全文获取类型
收费全文 | 4350篇 |
免费 | 527篇 |
国内免费 | 733篇 |
出版年
2023年 | 105篇 |
2022年 | 114篇 |
2021年 | 176篇 |
2020年 | 199篇 |
2019年 | 208篇 |
2018年 | 172篇 |
2017年 | 182篇 |
2016年 | 162篇 |
2015年 | 209篇 |
2014年 | 217篇 |
2013年 | 259篇 |
2012年 | 207篇 |
2011年 | 263篇 |
2010年 | 156篇 |
2009年 | 254篇 |
2008年 | 283篇 |
2007年 | 269篇 |
2006年 | 253篇 |
2005年 | 215篇 |
2004年 | 209篇 |
2003年 | 176篇 |
2002年 | 167篇 |
2001年 | 131篇 |
2000年 | 113篇 |
1999年 | 120篇 |
1998年 | 96篇 |
1997年 | 71篇 |
1996年 | 81篇 |
1995年 | 70篇 |
1994年 | 66篇 |
1993年 | 59篇 |
1992年 | 66篇 |
1991年 | 51篇 |
1990年 | 34篇 |
1989年 | 31篇 |
1988年 | 36篇 |
1987年 | 27篇 |
1986年 | 13篇 |
1985年 | 18篇 |
1984年 | 7篇 |
1983年 | 10篇 |
1982年 | 12篇 |
1981年 | 4篇 |
1980年 | 9篇 |
1979年 | 4篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1972年 | 3篇 |
排序方式: 共有5610条查询结果,搜索用时 15 毫秒
1.
《Saudi Journal of Biological Sciences》2017,24(6):1288-1293
The antimicrobial activity of plant extract of Peganum harmala, a medicinal plant has been studied already. However, knowledge about bacterial diversity associated with different parts of host plant antagonistic to different human pathogenic bacteria is limited. In this study, bacteria were isolated from root, leaf and fruit of plant. Among 188 bacterial isolates isolated from different parts of the plant only 24 were found to be active against different pathogenic bacteria i.e. Escherichia coli, Methicillin-resistant Staphylococcus aureus (MRSA), Enterococcus faecium, Enterococcus faecalis and Pseudomonas aeruginosa. These active bacterial isolates were identified on the basis of 16S rRNA gene analysis. Total population of bacteria isolated from plant was high in root, following leaf and fruit. Antagonistic bacteria were also more abundant in root as compared to leaf and fruit. Two isolates (EA5 and EA18) exhibited antagonistic activity against most of the targeted pathogenic bacteria mentioned above. Some isolates showed strong inhibition for one targeted pathogenic bacterium while weak or no inhibition for others. Most of the antagonistic isolates were active against MRSA, following E. faecium, P. aeruginosa, E. coli and E. faecalis. Taken together, our results show that medicinal plants are good source of antagonistic bacteria having inhibitory effect against clinical bacterial pathogens. 相似文献
2.
Lalitha Guruprasad 《Proteins》2020,88(11):1387-1393
Coronavirus disease 2019 (COVID-19) is a pandemic infectious disease caused by novel severe acute respiratory syndrome coronavirus-2 (SARS CoV-2). The SARS CoV-2 is transmitted more rapidly and readily than SARS CoV. Both, SARS CoV and SARS CoV-2 via their glycosylated spike proteins recognize the human angiotensin converting enzyme-2 (ACE-2) receptor. We generated multiple sequence alignments and phylogenetic trees for representative spike proteins of SARS CoV and SARS CoV-2 from various host sources in order to analyze the specificity in SARS CoV-2 spike proteins required for causing infection in humans. Our results show that among the genomes analyzed, two sequence regions in the N-terminal domain “MESEFR” and “SYLTPG” are specific to human SARS CoV-2. In the receptor-binding domain, two sequence regions “VGGNY“ and ”EIYQAGSTPCNGV” and a disulfide bridge connecting 480C and 488C in the extended loop are structural determinants for the recognition of human ACE-2 receptor. The complete genome analysis of representative SARS CoVs from bat, civet, human host sources, and human SARS CoV-2 identified the bat genome (GenBank code: MN996532.1) as closest to the recent novel human SARS CoV-2 genomes. The bat SARS CoV genomes (GenBank codes: MG772933 and MG772934) are evolutionary intermediates in the mutagenesis progression toward becoming human SARS CoV-2. 相似文献
3.
《Harmful algae》2017
In the last decades, the cyanobacterium Dolichospermum lemmermannii showed an increasing spread to Southern Europe, raising serious concerns due to its ability to produce cyanotoxins. The widening of its geographic distribution and the observation of strains showing high optimum temperature underline its ecological heterogeneity, suggesting the existence of different ecotypes. To investigate its biogeography, new isolates from different European water bodies, together with strains maintained by the Norwegian Institute for Water Research Culture Collection of Algae, were genetically characterised for the 16S rRNA gene and compared with strains obtained from public repositories. Geographic distance highly influenced the differentiation of genotypes, further suggesting the concurrent role of geographic isolation, physical barriers and environmental factors in promoting the establishment of phylogenetic lineages adapted to specific habitats. Differences among populations were also examined by morphological analysis and evaluating the toxic potential of single strains, which revealed the exclusive ability of North European strains to produce microcystins, whereas the populations in Southern Europe tested negative for a wide range of cyanotoxins. The high dispersion ability and the existence of toxic genotypes indicate the possible spread of harmful blooms in other temperate regions. 相似文献
4.
