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排序方式: 共有194条查询结果,搜索用时 15 毫秒
81.
海南岛西南地区某些药用植物微量元素的生物吸收和含量变化 总被引:2,自引:0,他引:2
本文研究海南岛西南地区12种不同类型药用植物微量元素的生物吸收系数(Ax)及其含量水平变化特性。 相似文献
82.
Distribution and conservation of mobile elements in the genus Drosophila 总被引:13,自引:1,他引:12
Essentially nothing is known of the origin, mode of transmission, and
evolution of mobile elements within the genus Drosophila. To better
understand the evolutionary history of these mobile elements, we examined
the distribution and conservation of homologues to the P, I, gypsy, copia,
and F elements in 34 Drosophila species from three subgenera. Probes
specific for each element were prepared from D. melanogaster and hybridized
to genomic DNA. Filters were washed under conditions of increasing
stringency to estimate the similarity between D. melanogaster sequences and
their homologues in other species. The I element homologues show the most
limited distribution of all elements tested, being restricted to the
melanogaster species group. The P elements are found in many members of the
subgenus Sophophora but, with the notable exception of D. nasuta, are not
found in the other two subgenera. Copia-, gypsy-, and F-element homologues
are widespread in the genus, but their similarity to the D. melanogaster
probe differs markedly between species. The distribution of copia and P
elements and the conservation of the gypsy and P elements is inconsistent
with a model that postulates a single ancient origin for each type of
element followed by mating-dependent transmission. The data can be
explained by horizontal transmission of mobile elements between
reproductively isolated species.
相似文献
83.
我国“三北”地区由于自然条件的限制,森林资源极为贫乏,而灌丛植被却有着广泛地分布。在复杂的生境类型中,存在着相应多种多样的天然灌丛,从高寒落叶阔叶灌丛到温性落叶阔叶灌丛,象半干旱地区草原沙地的锦鸡儿灌丛、柳灌丛以及干旱地区河岸冲积平原盐渍化草甸土上的柽柳灌丛等都是长期与当地自然条件相适应的近于原生植被,是相当稳定的群 相似文献
84.
Transcapillary CO2 exchange entails a transient perfusate CO2-HCO3(-)-H+ disequilibrium, leading to net loading or unloading of blood HCO3-. Perfusate reequilibration may or may not reach completion during the time of capillary transit, depending on the rate of intracapillary CO2-HCO3(-)-H+ reactions. Failure to reestablish equilibrium within the "open" capillary system leads to continued reequilibration in the "closed" postcapillary vasculature with resultant shifts in postcapillary perfusate PCO2, pH, and [HCO3-]. In the present study, we determined the effects of perfusate nonbicarbonate buffer capacity (beta) on intracapillary CO2-HCO3(-)-H+ reactions in isolated saline-perfused rat lungs. Effects of beta on the rate of transcapillary CO2 excretion (VCO2) and the magnitude of the postcapillary perfusate pH disequilibrium were measured as a function of luminal vascular carbonic anhydrase (CA) activity. The data indicate that beta markedly influenced the kinetics and dynamics of intravascular CO2-HCO3(-)-H+ reactions. beta affected VCO2 and the relative enhancement of VCO2 by luminal vascular CA. The data emphasize the inadequacies of using traditional "equilibrium" models of the CO2-HCO3(-)-H+ system to investigate capillary CO2 transport and exchange, even in organs (e.g., lungs) that contain significant luminal vascular CA activity. 相似文献
85.
The Opa Reservoir, established for water supply to the Obafemi Awolowo University community, over the years has received direct linkage to township drains as a result of the widening and dredging of its river channels. The current study aimed at documenting monthly heavy metal loads at its riverine, transition and lacustrine zones in 2012–2013. Most of the heavy metals, analysed using an atomic absorption spectrophotometer, occurred within wide ranges, with coefficients of variation ranging from 60% to 300%, although the differences in heavy metal loads between the different zones were statistically insignificant at both the surface and bottom levels. The overall order of dominance of the metals was Ni > Cu > Fe > Zn > Cd = Cr > Pb, with nickel concentration being very high throughout. Heavy metal total mean concentration was higher during the rainy season (1.889 mg l?1) than the dry season (1.503 mg l?1) irrespective of sampling sites, having highly significant seasonal differences (p < 0.001) in Ni, Cd, Fe and Cu concentrations. The mean concentrations of Zn (0.074 mg l?1), Fe (0.176 mg l?1), Cu (0.507 mg l?1) and Pb (0.004 mg l?1) were within the WHO acceptable limit. However, the recorded levels of toxic elements Cd (0.031 mg l?1) and Ni (0.905 mg l?1) pose potential health risk to water consumers. 相似文献
86.
