共查询到20条相似文献，搜索用时 671 毫秒
1.
B A Hills 《Science (New York, N.Y.)》1967,157(3791):942-943
An analysis is undertaken to determine the continuous distribution of circulation rates which would be necessary for a single tissue type to display a simple exponential response for the exchange of a solute with blood. It is shown that there can be no more than one real solution. This indicates that heterogeneous circulation cannot be invoked to explain anomalies to the popular assumption that blood perfusion is the process limiting the rate of blood-tissue exchange. 相似文献
2.
Savannas worldwide are vital for both socioeconomic and biodiversity values. In these ecosystems, management decisions are based on the perception that wildlife and livestock compete for food, yet there are virtually no experimental data to support this assumption. We examined the effects of wild African ungulates on cattle performance, food intake, and diet quality. Wild ungulates depressed cattle food intake and performance during the dry season (competition) but enhanced cattle diet quality and performance during the wet season (facilitation). These results extend our understanding of the context-dependent-competition-facilitation balance, in general, and are critical for better understanding and managing wildlife-livestock coexistence in human-occupied savanna landscapes. 相似文献
3.
渔业资源评估中的回顾性问题 总被引：9，自引：5，他引：4
回顾性问题是指,随着渔业数据逐年加入,渔业资源评估结果如资源量或捕捞死亡系数等出现系统性偏差。多种渔业资源应用不同评估模型进行渔业资源评估,其结果常存在回顾性问题,这使得渔业资源评估的结果存在不确定性,不利于渔业资源合理的管理与开发,因此,回顾性问题是目前渔业资源评估研究中的热点与难点之一。回顾性问题的度量目前主要采用MOHNρ,产生回顾性问题的原因主要可归纳为数据错误与模型假设错误。目前,尽管有多种方法用于诊断、分析回顾性问题的成因,但这些方法均存在局限性。针对回顾性问题,渔业资源评估学者根据其研究对象的特点提出了校正回顾性问题的方法,但目前仍缺乏通用的方法或手段以校正或避免回顾性问题。在渔业资源评估中,若结果存在回顾性问题,则表明数据或模型假设存在问题。因此进行回顾性分析,是评估数据质量及检查数据与模型假设是否一致的有效手段。 相似文献
4.
Bacterial spores: chemical sensitization to heat 总被引：8，自引：0，他引：8
Spore heat resistance is largely an inducible property, chemically reversible between a sensitive and resistant state. Therefore, the constitutive assumption and common practices based on it, such as direct testing of spores for heat resistance without prior treatment and the assumption of logarithmic death for spores in general, are no longer appropriate. A new approach is reported to the reduction of heating severity for a given survivor reduction of bacterial spores suspended in complex biological mixtures at their ordinary pH. Heating time advantages amount to severalfold and do not involve antimicrobial additives. 相似文献
5.
Zapolsky HS 《Science (New York, N.Y.)》1966,153(3736):635-638
The hypothesis that the quasistellar sources (quasars) are local objects moving with velocities close to the speed of light is examined. Provided there is no observational cutoff on apparent bolometric magnitude for the quasars, the transverse Doppler effect leads to the expectation of fewer blue shifts than red shifts for an isotropic distribution of velocities. Such a distribution also yields a function N(z), the number of objects with red shift less than z which is not inconsistent with the present data. On the basis of two extreme assumptions concerning the origin of such rapidly moving sources, we computed curves of red shift plotted against magnitude. In particular, the curve obtained on the assumption that the quasars originated from an explosion in or nearby our own galaxy is in as good agreement with the observations as the curve of cosmological red shift plotted against magnitude. 相似文献
6.
Population experiments with Anolis lizard species demonstrate a relation between the amount of between-species competition and the degree of interspecific resource partitioning (the more the partitioning the less the competition). Specifically, the amount of resource partitioning between the two species (Anolis gingivinus and Anolis wattsi pogus) on the island of St. Maarten is less than that between the two species (Anolis bimaculatus and Anolis wattsi schwartzi) on the island of St. Eustatius. The presence of Anolis wattsi both lowers the growth rates and raises the perch heights of Anolis gingivinus individuals. In contrast, Anolis wattsi has no effect on Anolis bimaculatus. Thus, when there is less resource partitioning, Anolis wattsi has a greater competitive effect. This verifies, for these species, a central assumption of competition theory: the strength of between-species competition is inversely related to the amount of interspecific resource partitioning. 相似文献
7.
