全文获取类型
收费全文 | 2477篇 |
免费 | 213篇 |
国内免费 | 111篇 |
出版年
2024年 | 7篇 |
2023年 | 35篇 |
2022年 | 38篇 |
2021年 | 71篇 |
2020年 | 81篇 |
2019年 | 97篇 |
2018年 | 78篇 |
2017年 | 85篇 |
2016年 | 82篇 |
2015年 | 105篇 |
2014年 | 124篇 |
2013年 | 143篇 |
2012年 | 101篇 |
2011年 | 104篇 |
2010年 | 94篇 |
2009年 | 142篇 |
2008年 | 148篇 |
2007年 | 160篇 |
2006年 | 114篇 |
2005年 | 110篇 |
2004年 | 107篇 |
2003年 | 90篇 |
2002年 | 65篇 |
2001年 | 64篇 |
2000年 | 69篇 |
1999年 | 65篇 |
1998年 | 41篇 |
1997年 | 39篇 |
1996年 | 53篇 |
1995年 | 27篇 |
1994年 | 28篇 |
1993年 | 21篇 |
1992年 | 28篇 |
1991年 | 35篇 |
1990年 | 28篇 |
1989年 | 21篇 |
1988年 | 17篇 |
1987年 | 15篇 |
1986年 | 12篇 |
1985年 | 12篇 |
1984年 | 11篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1978年 | 8篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有2801条查询结果,搜索用时 93 毫秒
61.
Summary Foundresses of two species of Japanese paper wasps,Polistes chinensis antennalis andP. jadwigae, attacked other colonies of the same species. A foundress ofP. chinensis antennalis visited two nests of the same species, and ate larvae from them, while two foundresses ofP. jadwigae each visited a nest of the same species, eating larvae and pupae even when the foundress of the attacked nest was on her
nest. In addition, a foundress ofP. jadwigae distributed flesh balls thus obtained among their larvae. Discussion was made on the adaptive significance of the inter-colonial
cannibalism. It was considered that, at first, it increases the foraging efficiency and secondly it plays a role in regulating
population density. 相似文献
62.
David D. Ackerly 《Journal of Biogeography》2009,36(7):1221-1233
This paper addresses some of the conceptual issues involved in the analysis of the age and origin of mediterranean‐climate plant taxa, paying particular attention to three topics: (1) the importance of an explicit time frame in the definition of biogeographical origins, (2) the distinction between the age of traits and the age of taxa, and (3) the idea of mediterranean‐type ecosystems as environmental islands. (1) In California, recent analyses demonstrate that the diversity of species derived from different biogeographical origins is significantly correlated with temperature and precipitation gradients. These patterns support the hypothesis that niche conservatism is an important factor structuring modern diversity gradients. However, depending on how far back in time one looks, a species may be assigned to different origins; future discussions of biogeographical origins need to address the appropriate time frame for analysis. (2) Past research has demonstrated distinctive trait syndromes among woody plants of the Mediterranean, Chile, California and Mexico, and proposed that the syndromes are associated with lineages of different age in these floras. Reanalysis of individual traits demonstrates greater variability among regions than previously reported. The classification of plants into ‘old’ and ‘new’ genera is re‐evaluated, and it is suggested that greater attention be paid to the age of traits, rather than to the age of taxa, especially at an arbitrary rank such as genus. (3) The idea of mediterranean‐climate regions as ‘climatic islands’ is examined. Space–time diagrams of climate enable one to view the emergence of distinctive climatic regions in a continental context. The terms ‘synclimatic’ and ‘anticlimatic’ are proposed, referring to migration routes that parallel climate contours in space and time versus those that cross contours (including the case of geographic stasis in the face of climate change), respectively. Mediterranean‐climate regions have served as important case studies in plant ecology and evolution, and merit continued close examination in the light of continued advances in phylogenetics and palaeoecology. 相似文献
63.
ObjectivesTo describe quitting experiences of cancer patients in a Cancer Center in Jordan; to study patients’ perceptions regarding the process of smoking cessation; and to provide insights about patients in this difficult setting in order to inform oncology practitioners with regards to how improve perceptions and skills related to quitting.MethodsAn Arabic cross-sectional questionnaire was developed to evaluate smoking and quitting behaviors in the context of cancer. The tool used as its framework the Theoretical Domains Framework to capture quitting perceptions of cancer patients who smoke, as well as social, environmental, and system-level factors that influence quitting. Eligible patients who were treated at the Center (both in-patient and out-patient settings) and who were current smokers or who smoked up to the time of cancer diagnosis were eligible. Patients were interviewed between July, 2018 and January 2020 using two versions of the questionnaire: an ‘ex-smokers’ version, and a ‘current smokers’ version.ResultsOnly a third of subjects (104/350) had been smoke-free for at least 30 days. Both smokers and ex-smokers generally felt that quitting was important, but mean importance and confidence scores (out of 10) were significantly lower in current smokers (8.2 versus 9.1, p-value=0.002; 6.4 versus 8.7, p-value=0.000). Roughly 31% of subjects believed smoking harms were exaggerated and that smoking was not an addiction. About 62% of subjects agreed quitting required skills, and 78.5% felt the steps to quit were clear, but across several listed strategies for quitting, use of these was limited (even in ex-smokers). Among current smokers, roughly a third exhibited forms of cessation fatigue.ConclusionJordanian cancer patients who smoke present with limited knowledge about the quitting process. Even when some success is observed, low rates of utilization of specific quitting strategies were observed, highlighting the need for better counseling about quitting. 相似文献
64.
