全文获取类型
收费全文 | 36400篇 |
免费 | 4964篇 |
国内免费 | 20172篇 |
专业分类
61536篇 |
出版年
2024年 | 474篇 |
2023年 | 1263篇 |
2022年 | 2051篇 |
2021年 | 2305篇 |
2020年 | 2065篇 |
2019年 | 2354篇 |
2018年 | 1638篇 |
2017年 | 1532篇 |
2016年 | 1664篇 |
2015年 | 2383篇 |
2014年 | 3293篇 |
2013年 | 2881篇 |
2012年 | 4019篇 |
2011年 | 3849篇 |
2010年 | 3120篇 |
2009年 | 3178篇 |
2008年 | 3494篇 |
2007年 | 3308篇 |
2006年 | 3061篇 |
2005年 | 2535篇 |
2004年 | 1933篇 |
2003年 | 1636篇 |
2002年 | 1489篇 |
2001年 | 1380篇 |
2000年 | 1307篇 |
1999年 | 807篇 |
1998年 | 451篇 |
1997年 | 242篇 |
1996年 | 226篇 |
1995年 | 175篇 |
1994年 | 180篇 |
1993年 | 129篇 |
1992年 | 115篇 |
1991年 | 113篇 |
1990年 | 100篇 |
1989年 | 90篇 |
1988年 | 81篇 |
1987年 | 60篇 |
1986年 | 56篇 |
1985年 | 74篇 |
1984年 | 41篇 |
1983年 | 49篇 |
1982年 | 59篇 |
1981年 | 31篇 |
1980年 | 13篇 |
1958年 | 21篇 |
1957年 | 19篇 |
1955年 | 16篇 |
1954年 | 19篇 |
1950年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
HAN-QI YANG HONG WANG DE-ZHU LI 《Botanical journal of the Linnean Society. Linnean Society of London》2008,156(3):411-423
The foliage leaf epidermis of 35 species representing 12 key genera of woody bamboos of the Asian tropics was investigated using light and scanning electron microscopy. The results indicated that papillae forms and distributional patterns around the stomatal apparatus of the abaxial foliage leaf epidermis were usually constant and were of great taxonomic significance at the specific and generic levels. However, papillae characters were not suitable for dividing subtribes within woody bamboos of the Asian tropics. On the basis of papillae characters, Schizostachyum s.s. and Cephalostachyum were confirmed, but their delimitations should be modified. The transfer of Leptocanna chinensis and Schizostachyum sanguineum into Cephalostachyum was supported, and Cephalostachyum virgatum and C . pergracile were confirmed to be members of Schizostachyum s.s. The subtribe Racemobambosinae did not obtain support and Racemobambos appeared to be better placed in subtribe Bambusinae. Neomicrocalamus was supported as a close relative and better treated as a synonym of Racemobambos . Gigantochloa was closely related to Dendrocalamus . © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 156 , 411–423. 相似文献
2.
多种天敌控制多种害虫的模糊数学模型 总被引:2,自引:0,他引:2
从生态经济学的原理出发,利用模糊集理论,探讨如何评价天敌控制多种害虫的为害作用,提出了多种天敌控制多种害虫的模糊数学模型。同时又得到了天敌对害虫的控制能力C,影响率G,影响强度H,及权数ω(x)的计算公式,以及在天敌作用下安全度的讨论与分析。 相似文献
3.
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
4.
In searching for differentially expressed genes in human uterine leiomyomas (ULs), suppression sub-tractive hybridization was used to construct an UL up-regulated library, which turned out to represent 88genes. After two rounds of screening by reverse Northern analysis, twenty genes were proved to be up-regulated, including seventeen known genes and three genes with unknown function. All these genes werefirstly associated with UL. Three genes with notable difference were selected for Northern confirmationOur results proved the authenticity of the twenty genes. One gene named Phospholipase A2 (PLA2) showedup-regulation in 4/6 of the patients and investigation of tissue distribution indicated that it had obviousexpression in prostate, testis, liver, heart and skeletal muscle. 相似文献
5.
