The trichotomosulcate asparagoids: pollen morphology of Hemerocallidaceae in relation to systematics and pollination biology

We examined pollen of 19 genera of Hemerocallidaceae by using scanning electron microscopy (SEM), and one genus (Dianella) by using transmission electron microscopy (TEM). Pollen was generally small in size, with a rounded triangular outline when hydrated, and a characteristic three-armed aperture, a distal trichotomosulcus. The pollen surface was finely sculptured and the exine was thin. Microreticulate pollen is a potential synapomorphy for several species of the ‘crown phormioid’ subclade recognised in molecular analyses. Perforate and fossulate pollen supports a relationship between several species of Dianella. Microrugulate pollen is more frequent in the johnsonioids than in the phormioids. Hemerocallis is distinguished by elongated monosulcate pollen, a relatively thick exine with a pronounced reticulate surface, and large globules of attached pollenkitt. We hypothesise that Hemerocallidaceae are ancestrally buzz-pollinated, and their pollen morphology is an adaptation to this pollination type. A reversal to butterfly or moth pollination occurred in Hemerocallis, with associated changes in pollen morphology.Carol A. Furness, John G. Conran, Thomas Gregory and Paula J. Rudal

A survey and comparison of several friction force models for dynamic analysis of multibody mechanical systems

This study is aimed at examining and comparing several friction force models dealing with different friction phenomena in the context of multibody system dynamics. For this purpose, a comprehensive review of present literature in this field of investigation is first presented. In this process, the main aspects related to friction are discussed, with particular emphasis on the pure dry sliding friction, stick–slip effect, viscous friction and Stribeck effect. In a simple and general way, the friction force models can be classified into two main groups, namely the static friction approaches and the dynamic friction models. The former group mainly describes the steady-state behavior of friction force, while the latter allows capturing more properties by using extra state variables. In the present study, a total of 21 different friction force models are described and their fundamental physical and computational characteristics are discussed and compared in details. The application of those friction models in multibody system dynamic modeling and simulation is then investigated. Two multibody mechanical systems are utilized as demonstrative application examples with the purpose of illustrating the influence of the various frictional approaches on the dynamic response of the systems. From the results obtained, it can be stated that both the choice of the friction force model and friction parameters involved can significantly affect the simulated/ modeled dynamic response of mechanical systems with friction.The first author expresses his gratitude to the Portuguese Foundation for Science and Technology through the PhD grant (PD/BD/114154/2016). This work has been supported by the Portuguese Foundation for Science and Technology with the reference project UID/EEA/04436/2013, by FEDER funds through the COMPETE 2020—Programa Operacional Competitividade e Internacionalização (POCI) with the reference project POCI-01-0145-FEDER-006941.info:eu-repo/semantics/publishedVersio