全文获取类型
收费全文 | 500篇 |
免费 | 42篇 |
出版年
2023年 | 10篇 |
2022年 | 1篇 |
2021年 | 24篇 |
2020年 | 11篇 |
2019年 | 20篇 |
2018年 | 16篇 |
2017年 | 12篇 |
2016年 | 26篇 |
2015年 | 42篇 |
2014年 | 50篇 |
2013年 | 40篇 |
2012年 | 55篇 |
2011年 | 44篇 |
2010年 | 21篇 |
2009年 | 21篇 |
2008年 | 32篇 |
2007年 | 17篇 |
2006年 | 20篇 |
2005年 | 8篇 |
2004年 | 11篇 |
2003年 | 7篇 |
2002年 | 7篇 |
2001年 | 3篇 |
2000年 | 2篇 |
1998年 | 4篇 |
1997年 | 2篇 |
1996年 | 1篇 |
1995年 | 2篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1985年 | 5篇 |
1984年 | 6篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1974年 | 1篇 |
1972年 | 1篇 |
1967年 | 1篇 |
1957年 | 1篇 |
1948年 | 1篇 |
排序方式: 共有542条查询结果,搜索用时 15 毫秒
71.
Increased mercury in forest soils under elevated carbon dioxide 总被引:1,自引:0,他引:1
Fossil fuel combustion is the primary anthropogenic source of both CO2 and Hg to the atmosphere. On a global scale, most Hg that enters ecosystems is derived from atmospheric Hg that deposits
onto the land surface. Increasing concentrations of atmospheric CO2 may affect Hg deposition to terrestrial systems and storage in soils through CO2-mediated changes in plant and soil properties. We show, using free-air CO2 enrichment (FACE) experiments, that soil Hg concentrations are almost 30% greater under elevated atmospheric CO2 in two temperate forests. There were no direct CO2 effects, however, on litterfall, throughfall or stemflow Hg inputs. Soil Hg was positively correlated with percent soil organic
matter (SOM), suggesting that CO2-mediated changes in SOM have influenced soil Hg concentrations. Through its impacts on SOM, elevated atmospheric CO2 may increase the Hg storage capacity of soils and modulate the movement of Hg through the biosphere. Such effects of rising
CO2, ones that transcend the typically studied effects on C and nutrient cycling, are an important next phase for research on
global environmental change. 相似文献
72.
Ecological differentiation is a major contributor to the generation and maintenance of biological diversity. We investigated habitat differentiation between and within sites in the fully cross-fertile and hybridizing Silene dioica and S. latifolia using amplified fragment length polymorphisms (AFLP) profiles and corresponding vegetation relevés around individual plants. Nineteen study sites in the Swiss Alps included pure sites and contact sites (both taxa present within 30 m). In pure sites and at contact sites, the two taxa showed consistently differentiated AFLP banding patterns across regions but few discriminating bands. This indicates that although the two taxa are weakly differentiated, current introgression has not led to genome-wide admixture. Only three putative early generation hybrids were detected at contact sites. The habitats of the two taxa differed between pure sites with S. dioica occurring in moister, colder, and less-disturbed sites than S. latifolia. However, asymmetric habitat overlap was evident within contact sites found in intermediate conditions that were more similar to S. latifolia sites. This situation might favor introgression from S. dioica into S. latifolia. Evidence for habitat-genotype associations within contact sites was weak making habitat-mediated selection against intermediate phenotypes of hybrids unlikely in the contact sites investigated. We suggest that other reproductive barriers together with dispersal limitation contribute to the rarity of early generation hybrids. 相似文献
73.
Thierry Cheutin Marie-Fran?oise O'Donohue Adrien Beorchia Christophe Klein Hervé Kaplan Dominique Ploton 《The journal of histochemistry and cytochemistry》2003,51(11):1411-1423
The monoclonal antibody (MAb) Ki-67 is routinely used in clinical studies to estimate the growth fraction of tumors. However, the role of pKi-67, the protein detected by the Ki-67 MAb, remains elusive, although some biochemical data strongly suggest that it might organize chromatin. To better understand the functional organization of pKi-67, we studied its three-dimensional distribution in interphase cells by confocal microscopy and electron tomography. FluoroNanogold, a single probe combining a dense marker with a fluorescent dye, was used to investigate pKi-67 organization at the optical and ultrastructural levels. Observation by confocal microscopy followed by 3D reconstruction showed that pKi-67 forms a shell around the nucleoli. Double labeling experiments revealed that pKi-67 co-localizes with perinucleolar heterochromatin. Electron microscopy studies confirmed this close association and demonstrated that pKi-67 is located neither in the fibrillar nor in the granular components of the nucleolus. Finally, spatial analyses by electron tomography showed that pKi-67 forms cords 250-300 nm in diameter, which are themselves composed of 30-50-nm-thick fibers. These detailed comparative in situ analyses strongly suggest the involvement of pKi-67 in the higher-order organization of perinucleolar chromatin. 相似文献
74.
