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61.
拐芹根化学成分研究Ⅱ 总被引:3,自引:0,他引:3
从伞型科当归属植物拐芹(Angelica polymorpha Maxim)的根及根茎中又分得4个结晶性化合物。经物理常数测定、光谱分析,分别鉴定为欧前胡素Ⅰ,异氧化前胡内酯Ⅱ,Pabulenol Ⅲ,Phellopterin Ⅳ。 相似文献
62.
Bacterioplankton Dynamics in the McMurdo Dry Valley Lakes, Antarctica: Production and Biomass Loss over Four Seasons 总被引:4,自引:0,他引:4
Abstract Research of the microbial ecology of McMurdo Dry Valley lakes has concentrated primarily on phototrophs; relatively little is known about the heterotrophic bacterioplankton. Bacteria represent a substantial proportion of water column biomass in these lakes, comprising 30 to 60% of total microplankton biomass. Bacterial production and cell numbers were measured 3 to 5 times, within four Antarctic seasons (October to January), in Lakes Fryxell, Hoare, and Bonney. The winter-spring transition (September to October) was included during one year. Lake Fryxell was the most productive, but variable, lake, followed by Lakes Bonney and Hoare. Bacterial production ranged from 0 to 0.009 μg C ml-1 d-1; bacterial populations ranged from 3.2 x 10(4) to 4.4 x 10(7) cells ml-1. Bacterial production was always greatest just below the ice cover at the beginning of the season. A second maximum developed just above the chemocline of all the lakes, as the season progressed. Total bacterioplankton biomass in the lakes decreased as much as 88% between successive sampling dates in the summer, as evidenced by areal integration of bacterial populations; the largest decreases in biomass typically occurred in mid-December. A forward difference model of bacterial loss in the trophogenic zone and the entire water column of these lakes showed that loss rates in the summer reached 6.3 x 10(14) cells m-2 d-1 and 4.16 x 10(12) cells m-2 d-1, respectively. These results imply that bacteria may be a source of carbon to higher trophic levels in these lakes, through grazing. 相似文献
63.
Mandyam CD Crawford EF Eisch AJ Rivier CL Richardson HN 《Developmental neurobiology》2008,68(5):575-589
Hippocampal function and plasticity differ with gender, but the regulatory mechanisms underlying sex differences remain elusive and may be established early in life. The present study sought to elucidate sex differences in hippocampal plasticity under normal developmental conditions and in response to repetitive, predictable versus varied, unpredictable prenatal stress (PS). Adult male and diestrous female offspring of pregnant rats exposed to no stress (control), repetitive stress (PS-restraint), or a randomized sequence of varied stressors (PS-random) during the last week of pregnancy were examined for hippocampal proliferation, neurogenesis, cell death, and local microenvironment using endogenous markers. Regional volume was also estimated by stereology. Control animals had comparable proliferation and regional volume regardless of sex, but females had lower neurogenesis compared to males. Increased cell death and differential hippocampal precursor kinetics both appear to contribute to reduced neurogenesis in females. Reduced local interleukin-1beta (IL-1beta) immunoreactivity (IR) in females argues for a mechanistic role for the anti-apoptotic cytokine in driving sex differences in cell death. Prenatal stress significantly impacted the hippocampus, with both stress paradigms causing robust decreases in actively proliferating cells in males and females. Several other hippocampal measures were feminized in males such as precursor kinetics, IL-1beta-IR density, and cell death, reducing or abolishing some sex differences. The findings expand our understanding of the mechanisms underlying sex differences and highlight the critical role early stress can play on the balance between proliferation, neurogenesis, cell death, and hippocampal microenvironment in adulthood. 相似文献
64.
Root colonization by arbuscular mycorrhizae (AM) and dark septate endophytic (DSE) fungi in nitrogen amended and unamended
mixed tallgrass prairie communities were analyzed monthly over two growing seasons. Roots were stained with Trypan blue and
Sudan IV and fungal structures quantified using the modified magnified intersections method. Root length colonized (RLC) by
DSE exceeded AM colonization during early part of the growing season. Fungal colonization varied among the years and was greater
in 2003 than in 2002. Seasonal variation among the months within a growing season was observed in 2002 but not in 2003 for
both AM and DSE. AM fungi were most abundant during the peak growing season of dominant C4 vegetation while DSE were most abundant during the early part of the growing season. Hyperparasitism of AM hyphal coils by
melanized septate fungi was frequently observed and increased with AM coil frequency. Although nitrogen amendment had altered
the plant community composition, it had no impact on the colonization by AM or DSE fungi. 相似文献
65.
