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   180597篇
  免费   14314篇
  国内免费   45篇
  2018年   1808篇
  2017年   1756篇
  2016年   2248篇
  2015年   2137篇
  2014年   2934篇
  2013年   4185篇
  2012年   4797篇
  2011年   5305篇
  2010年   3714篇
  2009年   3350篇
  2008年   4852篇
  2007年   5127篇
  2006年   4820篇
  2005年   4539篇
  2004年   4687篇
  2003年   4590篇
  2002年   4569篇
  2001年   8375篇
  2000年   8572篇
  1999年   6281篇
  1998年   1931篇
  1997年   1965篇
  1996年   1746篇
  1995年   1672篇
  1992年   5194篇
  1991年   5360篇
  1990年   5132篇
  1989年   5114篇
  1988年   4727篇
  1987年   4480篇
  1986年   4072篇
  1985年   4230篇
  1984年   3407篇
  1983年   2911篇
  1982年   1929篇
  1981年   1728篇
  1980年   1716篇
  1979年   3210篇
  1978年   2544篇
  1977年   2278篇
  1976年   2080篇
  1975年   2546篇
  1974年   2883篇
  1973年   2802篇
  1972年   2650篇
  1971年   2411篇
  1970年   2116篇
  1969年   2022篇
  1968年   1906篇
  1967年   1740篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
Primary caregivers for victims of chronic illness and or trauma experience both positive and negative emotional consequences. These are broadly classified as compassion satisfaction (CS) and compassion fatigue (CF). Because one of the components of CF, burnout, varies with chronotype and sleep quality, we assessed the influence of chronobiological features on the broader constructs of CS and CF. Responses from primary ambulatory care oncology staff working dayshifts were assessed for potential relationships of chronotype and sleep quality with CS and CF using the professional quality of life scale. These were analyzed further in a multivariate model that included personality and job satisfaction as cofactors. We found that sleep quality was a key contributor to CS development and CF reduction. Morningness was positively linked to CS, but the univariate association was masked in the multivariate model. Job satisfaction (contingent rewards, nature of work and operating procedures) heavily influenced CS and CF development. Agreeableness and openness showed positive correlations with CS and negative with burnout, while emotional stability was linked to reduced CF. While job satisfaction and personality predictably played roles in the development of CS and CF, sleep quality and chronotype contributed significantly to benefits and negative consequences of providing care.  相似文献   
4.
5.
1. Organisms can respond to changing climatic conditions in multiple ways including changes in phenology, body size or morphology, and range shifts. Understanding how developmental temperatures affect insect life‐history timing and morphology is crucial because body size and morphology affect multiple aspects of life history, including dispersal ability, whereas phenology can shape population performance and community interactions. 2. It was experimentally assessed how developmental temperatures experienced by aquatic larvae affected survival, phenology, and adult morphology of dragonflies [Pachydiplax longipennis (Burmeister)]. Larvae were reared under three environmental temperatures: ambient, +2.5, and +5 °C, corresponding to temperature projections for our study area 50 and 100 years in the future, respectively. Experimental temperature treatments tracked naturally‐occurring variation. 3. Clear effects of temperature were found in the rearing environment on survival and phenology: dragonflies reared at the highest temperatures had the lowest survival rates and emerged from the larval stage approximately 3 weeks earlier than animals reared at ambient temperatures. There was no effect of rearing temperature on overall body size. Although neither the relative wing nor thorax size was affected by warming, a non‐significant trend towards an interaction between sex and warming in relative thorax size suggests that males may be more sensitive to warming than females, a pattern that should be investigated further. 4. Warming strongly affected survival in the larval stage and the phenology of adult emergence. Understanding how warming in the developmental environment affects later life‐history stages is critical to interpreting the consequences of warming for organismal performance.  相似文献   
6.
7.
T cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignant neoplasm of the lymphocyte precursors that suffered malignant transformation arresting the lymphoid cell differentiation. Clinical studies revealed monoor, more rarely, oligoclonal nature of the disease. A precise identification of malignant clone markers is both the crucial stage of early diagnostics and the essential prognostic factor for therapeutic treatment. Here we present an improved system for unbiased detection of lymphoblastic clones in bone marrow aspirates of T-ALL patients. The system based on multiplex PCR of rearranged T-cell receptor locus (TRB) and straightforward sequencing of the resulted PCR fragments. Testing of the system on genomic DNA from Jurkat cell line and four clinical bone marrow aspirates revealed a set of unique TRB rearrangements that precisely characterize each of tested samples. Therefore, the outcome of the system produces highly informative molecular genetic markers for further monitoring of minimal residual disease in T-ALL patients.  相似文献   
8.
Some epiphytic Hymenophyllaceae are restricted to lower parts of the host (<60 cm; 10–100 μmol photons m-2 s-1) in a secondary forest of Southern Chile; other species occupy the whole host height (≥10 m; max PPFD >1000 μmol photons m-2 s-1). Our aim was to study the photosynthetic light responses of two Hymenophyllaceae species in relation to their contrasting distribution. We determined light tolerance of Hymenoglossum cruentum and Hymenophyllum dentatum by measuring gas exchange, PSI and PSII light energy partitioning, NPQ components, and pigment contents. H. dentatum showed lower maximum photosynthesis rates (Amax) than H. cruentum, but the former species kept its net rates (An) near Amax across a wide light range. In contrast, in the latter one, An declined at PPFDs >60 μmol photons m-2 s-1. H. cruentum, the shadiest plant, showed higher chlorophyll contents than H. dentatum. Differences in energy partitioning at PSI and PSII were consistent with gas exchange results. H. dentatum exhibited a higher light compensation point of the partitioning of absorbed energy between photochemical Y(PSII) and non-photochemical Y(NPQ) processes. Hence, both species allocated energy mainly toward photochemistry instead of heat dissipation at their light saturation points. Above saturation, H. cruentum had higher heat dissipation than H. dentatum. PSI yield (YPSI) remained higher in H. dentatum than H. cruentum in a wider light range. In both species, the main cause of heat dissipation at PSI was a donor side limitation. An early dynamic photo-inhibition of PSII may have caused an over reduction of the Qa+ pool decreasing the efficiency of electron donation to PSI. In H. dentatum, a slight increase in heat dissipation due to acceptor side limitation of PSI was observed above 300 μmol photons m-2s-1. Differences in photosynthetic responses to light suggest that light tolerance and species plasticity could explain their contrasting vertical distribution.  相似文献   
9.
Location of thePoteriocrinus impressus Phillips 1836 holotype has not been known, but it was discovered in the collections of the Natural History Museum, London. Rather than belonging toBlothrocrinus, as suggested by more recent authors, this species should be reassigned toPoteriocrinites. Poteriocrinites impressus is redescribed and photographically illustrated for the first time.  相似文献   
10.
Poliovirus RNA species with nucleotides 564 to 571 deleted or with a secondary structure domain (positions 564 to 629) replaced by a shorter irregular oligonucleotide have been engineered previously; these RNAs have been considered quasi-infectious (yielding a single late revertant plaque) and dead, respectively (E. Pilipenko, A. Gmyl, Y. Svitkin, S. Maslova, A. Sinyakov, and V. Agol, Cell 68:119-131, 1992). By using large amounts of these RNAs for transfections, revertant clones with a great variety of genetic changes (point mutations, insertions of foreign sequences, short or extended deletions) were isolated. The pattern of these changes supported the notion that an appropriately spaced oligopyrimidine-AUG tandem is important for efficient poliovirus RNA translation. Structural features within and around this tandem modulated the initiation efficiency. The functional and genetic plasticities of the poliovirus genome are briefly discussed.  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