N Lübcker J Dabrowski TA Zengeya PJ Oberholster G Hall S Woodborne 《African Journal of Aquatic Science》2016,41(4):399-411
The alien invasive silver carp Hypophthalmichthys molitrix established a self-sustaining feral population in an oligotrophic impoundment, Flag Boshielo Dam, in South Africa. The ability of this population to persist in a dam with low algal biomass (median annual suspended chlorophyll a = 0.08 µg l?1), and limited access to rivers considered large enough for successful spawning, has implications for their invasive potential in other systems. Stomach content and stable isotope analysis were used to assess the trophic ecology of H. molitrix, which was then compared with indigenous Mozambique tilapia Oreochromis mossambicus, on a seasonal basis during 2011. Hypophthalmichthys molitrix are generalist filter feeders, with a diet consisting primarily of sediment, vegetative detritus, dinoflagellates and diatoms. The dominance of sediments in their stomachs suggests occasional benthic scavenging. However, H. molitrix occupied a higher trophic level (TL = 2.8) than expected, suggesting that this population subsidised their diet with an unidentified dietary constituent, characterised by enriched nitrogen values. Although the stomach contents indicated dietary overlap between H. molitrix and O. mossambicus, stable isotopes revealed fine-scale resource partitioning, despite both species occupying the same trophic level. Nonetheless, the persistence of this feral H. molitrix population in an oligotrophic impoundment highlights their phenotypic plasticity. 相似文献
87.
88.
Carolyn TA Herzig Ray W Waters Cynthia L Baldwin Janice C Telfer 《BMC evolutionary biology》2010,10(1):181
Background
The scavenger receptor cysteine rich (SRCR) domain is an ancient and conserved protein domain. CD163 and WC1 molecules are classed together as group B SRCR superfamily members, along with Spα, CD5 and CD6, all of which are expressed by immune system cells. There are three known types of CD163 molecules in mammals, CD163A (M130, coded for by CD163), CD163b (M160, coded for by CD163L1) and CD163c-α (CD163L1 or SCART), while their nearest relative, WC1, is encoded by a multigene family so far identified in the artiodactyl species of cattle, sheep, and pigs. 相似文献89.
Tao Du Jian Wang Heming Wang Xin Tian Qiang Yue Hiroki Tanikawa 《Journal of Industrial Ecology》2020,24(4):923-934
China is the largest producer and consumer of cement worldwide, and cement production entails the release of substantial carbon dioxide (CO2) emissions. As the cement sector is a crucial sector of the Chinese economy, understanding the role of supply‐ and demand‐side factors may help accelerate efforts to mitigate CO2 emissions. However, few studies have analyzed the critical factors affecting CO2 emissions in the sector based on a combined supply‐ and demand‐side perspective. In this study, we developed an integrated framework that included eleven indicators covering both the supply and demand sides. Results revealed that improving cement production technology cannot offset CO2 emissions from the growth in demand for cement. Improving technology on the supply side would considerably reduce CO2 emissions from Chinese cement production; nevertheless, the combination of rapid urbanization, GDP growth, and an ultra‐high fixed capital formation ratio on the demand side increased CO2 emissions nearly 25‐fold from 1990 to 2015. Notably, some demand‐side factors also had an effect that reduced CO2 emissions. The in‐use stock per unit of fixed capital formation and output per in‐use stock reduced CO2 emissions by 332 million metric tons, which is comparable to the contribution of technological progress. Based on these results, we examine why these demand‐side factors substantially influence CO2 emissions in the Chinese cement sector, and we provide recommendations for policy‐makers on carbon‐reduction measures in this CO2‐intensive sector. 相似文献
90.
Yang Heming Li Zhiguo Liu Kai Mao Huibing Song Changsheng Wang Jiqing 《Plasmonics (Norwell, Mass.)》2020,15(6):1967-1975
Plasmonics - A multi-layer structure consisting of metal and dielectric layers which allows coupling between surface plasmon polaritons and waveguide modes is studied by calculating reflectivity... 相似文献