Septate endophyte colonization and host responses of grasses and forbs native to a tallgrass prairie
Native tallgrass prairies support distinct dark septate endophyte (DSE) communities exemplified by Periconia macrospinosa and Microdochium sp. that were recently identified as common root symbionts in this system. Since these DSE fungi were repeatedly isolated
from grasses and forbs, we aimed to test their abilities to colonize different hosts. One Microdochium and three Periconia strains were screened for colonization and growth responses using five native grasses and six forbs in an in vitro system.
Previously published data for an additional grass (Andropogon gerardii) were included and reanalyzed. Presence of indicative inter- and intracellular structures (melanized hyphae, microsclerotia,
and chlamydospores) demonstrated that all plant species were colonized by the DSE isolates albeit to varying degrees. Microscopic
observations suggested that, compared to forbs, grasses were colonized to a greater degree in vitro. Host biomass responses
varied among the host species. In broad comparisons, more grass species than forbs tended to respond positively to colonization,
whereas more forb species tended to be non-responsive. Based on the suspected differences in the levels of colonization, we
predicted that tallgrass prairie grasses would support greater DSE colonization than forbs in the field. A survey of field-collected
roots from 15 native species supported this hypothesis. Our study supports the “broad host range” of DSE fungi, although the
differences in the rates of colonization in the laboratory and in the field suggest a greater compatibility between grasses
and DSE fungi. Furthermore, host responses to DSE range from mutualism to parasitism, suggesting a genotype-level interplay
between the fungi and their hosts that determines the outcome of this symbiosis. 相似文献
66.
In principle, there are two strategies for navigating a straight course. One is to use an external directional reference and
continually reorienting with reference to it, while the other is to infer body rotations from internal sensory information
only. We show here that, while the first strategy will enable an animal or mobile agent to move arbitrarily far away from
its starting point, the second strategy will not do so, even after an infinite number of steps. Thus, an external directional
reference—some form of compass—is indispensable for ensuring progress away from home. This limitation must place significant
constraints on the evolution of biological navigation systems. Some specific examples are discussed. An important corollary
arising from the analysis of compassless navigation is that the maximum expected displacement represents a robust measure
of the straightness of a path. 相似文献
67.
Heungsoon Felix Lee Mandyam M. Srinivasan Candace Arai Yano 《Flexible Services and Manufacturing Journal》2006,18(4):239-268
We consider the problem of simultaneously determining the number of machines (and/or workers), the assignment of tasks (and related tools and components) to these machines, and the number of jobs circulating in a flexible assembly system (FAS), to satisfy steady-state throughput requirements for a family of similar products at minimum cost. We focus on situations where there are precedence relations among the various tasks, as is common in assembly systems. We present a framework for solving this problem based on a heuristic decomposition approach which involves the solution of only a few types of sub-problems. We demonstrate the efficiency and effectiveness of the overall procedure using a number of example problems. 相似文献
68.
Our study was designed to examine how components of complex mixtures can
inhibit the binding of other components to receptor sites in the olfactory
system of the spiny lobster Panulirus argus. Biochemical binding assays
were used to study how two- to six-component mixtures inhibit binding of
the radiolabeled odorants taurine, L-glutamate and
adenosine-5'-monophosphate to a tissue fraction rich in dendritic membrane
of olfactory receptor neurons. Our results indicate that binding inhibition
by mixtures can be large and is dependent on the nature of the odorant
ligand and on the concentration and composition of the mixture. The binding
inhibition by mixtures of structurally related components was generally
predicted using a competitive binding model and binding inhibition data for
the individual components. This was not the case for binding inhibition by
most mixtures of structurally unrelated odorants. The binding inhibition
for these mixtures was generally smaller than that for one or more of their
components, indicating that complex binding interactions between components
can reduce their ability to inhibit binding. The magnitude of binding
inhibition was influenced more by the mixture's precise composition than by
the number of components in it, since mixtures with few components were
sometimes more inhibitory than mixtures with more components. These
findings raise the possibility that complex binding interactions between
components of a mixture and their receptors may shape the output of
olfactory receptor neurons to complex mixtures.
相似文献
69.
70.
Alvaro CD Faria Agnaldo J Lopes José M Jansen Pedro L Melo 《Biomedical engineering online》2009,8(1):22-10