and histopathological features and then analyzed using real-time PCR in order to determine the expression levels of TGF-β1, FoxP3, CTLA4, IKZF2, IL-10, T-bet, IFN-γ, GATA3, IL-4, IL-5, IL-13, IL-33, RORC, IL-17A, IL-17F, and IL-22 and to compare these levels to those of healthy control skin (N = 12). The results showed an increased expression of FoxP3, CTLA4, TGF-β1, IL-10, T-bet, IL-17F, and IL-17A in lesions, while GATA3 and IL-4 levels were found to be lower in diseased skin than in the control group. When the clinical forms were compared, TGF-β1 was found to be highly expressed in patients with a single localized lesion while IL-5 and IL-17A levels were higher in patients with multiple/disseminated lesions. These results demonstrate the occurrence of mixed T helper responses and suggest the dominance of regulatory T cell activity, which could inhibit Th-dependent protective responses to intracellular fungi such as L. loboi. Therefore, Tregs may play a key role in JLD pathogenesis.  相似文献   
8.
Auditory cues can create the illusion of self-motion (vection) in the absence of visual or physical stimulation. The present study aimed to determine whether auditory cues alone can also elicit motion sickness and how auditory cues contribute to motion sickness when added to visual motion stimuli. Twenty participants were seated in front of a curved projection display and were exposed to a virtual scene that constantly rotated around the participant''s vertical axis. The virtual scene contained either visual-only, auditory-only, or a combination of corresponding visual and auditory cues. All participants performed all three conditions in a counterbalanced order. Participants tilted their heads alternately towards the right or left shoulder in all conditions during stimulus exposure in order to create pseudo-Coriolis effects and to maximize the likelihood for motion sickness. Measurements of motion sickness (onset, severity), vection (latency, strength, duration), and postural steadiness (center of pressure) were recorded. Results showed that adding auditory cues to the visual stimuli did not, on average, affect motion sickness and postural steadiness, but it did reduce vection onset times and increased vection strength compared to pure visual or pure auditory stimulation. Eighteen of the 20 participants reported at least slight motion sickness in the two conditions including visual stimuli. More interestingly, six participants also reported slight motion sickness during pure auditory stimulation and two of the six participants stopped the pure auditory test session due to motion sickness. The present study is the first to demonstrate that motion sickness may be caused by pure auditory stimulation, which we refer to as “auditorily induced motion sickness”.  相似文献   
9.
Summary C. sitophila strain TFB-27441 showed 2–3 times higher lignolytic activity thanPhanerochaete chrysosporium (BKM-F-1767 strain). Lignin had a marked effect on the ligninase activity indicating that some induction or activation mechanism is involved in lignin degradation byC. sitophila.  相似文献   
10.
We have studied by X-ray diffraction fibers of complexes of poly(dG).poly(dC) with N-alpha-acetyl-L-arginine ethylamide. Although these polynucleotides favour the A form of DNA, in this complex it is never found, thus confirming that arginine prevents the appearance of this form of DNA. At high relative humidity the B form is present. Upon dehydration two new structures appear. One of them is a triple helix, most likely formed by poly(dC+).poly(dG).poly(dC). The other structure found also has features which indicate a multistranded conformation.  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