林正华提出的一个求解非线性方程组的混合牛顿与二阶拟牛顿迭法是有效的，但有缺点，本文中提出了一个改进的算法：在每一迭代步，只需求解线性方程组的近似解。在合理的假设下证明了本算法具有与林算法相同的收敛性质。 相似文献
8.
Cyclic adenosine monophosphate: selective increase in caudate nucleus after administration of L-dopa
Treatment with the dopamine precursor L-dopa produced a significant accumulation of adenosine 3',5'-monophosphate (cyclic AMP) in the caudate nucleus of the rat. In contrast, there was no change in the amount of cyclic AMP in the cerebellum. Accumulation of cyclic AMP in the caudate nucleus after administration of L-dopa was prevented by prior treatment with the decarboxylase inhibitor RO 4-4602. These observations and those in other laboratories support the assumption that dopamine formed from L-dopa selectively activates striatal adenylate cyclase. The in vivo activation of adenylate cyclase after treatment with L-dopa may be a useful model for studying neurological and psychiatric disorders that are thought to involve the dopaminergic system of the brain. 相似文献
9.
In Tetrahymena thermophila the major argument for the existence of diploid subunits has been that some loci show a delay in the accumulation of stable subclones during macronuclear assortment. This delay is based on the assumption that throughout the life cycle there are 45 subunits. We find that for at least 50 fissions after conjugation there is sufficient DNA for 66 haploid subunits. These additional subunits early in the life cycle are sufficient to explain the observed accumulation of stable subclones in all instances. This removes the need to invoke diploidy to explain assortment, thus resolving the question of subunit ploidy in favor of haploidy. 相似文献
10.
Haas AR 《Science (New York, N.Y.)》1917,46(1193):462-464
When Laminaria is exposed to the action of anesthetics (in sufficient concentration to produce any result) there is an increase in respiration. This may be followed by a decrease if the reagent is sufficiently toxic. No decrease is observed with low concentrations which are not toxic. These facts contradict the theory of Verworn that anesthesia is a kind of asphyxia, for his view is based upon the assumption that anesthetics decrease respiration. 相似文献
11.
Hough SE 《Science (New York, N.Y.)》1995,267(5195):211-213
Although there is debate on the maximum size of earthquake that is possible on any of several known fault systems in the greater Los Angeles metropolitan region, it is reasonable to assume that the distribution of earthquakes will follow a fractal distribution of rupture areas. For this assumption and an overall slip-rate for the region of approximately 1 centimeter per year, roughly one magnitude 7.4 to 7.5 event is expected to occur every 245 to 325 years. A model in which the earthquake distribution is fractal predicts that, additionally, there should be approximately six events in the range of magnitude 6.6 in this same span of time, a higher rate than has occurred in the historic record. 相似文献
12.
在原四杆机构空间模型理论基础上，建立了四杆运动链空间模型，北给出了四杆运动甸宾标准尺寸型及其关系式，在空间模型中，根据有无整周转动杆件又将其分成若干子空间，为进一步系统分析打下基础，该空间模型的建立，将使四杆机构的性能图谱大为减少，并可实现一图谱多用，为设计人员提供方便。 相似文献
13.
剪跨比对异形柱受力性能影响初探 总被引：4，自引：0，他引：4
剪跨比是决定异形截面柱截面特性的主要因素。当剪跨比较大时，截面正应变分布符合平截面假定，随着剪跨比的减小应变分布向曲面方向发展，异形柱的受力性能越来越接近短肢剪力墙。 相似文献
14.
The philosophy of the storage of high-grade radioactive wastes in salt seems to be based on the assumption that the salt glaciers of Iran are no longer moving. Monitoring the movements of markers painted onto one of the salt glaciers suggests that the glacier deforms elastically as a result of temperature changes most of the time but flows plastically when it is sufficiently wet during each annual rainy season. 相似文献
15.
农户风险处理策略分析 总被引：27，自引：1，他引：27
本文以农户内部具有有效的风险防范和处理机制为基本前提，归纳和描述了几种农户风险处理策略及其特点。农户的风险管理策略是理性的，小规模农户的防范和处理风险的策略也是有效的。农户的风险处理策略对保障家庭经济奠定起到了重要作用。 相似文献
16.
