Ferns may descend from the first vascular plants.
Almost all land plants, including flowering plants, conifers like pine trees, ferns, gingkoes and cycads, are classified as vascular plants, or tracheophytes. They all have xylem and phloem, rigid tissues that transport water and nutrients. Because these structures facilitate upward growth towards sunlight, the tracheophytes have greatly succeeded and diversified on land. Most evidence identifies the members of the phylum Rhyniophyta as the first vascular plants. Rhyniophytes arose about 425 million years ago and went extinct around 380 million years ago.
Discovery
The phylum Rhyniophyta is named after Scotland's Rhynie Chert, where British paleobotanists William Henry Lang and Robert Kidston discovered the first complete, clear, abundant fossils of these plants in 1917. Paleobotanists are scientists who study ancient plant life. Composed mostly of silicon dioxide, chert is a type of rock that is sometimes found in layered sediments containing well-preserved fossils. Kidston and Lang were astonished to find vascular plant fossils in such old rock.
Characteristics
Rhyniophyta species were quite similar. Instead of roots, they put down rhizomes, water-absorbing stems that extended horizontally under the ground. Rhyniophytes clearly contained both xylem and phloem. Their stems grew in a dichotomous pattern, meaning that the apex or growing tip of each stem divided into two equally sized branches. These branches ended in sporangia, spore-forming structures. Like ferns and mosses today, rhyniophytes reproduced by spores instead of seeds. They probably grew no higher than 50 centimeters.
Environment
Rhyniophytes existed during the mid-Silurian to early Devonian geological periods, when life first moved from the ocean onto the land and began transforming it. The sea teemed with fish, but land vertebrates were far in the future, as were flowering plants like trees and grasses. Rhyniophytes shared the land with bacteria, algae mats, fungi, short, spore-producing nonvascular plants, arachnids (relatives of spiders) and centipedes. William Purves pictures a landscape "of bare ground, with stands of rhyniophytes in low-lying moist areas." That ground was likely reddish because it lacked an accumulation of organic matter.
Ancestors and Descendants
Land plants like Rhynophytia evolved from green algae in the ocean. Some present-day plants, such as ferns and horsetails, may in turn be descendants of Rhynophytia. So might the progymnosperms, the long-extinct possible ancestors of seed plants. Tracing the lines of ancestry is complicated by the fact that no Rhynophytia live today. In addition, the genus and species breakdown of the phylum has changed over time with new fossil discoveries. However, the genus Cooksonia and the species Rhynia gwynne-vaughanii, for example, have been consistently classified as Rhyniophyta since the early 20th century.
Tags: vascular plants, first vascular, first vascular plants, flowering plants, million years, phylum Rhyniophyta, plants like
