Periodická tabulka prvků má dlouhou historii, která je současně cestou předchemického a chemického bádání. Byla to cesta, jejímž cílem bylo vnést řád do světa hmoty. Zformování pojmu "chemický prvek" a následně atomové teorie vedlo k moderni chemii. Bylo jen otázkou času, kdy se podaří vnést systém do narůstajícího počtu známých prvků, až nakonec periodický zákon zformuloval Mendělejev. Dnes je otázkou nejen pokračování periodické tabulky a hledání případných "ostrovů stability", ale samozřejmě i naprosto zásadní problém - končí tabulka někde? and Vladimír Karpenko.
The ultrastructure and dimensions of chloroplasts in leaf mesophyll cells were quantitatively examined in three parental inbred lines of maize (Zea mays L.) and their four hybrids subjected to two types of four-week low-temperature (LT) treatment: the abrupt onset of chilling temperatures ("severe chilling", SC) and the gradual, more moderate one ("moderate chilling", MC). The relationship between the response of individual genotypes to one or the other type of chilling was analyzed as well as the possibility to predict the behaviour of chloroplasts in hybrids from that of their parents. Although selected parameters of chloroplast ultrastructure (e.g. volume densities of granal and intergranal thylakoids, plastoglobuli, and peripheral reticulum) and dimensions changed due to the exposure of maize plants to LT, no general pattern of such changes was found for this species due to the observed intraspecific variability. The response of some genotype to SC could not be predicted from its behaviour under MC (and vice versa) and no clear rules could be applied for the inheritance of chloroplast response to chilling in the general sense. Thus, great caution should be always taken when interpreting the results of studies aimed at the dissection of chloroplast ultrastructure as affected by LT, particularly in case such studies are made with one genotype or under one type of chilling only. and D. Holá ... [et al.].
The paper deals with stress concentration in inclined bars and beams, where the shoulder radius is often not prescribed in the detail drawings. The commonly accepted notch definition as a stepwise change of the beam cross section, as well as a lack of
nomograms or other data on stress concentration in inclined shoulders in the available literature ([1], [2], [3]) support the assumption of negligible stress concentration in inclined shoulders. Several failures of shaft-like components with inclined shoulders made us to investigate the stress concentration in these shoulders. Computational modelling confirmed a rather high stress concentration even in shoulders with a very low inclination β angle. Even in the case of β = 30°, the stress concentration factor is only slightly lower than in a comparable perpendicular (β = 90°) shoulder. Therefore nomograms for evaluation of stress concentration factors in inclined bar and beam shoulders under basic loading types were created and published in the paper.