全文获取类型
收费全文 | 4064篇 |
免费 | 403篇 |
国内免费 | 378篇 |
出版年
2024年 | 33篇 |
2023年 | 114篇 |
2022年 | 145篇 |
2021年 | 152篇 |
2020年 | 157篇 |
2019年 | 182篇 |
2018年 | 167篇 |
2017年 | 160篇 |
2016年 | 170篇 |
2015年 | 182篇 |
2014年 | 245篇 |
2013年 | 278篇 |
2012年 | 172篇 |
2011年 | 245篇 |
2010年 | 224篇 |
2009年 | 246篇 |
2008年 | 258篇 |
2007年 | 249篇 |
2006年 | 194篇 |
2005年 | 172篇 |
2004年 | 131篇 |
2003年 | 122篇 |
2002年 | 100篇 |
2001年 | 77篇 |
2000年 | 53篇 |
1999年 | 60篇 |
1998年 | 46篇 |
1997年 | 71篇 |
1996年 | 36篇 |
1995年 | 38篇 |
1994年 | 47篇 |
1993年 | 33篇 |
1992年 | 43篇 |
1991年 | 39篇 |
1990年 | 24篇 |
1989年 | 18篇 |
1988年 | 17篇 |
1987年 | 14篇 |
1986年 | 19篇 |
1985年 | 18篇 |
1984年 | 20篇 |
1983年 | 22篇 |
1982年 | 19篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1977年 | 6篇 |
1976年 | 2篇 |
1953年 | 2篇 |
排序方式: 共有4845条查询结果,搜索用时 31 毫秒
121.
122.
123.
A high‐resolution approach for the spatiotemporal analysis of forest canopy space using terrestrial laser scanning data
下载免费PDF全文
![点击此处可从《Ecology and evolution》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Carsten Hess Werner Härdtle Matthias Kunz Andreas Fichtner Goddert von Oheimb 《Ecology and evolution》2018,8(13):6800-6811
Forest canopies and tree crown structures are of high ecological importance. Measuring canopies and crowns by direct inventory methods is time‐consuming and of limited accuracy. High‐resolution inventory tools, in particular terrestrial laser scanning (TLS), is able to overcome these limitations and obtain three‐dimensional (3D) structural information about the canopy with a very high level of detail. The main objective of this study was to introduce a novel method to analyze spatiotemporal dynamics in canopy occupancy at the individual tree and local neighborhood level using high‐resolution 3D TLS data. For the analyses, a voxel grid approach was applied. The tree crowns were modeled through the combination of two approaches: the encasement of all crown points with a 3D α‐shape, which was then converted into a voxel grid, and the direct voxelization of the crown points. We show that canopy occupancy at individual tree level can be quantified as the crown volume occupied only by the respective tree or shared with neighboring trees. At the local neighborhood level, our method enables the precise determination of the extent of canopy space filling, the identification of tree–tree interactions, and the analysis of complementary space use. Using multitemporal TLS data recordings, this method allows the precise detection and quantification of changes in canopy occupancy through time. The method is applicable to a wide range of investigations in forest ecology research, including the study of tree diversity effects on forest productivity or growing space analyses for optimal tree growth. Due to the high accuracy of this novel method, it facilitates the precise analyses even of highly plastic individual tree crowns and, thus, the realistic representation of forest canopies. Moreover, our voxel grid framework is flexible enough to allow for the inclusion of further biotic and abiotic variables relevant to complex analyses of forest canopy dynamics. 相似文献
124.
Using ethane as a marker for peroxidative damage to membranes by reactive oxygen species (ROS) we examined the injury of rice
seedlings during submergence in the dark. It is often expressed that membrane injury from ROS is a post-submergence phenomenon
occurring when oxygen is re-introduced after submergence-induced anoxia. We found that ethane production, from rice seedlings
submerged for 24–72 h, was stimulated to 4–37 nl gFW−1, indicating underwater membrane peroxidation. When examined a week later the seedlings were damaged or had died. On de-submergence
in air, ethane production rates rose sharply, but fell back to less than 0.1 nl gFW−1 h−1 after 2 h. We compared submergence-susceptible and submergence-tolerant cultivars, submergence starting in the morning (more
damage) and in the afternoon (less damage) and investigated different submergence durations. The seedlings showed extensive
fatality whenever total ethane emission exceeded about 15 nl gFW−1. Smaller amounts of ethane emission were linked to less extensive injury to leaves. Partial oxygen shortage (O2 levels <1%) imposed for 2 h in gas phase mixtures also stimulated ethane production. In contrast, seedlings under anaerobic
gas phase conditions produced no ethane until re-aerated: then a small peak was observed followed by a low, steady ethane
production. We conclude that damage during submergence is not associated with extensive anoxia. Instead, injury is linked
to membrane peroxidation in seedlings that are partially oxygen deficient while submerged. On return to air, further peroxidation
is suppressed within about 2 h indicating effective control of ROS production not evident during submergence itself. 相似文献
125.
