Well, considering this weekend is Robbie Burns' 250th Birthday anniversary, I thought it would be apropos to write about an important little plant that was found in Rhynia, Scotland.
Rhynia is north of Edinburgh and west of Aberdeen, and it is here that Rhynie Chert is found. This rock was formed when silica rich water from geysers evaporated and hardened. Within this rock, plants were fossilized with amazing detail and it is the first described vascular plant. It comes from the Devonian era, but it was not the first discovered vascular plant, as some poor guy named Dawson found an earlier vascular plant before these guys, but no one believed him.
In Canada Dawson found a fossilized plant from the Silurian era which is evidence for early vascularization of plants. Everyone thought at the time that the plant was too old to have vascular tissue (not believing that it could be evolved from that time), but he was eventually vindicated after Kidston and Lang found Rhynia gwynne-vaughanii in 1917. The plant Dawson found is currently the earliest known vascular plant from 430 MYA (million years ago) called Cooksonia sp.
In the chert are compressions of these Rhynia gwynne-vaughanii plants, the darker areas that clearly show the stems and sporangia at the terminal positions (tops) of the stems. In some compressions, annular thickenings of the secondary cell wall in xylem can be seen, elevating the status of this plant to "vascular". The cross section shows that this plant had stomata, basal patterning, but the stele (vasculature arrangement) is quite noticeable in the middle. The plant seen in the chert is the sporophyte, with recently found smaller gametophytes that are not dependent on the sporophyte like some derived plants (A pattern found is the more derived; the smaller the gametophyte). Also, more basal plants have sporophytes dependent on the gametophyte indicating that this plant may be a transition plant. This plant is known to be a sporophyte because of the sporangia on top of stems. Inside the sporangia, tetrads of spores have been found, indicating that meiosis has occurred (meiosis only in diploid; and therefore sporophyte).
Paleobotany is an interesting field filled with excitement, as new discoveries are being made and interpreted all of the time. Fieldwork and lab work combine to shed new light on plant beginnings. Most of all, it makes it difficult to decide which field I want to be in! I want to thank my prof R. Stockey (great lecture!) ; and the following ref's on the net:
Down the River Unto the Sea
11 hours ago