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   11898篇
  免费   1732篇
  国内免费   4169篇
  2024年   26篇
  2023年   284篇
  2022年   472篇
  2021年   757篇
  2020年   688篇
  2019年   759篇
  2018年   581篇
  2017年   524篇
  2016年   658篇
  2015年   892篇
  2014年   1107篇
  2013年   1006篇
  2012年   1324篇
  2011年   1148篇
  2010年   893篇
  2009年   853篇
  2008年   853篇
  2007年   766篇
  2006年   657篇
  2005年   589篇
  2004年   485篇
  2003年   416篇
  2002年   399篇
  2001年   260篇
  2000年   252篇
  1999年   183篇
  1998年   80篇
  1997年   74篇
  1996年   56篇
  1995年   62篇
  1994年   54篇
  1993年   41篇
  1992年   72篇
  1991年   53篇
  1990年   58篇
  1989年   59篇
  1988年   36篇
  1987年   40篇
  1986年   35篇
  1985年   41篇
  1984年   34篇
  1983年   23篇
  1982年   20篇
  1981年   10篇
  1980年   14篇
  1977年   9篇
  1972年   11篇
  1971年   9篇
  1970年   9篇
  1969年   9篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Merkel cell tumor is an unusual, aggressive malignancy of skin that has been considered to be derived from cutaneous Merkel cells. We are reporting a case of Merkel cell tumor overlying the left scapula with metastases to the thoracic spine and pleura. The tumor was found incidentally in a 23-year-old pregnant black woman. The tumor recurred locally 5 months after initial wide excision. Subsequently, a second wide excision of the recurrent tumor with ipsilateral axillary dissection was performed. The course of the disease was complicated by local recurrence and formation of distant metastases to pleura and spine. At the end-stage of the disease, the patient was found to have a cardiac murmur, and echocardiography revealed a mass in the anterior wall of the right ventricle that was suspicious for a metastatic lesion. The patient expired from extensive distant metastases 23 months after diagnosis.  相似文献   
3.
To report the results of a systematic review and meta-analysis examining whether weight loss interventions improve body image in obese/overweight individuals. Medline, Current Contents, and the Cochrane database were searched to identify studies involving obese/overweight adults who were enrolled in weight loss interventions in which body image was quantitatively assessed. Outcomes assessed were changes in body shape concern, body size dissatisfaction, and body satisfaction (intervention vs comparator/control group). Seven studies were included in the systematic review (4 in the meta-analysis). All but 1 study involved female participants only. The type of weight loss intervention varied between studies as did the type of control/comparator group. In 3 studies, there was no significant difference in body image outcomes, whereas in 4 studies, improvement in body image was significantly more pronounced in the intervention vs the control/comparator group. Meta-analysis revealed that improvements in body shape concern (standardized difference: -0.52; 95% confidence interval [CI]: -1.04 to 0.00), body size dissatisfaction (standardized difference: -0.66; 95% CI: -0.88 to -0.45), and body satisfaction (standardized difference: 0.74; 95% CI: 0.09 to 1.38) significantly favored the intervention over the comparator/control group (P<0.05). The results of this systematic review/meta-analysis lend support to the notion that weight loss interventions may improve body image. This is a noteworthy finding that has clear clinical applicability because body image affects psychological well-being and the ability of an individual to maintain weight loss. Future research should determine which weigh loss interventions are associated with optimal improvements in body image and maintenance of weight loss.  相似文献   
4.
Waterlogging is one of the major stresses limiting crop production worldwide. The understanding of the mechanisms of plant adaptations to waterlogging stress helps improve plant tolerance to stress. In this study, physiological responses and morpho-anatomical adaptations of seven different barley genotypes were investigated under waterlogging stress. The results showed that the waterlogging-tolerant varieties (TX9425, Yerong, TF58) showed less reduction in plant height, SPAD (soil–plant analyses development analyses) value, tillers, shoot and root biomasses than did the waterlogging-sensitive varieties (Franklin, Naso Nijo, TF57). Under waterlogging stress condition, the tolerant genotypes also showed a much larger number of adventitious roots than did the sensitive genotypes. More intercellular spaces and better integrated chloroplast membrane structures were observed in the leaves of the waterlogging-tolerant cultivars, which is likely due to increased ethylene content, decreased ABA content and less accumulation of O2.?. The ability to form new adventitious roots and intercellular spaces in shoots can also be used as selection criteria in breeding barley for waterlogging tolerance.  相似文献   
5.
N Patil  E Lacy  M V Chao 《Neuron》1990,4(3):437-447
Transgenic mice carrying multiple copies of the human NGF receptor gene have been generated. Using a monoclonal antibody specific for the human receptor, we have detected specific expression in cholinergic neurons in the basal forebrain and Purkinje cells in the cerebellum during the postnatal period. Expression in the PNS was exemplified by immunostaining of sympathetic and sensory neurons during an early embryonic age. Transection of the sciatic nerve in transgenic animals resulted in induction of human NGF receptors, indicating that the inserted gene can be appropriately regulated. These transgenic mice will provide an opportunity to study the elements regulating the NGF receptor. Furthermore, the ability to obtain specific expression in transgenic mice will permit directed expression of heterologous genes in discrete cells important in the cholinergic septal-hippocampal pathway and the PNS.  相似文献   
6.
