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21.
22.
Enzymatic synthesis of glutathione 总被引:8,自引:0,他引:8
23.
The introduction of exotic species to ecosystems can have severe consequences for populations of native organisms, but logistical limitations and shortage of historical data often hinder attempts to quantify the ecological implications of such relationships. The establishment and rapid expansion of Ring‐necked Parakeets Psittacula krameri in England therefore presents a rare opportunity to apply novel analytical methods to existing extensive national bird monitoring data from the UK Breeding Bird Survey for an invasive species. A previous study from Belgium suggests that Ring‐necked Parakeets have the potential to reduce the abundance of Eurasian Nuthatch Sitta europaea through competition for nesting cavities. Our analysis provides no evidence for a significant impact through competition on Nuthatch populations or those of any other cavity‐nesting species within the Parakeet’s current range in the UK. However, we cannot exclude the possibility that competitive exclusion could be occurring at a minority of sites at which availability of nest cavities is limiting. This may yet have significant implications for future conflict if Parakeets continue to increase in numbers and range. 相似文献
24.
PETER JOHNSTON 《Zoological Journal of the Linnean Society》2011,163(3):959-982
There has been little agreement on the phylogeny of palaeognathous birds, with major differences amongst and between results from morphological and molecular data. Two recently published phylogenies using nuclear and mitochondrial DNA have substantial agreement in overall topology, with the ostrich as sister group of all other extant palaeognaths and a kiwi‐emu‐cassowary clade. Here I report a morphological phylogeny based mainly on new characters from the tongue apparatus and cranial osteology, with a theoretical ancestor as outgroup. A new interpretation of the evolution of the avian palate is included. This phylogeny is very similar to these recent molecular results; this is the first report of such congruence, and offers a credible basis for understanding the evolution of this clade. This phylogeny is fully consistent with a Gondwana vicariance model of evolution. Dates attributed from known geological events place the first extant radiation (ostrich) in the mid‐Cretaceous, and offer a means of calibration of future molecular clock investigations. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 163 , 959–983. 相似文献
25.
Uptake of CO2 by aquatic vegetation 总被引:15,自引:10,他引:5
Abstract Photosynthesis by aquatic plants based on the supply of CO2 from air-equilibrated solutions may be limited by the low diffusion coefficient of CO2 in water. For plants in which the transport of CO2 from the bulk medium is by diffusion, and the initial carboxylation uses RUBISCO, CO2 supply can be increased by growth in habitats with fast water flow over the surface (reducing unstirred layer thickness), or with heterotrophically-augmented CO2 levels, including the direct use of sediment CO2. Many aquatic plants using RUBISCO as their initial carboxylase counter the limitations on CO2 supply via the operation of biophysical CO2 concentrating mechanisms which are based on active transport of HCO?3, CO2 or H+ at the plasmalemma, and use bulk-phase HCO?3 or CO2 as the C source. A final group of aquatic plants use biochemical CO2 concentrating mechanisms based on auxiliary carboxylation by PEPc: C4-like and Crassulacean Acid Metabolism–like processes are involved. These various mechanisms for increasing CO2 supply to RUBISCO also help to offset the low specific reaction rate of aquatic plant RUBISCOs at low [CO2] and low [CO2]: [CO2]. In addition to overcoming restrictions on CO2 supply, the various methods of increasing inorganic C availability may also be important in alleviating shortages of nitrogen or photons. 相似文献
26.
27.
Variability of DNA Content in Individual Cells of <Emphasis Type="Italic">Bacillus</Emphasis> 总被引:4,自引:0,他引:4
THE average content of DNA in Bacillus spores is unaffected by the growth medium and is constant for each species1. Within the Bacillus group, however, the average amount of DNA per spore varies from species to species1. A cell of B. cereus contains on average twice as much DNA as the spore and during sporulation this DNA is divided equally between the spore and the sporangium2. Estimates3–5 of genome size vary from 1.3 × 109 to 10 × 109 daltons and the number of genomes per cell from 2 to 44–8. Some of these variations may be associated with differences within the cells rather than differences of methodolqgy; spores within a population vary not only in size and shape, but also in their content of stainable chromatin9. Moreover, in ‘Renografm’ density gradients10, spores band within a range of densities. If spores are taken from a narrow part of this range, regrown and rebanded, the original pattern of dispersal occurs, suggesting that spores in the same population normally show variation in density as well as in size (A. I. Aronson, personal communication). 相似文献
28.
The resistance and capacitance of the membranes of Nitella translucenshave been measured by direct current and alternating currentmethods. Current of the order of 10-7 amp. was injected intothe cell by means of a conventional Ag, AgCl-3N KCl glass microelectrodeinserted into the vacuole of the cell. The change of potentialacross the membrane was recorded by two other internal microelectrodeswhich had been inserted into the cell at known distances fromthe current-injecting electrode. In the direct-current experimentsthe input current was in the form of a square pulse, while sinusoidalcurrents of frequency 25 cycles per second were used in thealternating current experiments. The cell was treated as a shortlength of coaxial cable and from the measurements the followingparameters could be obtained: the space constant (), the membraneresistance (Rm) and the membrane capacitance (Cm). The valuesof Rm ranged from 6.7 to 36 K ohm cm.2 (mean of 21.4 K ohm cm.2)and those of ranged from 1.5 to 5.7 cm. (mean of 2.6 cm.).The capacitance value was about I µF cm.-2 These results are discussed within the framework of our knowledgeof these parameters for other cells, particularly plant cells.The measured electrical resistance is shown to be at least tentimes less than the value estimated from the passive fluxesof the principal ions K, Na, and Cl. It is suggested that thisdiscrepancy, which is usually attributed to non-independentmovement of these ions, could be partially explained on electro-osmoticgrounds. The value of the capacitance is very close to thatwhich is usually obtained for other cell membranes. One exceptionallylow value for Nitella has been quoted in the literature. Thereason for the gross error in this particular measurement isgiven. 相似文献
29.
30.
P. W. HATTERSLEY L. WATSON F.L.S C. R. JOHNSTON 《Botanical journal of the Linnean Society. Linnean Society of London》1982,84(4):265-272
The leaf blade anatomy of all species of Neurachne, Paraneurachne and Thyridolepis has been examined, with special reference to features reliably indicative of photosynthetic pathway. Neurachne alopecuroidea, N. queenslandica and N. tenuifolia exhibit C3 anatomy, as do all three Thyridolepis species. Paraneurachne muelleri, N. munroi and N. minor all have C4 anatomy, of the rare ' Alloteropsis-type'. However, all the species exhibit unusual anatomical features, and N. lanigera cannot be confidently assigned as C3 or C, on anatomical grounds alone. Taxonomic considerations confirm that the three genera are closely related to one another in the context of the eu-panicoid assemblage as a whole, and that the genus Neurachne itself, although it exhibits variation in photosynthetic pathway, is not amenable to further taxonomic subdivision. 相似文献