全文获取类型
收费全文 | 9532篇 |
免费 | 1450篇 |
国内免费 | 2297篇 |
出版年
2023年 | 279篇 |
2022年 | 400篇 |
2021年 | 476篇 |
2020年 | 422篇 |
2019年 | 466篇 |
2018年 | 478篇 |
2017年 | 469篇 |
2016年 | 545篇 |
2015年 | 463篇 |
2014年 | 625篇 |
2013年 | 761篇 |
2012年 | 513篇 |
2011年 | 529篇 |
2010年 | 379篇 |
2009年 | 504篇 |
2008年 | 557篇 |
2007年 | 524篇 |
2006年 | 526篇 |
2005年 | 510篇 |
2004年 | 441篇 |
2003年 | 384篇 |
2002年 | 335篇 |
2001年 | 247篇 |
2000年 | 269篇 |
1999年 | 206篇 |
1998年 | 196篇 |
1997年 | 251篇 |
1996年 | 156篇 |
1995年 | 156篇 |
1994年 | 123篇 |
1993年 | 118篇 |
1992年 | 126篇 |
1991年 | 110篇 |
1990年 | 91篇 |
1989年 | 81篇 |
1988年 | 66篇 |
1987年 | 65篇 |
1986年 | 73篇 |
1985年 | 46篇 |
1984年 | 54篇 |
1983年 | 39篇 |
1982年 | 49篇 |
1981年 | 29篇 |
1980年 | 21篇 |
1979年 | 13篇 |
1978年 | 23篇 |
1977年 | 17篇 |
1976年 | 19篇 |
1975年 | 14篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 13 毫秒
1.
2.
In the current work, the quantification of different poly(ethylene glycol) (PEG)–potassium phosphate/sodium citrate aqueous two-phase system (ATPS) phase-forming components was investigated by using conductivity and refractive index measurements. For this purpose, refractive index and conductivity calibration curves were obtained for ATPS at different pH values in the presence of different bovine serum albumin (BSA) concentrations. Whereas BSA had no effect on the conductivity, it had a considerable effect on the refractive index. Finally, a convenient dilution of the samples prior to the ATPS constituent determination is needed to ensure no significant influence from BSA. 相似文献
3.
运用相邻格子法及聚集度指标测定法,在植被调查的基础上,对云南普洱不同恢复阶段(恢复15a、30a和老龄林)季风常绿阔叶林的刺栲(Castanopsis hystrix)、短刺栲(Castanopsis echidnocarpa)和红木荷(Schima wallichii)3个优势物种进行种群空间分布格局研究,以阐明种群分布格局与生境条件、种群年龄结构的动态关系,揭示种群发育过程中空间动态与生物学机理。结果发现:在所有恢复阶段内,刺栲、短刺栲、红木荷均为聚集分布。在不同尺度下,刺栲在原始林中10m×10m尺度下为均匀分布,而在恢复15年中20m×20m尺度为随机分布;红木荷在10m×20m尺度下为均匀分布,在其它所有恢复阶段和尺度下3个物种均为聚集分布。研究结果表明,不同恢复阶段和不同尺度下刺栲、短刺栲和红木荷3个优势物种绝大多数为聚集分布,说明聚集分布为3个优势种的基本属性。 相似文献
4.
选取我国东北、西南、西北及中北部地区的10个典型湖泊,调查了表层沉积物生物硅含量变化情况,并通过区域对比分析了生物硅含量变化的原因,寻找我国湖泊生物硅变化的空间规律。结果表明,西北及中北部地区湖泊生物硅含量平均值整体低于东北和西南地区湖泊,可能是由于西北及中北部地区较低的温度和较少的降水量导致硅藻生长受限,从而造成生物硅含量偏低。东北湖泊与西南湖泊生物硅含量相对较高,但其形成原因不同。东北2个湖泊均靠近人类聚居区,属富营养型湖泊,虽然东北地区年均温较低,但充足的营养盐为硅藻生长提供了必要因素。而西南地区湖泊大多为中贫营养型湖泊,其生物硅含量整体较高可能主要归因于较高的温度和较多的降水。 相似文献
5.
This paper introduces the vectorial Kappa (κv) that one can utilize to assess congruence between two vectorial mosaics. The vectorial Kappa extends for vectorial mosaics the approach of the so-called Cohen's Kappa index, commonly used to compare raster mosaics. By comparing both approaches, we aim to demonstrate how efficient and convenient a vector-based congruence may be when working on vectorial mosaics. 相似文献
6.
7.
