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   7444篇
  免费   714篇
  国内免费   3篇
  8161篇
  2022年   52篇
  2021年   114篇
  2020年   51篇
  2019年   54篇
  2018年   79篇
  2017年   91篇
  2016年   143篇
  2015年   265篇
  2014年   288篇
  2013年   393篇
  2012年   446篇
  2011年   404篇
  2010年   291篇
  2009年   257篇
  2008年   389篇
  2007年   406篇
  2006年   387篇
  2005年   364篇
  2004年   369篇
  2003年   380篇
  2002年   354篇
  2001年   99篇
  2000年   59篇
  1999年   88篇
  1998年   127篇
  1997年   80篇
  1996年   72篇
  1995年   78篇
  1994年   52篇
  1993年   75篇
  1992年   66篇
  1991年   61篇
  1990年   66篇
  1989年   52篇
  1988年   52篇
  1987年   83篇
  1986年   62篇
  1985年   66篇
  1984年   80篇
  1983年   55篇
  1982年   60篇
  1981年   74篇
  1980年   57篇
  1979年   45篇
  1978年   58篇
  1977年   48篇
  1976年   48篇
  1974年   61篇
  1973年   64篇
  1972年   40篇
排序方式: 共有8161条查询结果,搜索用时 15 毫秒
71.
Despite some highly visible projects that have resulted in environmental benefits, recent efforts to quantify the number and distribution of river restoration projects revealed a paucity of written records documenting restoration outcomes. Improving restoration designs and setting watershed priorities rely on collecting and making accessible this critical information. Information within the unpublished notes of restoration project managers is useful but rarely documents ecological improvements. This special section of Restoration Ecology is devoted to the current state of knowledge on river restoration. We provide an overview of the section’s articles, reflecting on lessons learned, which have implications for the implementation, legal, and financing frameworks for restoration. Our reflections are informed by two databases developed under the auspices of the National River Restoration Science Synthesis project and by extensive interactions with those who fund, implement, and permit restoration. Requiring measurable ecological success criteria, comprehensive watershed plans, and tracking of when and where restoration projects are implemented are critical to improving the health of U.S. waters. Documenting that a project was put in the ground and stayed intact cannot be equated with ecological improvements. However, because significant ecological improvements can come with well‐designed and ‐implemented stream and river restorations, a small investment in documenting the factors contributing to success will lead to very large returns in the health of our nation’s waterways. Even projects that may appear to be failures initially can be turned into success stories by applying the knowledge gained from monitoring the project in an adaptive restoration approach.  相似文献   
72.
ABSTRACT Point count surveys are widely used for monitoring songbird populations, but little is known of the effect of the observer on songbird behavior during point counts. We used a novel, wireless array of recorders to determine the location of singing birds with and without the presence of an observer. The array consisted of seven autonomous recording units synchronized to Global Positioning System (GPS) clocks, set around the perimeter of a 50‐m‐radius circle with one in the middle. Units were set to record automatically from half an hour before dawn until 10:00 each morning. We sampled 26 different locations in old fields at the Prince Edward Point National Wildlife Area in eastern Ontario between 1 June and 4 July 2007. Position estimates derived from a time‐lag cross‐correlation algorithm had a mean error of 1.7 m within 50 m and 5.6 m at 100 m from the center of the array. We found no difference in the positions of birds when an observer was present or absent. We also found no difference in the number of individuals or species detected or in the onset of singing. Our results suggest that, at least in the community we studied, observers conducting point counts do not cause significant changes in bird behavior.  相似文献   
73.
During the past two years a pilot project was conducted in which 19 inactive physicians were retrained in preparation for resumption of active practice. The initial program consisted of a flexible training program of six months to one year patterned after conventional internship-residency concepts. During the second year the program was modified by providing an initial condensed indoctrination period of two months'' duration especially designed for this purpose, followed by a preceptorship type of training.The project was considered successful in permitting trainees to enter some form of active medical work, or to enroll in formal specialty training. The observations made by the faculty of the program and its accomplishments are discussed in the light of the effort expended and the cost of the project.  相似文献   
74.
Autophagy is a vesicular trafficking pathway that regulates the degradation of aggregated proteins and damaged organelles. Initiation of autophagy requires several multiprotein signaling complexes, such as the ULK1 kinase complex and the Vps34 lipid kinase complex, which generates phosphatidylinositol 3-phosphate [PtdIns(3)P] on the forming autophagosomal membrane. Alterations in autophagy have been reported for various diseases, including myopathies. Here we show that skeletal muscle autophagy is compromised in mice deficient in the X-linked myotubular myopathy (XLMTM)-associated PtdIns(3)P phosphatase myotubularin (MTM1). Mtm1-deficient muscle displays several cellular abnormalities, including a profound increase in ubiquitin aggregates and abnormal mitochondria. Further, we show that Mtm1 deficiency is accompanied by activation of mTORC1 signaling, which persists even following starvation. In vivo pharmacological inhibition of mTOR is sufficient to normalize aberrant autophagy and improve muscle phenotypes in Mtm1 null mice. These results suggest that aberrant mTORC1 signaling and impaired autophagy are consequences of the loss of Mtm1 and may play a primary role in disease pathogenesis.  相似文献   
75.
76.