126.
A validated, simple and sensitive HPLC method was developed for the simultaneous determination of lipoperoxidation relevant reactive aldehydes: glyoxal (GO), acrolein (ACR), malondialdehyde (MDA), and 4-hydroxy-2-nonenal (HNE) in human serum. The studied aldehydes were reacted with 2,2′-furil to form fluorescent difurylimidazole derivatives that were separated on a C18 column using gradient elution and fluorescence detection at excitation and emission wavelengths of 250 and 355 nm, respectively. The method showed good linearity over the concentration ranges of 0.100–5.00, 0.200–10.0, 0.200–40.0, and 0.400–10.0 nmol/mL for GO, ACR, HNE, and MDA, respectively, with detection limits ranging from 0.030 to 0.11 nmol/mL. The percentage RSD of intraday and interday precision did not exceed 5.0 and 6.2%, respectively, and the accuracy (%found) ranged from 95.5 to 103%. The proposed method was applied for monitoring the four aldehydes in sera of healthy, diabetic, and rheumatic human subjects with simple pretreatment steps and without interference from endogenous components. By virtue of its high sensitivity and accuracy, our method enabled detection of differences between analytes concentrations in sera of human subjects under different clinical conditions. 相似文献
127.
探索目的基因在转基因白桦杂交子代群体中的传递规律,为后续进一步跟踪调查,实现转基因白桦的聚合育种提供帮助。以1年生BpAP1、TabZIP转基因白桦杂交T1代和5年生BpGH3.5、BpCCR转基因白桦为研究对象,测定杂交子代种子千粒质量、发芽率和发芽势等活力指标,同时采用PCR扩增法检测目的基因,调查杂交T2代白桦苗高。杂交子代各家系间种子千粒质量、种子活力指标和苗高等在0.01水平上差异显著,各性状的家系遗传力均高于85.00%。研究结果表明,目的基因在T2代群体中的传递规律不符合孟德尔遗传规律,雌、雄配子目的基因的传递效率分别为30.00%、50.00%左右,同时未获得3目的基因聚合的杂交T2代个体。目的基因的随机插入降低了白桦杂交T2代种子活力,未获得3基因聚合的白桦杂交子代,目的基因的聚合对杂交子代个体的生长发育产生了不利影响,甚至产生致死现象。 相似文献
128.
Isothermal and rapid amplification detection of nucleic acids is an important technology in environmental monitoring, foodborne pathogen detection, and point-of-care clinical diagnostics. Here we have developed a novel method of isothermal signal amplification for single-stranded DNA (ssDNA) detection. The ssDNA target could be used as an initiator, coupled with a double-nicked molecular beacon, to originate amplification cycles, achieving cascade signal amplification. In addition, the method showed good specificity and strong anti-jamming capability. Overall, it is a one-pot and isothermal strand displacement amplification method without the requirement of a stepwise procedure, which greatly simplifies the experimental procedure and decreases the probability of contamination of samples. With its advantages, the method would be very useful to detect nucleic acids in point-of-care or field use. 相似文献
129.
Rapid and effective detection of anthrax spores in soil by PCR 总被引:2,自引:0,他引:2
Cheun HI Makino SI Watarai M Erdenebaatar J Kawamoto K Uchida I 《Journal of applied microbiology》2003,95(4):728-733
AIMS: To detect Bacillus anthracis DNA from soil using rapid and simple procedures. METHODS AND RESULTS: Various amounts of B. anthracis Pasteur II spores were added artificially to 1 g of soil, which was then washed with ethanol and sterile water. Enrichment of the samples in trypticase soy broth was performed twice. A DNA template was prepared from the second enrichment culture using a FastPrep instrument. The template was then used for nested and real-time polymerase chain reaction (PCR) with B. anthracis-specific primers, to confirm the presence of B. anthracis chromosomal DNA and the pXO1/pXO2 plasmids. CONCLUSIONS: One cell of B. anthracis in 1 g of soil could be detected by nested and real-time PCR. The usefulness of the PCR method using field samples was also confirmed. SIGNIFICANCE AND IMPACT OF THE STUDY: The results indicate that this could be a useful method for detecting anthrax-spore contaminated soil with high sensitivity. Its application could have great impact on the progress of epidemiological surveillance. 相似文献
130.