Massimo Mezzavilla Annamaria Iorio Marco Bobbo Angela D'Eustacchio Marco Merlo Paolo Gasparini Sheila Ulivi Gianfranco Sinagra 《Gene》2014
Background
Recent studies suggested that resting heart rate (RHR) might be an independent predictor of cardiovascular mortality and morbidity. Nonetheless, the interrelation between RHR and cardiovascular diseases is not clear. In order to resolve this puzzle, the importance of genetic determinants of RHR has been recently suggested, but it needs to be further investigated.Objective
The aim of this study was to estimate the contribution of common genetic variations on RHR using Genome Wide Association Study.Methods
We performed a Genome Wide Association Study in an isolated population cohort of 1737 individuals, the Italian Network on Genetic Isolates — Friuli Venezia Giulia (INGI-FVG). Moreover, a haplotype analysis was performed. A regression tree analysis was run to highlight the effect of each haplotype combination on the phenotype.Results
A significant level of association (p < 5 × 10− 8) was detected for Single Nucleotide Polymorphisms (SNPs) in two genes expressed in the heart: MAML1 and CANX. Founding that the three different variants of the haplotype, which encompass both genes, yielded a phenotypic correlation. Indeed, a haplotype in homozygosity is significantly associated with the lower quartile of RHR (RHR ≤ 58 bpm). Moreover no significant association was found between cardiovascular risk factors and the different haplotype combinations.Conclusion
Mastermind-like 1 and Calnexin were found to be associated with RHR. We demonstrated a relation between a haplotype and the lower quartile of RHR in our populations. Our findings highlight that genetic determinants of RHR may be implicated in determining cardiovascular diseases and could allow a better risk stratification. 相似文献8.
J. J. Windig 《Journal of evolutionary biology》1994,7(6):665-695
The genetic basis of the dry-wet season polyphenism of wing pattern in response to temperature shown by Bicyclus anynana was studied, using a split-family design over four temperatures. Reaction norms crossed, but were only linear in the three highest temperatures, and only when larval development time was used as the environmental axis. Significant full-sib additive variances (VA) and heritabilities (h2) for plasticity were found using slopes of reaction norms in a bootstrap procedure. Heritabilities were lower in intermediate temperatures, mainly due to differences in the residual variances (VR). There was no clear trend in VA across temperatures, contrary to the expectation that VA would have been depleted by natural selection at the extreme temperatures and not depleted at the intermediate temperatures which occur less frequently in the field. Unpredictability in the onset of the following season at intermediate temperatures might lead to selection for diverse flresponses resulting in relatively high VRs. Theoretical models linking reaction norms to genetic parameters in separate environments were difficult to apply in this study, particularly because they are based on the assumption that VRs are constant. However the reaction norm approach combined with quantitative genetics provided a valuable insight into the evolution of the observed polyphenism. 相似文献
9.
Dr. P. K. Khosla O. P. Toky R. P. Bisht S. Hamidullah 《Trees - Structure and Function》1995,10(2):108-113
In the mid-western Himalaya (altitude 1350 m, rainfall 1100 mm), multipurpose trees found as escapees in agricultural fields or naturally growing in the forests, play an important role in providing fuel, fooder and small timber to the farmers. Shoot elogation, and tree architecture of 4 year old trees of Grewia optiva, Robinia pseudoacacia and Celtis australis (early successionals), and Quercus leucotrichophora, Q. glauca and Ilex odorata (late successionals), were analyzed. All the late successional species showed a proleptic type of bud and branch production, while the early successional trees made growth through syllepsis. The shoot elongation differed significantly (P <0.05) with the crown position, and ranged from 11 to 30 cm in different species. Early successional species tended to maintain a comparatively narrow crown and showed a significantly (P <0.05) higher ramification ratio, and multilayered canopy. The late successionals, in contrast, showed a wide crown with monolayered canopy, adapted to the weak light intensity. There was only one flush of leaves in Q. leucotrichophora and Q. glauca while in the rest of the species there were two distinct flush periods. The results are important for the management of agroforestry trees. 相似文献
10.
Abstract The gene-protein database was used to obtain the two-dimensional polyacrylamide gel coordinates of proteins phosphorylated in extracts of Escherichia coli including those phosphorylated by eukaryotic-like kinase activities. These suggest that the phosphoproteins correspond to, or co-migrate with, the product of an open reading frame at 1.3 min (Orf80), Enzyme 1 of the phosphoenolpyruvate-dependent phosphotransferase system (Ptsl), the tRNA synthetase for histidine (HisS), and proteins involved in the response to carbon starvation and quinone treatment. 相似文献