全文获取类型
收费全文 | 435篇 |
免费 | 40篇 |
国内免费 | 18篇 |
专业分类
493篇 |
出版年
2023年 | 5篇 |
2022年 | 3篇 |
2021年 | 5篇 |
2020年 | 9篇 |
2019年 | 5篇 |
2018年 | 3篇 |
2017年 | 8篇 |
2016年 | 7篇 |
2015年 | 16篇 |
2014年 | 18篇 |
2013年 | 18篇 |
2012年 | 24篇 |
2011年 | 21篇 |
2010年 | 20篇 |
2009年 | 8篇 |
2008年 | 19篇 |
2007年 | 22篇 |
2006年 | 20篇 |
2005年 | 12篇 |
2004年 | 19篇 |
2003年 | 11篇 |
2002年 | 10篇 |
2001年 | 17篇 |
2000年 | 10篇 |
1999年 | 11篇 |
1998年 | 13篇 |
1997年 | 7篇 |
1996年 | 6篇 |
1995年 | 5篇 |
1993年 | 6篇 |
1992年 | 6篇 |
1991年 | 5篇 |
1990年 | 7篇 |
1989年 | 6篇 |
1988年 | 9篇 |
1987年 | 8篇 |
1986年 | 11篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1980年 | 7篇 |
1979年 | 8篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1975年 | 5篇 |
1973年 | 5篇 |
1972年 | 7篇 |
1970年 | 3篇 |
1931年 | 2篇 |
排序方式: 共有493条查询结果,搜索用时 15 毫秒
11.
热休克蛋白gp96是热休克蛋白90家族成员,能够引起非特异性和特异性免疫反应。得到大量高纯度的蛋白质是研究开发gp96的关键。然而重组的gp96容易在E.coli中降解,并在一定条件下形成多聚体。实验先将人gp96基因克隆到pET-30a载体上并在E.coli Blstar中表达,再经过亲和层析、阴离子交换、分子筛分别纯化gp96。最终去掉了大部分的降解片段和多聚体,得到一定量的可溶性gp96,为进一步研究其结构和功能打下一定的基础。 相似文献
12.
Effects of cooling water discharge on the structure and dynamics of epilithic algal communities in the northern Baltic 总被引:1,自引:0,他引:1
The Forsmark Biotest Basin is a shallow coastal ecosystem that receives brackish cooling-water discharge from a nuclear power
plant. The effects of the discharge on epilithic algal communities were investigated by analysing samples taken every third
week throughout one year at 11 sites differentially affected by temperature and/or flow rate enhancement. Community variation
was summarized in a canonical correspondence analysis (CCA) of species abundances as a function of site and date. The temperature
increase favoured blue-green algae at the expense of red and brown algae. Blue-green algae were however abundant in summer
in stagnant water, whether heated or not, and some red and brown algae became abundant in winter in heated sites with flowing
water. Green algae and diatoms increased in biomass in the heated sites, but not in relative cover-abundance. The absence
of ice and snow cover at sites with heated and/or flowing water caused autumn species to persist into winter, because of the
higher light intensity (compared with natural conditions) and the absence of the mechanical abrasion by ice. The thermal discharge
lowered species diversity (Shannon-Weaver index) both in summer and winter at sites with flowing water, but not at sites with
quiescent or stagnant water. CCA showed alternate periods of stability and rapid change within the seasonal cycle. Individual
species were placed according to their optimum; red and brown algae in winter/spring, green algae in spring/summer, blue-green
algae in summer, and diatoms at various times. Exceptions to this pattern were species endo- or epiphytic on species of a
different group. Analysis of the effects of temperature, flow rate and ice cover on the seasonal pattern of particular species
showed that different species respond in individualistic ways to different combinations of these environmental variables. 相似文献
13.
14.
The cost of managing 114 adult haemophiliacs in the west of Scotland was assessed for the period 1 March 1971 to 28 February 1974. Altogether 23 of them (20%) accounted for 80% of the resources used. The cost of hospital treatment of these patients during the period was compared with the predicted cost of home treatment, given the availability of freeze-dried factor VIII concentrate in sufficient amounts. We calculate that adequate on-demand home treatment would cost only 16% more than the present treatment, which is substantially less efficient. 相似文献
15.
Alessio Collalti Mark G. Tjoelker Günter Hoch Annikki Mkel Gabriele Guidolotti Mary Heskel Giai Petit Michael G. Ryan Giovanna Battipaglia Giorgio Matteucci Iain Colin Prentice 《Global Change Biology》2020,26(3):1739-1753
Two simplifying hypotheses have been proposed for whole‐plant respiration. One links respiration to photosynthesis; the other to biomass. Using a first‐principles carbon balance model with a prescribed live woody biomass turnover, applied at a forest research site where multidecadal measurements are available for comparison, we show that if turnover is fast the accumulation of respiring biomass is low and respiration depends primarily on photosynthesis; while if turnover is slow the accumulation of respiring biomass is high and respiration depends primarily on biomass. But the first scenario is inconsistent with evidence for substantial carry‐over of fixed carbon between years, while the second implies far too great an increase in respiration during stand development—leading to depleted carbohydrate reserves and an unrealistically high mortality risk. These two mutually incompatible hypotheses are thus both incorrect. Respiration is not linearly related either to photosynthesis or to biomass, but it is more strongly controlled by recent photosynthates (and reserve availability) than by total biomass. 相似文献
16.