Transgenic mice are widely used in biomedical research to study gene expression, developmental biology, and gene therapy models. Bacterial artificial chromosome (BAC) transgenes direct gene expression at physiological levels with the same developmental timing and expression patterns as endogenous genes in transgenic animal models. We generated 707 transgenic founders from 86 BAC transgenes purified by three different methods. Transgenesis efficiency was the same for all BAC DNA purification methods. Polyamine microinjection buffer was essential for successful integration of intact BAC transgenes. There was no correlation between BAC size and transgenic rate, birth rate, or transgenic efficiency. A narrow DNA concentration range generated the best transgenic efficiency. High DNA concentrations reduced birth rates while very low concentrations resulted in higher birth rates and lower transgenic efficiency. Founders with complete BAC integrations were observed in all 47 BACs for which multiple markers were tested. Additional founders with BAC fragment integrations were observed for 65% of these BACs. Expression data was available for 79 BAC transgenes and expression was observed in transgenic founders from 63 BACs (80%). Consistent and reproducible success in BAC transgenesis required the combination of careful DNA purification, the use of polyamine buffer, and sensitive genotyping assays.  相似文献   
77.
In recognition, discriminators use sensory information to make decisions. For example, honeybee (Apis mellifera) entrance guards discriminate between nest-mates and intruders by comparing their odours with a template of the colony odour. Comb wax plays a major role in honeybee recognition. We measured the rejection rates of nest-mate and non-nest-mate worker bees by entrance guards before and after a unidirectional transfer of wax comb from a 'comb donor' hive to a 'comb receiver' hive. Our results showed a significant effect that occurred in one direction. Guards in the comb receiver hive became more accepting of non-nest-mates from the comb donor hive (rejection decreased from 70 to 47%); however, guards in the comb donor hive did not become more accepting of bees from the comb receiver hive. These data strongly support the hypothesis that the transfer of wax comb increases the acceptance of non-nest-mates not by changing the odour of the bees, but by changing the template used by guards.  相似文献   
78.
Summary   All forests, including commercial plantations, provide a range of habitats for conserving and enhancing elements of native biodiversity. However, the biodiversity values of commercial plantations will depend on the management practices adopted on site, as well as the landscape context of the plantation. The present study describes a generic, quantitative method for assessing the potential biodiversity benefits that might be derived from a plantation, depending on the management practices adopted. This method is based on existing ecological design and management principles. The Plantation Biodiversity Benefits Score (PBBS) was designed to be repeatable and practical to apply. The method can be used either as a stand-alone tool or as part of an integrated framework to assess and compare the commercial and environmental benefits that can be derived from different layouts, management practices and locations of plantations anywhere in Australia.  相似文献   
79.
The purpose ofthis study was to test the hypothesis that regulated body temperatureis decreased in the preovulatory phase in eumenorrheic women. Six womenwere studied in both the preovulatory phase (Preov-2;days 9-12), which was 1-2days before predicted ovulation when 17-estradiol(E2) was estimated to peak, andin the follicular phase (F; days2-6). The subjects walked on a treadmill (~225W · m2)in a warm chamber (ambient temperature = 30°C; dew-pointtemperature = 11.5°C) while heavily clothed.E2, esophageal temperature(Tes), local skin temperatures,and local sweating rate were measured. The estimate of when theE2 surge would occur was correctfor four of six subjects. In these four subjects,E2 increased(P  0.05) from 42.0 ± 24.5 pg/mlduring F to 123.2 ± 31.3 pg/ml during Preov-2. RestingTes was 37.02 ± 0.20°Cduring F and 36.76 ± 0.28°C during Preov-2(P  0.05). TheTes threshold for sweating wasdecreased (P  0.05) from 36.88 ± 0.27°C during F to 36.64 ± 0.35°C during Preov-2. Both meanskin and mean body temperatures were decreased during rest in Preov-2group. The hypothesis that regulated body temperature is decreasedduring the preovulatory phase is supported.

  相似文献   
80.
The potential influence of underlying differences in relative leukocyte distributions in studies involving blood-based profiling of DNA methylation is well recognized and has prompted development of a set of statistical methods for inferring changes in the distribution of white blood cells using DNA methylation signatures. However, the extent to which this methodology can accurately predict cell-type proportions based on blood-derived DNA methylation data in a large-scale epigenome-wide association study (EWAS) has yet to be examined. We used publicly available data deposited in the Gene Expression Omnibus (GEO) database (accession number GSE37008), which consisted of both blood-derived epigenome-wide DNA methylation data assayed using the Illumina Infinium HumanMethylation27 BeadArray and complete blood cell (CBC) counts among a community cohort of 94 non-diseased individuals. Constrained projection (CP) was used to obtain predictions of the proportions of lymphocytes, monocytes and granulocytes for each of the study samples based on their DNA methylation signatures. Our findings demonstrated high consistency between the average CBC-derived and predicted percentage of monocytes and lymphocytes (17.9% and 17.6% for monocytes and 82.1% and 81.4% for lymphocytes), with root mean squared error (rMSE) of 5% and 6%, for monocytes and lymphocytes, respectively. Similarly, there was moderate-high correlation between the CP-predicted and CBC-derived percentages of monocytes and lymphocytes (0.60 and 0.61, respectively), and these results were robust to the number of leukocyte differentially methylated regions (L-DMRs) used for CP prediction. These results serve as further validation of the CP approach and highlight the promise of this technique for EWAS where DNA methylation is profiled using whole-blood genomic DNA.  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