RHYNIE PLANTS

several genera of fossil plants uncovered near Rhynie, Aberdeen, Scot., of significance in tracing the evolution of vascular plants (plants with special cells that conduct water and food). The rocks containing these fossils are of Devonian age (the Devonian Period lasted from 408 to 360 million years ago) and are part of a geological formation called the Old Red Sandstone. Three genera are represented by almost perfectly preserved fossils of virtually entire plants: Rhynia and Horneophyton, of the family Rhyniaceae; and Asteroxylon, of the family Asteroxylaceae. Rhynia, often regarded as the most primitive vascular plant, is not necessarily the one from which later plants evolved. In fact, there is evidence that Rhynia evolved from vascular plants that extend back into the Silurian Period (which lasted from 438 to 408 million years ago). Slender leafless stems arose from a horizontal underground stem (rhizome). In R. gwynne-vaughani the erect shoots were about 17 centimetres (7 inches) high; in R. major they were about three times that height. Rhynia grew in dense cover with Horneophyton and Asteroxylon. Asteroxylon, which grew to about 50 cm (20 in.), had erect stems clothed completely with scalelike leaves. Rhynia and Horneophyton reproduced by means of spores borne in spore cases at the tips of stems. Asteroxylon had kidney-shaped spore cases borne laterally, directly on the main axes.