Catherine Morfopoulos Iain C. Prentice Trevor F. Keenan Pierre Friedlingstein Belinda E. Medlyn Josep Pe?uelas Malcolm Possell 《Annals of botany》2013,112(7):1223-1238
Background and Aims
Isoprene is the most important volatile organic compound emitted by land plants in terms of abundance and environmental effects. Controls on isoprene emission rates include light, temperature, water supply and CO2 concentration. A need to quantify these controls has long been recognized. There are already models that give realistic results, but they are complex, highly empirical and require separate responses to different drivers. This study sets out to find a simpler, unifying principle.Methods
A simple model is presented based on the idea of balancing demands for reducing power (derived from photosynthetic electron transport) in primary metabolism versus the secondary pathway that leads to the synthesis of isoprene. This model''s ability to account for key features in a variety of experimental data sets is assessed.Key results
The model simultaneously predicts the fundamental responses observed in short-term experiments, namely: (1) the decoupling between carbon assimilation and isoprene emission; (2) a continued increase in isoprene emission with photosynthetically active radiation (PAR) at high PAR, after carbon assimilation has saturated; (3) a maximum of isoprene emission at low internal CO2 concentration (ci) and an asymptotic decline thereafter with increasing ci; (4) maintenance of high isoprene emissions when carbon assimilation is restricted by drought; and (5) a temperature optimum higher than that of photosynthesis, but lower than that of isoprene synthase activity.Conclusions
A simple model was used to test the hypothesis that reducing power available to the synthesis pathway for isoprene varies according to the extent to which the needs of carbon assimilation are satisfied. Despite its simplicity the model explains much in terms of the observed response of isoprene to external drivers as well as the observed decoupling between carbon assimilation and isoprene emission. The concept has the potential to improve global-scale modelling of vegetation isoprene emission. 相似文献17.
Uxmal and Tulum are two important Mayan sites in the Yucatan peninsula. The buildings are mainly composed of limestone and grey/black discoloration is seen on exposed walls and copious greenish biofilms on inner walls. The principal microorganisms detected on interior walls at both Uxmal and Tulum were cyanobacteria; heterotrophic bacteria and filamentous fungi were also present. A dark‐pigmented mitosporic fungus and Bacillus cereus, both isolated from Uxmal, were shown to be acidogenic in laboratory cultures. Cyanobacteria belonging to rock‐degrading genera Synechocystis and Gloeocapsa were identified at both sites. Surface analysis previously showed that calcium ions were present in the biofilms on buildings at Uxmal and Tulum, suggesting the deposition of biosolubilized stone. Apart from their potential to degrade the substrate, the coccoid cyanobacteria supply organic nutrients for bacteria and fungi, which can produce organic acids, further increasing stone degradation. 相似文献
18.
The morphology of the teliospores of Puccinia smyrnii has been variously described as warted, or reticulate, or a combination of both patterns. Spores were examined by LM and SEM, and shown to be irregularly warted. The sequence of development of the spores was examined by TEM. Four phases of wall differentiation were recognised. The ornamentation results from a differential deposition of secondary wall components, which are concentrated into invaginations of the cytoplasm. The subsequent exsertion of these invaginations, and concomitant disappearance of the primary wall, reveal the irregular warts of the mature spore. This mode of ornament formation is compared with other rust spore forms, and contrasted with that already outlined for Puccinia chaerophylli, a truly reticulatespored Umbelliferous rust. Combined SEM and TEM observations suggest an explanation for the erroneous LM interpretations. 相似文献
19.
Arco J. van Strien Tim Termaat Vincent Kalkman Marijn Prins Geert De Knijf Anne-Laure Gourmand Xavier Houard Brian Nelson Calijn Plate Stephen Prentice Eugenie Regan David Smallshire Cédric Vanappelghem Wouter Vanreusel 《Biodiversity and Conservation》2013,22(3):673-686
There is limited information available on changes in biodiversity at the European scale, because there is a lack of data from standardised monitoring for most species groups. However, a great number of observations made without a standardised field protocol is available in many countries for many species. Such opportunistic data offer an alternative source of information, but unfortunately such data suffer from non-standardised observation effort and geographical bias. Here we describe a new approach to compiling supranational trends using opportunistic data which adjusts for these two major imperfections. The non-standardised observation effort is dealt with by occupancy modelling, and the unequal geographical distribution of sites by a weighting procedure. The damselfly Calopteryx splendens was chosen as our test species. The data were collected from five countries (Ireland, Great Britain, the Netherlands, Belgium and France), covering the period 1990–2008. We used occupancy models to estimate the annual number of occupied 1 × 1 km sites per country. Occupancy models use presence-absence data, account for imperfect detection of species, and thereby correct for between-year variability in observation effort. The occupancy models were run per country in a Bayesian mode of inference using JAGS. The occupancy estimates per country were then aggregated to assess the supranational trend in the number of occupied 1 × 1 km2. To adjust for the unequal geographical distribution of surveyed sites, we weighted the countries according to the number of sites surveyed and the range of the species per country. The distribution of C. splendens has increased significantly in the combined five countries. Our trial demonstrated that a supranational trend in distribution can be derived from opportunistic data, while adjusting for observation effort and geographical bias. This opens new perspectives for international monitoring of biodiversity. 相似文献
20.