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21.
R. Habib 《Plant and Soil》1988,108(2):267-274
A method is proposed for estimating the total length of a root system from sub-samples. This method is based on the measurement
of the length and diameter of small pieces of roots, and on the measurements of the bulk density of root sub-samples. It is
assumed that roots are cylinders with a given bulk density. The length and diameter of small root pieces are measured by image
analysis. A weighted quadratic mean (W.Q.M.) root diameter is then calculated and used in estimating the root length. This
W.Q.M. diameter is defined as the real mean diameter of an equivalent single root with the same length and volume as the tested
root system. The accuracy of prediction is demonstrated for one theoretical root system. The standard deviation of estimation
can be calculated using sampling simulations. 相似文献
22.
Bhabesh Sen Franklin A. Graybill Naitee Ting 《Biometrical journal. Biometrische Zeitschrift》1992,34(3):259-274
The model considered in this article is the two-factor nested unbalanced variance component model: for p = 1, 2, …, P; q = 1, 2, …, Qp; and r = 1, 2, …, Rpq. The random variables Ypqr are observable. The constant μ is an unknown parameter, and Ap, Bpq and Cpqr are (unobservable) normal and independently distributed random variables with zero means and finite variances σ2A, σ2B, and σ2C, respectively. Approximate confidence intervals on ?A and ?B using unweighted means are derived, where The performance of these approximate confidence intervals are evaluated using computer simulation. The simulated results indicate that these proposed confidence intervals perform satisfactorily and can be used in applied problems. 相似文献
23.
Guillermo Ortega Diego A. Golombek Dino Otero Lilia Romanelli Daniel P. Cardinali 《Chronobiology international》1992,9(2):137-147
The two-oscillator model of human circadian rhythmicity was analyzed when a zeitgeber relative intensity of 1, 0.5, or 0.1 was introduced into the equations. Fourier analysis was compared with dynamic analysis such as attractor reconstruction or Liapunov exponent calculation. After a 50 or 90% reduction in zeitgeber intensity, the dynamics of the system became equivalent and differed significantly from those of a system with maximal zeitgeber intensity. When 10% aleatory noise was added to the data, the analysis was still applicable, and the results obtained were essentially the same as in the absence of noise. Dynamic analysis could thus provide a distinct classification for periodic data, based on the type of analysis. 相似文献
24.
Summary As clonal plants grow they move through space. The movement patterns that result can be complex and difficult to interpret without the aid of models. We developed a stochastic simulation model of clonal growth in the tall goldenrod, Solidago altissima. Our model was calibrated with field data on the clonal expansion of both seedlings and established clones, and model assumptions were verified by statistical analyses.When simulations were based on empirical distributions with long rhizome lengths, there was greater dispersal, less leaf overlap, and less spatial aggregation than when simulations were based on distributions with comparatively short rhizome lengths. For the field data that we utilized, variation in rhizome lengths had a greater effect than variation for either branching angles or rhizome initiation points (see text). We also found that observed patterns of clonal growth in S. altissima did not cause the formation of fairy rings. However, simulations with an artificial distribution of branching angles demonstrate that fairy rings can result solely from a plant's clonal morphology.Stochastic simulation models that incorporated variation in rhizome lengths, branching angles, and rhizome initiation points produced greater dispersal and less leaf overlap than deterministic models. Thus, variation for clonal growth parameters may increase the efficiency of substrate exploration by increasing the area covered and by decreasing the potential for intraclonal competition. We also demonstrated that ramet displacements were slightly, but consistently lower in stochastic simulation models than in random-walk models. This difference was due to the incorporation of details on rhizome bud initiation into stochastic simulation models, but not random-walk models. We discuss the advantages and disadvantages of deterministic, stochastic simulation, and random-walk models of clonal growth. 相似文献
25.
Reduced orthostatic tolerance following 4 h head-down tilt 总被引:2,自引:0,他引:2
Gary C. Butler Huacheng Xing David R. Northey Richard L. Hughson 《European journal of applied physiology and occupational physiology》1991,62(1):26-30
The cardiovascular responses to a 10-min 1.22 rad (70 degrees) head-up tilt orthostatic tolerance test (OST) was observed in eight healthy men following each of a 5-min supine baseline (control), 4 h of 0.1 rad (6 degrees) head-down tilt (HDT), or 4 h 0.52 rad (30 degrees) head-up tilt (HUT). An important clinical observation was presyncopal symptoms in six of eight subjects following 4 h HDT, but in no subjects following 4 h HUT. Immediately prior to the OST, there were no differences in heart rate, stroke volume, cardiac output, mean arterial pressure and total peripheral resistance for HDT and HUT. However, stroke volume and cardiac output were greater for the control group. Mean arterial pressure for the control group was less than HDT but not HUT. Over the full 10-min period of OST, the mean arterial pressure was not different between groups. Heart rate increased to the same level for all three treatments. Stroke volume decreased across the full time period for control and HDT, but only at 3 and 9 min for HUT. There was a higher total peripheral resistance in the HDT group than control or HUT. The pre-ejection period to left ventricular ejection time ratio was less in HDT than for control or HUT groups. These data indicate a rapid adaptation of the cardiovascular system to 4 h HDT that appears to be inappropriate on reapplication of a head to foot gravity vector. We speculate that the cause of the impaired orthostatic tolerance is decreased tone in venous capacitance vessels so that venous return is inadequate. 相似文献
26.