65.
Phosphorylation, one of the important protein post-translational modifications, is involved in many essential cellular processes. Site-specifical and homogeneous phosphoproteins can be used as probes for elucidating the protein phosphorylation network and as potential therapeutics for interfering their involved biological events. However, the generation of phosphoproteins has been challenging owing to the limitation of chemical synthesis and protein expression systems. Despite the pioneering discoveries in phosphoprotein synthesis, over the past decade, great progresses in this field have also been made to promote the biofunctional exploration of protein phosphorylation largely. Therefore, in this review, we mainly summarize recent advances in phosphoprotein synthesis, which includes five sections: 1) synthesis of the nonhydrolyzable phosphorylated amino acid mimetic building blocks, 2) chemical total and semisynthesis strategy, 3) in-cell and in vitro genetic code expansion strategy, 4) the late-stage modification strategy, 5) nonoxygen phosphoprotein synthesis. 相似文献
66.
67.
Keith Sumption Theodore J. D. Knight-Jones Melissa McLaws David J. Paton 《Proceedings. Biological sciences / The Royal Society》2020,287(1938)
Foot-and-mouth disease (FMD) is an extremely infectious viral infection of cloven-hoofed animals which is highly challenging to control and can give rise to national animal health crises, especially if there is a lack of pre-existing immunity due to the emergence of new strains or following incursions into disease-free regions. The 2001 FMD epidemic in the UK was on a scale that initially overwhelmed the national veterinary services and was eventually controlled by livestock lockdown and slaughter on an unprecedented scale. In 2020, the rapid emergence of COVID-19 has led to a human pandemic unparalleled in living memory. The enormous logistics of multi-agency control efforts for COVID-19 are reminiscent of the 2001 FMD epidemic in the UK, as are the use of movement restrictions, not normally a feature of human disease control. The UK experience is internationally relevant as few countries have experienced national epidemic crises for both diseases. In this review, we reflect on the experiences and lessons learnt from UK and international responses to FMD and COVID-19 with respect to their management, including the challenge of preclinical viral transmission, threat awareness, early detection, different interpretations of scientific information, lockdown, biosecurity behaviour change, shortage of testing capacity and the choices for eradication versus living with infection. A major lesson is that the similarity of issues and critical resources needed to manage large-scale outbreaks demonstrates that there is benefit to a ‘One Health’ approach to preparedness, with potential for greater cooperation in planning and the consideration of shared critical resources. 相似文献
68.
阴道微生态是由阴道的局部解剖结构、周期性的内分泌变化、阴道局部免疫系统和阴道内微生物菌群共同组成的阴道环境和生态系统。多项研究证实阴道微生态失调通过局部炎症因子释放、黏膜免疫应答的改变和局部代谢变化,可引起早产的发生。阴道内小分子物质如糖类、短链脂肪酸和胺类通过代谢路径和代谢产物在阴道微生态失调与早产的发病机制起作用。近年来,阴道微生态的理念得到重视,治疗方法由传统的抗生素治疗转向了综合治疗,目的是恢复正常阴道菌群和阴道上皮黏膜免疫系统。但阴道微生态与宿主之间存在复杂的相互作用,因此仍需要进一步的研究。 相似文献
69.
Xiaohan Sun Feng Wang Rong Cui Xiao Liu Xiangxiang Li Jibin Dong Lu Sun Siqi Qin Renqing Wang Peiming Zheng Hui Wang 《Ecology and evolution》2020,10(12):5270-5280
Vitex negundo L. var. heterophylla (Franch.) Rehder (Lamiaceae) is an important tree species for soil and water conservation, yet the reproductive ecology of this species remains to be elucidated. To investigate the reproductive traits of V. negundo var. heterophylla, the phenology, morphological characteristics (a suite of characters was assessed: floral morphology, nectar production, pollen viability, and stigma receptivity) and mating system of this species were systematically revealed for the first time in this study. Phenological observations, morphological measurements, and nectar production analysis were conducted during anthesis. Pollen viability and stigma receptivity at different flowering stages were measured by biochemical methods. Finally, genetic analysis based on SSR markers was used to reveal the mating system; outcrossing index and pollen‐ovule ratio were also calculated to help analysis. V. negundo var. heterophylla showed several obvious characteristics of outcrossing, such as abundant and attractive flowers, secreting nectar, and emitting scent. In addition, mechanisms such as homogamy and a short anther‐stigma distance that can promote self‐fertilization were also identified in this species. The coexistence of selfing and outcrossing characteristics demonstrates a predominantly outcrossed mixed mating system (outcrossing rate, t = 95%). The scientific information provided by this study may contribute to conservation of V. negundo var. heterophylla from a reproductive perspective. 相似文献
70.
以68种蕨类植物和2种石松类植物的rps12基因为对象,在系统发育背景下,结合最大似然法,使用HyPhy和PAML软件对该基因进行进化速率和适应性进化研究。结果显示:位于IR区的外显子2~3,其替换率明显降低,rps12基因编码序列的替换率也随之降低,且rps12基因密码子第3位的GC含量明显升高;在蕨类植物的进化过程中,3′-rps12更倾向定位于IR区,以保持较低的替换率;rps12基因编码的123个氨基酸位点中,共检测到4个正选择位点和116个负选择位点。研究结果表明基因序列进入到IR区后,显示出降低的替换率;强烈的负选择压力表明RPS12蛋白的高度保守性以及rps12基因的功能和结构已经趋于稳定。 相似文献