A prototypical operating statement similar to that used by business firms has been shown to be a useful decision-making tool for a community choosing a solid waste management system. When applied to resource recovery, it highlights the economics of recovery and the values of the input parameters necessary to achieve economic viability, whether in the case of public or private ownership (23). In most communities, refuse processing to recover material resources must be based on more than one source of revenue. In addition to the revenues from the sale of by-products, there must be revenues from processing the incoming refuse and from a user, or dump, fee. In the first case discussed, that of materials recovery by a front end system, resource recovery is shown to be economically feasible for those communities in which the present cost of disposal is relatively high. The indifferent community was one having a current cost of $7.72 per ton; more accurately, this would be the cost for the near-term future. It is not necessary that current costs be used, since many communities are merely "dumping" their refuse. The indifference decision should be based on the cost of an environmentally sound alternative. Energy recovery from municipal solid waste can increase the number of communities in which resource recovery will be an economic adjunct to a solid waste management system. The analysis presented here was based on the assumption that the value of the fuel recovered exactly offset the additional capital and operating costs of the utility which burns it. There could be costs above and beyond this; similarly, there could be a saving by taking into account the economic value of the organic fraction as fuel. However, it is believed that the assumption under which the materials-plus-energy case was analyzed seems to be realistic at this time. 相似文献
17.
为探究不同程度的地块连片对规模户土地生产率的影响,将连片地块划分为完全连片和适度连片两种类型,以湖北省两县区水稻规模户的调查数据为例分析其地块连片状况,并运用内生转换回归模型测算不同程度的地块连片对土地生产率的影响效应.结果显示:无论何种程度的地块连片均有助于提高规模户的土地生产率,具体表现为:在反事实假设下,实际已将... 相似文献
18.
Agronomists use overlaying protein and yield maps to identify factors limiting cereal crop growth and development. Management decisions can be derived from knowing what and where these limiting factors are. In using protein and yield in this manner, there is an assumption that a physiologically or biologically significant relationship exists between grain protein and grain yield at the local level. In this paper, we investigate whether within-field yield and protein data support this relationship. The protein-yield relationship was modelled using weighted regression with global and local neighbourhoods in both 1-D and 2-D spatial location frameworks. The results from both the 1-D and 2-D analyses showed that the relationships between protein and yield are significant at both the macro (field level) (r2=0.25) and the micro-scale (local within field level) (r2=0.69). The assumption of a significant local relationship between protein and yield is supported by these data, suggesting that management decisions may be determined using such a relationship. 相似文献
19.
Westenberg AA 《Science (New York, N.Y.)》1972,177(4045):255-256
A recent laboratory measurement of the ratio of the rate constant for the reaction CO + HO (2) --> C0(2) + OH relative to that for H + HO(2) --> 2OH indicates that the former reaction is probably faster than CO + OH --> CO(2) + H. On this basis a simple analysis is given showing that the calculated lifetime of nitric oxide in polluted atmospheres would be appreciably longer than that estimated on the assumption that the carbon monoxide-hydroperoxyl reaction may be neglected. A fast carbon monoxide-hydroperoxyl reaction implies that the cyclic consumption of carbon monoxide (an atmospheric sink) could occur even with no nitric oxide present. 相似文献
20.
This is an exploratory study on mental accounting and food budgeting of agricultural households, in which we assumed that agricultural households may have a mental account for consumption of their self-produced food. Accordingly, they may reserve a certain quantity of self-produced food as a set budget for own consumption, implying that they may keep on consuming their own produce until they have consumed the quantity set for the mental budget. By making the mental accounting assumption, we hypothesized that the consumption of self-produced food is independent of market price. Also, we hypothesized that the consumption of self-produced food is increasing in the quantity of production if production is lower than the set budget, and independent of the quantity of production if production exceeds the set budget. By applying a double-log demand model and using survey data from six poor rural counties in China, we tested these hypotheses for five food items, which are rice, flour, potatoes, pork, and eggs. We found that the hypothesis of no significant effect of price holds for flour, potatoes, and pork if production is lower than the set budget, and for rice, pork, and eggs if production is higher than the set budget. Production has a significant positive effect on consumption of selfproduced food but with a much greater influence when production is lower than the set budget for all five food items. These findings partly support our assumption of mental accounting of self-produced food. Limitations, policy implications, and possible future studies are discussed. 相似文献