Aim We propose a phylogenetic hypothesis for the marine‐derived sciaenid genus Plagioscion in the context of geomorphology and adaptation to freshwaters of South America, and assess the extent to which contemporary freshwater hydrochemical gradients influence diversification within a widely distributed Plagioscion species, Plagioscion squamosissimus. Location Amazon Basin and South America. Methods Using nuclear and mitochondrial DNA sequence data, phylogenetic analyses were conducted on the five nominal Plagioscion species, together with representatives from Pachyurus and Pachypops, using character and model‐based methods. Genealogical relationships and population genetic structure of 152 P. squamosissimus specimens sampled from the five major rivers and three hydrochemical settings/‘colours’ (i.e. white, black and clear water) of the Amazon Basin were assessed. Results Phylogenetic analyses support the monophyly of Plagioscion in South America and identify two putative cryptic species of Plagioscion. Divergence estimates suggest that the Plagioscion ancestor invaded South America via a northern route during the late Oligocene to early Miocene. Within P. squamosissimus a strong association of haplotype and water colour was observed, together with significant population structure detected between water colours. Main conclusions Our analyses of Plagioscion are consistent with a biogeographic scenario of early Miocene marine incursions into South America. Based on our phylogenetic results, the fossil record, geomorphological history and distributional data of extant Plagioscion species, we propose that marine incursions into western Venezuela between the late Oligocene and early Miocene were responsible for the adaptation to freshwaters in Plagioscion species. Following the termination of the marine incursions during the late Miocene and the establishment of the modern Amazon River, Plagioscion experienced a rapid diversification. Plagioscion squamosissimus arose during that time. The formation of the Amazon River probably facilitated population and range expansions for this species. Further, the large‐scale hydrochemical gradients within the Amazon Basin appear to be acting as ecological barriers maintaining population discontinuities in P. squamosissimus even in the face of gene flow. Our results highlight the importance of divergent natural selection through time in the generation and maintenance of sciaenid diversity in Amazonia.  相似文献   
7.
8.
大豆下胚轴可溶性蛋白中钙激活的蛋白激酶   总被引:6,自引:0,他引:6  
大豆(Glycine m ax L.) 下胚轴可溶性蛋白提取液进行自磷酸化,以SDS-PAGE电泳分析其标记产物时发现,当有较高浓度的Ca2+ 存在于反应液中时,有一条18 kD蛋白带被高强度标记,同时也可观察到另一条标记强度不高的67 kD蛋白带. 当反应时间延长到15 或30m in 时,它们的标记强度都逐渐减弱,最终从放射自显影底片上消失;在反应液中加入钙螯合剂EGTA 时,则只有67 kD 被高强度标记;在磷酸化反应过程中加入非标记ATP,蛋白中的32P逐渐被非标记磷取代,表明反应体系处于磷酸化-脱磷酸化的平衡过程中,并有结果显示这一过程是钙依赖性的. 组蛋白H1 可以使反应进程加快,表明提取液中的蛋白激酶可以利用它作为底物. 综合结果表明,18 kD和67 kD蛋白可能是具有自磷酸化能力且对Ca2+ 敏感的蛋白激酶,它们对Ca2+ 的不同反应,使得钙信号的传递更具可控性  相似文献   
9.
胡云锋  高戈 《生态学报》2020,40(21):7805-7815
当前,城市景观生态风险研究缺少科学合理、方便实用的评估框架。作者基于景观生态风险评估基本范式,明确了城市景观生态服务价值的测算方法,分析了引起生态损害的自然因素和人类活动因素,形成了城市景观生态风险评估的技术框架和参数体系;继而以北京天坛地区为研究区,开展了典型城市景观生态风险的定量评估。结果表明:(1)天坛地区景观生态价值总量约为2.41亿元。区域的历史文化价值最高,教育和美学景观价值紧随其后。(2)城市景观生态受损概率呈现"北高南低"的空间分布格局。生态受损概率的高值区面积占整个区域总面积的22.2%,主要分布在珠市口、磁器口和崇文门附近区域。(3)城市景观生态风险呈现"北低南高"的空间分布格局。高风险区主要分布在天坛公园内的文物建筑周边。本研究提供了一个可参考的城市景观生态风险评估应用框架,对生态风险评估中的不确定性进行了讨论,研究针对天坛案例区的具体结论有助于城市管理者避免潜在的风险。  相似文献   
10.
Calcium, as the most widely accepted messenger, plays an important role in plant stress responses through calcium-dependent signaling pathways. The calmodulin-like family genes (CMLs) encode Ca2+ sensors and function in signaling transduction in response to environmental stimuli. However, until now, the function of plant CML proteins, especially soybean CMLs, is largely unknown. Here, we isolated a Glycine soja CML protein GsCML27, with four conserved EF-hands domains, and identified it as a calcium-binding protein through far-UV CD spectroscopy. We further found that expression of GsCML27 was induced by bicarbonate, salt and osmotic stresses. Interestingly, ectopic expression of GsCML27 in Arabidopsis enhanced plant tolerance to bicarbonate stress, but decreased the salt and osmotic tolerance during the seed germination and early growth stages. Furthermore, we found that ectopic expression of GsCML27 decreases salt tolerance through modifying both the cellular ionic (Na+, K+) content and the osmotic stress regulation. GsCML27 ectopic expression also decreased the expression levels of osmotic stress-responsive genes. Moreover, we also showed that GsCML27 localized in the whole cell, including cytoplasm, plasma membrane and nucleus in Arabidopsis protoplasts and onion epidermal cells, and displayed high expression in roots and embryos. Together, these data present evidence that GsCML27 as a Ca2+-binding EF-hand protein plays a role in plant responses to bicarbonate, salt and osmotic stresses.  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