全文获取类型
收费全文 | 207篇 |
免费 | 20篇 |
出版年
2022年 | 2篇 |
2020年 | 3篇 |
2019年 | 3篇 |
2018年 | 7篇 |
2017年 | 4篇 |
2016年 | 8篇 |
2015年 | 13篇 |
2014年 | 9篇 |
2013年 | 7篇 |
2012年 | 8篇 |
2011年 | 8篇 |
2010年 | 10篇 |
2009年 | 6篇 |
2008年 | 11篇 |
2007年 | 14篇 |
2006年 | 19篇 |
2005年 | 6篇 |
2004年 | 7篇 |
2003年 | 3篇 |
2002年 | 11篇 |
2000年 | 3篇 |
1999年 | 8篇 |
1998年 | 7篇 |
1997年 | 2篇 |
1996年 | 1篇 |
1995年 | 4篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1990年 | 1篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1972年 | 2篇 |
1971年 | 2篇 |
1970年 | 1篇 |
1969年 | 2篇 |
1965年 | 1篇 |
1963年 | 1篇 |
1947年 | 1篇 |
排序方式: 共有227条查询结果,搜索用时 15 毫秒
1.
In this work, incorporation of plasmid DNA, pre-complexed with PEI, into polyelectrolyte multilayers has been studied to further develop platforms for local gene delivery. Polyplex embedding in synthetic and naturally degradable architectures was efficient for transfection of human hepato-cellular carcinoma cells. 相似文献
2.
Cytokinesis in the coenocytic green alga Protosiphon botryoides (Kütz.) Klebs was studied with transmission electron microscopy. In vegetative cells, nuclei with associated basal bodies and dictyosomes are scattered throughout the cytoplasm. Mature cells may develop either multinucleate resting spores (coenocysts) or uninucleate zoospores. Cytokinesis may be preceded by contraction of the protoplast due to the disintegration of vacuoles that are present in larger, siphonous cells. The formation of coenocysts in ageing, siphonous cells, is signalled by cleavage of the chloroplast and the development of arrays of phycoplast microtubules in one or more transversely oriented planes through the cell. Nuclei with associated basal apparatuses stay dispersed throughout the cytoplasm; the basal bodies apparently are not involved in organization of the phycoplast. The plasma membrane invaginates, resulting in a centripetal cleavage of the protoplast into two or more multinucleate daughter protoplasts. Simultaneously, wall material is deposited along the outside of the daughter protoplasts by dictyosome-derived vesicles, and finally two or more thick-walled coenocysts are formed. The formation of zoospores, on the other hand, is signalled by clustering of the nuclei in one or more groups depending on the shape of the mother cell. The nuclei become arranged with the associated basal apparatuses facing toward the center of the cluster. Bundles of phycoplast microtubules develop between the nuclei, radiating from the center of a cluster toward the plasma membrane; basal apparatuses or associated structures apparently are involved in organization of the phycoplast. Cleavage furrows grow out centrifugally along these bundles of micro-tubules, fed by dictyosome-derived vesicles. No wall material is deposited. An additional mitotic division occurs during cleavage, and finally numerous uninucleate, wall-less, biflagellate zoospores are formed. The ultrastructural features of the two different types of cytoplasmic cleavage associated with two different types of daughter cells have not previously been reported for chlorophycean algae. 相似文献
3.
4.
5.
The impact of hydrological manipulation of an unfertilised, Dutch peat grassland area on plant species composition on ditch banks and in ditchwater was studied. The hydrological manipulation involved raising the groundwater level by admitting nutrient-poor, Ca-rich groundwater in one compartment, and by retaining precipitation in another compartment. A third compartment served as control.The plant species composition showed significant correlations with the following hydrochemical parameters: in bank vegetation with K+ concentration, and winter and summer groundwater levels, water depth and elevation; in aquatic vegetation with pH, the concentrations of Cl- organic-C and NH4
+ water temperature and elevation.The number of terrestrial plant species increased after compartmentation from 97 to 122; 16 submerged and floating species were found. Most new terrestrial species probably emerged in response to cessation of fertilization and biomass removal, since they showed no preference for any compartment. Five species showed preference for the groundwater compartment and two for the rainwater compartment. Of the new terrestrial plants, seven were relatively rare: Carex panicea, C. oederi, C. pallescens, C. vesicaria, Galium uliginosum, Juncus acutiflorus and Stellaria uliginosa. Of the aquatic plants, eight were relatively rare: Chara globularis, two Callitriche spp., four potamogetonaceae and Ranunculus circinatus.This study indicates that hydrological manipulation of grassland systems in which fertilisation has ceased has profound effects on the vegetation in the ditches of these systems. Bank vegetation responds more slowly and to other hydrochemical factors than aquatic vegetation. Short-term responses in terms of increase in diversity of vegetation pattern and species richness are promising. Long-term responses are not yet known. 相似文献
6.
