全文获取类型
收费全文 | 719篇 |
免费 | 63篇 |
出版年
2022年 | 4篇 |
2021年 | 5篇 |
2020年 | 8篇 |
2019年 | 7篇 |
2018年 | 18篇 |
2017年 | 7篇 |
2016年 | 13篇 |
2015年 | 30篇 |
2014年 | 32篇 |
2013年 | 32篇 |
2012年 | 48篇 |
2011年 | 48篇 |
2010年 | 34篇 |
2009年 | 33篇 |
2008年 | 36篇 |
2007年 | 34篇 |
2006年 | 36篇 |
2005年 | 26篇 |
2004年 | 40篇 |
2003年 | 45篇 |
2002年 | 28篇 |
2001年 | 15篇 |
2000年 | 7篇 |
1999年 | 12篇 |
1998年 | 23篇 |
1997年 | 6篇 |
1996年 | 12篇 |
1995年 | 7篇 |
1994年 | 5篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1991年 | 7篇 |
1990年 | 9篇 |
1989年 | 4篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 6篇 |
1984年 | 12篇 |
1983年 | 7篇 |
1982年 | 10篇 |
1981年 | 8篇 |
1980年 | 3篇 |
1979年 | 8篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1973年 | 4篇 |
1972年 | 2篇 |
1956年 | 1篇 |
排序方式: 共有782条查询结果,搜索用时 26 毫秒
141.
Allocation of resources within mountain birch canopy after simulated winter browsing 总被引:3,自引:0,他引:3
As a response to browsing, birches are known to produce fewer but larger, more nutritious leaves, with enhanced palatability for herbivores. We simulated winter browsing in ramets of mountain birch ( Betula pubescens ssp. czerepanovii ) to find out whether it decreases subsequent foliage biomass and alters the number and type of shoots. After removal of a considerable proportion of buds (up to 35%) in late winter, the birches were able to compensate for the lost leaf biomass in the following summer; there were no differences in total leaf biomass between winter-clipped and control ramets. This indicates that foliage growth was limited by the total amount of stored resources, not by the number of buds. Depending on the position of the buds removed, different mechanisms were responsible for the compensation. After removal of apical buds, the number of leaves decreased significantly but leaves were larger than in control ramets. Removal of the same mass of basal buds – containing similar amount of carbohydrates and proteins as in the treatment removing apical buds – activated dormant buds, especially in apical locations, so that leaf number was similar as in the controls; consequently, size of individual leaves increased only slightly. Thus, while the total leaf biomass in a tree seems to be limited by resources from source organs, the distribution of resources among different canopy sections is controlled by their relative sink strengths. In terms of leaf biomass, apical parts are able to compensate for bud loss by increasing shoot number, basal parts only by increasing leaf size. 相似文献
142.
Sanni Saarinen Heta Moustgaard Hanna Remes Riikka Sallinen Pekka Martikainen 《PLoS medicine》2022,19(8)
BackgroundAlthough intrahousehold transmission is a key source of Coronavirus Disease 2019 (COVID-19) infections, studies to date have not analysed socioeconomic risk factors on the household level or household clustering of severe COVID-19. We quantify household income differences and household clustering of COVID-19 incidence and severity.Methods and findingsWe used register-based cohort data with individual-level linkage across various administrative registers for the total Finnish population living in working-age private households (N = 4,315,342). Incident COVID-19 cases (N = 38,467) were identified from the National Infectious Diseases Register from 1 July 2020 to 22 February 2021. Severe cases (N = 625) were defined as having at least 3 consecutive days of inpatient care with a COVID-19 diagnosis and identified from the Care Register for Health Care between 1 July 2020 and 31 December 2020. We used 2-level logistic regression with individuals nested within households to estimate COVID-19 incidence and case severity among those infected.Adjusted for age, sex, and regional characteristics, the incidence of COVID-19 was higher (odds ratio [OR] 1.67, 95% CI 1.58 to 1.77, p < 0.001, 28.4% of infections) among individuals in the lowest household income quintile than among those in the highest quintile (18.9%). The difference attenuated (OR 1.23, 1.16 to 1.30, p < 0.001) when controlling for foreign background but not when controlling for other household-level risk factors. In fact, we found a clear income gradient in incidence only among people with foreign background but none among those with native background. The odds of severe illness among those infected were also higher in the lowest income quintile (OR 1.97, 1.52 to 2.56, p < 0.001, 28.0% versus 21.6% in the highest quintile), but this difference was fully attenuated (OR 1.08, 0.77 to 1.52, p = 0.64) when controlling for other individual-level risk factors—comorbidities, occupational status, and foreign background. Both incidence and severity were strongly clustered within households: Around 77% of the variation in incidence and 20% in severity were attributable to differences between households. The main limitation of our study was that the test uptake for COVID-19 may have differed between population subgroups.ConclusionsLow household income appears to be a strong risk factor for both COVID-19 incidence and case severity, but the income differences are largely driven by having foreign background. The strong household clustering of incidence and severity highlights the importance of household context in the prevention and mitigation of COVID-19 outcomes.Sanni Saarinen and colleagues explore the association between income differences and COVID-19 incidence and severity among people with foreign and native background in Finland. 相似文献
143.