6.
7.
Allen Ho Ning Lin Nareerat Charoenvimolphan Mary Stanley Kai U. Frerichs Arthur L. Day Rose Du 《PloS one》2014,9(4)
The rupture risk of unruptured intracranial aneurysms is known to be dependent on the size of the aneurysm. However, the association of morphological characteristics with ruptured aneurysms has not been established in a systematic and location specific manner for the most common aneurysm locations. We evaluated posterior communicating artery (PCoA) aneurysms for morphological parameters associated with aneurysm rupture in that location. CT angiograms were evaluated to generate 3-D models of the aneurysms and surrounding vasculature. Univariate and multivariate analyses were performed to evaluate morphological parameters including aneurysm volume, aspect ratio, size ratio, distance to ICA bifurcation, aneurysm angle, vessel angles, flow angles, and vessel-to-vessel angles. From 2005–2012, 148 PCoA aneurysms were treated in a single institution. Preoperative CTAs from 63 patients (40 ruptured, 23 unruptured) were available and analyzed. Multivariate logistic regression revealed that smaller volume (p = 0.011), larger aneurysm neck diameter (0.048), and shorter ICA bifurcation to aneurysm distance (p = 0.005) were the most strongly associated with aneurysm rupture after adjusting for all other clinical and morphological variables. Multivariate subgroup analysis for patients with visualized PCoA demonstrated that larger neck diameter (p = 0.018) and shorter ICA bifurcation to aneurysm distance (p = 0.011) were significantly associated with rupture. Intracerebral hemorrhage was associated with smaller volume, larger maximum height, and smaller aneurysm angle, in addition to lateral projection, male sex, and lack of hypertension. We found that shorter ICA bifurcation to aneurysm distance is significantly associated with PCoA aneurysm rupture. This is a new physically intuitive parameter that can be measured easily and therefore be readily applied in clinical practice to aid in the evaluation of patients with PCoA aneurysms. 相似文献
8.
Investigation into the association of insurance status with the outcomes of patients undergoing neurosurgical intervention has been limited: this is the first nationwide study to analyze the impact of primary payer on the outcomes of patients with aneurysmal subarachnoid hemorrhage who underwent endovascular coiling or microsurgical clipping. The Nationwide Inpatient Sample (2001–2010) was utilized to identify patients; those with both an ICD-9 diagnosis codes for subarachnoid hemorrhage and a procedure code for aneurysm repair (either via an endovascular or surgical approach) were included. Hierarchical multivariate regression analyses were utilized to evaluate the impact of primary payer on in-hospital mortality, hospital discharge disposition, and length of hospital stay with hospital as the random effects variable. Models were adjusted for patient age, sex, race, comorbidities, socioeconomic status, hospital region, location (urban versus rural), and teaching status, procedural volume, year of admission, and the proportion of patients who underwent ventriculostomy. Subsequent models were also adjusted for time to aneurysm repair and time to ventriculostomy; subgroup analyses evaluated for those who underwent endovascular and surgical procedures separately. 15,557 hospitalizations were included. In the initial model, the adjusted odds of in-hospital mortality were higher for Medicare (OR 1.23, p<0.001), Medicaid (OR 1.23, p<0.001), and uninsured patients (OR 1.49, p<0.001) compared to those with private insurance. After also adjusting for timing of intervention, Medicaid and uninsured patients had a reduced odds of non-routine discharge (OR 0.75, p<0.001 and OR 0.42, p<0.001) despite longer hospital stays (by 8.35 days, p<0.001 and 2.45 days, p = 0.005). Variations in outcomes by primary payer–including in-hospital post-procedural mortality–were more pronounced for patients of all insurance types who underwent microsurgical clipping. The observed differences by primary payer are likely multifactorial, attributable to varied socioeconomic factors and the complexities of the American healthcare delivery system. 相似文献
9.
10.