Delayed Germination of Seeds: A Look at the Effects of Adult Longevity, the Timing of Reproduction, and Population Age/Stage Structure

The effects of adult longevity, the timing of reproduction, and population age/stage structure on the evolution of seed dormancy are explored in both constant and variable environment models. In the constant environment models complete germination is the evolutionarily stable strategy (ESS) regardless of adult longevity. Incorporating a cost of reproduction on subsequent survival does not alter this result. In contrast, in a variable environment changes in adult longevity can exert a strong selection pressure against seed dormancy. Incorporating a cost of reproduction for iteroparous species reduces adult longevity, which selects for more seed dormancy. The magnitude of the change in ESS germination probability depends on several factors, including which life-history stage is variable (e.g., fecundity, seedling survival), whether seeds can detect favorable sites for establishment, and the age/stage structure of the population. In general, increases in adult longevity select against seed dormancy, but exceptions to this pattern are discussed. The idea that established plant traits are uncoupled from those of the regenerative phase, as assumed by J. P. Grime's competition-stress-ruderal model, is considered critically

Taxation and the Household

Previous analyses of demand systems and the welfare effects of taxing male and female labour supplies suppress the analysis of household resource allocation by assuming a household utility function. To analyse the implications of assuming this is not the case, we construct a simple but fairly general model of household resource allocation and use the properties of the equilibrium of this model to characterise the effects of tax policy on individual utilities, as determined by the household resource allocation proces

A Fundamental Domain for V_3

We describe a fundamental domain for the punctured Riemann surface $V_{3,m}$ which parametrises (up to M\"obius conjugacy) the set of quadratic rational maps with numbered critical points, such that the first critical point has period three, and such that the second critical point is not mapped in $m$ iterates or less to the periodic orbit of the first. This gives, in turn, a description, up to topological conjugacy, of all dynamics in all type III hyperbolic components in $V_{3}$, and gives indications of a topological model for $V_{3}$, together with the hyperbolic components contained in it.Comment: 120 page

Null Models and Dispersal Distributions: A Comment on an Article by Caley

[FIRST PARAGRAPH] In a recent article Caley (1991) outlined a null model for dispersal distributions against which he suggested empirical data should be compared. He first presented Waser's geometric model (Waser 1985), which can be derived as follows: Dispersing individuals move in a straight line from the natal site and settle in the first unoccupied site they encounter. If unoccupied sites occur independently at random with probability t as a result of turnover within the habitat, then the distribution of dispersal distances will follow a geometric distribution in which the probability of settling at distance i is given by p(i) = t(l - t)' for i = 0, 1,2,3,. . . continues.

Hairdressing in groups: a survey of combings and formal languages

A group is combable if it can be represented by a language of words satisfying a fellow traveller property; an automatic group has a synchronous combing which is a regular language. This article surveys results for combable groups, in particular in the case where the combing is a formal language.Comment: 17 pages. Published copy, also available at http://www.maths.warwick.ac.uk/gt/GTMon1/paper24.abs.htm

Deformation and rupture of stainless steel under cyclic, torsional creep

Copyright 2008 @ Engineering Integrity Society.Recent results from a long-term, strain-limited, cyclic creep test program upon stainless steel tubes are given. The test conditions employed were: constant temperature 500 °C, shear stress Ƭ = ± 300 MPa and shear strain limits ƴ = ± 4%. It is believed that a cyclic creep behaviour for the material has been revealed that has not been reported before in the literature. That is, the creep curves for stainless steel under repeated, shear stress reversals shows two basic square root dependencies: one upon time and the other upon cycle number. Consequently, the combined effect is such that the shear creep strain depends upon the square root of the product of cycle number and the time elapsed within that cycle. Despite extended times of cycling, with the test running into a period of over a year, no secondary or tertiary creep stages were ever observed within individual creep curves. Thus both the forward and reversed creep curves were exclusively primary in nature, within which the only visible evidence of a slow degradation of the deforming material was that the creep interval reduced successively between the imposed strain limits. However, it was found that the creep curve, when plotted within axes of cumulative creep strain and time, did recover a "pseudo-tertiary" stage. This stage concords with earlier results that showed tertiary creep to be a dominant contributor to the creep curve for this material under a steady torque. Given either the tensile ductility of the material or, a tensile creep rupture time, it is shown how final failure is predicted from the phenomenological square root law and an equivalence criterion