Aedes aegypti: model for blood finding strategy and prediction of parasite manipulation 总被引:1,自引:0,他引:1
Aedes aegypti mosquitoes salivate during intradermal probing of vertebrate prey before ingesting blood (Griffiths and Gordon 1952). Nonsalivating mosquitoes locate blood more slowly; this difference was ascribed to an anti-platelet activity found in the mosquito's saliva (Ribeiro et al. 1984). Mosquitoes infected with Plasmodium gallinaceum suffer pathology that specifically impairs saliva anti-hemostatic activity but without reducing volume of output (Rossignol et al. 1984). The complexity of the feeding apparatus of mosquitoes provides opportunity for a variety of strategies in which pathogens may produce specific lesions that enhance their transmission, but the variables that affect the duration of probing by mosquitoes have not been defined. We sought to resolve this complexity by identifying and quantifying relevant parameters of probing behavior. Mosquitoes thrust their mouthparts repeatedly through their host's skin while searching for blood. Female A. aegypti thrust at 7-sec intervals. If this search results in success, feeding ensues. Alternatively, the mosquito "desists," the mouthparts stylets are withdrawn, and the mosquito attempts to feed at another site. Even after previous desistance, the probability of finding blood remains undiminished. Functions for the probability of feeding success and desistance over time were derived using data from observations on 300 mosquitoes. The probability of feeding success was interpreted as being a function of the density of vessels in the skin, their geometric distribution, and the conditions locally affecting hemostasis. During each probe, the probability of desisting increased linearly with time, and after desisting once, mosquitoes tended to desist more rapidly. A model was developed incorporating Monte Carlo simulation which closely fit observed data. By changing values for the several parameters of the probability functions, we predicted modes in which parasites may manipulate their hosts to enhance transmission, both to and from the vector. In particular, parasite strategies in the vector would include induced salivary pathology; increased duration of probing thrusts; decreased desistance time; and inhibited phagoreception. Predicted parasite strategies in the reservoir host would include increased skin vascular volume and impaired host hemostasis. Our model supports the hypothesis of a mutualistic interaction of malaria and mosquitoes. 相似文献
27.
Robert J. Kosinski 《Hydrobiologia》1984,119(2):139-148
Several methods for the estimation of the reaeration coefficient were compared by determining the ability of the methods to recover the correct K value from a computer-simulated stream oxygen record affected by a variety of non-ideal conditions. Noisy data and long observation intervals were not a serious problem for most methods. Saturating photosynthesis, fluctuating light intensity, afternoon depression and temperature variation caused failures by some methods but were well handled by others. Serious impairment of all methods occurred with low productivity or high K. In general, the best-performing methods were the modified hysteresis, nighttime regression, daytime regression, Odum and Hornberger-Kelly daytime methods. 相似文献
28.
The conversion of aphid prey tissue (Acyrthosiphon pisum Harris) into predator biomass (immature life stages ofPropylaea 14-punctata L. andCoccinella 7-punctata L.) is calculated by plotting weight gain against assimilation (i.e. consumption minus egestion). This concept is added to
the metabolic pool model byGutierrez
et al. (1981) that enables the simulation of growth and development of a predator on a physiological basis. Physiological time is
expressed in daydegrees above lower development thresholds for both species. Visual examination of observed and calculated
values showed that the model satisfactorily describes the growth patterns of the above predators.
相似文献
29.
I. Lavorel 《Photosynthesis research》1986,9(1-2):273-283
The purpose of this note is to illustrate the feasibility of simulating kinetic systems, such as commonly encountered in photosynthesis research, using the Monte Carlo (MC) method. In this approach, chemical events are considered at the molecular level where they occur randomly and the macroscopic kinetic evolution results from averaging a large number of such events. Their repeated simulation is easily accomplished using digital computing. It is shown that the MC approach is well suited to the capabilities and resources of modern microcomputers. A software package is briefly described and discussed, allowing a simple programming of any kinetic model system and its resolution. The execution is reasonably fast and accurate; it is not subject to such instabilities as found with the conventional analytical approach.Abbreviations MC
Monte Carlo
- RN
random number
- PSU
photosynthetic unit
Dedicated to Prof. L.N.M. Duysens on the occasion of his retirement. 相似文献
30.
J. R. Krummel R. V. O'Neill J. B. Mankin 《Human ecology: an interdisciplinary journal》1986,14(1):117-130
Regional analyses of the interaction between human populations and natural resources must integrate landscape scale environmental problems. An approach that considers human culture, environmental processes, and resource needs offers an appropriate methodology. With this methodology, we analyze problems of food availability in African cattle-keeping societies. The analysis interrelates cattle biomass, forage availability, milk and blood production, crop yields, gathering, food subsidies, population, and variable precipitation. While an excess of cattle leads to overgrazing, cattle also serve as valuable food storage mechanisms during low rainfall periods. Food subsidies support higher population levels but do not alter drought-induced population fluctuations. Variable precipitation patterns require solutions that stabilize year-to-year food production and also address problems of overpopulation. 相似文献