Latitudinal gradients in species diversity are well established for a number of plant and animal taxa. Both historical and present-day environmental factors have been suggested to be responsible for observed patterns. We tested the hypothesis that current environmental features of the environment (primary productivity and regional landscape structure) may explain the longitudinal variation in species richness of mammalian herbivores in the Holarctic boreal zone. Mammalian herbivore species diversity was strongly correlated with a number of environmental variables measured. We reduced the data set by a principal components analysis (PCA), and found that in the Palearctic, species richness is positively related to warm climate (high temperature sum), the number of hardwood species, and the area of boreal forest. In the Nearctic, species richness increases as the length of the growing season and the number of coniferous tree species increase. Thus indirect measures of primary productivity as well as tree species number may accurately predict species richness in mammalian herbivores. In addition, there seems to be a strong species-area effect at the regional level. The larger and more homogeneous in terms of forest coverage a boreal section is, the more species coexist there. 相似文献
144.
Jussi Jyväsjärvi Hannu Marttila Pekka M. Rossi Pertti Ala‐Aho Bo Olofsson Jakob Nisell Birgitta Backman Jari Ilmonen Risto Virtanen Lauri Paasivirta Ritva Britschgi Bjørn Kløve Timo Muotka 《Global Change Biology》2015,21(12):4561-4569
Interest in climate change effects on groundwater has increased dramatically during the last decade. The mechanisms of climate‐related groundwater depletion have been thoroughly reviewed, but the influence of global warming on groundwater‐dependent ecosystems (GDEs) remains poorly known. Here we report long‐term water temperature trends in 66 northern European cold‐water springs. A vast majority of the springs (82%) exhibited a significant increase in water temperature during 1968–2012. Mean spring water temperatures were closely related to regional air temperature and global radiative forcing of the corresponding year. Based on three alternative climate scenarios representing low (RCP2.6), intermediate (RCP6) and high‐emission scenarios (RCP8.5), we estimate that increase in mean spring water temperature in the region is likely to range from 0.67 °C (RCP2.6) to 5.94 °C (RCP8.5) by 2086. According to the worst‐case scenario, water temperature of these originally cold‐water ecosystems (regional mean in the late 1970s: 4.7 °C) may exceed 12 °C by the end of this century. We used bryophyte and macroinvertebrate species data from Finnish springs and spring‐fed streams to assess ecological impacts of the predicted warming. An increase in spring water temperature by several degrees will likely have substantial biodiversity impacts, causing regional extinction of native, cold‐stenothermal spring specialists, whereas species diversity of headwater generalists is likely to increase. Even a slight (by 1 °C) increase in water temperature may eliminate endemic spring species, thus altering bryophyte and macroinvertebrate assemblages of spring‐fed streams. Climate change‐induced warming of northern regions may thus alter species composition of the spring biota and cause regional homogenization of biodiversity in headwater ecosystems. 相似文献
145.
Savijoki K Lietzén N Kankainen M Alatossava T Koskenniemi K Varmanen P Nyman TA 《Journal of proteome research》2011,10(8):3460-3473
The present study reports an in-depth proteome analysis of two Lactobacillus rhamnosus strains, the well-known probiotic strain GG and the dairy strain Lc705. We used GeLC-MS/MS, in which proteins are separated using 1-DE and identified using nanoLC-MS/MS, to generate high-quality protein catalogs. To maximize the number of identifications, all data sets were searched against the target databases using two search engines, Mascot and Paragon. As a result, over 1600 high-confidence protein identifications, covering nearly 60% of the predicted proteomes, were obtained from each strain. This approach enabled identification of more than 40% of all predicted surfome proteins, including a high number of lipoproteins, integral membrane proteins, peptidoglycan associated proteins, and proteins predicted to be released into the extracellular environment. A comparison of both data sets revealed the expression of more than 90 proteins in GG and 150 in Lc705, which lack evolutionary counterparts in the other strain. Differences were noted in proteins with a likely role in biofilm formation, phage-related functions, reshaping the bacterial cell wall, and immunomodulation. The present study provides the most comprehensive catalog of the Lactobacillus proteins to date and holds great promise for the discovery of novel probiotic effector molecules. 相似文献
146.
