全文获取类型
收费全文 | 6078篇 |
免费 | 264篇 |
国内免费 | 1608篇 |
专业分类
7950篇 |
出版年
2024年 | 17篇 |
2023年 | 90篇 |
2022年 | 121篇 |
2021年 | 162篇 |
2020年 | 149篇 |
2019年 | 274篇 |
2018年 | 242篇 |
2017年 | 224篇 |
2016年 | 209篇 |
2015年 | 272篇 |
2014年 | 310篇 |
2013年 | 419篇 |
2012年 | 416篇 |
2011年 | 399篇 |
2010年 | 276篇 |
2009年 | 395篇 |
2008年 | 402篇 |
2007年 | 421篇 |
2006年 | 390篇 |
2005年 | 356篇 |
2004年 | 310篇 |
2003年 | 313篇 |
2002年 | 310篇 |
2001年 | 207篇 |
2000年 | 206篇 |
1999年 | 180篇 |
1998年 | 127篇 |
1997年 | 79篇 |
1996年 | 110篇 |
1995年 | 132篇 |
1994年 | 82篇 |
1993年 | 65篇 |
1992年 | 57篇 |
1991年 | 43篇 |
1990年 | 32篇 |
1989年 | 25篇 |
1988年 | 21篇 |
1987年 | 23篇 |
1986年 | 14篇 |
1985年 | 10篇 |
1984年 | 15篇 |
1983年 | 12篇 |
1982年 | 15篇 |
1981年 | 6篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有7950条查询结果,搜索用时 15 毫秒
231.
Le Zhang Sun-Ok Fernandez-Kim Tina L. Beckett Dana M. Niedowicz Katharina Kohler Kalavathi Dasuri Annadora J. Bruce-Keller M. Paul Murphy Jeffrey N. Keller 《生物化学与生物物理学报:疾病的分子基础》2019,1865(9):2157-2167
Alzheimer's disease (AD) is the most common age-related neurodegenerative disease, while obesity is a major global public health problem associated with the metabolic disorder type 2 diabetes mellitus (T2DM). Chronic obesity and T2DM have been identified as invariant risk factors for dementia and late-onset AD, while their impacts on the occurrence and development of AD remain unclear. As shown in our previous study, the diabetic mutation (db, Leprdb/db) induces mixed or vascular dementia in mature to middle-aged APPΔNL/ΔNL x PS1P264L/P264L knock-in mice (db/AD). In the present study, the impacts of the db mutation on young AD mice at 10 weeks of age were evaluated. The db mutation not only conferred young AD mice with severe obesity, impaired glucose regulation and activated mammalian target of rapamycin (mTOR) signaling pathway in the mouse cortex, but lead to a surprising improvement in memory. At this young age, mice also had decreased cerebral Aβ content, which we have not observed at older ages. This was unlikely to be related to altered Aβ synthesis, as both β- and γ-secretase were unchanged. The db mutation also reduced the cortical IL-1β mRNA level and IBA1 protein level in young AD mice, with no significant effect on the activation of microglia and astrocytes. We conclude that the db mutation could transitorily improve the memory of young AD mice, a finding that may be partially explained by the relatively improved glucose homeostasis in the brains of db/AD mice compared to their counterpart AD mice, suggesting that glucose regulation could be a strategy for prevention and treatment of neurodegenerative diseases like AD. 相似文献
232.
Xiang Nong Sheng-Nan Zhong Si-Min Li Yao-Jun Yang Zi Liang Yue Xie 《Saudi Journal of Biological Sciences》2019,26(5):1032-1036
mtDNA COII gene sequences were identified and analyzed using different types of software, namely, MEGA5.0, DNAMAN, and DnaSP5.0 in four Chinese provinces, namely, Sichuan, Zhejiang, Guizhou and Shanghai. Analysis of molecular genetic variation and its genetic structure and differentiation, combined with NJ tree, MP tree analysis and analysis of molecular variance (AMOVA), at Fst = 0.0582 conclude that the genetic differentiation is low, gene flow is Nm = 8.0911, and gene exchange is sufficient. However, for the geographic populations of Pseudoregma bambucicola in the four provinces, their gene exchange is relatively weak at Nm = 0.8284, whereas the genetic differentiation is high at Fst = 0.3764. Based on the data, total nucleotide diversity between the populations is 0.00158 ± 0.00021. The results showed that the total population of Tajima’s D and Fu’s Fs results are D = ?0.885 and Fs = 0.226, respectively. The experimental numerical results showed that this total population is not significant (P > 0.10), indicating that nine different geographic populations are short-term. No expansion occurred in the internal population. This study provided a theoretical and practical basis for the comprehensive prevention and control of P. bambucicola. 相似文献
233.