针对神农架川金丝猴生境基础研究中乔木树种大范围分布数据难以获取问题,尝试利用多源多时相遥感数据结合专家知识分层次实现树种识别。首先采用冬季Landsat8/OLI数据根据物侯特性分层提取常绿、落叶林的地域范围;进而依据夏季Worldview-2高分遥感影像的实地乔木样本的光谱特征分层次完成常绿树种(巴山冷杉、华山松、青$、刺叶栎)和落叶树种(红桦、日本落叶松、米心水青冈、漆树、锐齿槲栎、椅杨)的识别;并通过实地植被样方及专家知识通过高程数据完成分类结果的修正;最后结合GIS对主要优势树种的地形及地域分布特征进行了空间分析。实验精度表明常绿林中巴山冷杉、华山松、刺叶栎、虫害华山松整体精度较高,落叶林中红桦、漆树等识别精度相对较高,部分树种如椅杨、锐齿槲栎识别精度较低;总体上常绿树种的精度要优于落叶树种。从植物地理学、遥感、GIS三者相结合的角度,将多源、多时相遥感数据与物种物候特性、专家知识进行有效整合,提出了一种乔木树种识别的方法(1)提供了复杂山地环境的主要乔木优势种识别途径,且具有通用性;(2)完成了物种物候特性与遥感数据特性的整合利用,有效降低数据成本费用;(3)配合地面样方及专家知识修正结果,避免了过分依赖光谱特征引起的误判。这将为神农架川金丝猴栖息地保护与恢复提供更精确的数据依据。 相似文献
5.
Growth divergence – i.e. the expression of divergent growth trends of neighboring trees – has certain implications for dendrochronological research, for instance in the context of climate reconstructions but also in terms of estimating net ecosystem productivity. Thus, understanding the underlying mechanisms is essential to extend our fundamental dendroecological knowledge. In this context, the Picea genus plays an important role since several of its species were reported to exhibit growth divergence. Here, we investigate a well sampled Norway spruce (Picea abies (L.) Karst) data set for growth divergence comprising ring-width and Blue Intensity measurements from seven sites on Babia Góra Mountain, at the border between Poland and Slovakia. By means of Principal Component Gradient Analysis, inter-series correlations, and climate growth relationships, we are able to show that I) Norway spruce on Babia Góra expressed growth divergence since the 1970s, II) the definition of groups increased the strength of population signals and the stability of climate-growth relationships, and III) Blue Intensity appeared as a more robust proxy for environmental conditions. We discuss soil heterogeneity, genetics, and air pollution as possible underlying mechanisms, thereby indicating further research avenues to obtain a better understanding of growth divergence. 相似文献
6.
《Current biology : CB》2020,30(15):2951-2961.e5
7.
8.
地表硬化影响城市绿化树生长及生态服务功能的发挥,研究人工覆盖地表下城市绿化树关键生理过程具有重要意义.采用田间模拟试验方法,布设对照(自然地表)、透水硬化和不透水硬化3种覆盖地表,栽植北京2种典型绿化树白蜡和元宝枫,栽植后第4年野外测定其叶片光合及相关生理生态参数,分析人工覆盖地表对北京典型绿化树光合特性的影响.结果表明:白蜡和元宝枫光合作用对不同人工覆盖地表的响应存在差异,对于白蜡而言,透水硬化和不透水硬化地表均使其净光合速率显著降低,对于元宝枫而言,仅不透水硬化地表导致其净光合速率显著降低,透水硬化地表并未对其造成显著影响.透水硬化和不透水硬化地表的空气相对湿度和土壤体积含水率低于对照,植物可利用水分减少,叶片气孔导度下降是白蜡和元宝枫净光合速率降低的主要原因.透水硬化和不透水硬化地表使白蜡PSⅡ有效光化学量子产量(Fv′/Fm′)下降,不透水硬化地表使元宝枫PSⅡ实际光化学量子效率(ΦPSⅡ)和表观电子传递速率(ETR)升高. 相似文献
9.
为探究树木径向生长的影响因子,基于对长白山阔叶红松林25 hm2样地中3个常见树种红松、紫椴、蒙古栎连续5年的生长环监测数据,分析各树种的季节生长变化,比较树种间的生长速率差异,并分析初始径级、邻体树木竞争、土壤、地形因素等对不同树种径向生长的作用.结果表明: 3个树种径向生长的季节变化趋势一致,树木于5月末开始生长,7月是生长旺盛期,8月底生长开始减缓,于10月中下旬停止生长.3个树种的径向相对生长率存在显著差异,蒙古栎相对径向生长率显著高于红松和紫椴,该差异在小、中径级个体中表现更为显著.3个树种径向生长的主要影响因子不同,邻体树木的竞争显著抑制了红松和蒙古栎的径向生长,而紫椴的径向生长主要受初始径级、土壤和地形等因子的影响. 相似文献
10.
在浙江省临安市选择典型天然次生灌丛,分别进行封禁和目标树抚育,探讨灌丛恢复为乔木林的可能性.结果表明: 4年后,与未管护(对照)相比,封禁和目标树抚育后群落平均胸径分别提高1.3和2.6倍,平均高度分别提高0.5和1.1倍;目标树抚育林木出现了对照林分没有的4.5~8.5 cm径阶和4.5~8.5 m树高阶,形成了4 m高的新林层;灌木层物种丰富度和多样性指数没有因抚育而下降;封禁管理维持了群落的树种组成,遵循原有的演替方向;目标树抚育显著改变了群落的树种组成,提高了目的树种的重要值,近期有可能恢复成为针阔混交林群落.与封禁相比,目标树抚育在优势林木胸径和高度生长、树种组成改善等方面更能达到预想的目标.在有条件经营的情况下,可以选择目标树抚育模式对天然次生灌丛进行管理,从而达到加快群落恢复演替形成乔木林的目的. 相似文献