Nielsen J; Peixoto AA; Piccin A; Costa R; Kyriacou CP; Chalmers D 《Molecular biology and evolution》1994,11(6):839-853
The region of the clock gene period (per) that encodes a repetitive tract
of threonine-glycine (Thr-Gly) pairs has been compared between Dipteran
species both within and outside the Drosophilidae. All the non-
Drosophilidae sequences in this region are short and present a remarkably
stable picture compared to the Drosophilidae, in which the region is much
larger and extremely variable, both in size and composition. The
accelerated evolution in the repetitive region of the Drosophilidae appears
to be mainly due to an expansion of two ancestral repeats, one encoding a
Thr-Gly dipeptide and the other a pentapeptide rich in serine, glycine, and
asparagine or threonine. In some drosophilids the expansion involves a
duplication of the pentapeptide sequence, but in Drosophila pseudoobscura
both the dipeptide and the pentapeptide repeats are present in larger
numbers. In the nondrosophilids, however, the pentapeptide sequence is
represented by one copy and the dipeptide by two copies. These observations
fulfill some of the predictions of recent theoretical models that have
simulated the evolution of repetitive sequences.
相似文献
7.
Element budgets of a heavily damaged Norway spruce ecosystem at Hohe Matzen in the Fichtel Mountains/FRG were studied over 3 years. The trees show severe symptoms of decline and Mg deficiency. The soil is a typic Dystrochrept derived from granite with sandy texture, high stone content, and low base saturation. The budgets show high releases of N, S and Al from the ecosystem as a result of input, buffering and turnover processes. After an increase of proton fluxes in the organic surface layer, a strong reduction of protons in the B horizon was found. This process was accompanied by the release of Al, whereas reactive Al(OH)3 was exhausted in the A horizon. The low ANC is also shown in pH-stat.-titrations. The data indicate a strong mineralisation in the humus layer, which results in a net release of NH4, SO4 and TOC. Nitrification takes place mainly in the A horizon. With respect to the N-budget, the ecosystem is approaching the state of N saturation. The processes of N turnover lead to an internal proton production exceeding the atmospheric input, and thus contributing to soil acidification. 相似文献
8.
Main objective of this study was to test the effects of Mg(OH)2-fertilization in a Norway spruce ecosystem showing severe symptoms of Mg-deficiency.The site is characterized by high atmospheric inputs with deposition rates of 1.25 kg H, 42 kg S, and 32 kg N per ha and year. The typic Dystrochrept derived from granite is acidified down to greater depths. The pH-values in soil solution of the organic surface layer and the upper mineral soil are around 3.5. Concentrations of Al, SO4
2-, and especially NO3
- and DOC are very high. The element balance indicates a significant influence of N-inputs and processes of N-turnover on the chemical status of the soil and probably on tree nutrition. Nitrification in the upper mineral soil leads to a transformation of a major part of NH4
+ into NO3
-, which is quantitatively leached, resulting in an ecosystem-internal H+-production of 1.8 keq ha-1yr-1. NO3
- and SO4
2- govern the seepage output from the ecosystem.Mg(OH)2 fertilization resulted in manifold increased Mg2+ concentrations in soil solution down to 70 cm soil depth and to a significant increase of pH down to 25 cm mineral soil depth. Nitrate concentrations were elevated after fertilization, but decreased within 15 months below the level of the control plot. As a mean over the whole experimental period, N-output was not increased by fertilization. Despite an elevated internal proton production due to nitrification, acid buffering in the soil was clearly increased, but enhanced Al-mobilization was not observed. Mg/Al- and Ca/H-ratios in soil solution indicate much more favourable conditions for fine root growth. Fertilization also increased the amount of exchangeable Mg down to 40cm mineral soil depth. Mg contents in current-year needles increased after three vegetation periods. Thirty months after application, only 10% and 4% of the fertilized Mg had left the organic surface layer and the mineral soil with seepage water output, respectively. 相似文献
9.
10.
Stefano Potter Kylen Solvik Angela Erb Scott J. Goetz Jill F. Johnstone Michelle C. Mack James T. Randerson Miguel O. Romn Crystal L. Schaaf Merritt R. Turetsky Sander Veraverbeke Xanthe J. Walker Zhuosen Wang Richard Massey Brendan M. Rogers 《Global Change Biology》2020,26(3):1592-1607
Fire is a primary disturbance in boreal forests and generates both positive and negative climate forcings. The influence of fire on surface albedo is a predominantly negative forcing in boreal forests, and one of the strongest overall, due to increased snow exposure in the winter and spring months. Albedo forcings are spatially and temporally heterogeneous and depend on a variety of factors related to soils, topography, climate, land cover/vegetation type, successional dynamics, time since fire, season, and fire severity. However, how these variables interact to influence albedo is not well understood, and quantifying these relationships and predicting postfire albedo becomes increasingly important as the climate changes and management frameworks evolve to consider climate impacts. Here we developed a MODIS‐derived ‘blue sky’ albedo product and a novel machine learning modeling framework to predict fire‐driven changes in albedo under historical and future climate scenarios across boreal North America. Converted to radiative forcing (RF), we estimated that fires generate an annual mean cooling of ?1.77 ± 1.35 W/m2 from albedo under historical climate conditions (1971–2000) integrated over 70 years postfire. Increasing postfire albedo along a south–north climatic gradient was offset by a nearly opposite gradient in solar insolation, such that large‐scale spatial patterns in RF were minimal. Our models suggest that climate change will lead to decreases in mean annual postfire albedo, and hence a decreasing strength of the negative RF, a trend dominated by decreased snow cover in spring months. Considering the range of future climate scenarios and model uncertainties, we estimate that for fires burning in the current era (2016) the cooling effect from long‐term postfire albedo will be reduced by 15%–28% due to climate change. 相似文献