Surface characteristics of grasslands in Inner Mongolia as detected by micrometeorological measurements 总被引:1,自引:0,他引:1
A roving tower concept was used to compare a semi-arid grassland site in Inner Mongolia (China), which was fenced in 1979 and ungrazed thereafter (UG79) with differently grazed semi-arid steppe ecosystems. The study was conducted during three consecutive years characterised by contrasting precipitation. The different grazing intensities included continuously and moderately grazed (CG), winter grazed (WG), and heavily grazed (HG). Here, we compare the energy fluxes and surface parameters that characterise the differently managed plots. The main focus is on sensible heat flux (H), available energy (AE), surface temperature (T ( s )), and surface albedo (alpha). Systematic errors were excluded by a side-to-side intercomparison of the instruments, and systematic climatic differences were minimised by the close distance between the fixed and the roving eddy covariance tower. Statistically, AE and T ( s ) were always significantly different between two simultaneously measured grazing intensities. Whereas AE was higher at UG79 in all years (mean difference of about 19Wm(-2)), T ( s ) was typically lower at UG79 (mean differences of 0.4 degrees C to about 2 degrees C). The exception was the end of the vegetation period in 2004 when T ( s ) was 0.6 degrees C higher at UG79 compared to CG. At UG79 alpha was typically significantly lower, and H was typically significantly higher. Consequently, latent heat fluxes (both as energy balance residual and directly measured) do not differ much between the different grazing intensities. It is concluded, that (1) the roving tower concept is able to detect differences due to grazing, (2) differences between the sites can be attributed to real surface differences, and (3) differences due to grazing intensities are small compared to interannual differences in surface fluxes. 相似文献
234.
中国下泥盆统脊椎动物化石组合层序 总被引:4,自引:4,他引:4
本文综合了滇、黔、川、桂等地的一些早泥盆世地层剖面中的脊椎动物化石的出露情况,粗略地描绘出了早期脊椎动物演化的一般特征,归纳成三个发展阶段。并以此为依据,讨论了下泥盆统的分层与对比。 相似文献
235.
首次记述采自中国云南省藤Calamus platyacanthoides上的尼泊尔扁片蜡蚧Platylecanium nepalense Takagi,1975.主要鉴别特征为:体缘光滑,气门刺4或5根,肛板合成菱形和无足.文中对雌成虫形态特征进行了详细描述及绘图. 相似文献
236.
研究了盲蝽科齿爪盲蝽亚科Deraeocorinae点盾盲蝽属AlloeotomusFieber中国种类 ;共记载 5个种 ,其中包括 2新种 (突肩点盾盲蝽A .humeralissp.nov.,云南点盾盲蝽A .yunnanensissp .nov.)和 1个新异名 (A .kerzhneriQi&Nonnaizab =A .montanusQi&Nonnaizab,syn.nov .)。新种模式标本均存于天津南开大学生物系昆虫标本室 相似文献
237.
238.
A field experiment was conducted to examine the morphological variations of Leymus chinensis along the climatic gradient of the North‐east China Transect (NECT), from 115° to 125° E, in the vicinity of 43.5° N, in north‐eastern China. Ten sites selected for plant sampling along the gradient have approximately uniform theoretical radiation, but differ in precipitation and geographical elevation. The results of analyses showed that vegetative and reproductive shoot heights, flag leaf lengths and widths and seed numbers per inflorescence increased from the west to the east with precipitation, but decreased with aridity. Leaf lengths and widths for most leaf types from 115° to 124° E exhibited little variations. Significant correlations of plant heights, flag leaf lengths and widths and seed numbers per inflorescence with large‐scale climatic variables (e.g. annual precipitation, aridity) and geographical variation (longitude) found in this study indicated that climatic factors have significant effects on some morphological traits of L. chinensis along the NECT. 相似文献
239.
卷叶锦鸡儿——锦鸡儿属(豆科)一新种 总被引:2,自引:0,他引:2
报道了产于蒙古、内蒙古、宁夏和甘肃的锦鸡儿属(豆科)一新种--卷叶锦鸡儿。该种因叶轴全部宿存,荚果里外均被毛,植株垫状而与Caragana tibetica Kom.相近,但因小叶内卷,管状,横切面呈"O"形,翼瓣耳圆钝而不同。 相似文献
240.