Summary The lipid levels and fatty acid composition of three fractions (free fatty acids, triacylglycerols and sterol/triterpenoid esters) extracted from the sapwood and heartwood of three stems of Pinus sylvestris were determined to investigate both seasonal changes in sapwood and possible metabolic changes related to heartwood formation. Seasonal changes were observed only in the amount of the free fatty acids in the sapwood: the level of free fatty acids was greatest at the beginning and end of the growing season. In the January and March samples the amount of the free fatty acid fraction in the sapwood was very small. The amount of the other fractions remained at the same level throughout the study. Marked seasonal changes in the fatty acid composition occurred only in the free fatty acid fraction of the sapwood: the saturation grade increased during the winter. 相似文献
147.
Separating water‐potential induced swelling and shrinking from measured radial stem variations reveals a cambial growth and osmotic concentration signal
下载免费PDF全文
![点击此处可从《Plant, cell & environment》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Tommy Chan Teemu Hölttä Frank Berninger Harri Mäkinen Pekka Nöjd Maurizio Mencuccini Eero Nikinmaa 《Plant, cell & environment》2016,39(2):233-244
The quantification of cambial growth over short time periods has been hampered by problems to discern between growth and the swelling and shrinking of a tree stem. This paper presents a model, which separates cambial growth and reversible water‐potential induced diurnal changes from simultaneously measured whole stem and xylem radial variations, from field‐measured Scots pine trees in Finland. The modelled growth, which includes osmotic concentration changes, was compared with (direct) dendrometer measurements and microcore samples. In addition, the relationship of modelled growth and dendrometer measurements to environmental factors was analysed. The results showed that the water‐potential induced changes of tree radius were successfully separated from stem growth. Daily growth predicted by the model exhibited a high correlation with the modelled daily changes of osmotic concentration in phloem, and a temperature dependency in early summer. Late‐summer growth saw higher dependency on water availability and temperature. Evaluation of the model against dendrometer measurements showed that the latter masked a true environmental signal in stem growth due to water‐potential induced changes. The model provides better understanding of radial growth physiology and offers potential to examine growth dynamics and changes due to osmotic concentration, and how the environment affects growth. 相似文献
148.
Jorma Palvimo Anitta Mahonen Pekka H. Menp 《Biochimica et Biophysica Acta (BBA)/Molecular Cell Research》1987,931(3)
Chromosomal high-mobility-group (HMG) proteins have been examined as substrates for calcium/phospholipid-dependent protein kinase C. Protein kinase C from rat brain phosphorylated efficiently both HMG 14 and HMG 17 derived from calf thymus and the reactions were calcium/phospholipid-dependent. About 1 mol of 32P was incorporated per mol of HMG 14 and HMG 17. Phosphopeptide mapping suggested that the same major site was phosphorylated in both proteins at serine. The apparent Km values for HMG 14 and HMG 17 were about 5 μM. HMG 14, HMG 17 and the five histone H1 subtypes prepared from rat thymus, liver and spleen were phosphorylated by the kinase. HMG 14 and HMG 17 from transformed human lymphoblasts (Wi-L2) were also phosphorylated in a calcium/phospholipid-dependent manner. HMG 1 and HMG 2 from the tissues examined were found to be poor substrates for the kinase. 相似文献
149.
Living biomass contains 45 to 60% carbon and 0.05 to 3% nitrogen, in dry weight. Like throughout Europe, the amount of living biomass in Finnish forests has increased on average over the last decades, largely because of changes in forest management. The storage of organic C and N in biomass has also increased.Changes in biomass vary between regions. Data were analysed on changes in the last 30–40 years in C and N storage in living trees in Finland, subdivided into 20 regions. Tree biomass increased in 17 regions, and decreased in 3 regions. The storage rate varied between -170 and +480 kg C ha-1 a-1, and between –0.5 and +1.2 kg N ha-1 a-1.Nitrogen accumulation in trees was less than 15% of atmospheric N deposition in all regions. Although the eventual increase of the nitrogen concentration in tree tissues was omitted, it is not possible that living biomass has been the major sink for atmospheric N deposition to forests. A hypothesis is presented that the main sink is litter layer and organic soil. Carbon can also be accumulating in soils essentially faster than hitherto estimated in analyses of carbon budgets of European forests.Died on September 2, 1994. 相似文献
150.
Pekka Varmanen Erkki Vesanto James L. Steele Airi Palva 《FEMS microbiology letters》1994,124(3):315-320
Abstract The putative ribosome binding sites preceding 32 of Lactobacillus delbrueckii subsp. lactis bacteriophage LL-H genes were compared. A highly conserved consensus sequence for the ribosome binding sites of LL-H genes was inferred, GAAAGGAG. This study included the characterization of the last nucleotides of the 3'-end of the 16S rRNA molecule from L. delbrueckii subsp. lactis and its comparison to the ribosome binding site consensus sequence. 相似文献