Adrien C. Finzi Evan H. DeLucia Jason G. Hamilton Daniel D. Richter William H. Schlesinger 《Oecologia》2002,132(4):567-578
Elevated concentrations of atmospheric CO2 increase plant biomass, net primary production (NPP) and plant demand for nitrogen (N). The demand for N set by rapid plant growth under elevated CO2 could be met by increasing soil N availability or by greater efficiency of N uptake. Alternatively, plants could increase their nitrogen-use efficiency (NUE), thereby maintaining high rates of growth and NPP in the face of nutrient limitation. We quantified dry matter and N budgets for a young pine forest exposed to 4 years of elevated CO2 using free-air CO2 enrichment technology. We addressed three questions: Does elevated CO2 increase forest NPP and the demand for N by vegetation? Is demand for N met by greater uptake from soils, a shift in the distribution of N between plants, microbes, and soils, or increases in NUE under elevated CO2? Will soil N availability constrain the NPP response of this forest as CO2 fumigation continues? A step-function increase in atmospheric CO2 significantly increased NPP during the first 4 years of this study. Significant increases in NUE under elevated CO2 modulated the average annual requirement for N by vegetation in the first and third growing seasons under elevated CO2; the average stimulation of NPP in these years was 21% whereas the average annual stimulation of the N requirement was only 6%. In the second and fourth growing seasons, increases in NPP increased the annual requirement for N by 27-33%. Increases in the annual requirement for N were largely met by increases in N uptake from soils. Retranslocation of nutrients prior to senescence played only a minor role in supplying the additional N required by trees growing under elevated CO2. NPP was highly correlated with between-plot variation in the annual rate of net N mineralization and CO2 treatment. This demonstrates that NPP is co-limited by C availability, as CO2 from the atmosphere, and N availability from soils. There is no evidence that soil N mineralization rates have increased under elevated CO2. The correlation between NPP and N mineralization rates and the increase in the annual requirement for N in certain years imply that soil N availability may control the long-term productivity response of this ecosystem to elevated CO2. Although we have no evidence suggesting that NPP is declining in response to >4 years of CO2 fumigation, if the annual requirement of N continues to be stimulated by elevated CO2, we predict that the productivity response of this forest ecosystem will decline over time. 相似文献
75.
Pierrick Fevrier Philippe Gugan Florent Yvergnaux Jean Pierre Callegari Laurent Dufoss Adrien Binet 《Journal of Molecular Catalysis .B, Enzymatic》2001,11(4-6):445-453
Preliminary investigations on the regioselectiviy of various lipases were performed. Ten commercial lipases from different origins, including three immobilized lipases, were tested by esterification reaction between caprylic acid and propyl or isopropyl alcohol in n-hexane. Reaction products were analyzed with a gas chromatograph. Best yields were obtained with immobilized lipase IM60 from Rhizomucor miehei. Therefore, this enzyme was chosen as biocatalyst for a second step of regioselectiviy study with propylene glycol which bears primary and secondary alcohol groups. It was shown, by using several solvents, that polarity could influence the product profile in situations in which multiple products of various polarities can be formed. Furthermore, the major role of silica gel in reaction mixture was established. 相似文献
76.
77.
78.
Michel Roberge Adrien R. Beaudoin 《Biochimica et Biophysica Acta (BBA)/General Subjects》1982,716(3):331-336
The pancreatic secretion of anesthetized pigs was collected by cannulation after pulse labeling with [3H]leucine. Collection at 5 min intervals started immediately post-pulse labeling up to 85 min. The volume, the protein content and the trichloroacetic acid-precipitable radioactivity of the juice were measued. The specific radioactivity of the secertory proteins was compared to that of a zymogen granule fraction isolated from the same animal. The latter was very much higher. Caerulein stimulation for 5 min at 80 min post-pulse caused a sharp drop in the specific activity of secretory proteins in the juice, to a level lower than that of the zymogen granule content. These data support the concept of more than one pool of secretory proteins in the pancreas and are incompatible with the concept that secretory proteins derive from an homogeneous granule compartment in a functionally homogeneous population of cells. To explain our results the hypothesis of a second intracellular route for the secretory proteins is proposed. 相似文献
79.
Hélène Rangé Christine Poitou Adrien Boillot Cécile Ciangura Sandrine Katsahian Jean-Marc Lacorte Sébastien Czernichow Olivier Meilhac Philippe Bouchard Catherine Chaussain 《PloS one》2013,8(3)
Epidemiological data indicate an association between periodontitis and obesity. The biological mechanisms of this relationship remain unclear. A cross-sectional study was conducted to evaluate the relationship between periodontitis and the common systemic inflammatory markers in 32 morbidly obese patients recruited in a Clinical Nutrition department. Periodontal condition was evaluated using pocket depth (PD) measurement, a classical clinical marker of ongoing periodontitis. Major periodontal risk factors were recorded (age, gender, diabetes and smoking status), as well as plasma levels of inflammatory markers (CRP, orosomucoid, IL-6) and adipokines (adiponectin, leptin). All patients included in the sample exhibited evidence of periodontitis, 16 of whom were diagnosed as having severe disease. Adjusted logistic regression analysis indicated that the severity of periodontitis was associated with the plasma level of orosomucoid (p<0.04) after adjustment for age, gender and smoking. Our study thus suggests that the severity of periodontitis, in morbidly obese patients, is associated with the increase of orosomucoid levels. 相似文献
80.
Formal models have linked prehistoric and historical instances of technological change (e.g., the Upper Paleolithic transition, cultural loss in Holocene Tasmania, scientific progress since the late nineteenth century) to demographic change. According to these models, cumulation of technological complexity is inhibited by decreasing— while favoured by increasing—population levels. Here we show that these findings are contingent on how complexity is defined: demography plays a much more limited role in sustaining cumulative culture in case formal models deploy Herbert Simon''s definition of complexity rather than the particular definitions of complexity hitherto assumed. Given that currently available empirical evidence doesn''t afford discriminating proper from improper definitions of complexity, our robustness analyses put into question the force of recent demographic explanations of particular episodes of cultural change. 相似文献