Probable Fertile Region of Asteroxylon mackiei K. and L.
Asteroxylon mackiei was first described from the Rhynie Chert by Kidston and Lang1, but the nature of its reproductive region has remained uncertain, since the peculiar leafless axes and small pear-shaped sporangia, which they provisionally assigned to the plant, have never been found connected to its leafy stems.
Evolutionary trends in safety factors against wind-induced stem failure.
We explore the hypothesis that the safety factor against wind-induced stem failure remained high during early land plant evolution despite an evolutionary increase in height with concomitant increases in wind-induced drag forces, bending stresses, and moments. This hypothesis was examined for 17 Paleozoic plant species assuming that each (1) existed in a densely packed community of conspecifics with equivalent height, (2) coped with the same wind profile (where ambient wind speed decreased toward ground level), but (3) had different within-canopy wind speeds depending on plant height and general morphology. Drag forces, stresses, and moments were computed, and a safety factor was calculated for each taxon using the quotient of its stem-tissue breaking stress and maximum wind-induced bending stress.The highest factors of safety were calculated among the most ancient rhyniophyte and zosterophyllophyte species examined (e.g., Rhynia and Gosslingia), and, on average, decreased among the taller and geologically younger species. The tallest species examined (e.g., Archaeopteris and Diaphorodendron) had safety factors equal to or higher than those of some of their presumed ancestors (e.g., Psilophyton and Leclercqia). These trends were statistically more robust among rhyniophytes and their presumed descendants.Even though the results comply with the hypothesis, numerous limitations of our protocol exist (e.g., the requirement for reliable whole-plant reconstructions). These are discussed in terms of our theory. Nonetheless, we believe our theory and protocol afford a reasonable opportunity to explore the effects of wind on early plant evolution.
Mineral element analyses of various tropical forages in Guatemala and their relationship to soil concentrations
Determination de la composition minerale de differents fourrages dans 3 regions d'elevage du Guatemala et etude des relations entre la chimie des sols et la composition minerale des fourrages