Rhyniophyta

Categories: Evolution; paleobotany; Plantae; seedless vascular plants; taxonomic groups

In 1908, Octave Lignier developed a model of what the sporophyte of the earliest vascular land plants might look like. The sporophyte is a diploid (2n) plant that produces spores in a sporangium, while its counterpart, the gametophyte, is a haploid (n) plant that bears the male (antheridia) and female (archegonia) sex organs. Lignier proposed that the first vascular land plants would consist of a forked, photosynthetic stem lacking both roots and leaves. At the point of branching, the fork looked like a capital Y. Running along the ground or just below the soil surface was a horizontal stem (rhizome) bearing hairlike filaments (rhizoids) on its lower surface that functioned in anchorage and absorption. Spores were produced either within the unmodified stem tips of some of the aerial branches or in a specialized reproductive structure, a thick-walled, elongate sporangium, that terminated some branch tips.

In 1912, some unique plant fossils were found in the Rhynie Chert (406 million to 401 million years old) in Scotland. The Rhynie Chert is a hot-spring chert formed when silica-saturated water from a hot spring flooded a low-lying marsh, seeped into the plants, and hardened within and about them. Because they are encased in chert, the Rhynie plants were the first fossil plants to yield both structural and anatomical data. Two of the plants, Rhynia major (now Aglaophyton) and R. gwynne-vaughanii, were reconstructed as near-perfect matches to Lignier’s model of the earliest vascular land plant. With their discovery, the science of paleobotany came of age.

As knowledge of the rhyniophytes increased, a revision of the group seemed in order. As originally defined, the Rhyniophyta included both vascular (tracheophytes Rhynia and Cooksonia pertoni) and nonvascular plants that are intermediate between bryophytes and vascular plants (Aglaophyton and Horneophyton lignieri). These plants are placed in two groups, the Rhyniophyta and Horneophyta, respectively.

Horneophyta or Protracheophytes

Aglaophyton‘s rhizome was in contact with the ground at intervals, and rhizoids were concentrated at those positions rather than spread evenly along the axis. The tissue of the rhizome contained a fungus, representing the earliest known example of a plant-fungus symbiosis. The vast majority of land plants require this symbiotic relationship to live. The land plant provides the fungus with shelter within its cells and access to the products of photosynthesis. The fungus absorbs water and minerals from the soil, which it passes on to the land plant.

This partnership is so important to living plants that scientists were not surprised to find it very early in the fossil history of the land plants. Dense clusters of buds (possibly dormant stem tips) were found on the rhizome at the base of the aerial stems. The aerial stem was naked and branched by forking into two daughter axes of equal size. Elongate sporangia were borne at the tips of some aerial branches. The water-conducting cells of Aglaophyton lack the internal wall thickenings characteristic of tracheids (thick-walled, dead cells found in the xylem of vascular land plants) and resemble the thin-walled, water-conducting cells (hydroids) of the mosses. Because of the absence of tracheids, Aglaophyton is considered a nonvascular plant and must be removed from the Rhyniophyta. A male reproductive structure (Lyonophyton rhyniensis) represents the gametophyte of Aglaophyton.

Another nonvascularized sporophyte found in the Rhynie Chert was Horneophyton. Instead of having obvious sporangia, the spores of Horneophyton were produced within the stem tip and released by means of a terminal pore. Multicellular projections were found on the stem, which terminated in stomates. The aerial stems did not grow from a rhizome. The aerial stems each terminated in a bulbous, nonvascularized structure called a corm. Several corms were found attached to one another, but no vascular tissue connected them. As in Aglaophyton, the water-conducting cells were thin-walled and lacked internal wall thickenings. Langiophyton mackiei represents the female gametophyte of Horneophyton.

When gametophytes are known in the protracheophytes, they have an anatomical and structural complexity similar to that of their corresponding sporophytes. In contrast, the gametophytes of vascular land plants are small and inconspicuous (often subterranean). Scientists know more about the gametophytes of the plants of the Rhynie Chert than they do about almost all other groups of fossil plants.

Rhynia

Rhynia gwynne-vaughanii was reconstructed as a smaller version of Aglaophyton. The aerial stem of Rhynia was naked and branched by forking. The rhizome bore rhizoids. Thick-walled water-conducting cells (tracheids) were present. Although Rhynia was reconstructed bearing sporangia, none were originally found attached to the aerial stems. In addition to lacking sporangia, Rhynia had bumps along the aerial stem that were interpreted as female reproductive structures (archegonia). As a result, Rhynia was reinterpreted as the gametophyte of Aglaophyton. Subsequently, sporangia were found attached to Rhynia, making the plant a sporophyte once more. The sporangia were borne laterally along the stem on short side branches, and they were shed after releasing their spores. The gametophyte of Rhynia is unknown.

The small, circular vascular strands and fleshy stems associated with the rhyniophytes and horneophytes indicate that these plants did not grow very tall. Water pressing against the inside of the cell walls in their stems (turgor pressure) held these plants upright.

Cooksonia

The oldest vascular land plant (412 million to 406 million years old) and only cooksonioid with identifiable vascular tissue is Cooksonia pertoni. The aerial stems of Cooksonia branched by forking and bore at their branch tips sporangia shaped like kidney beans. Plants whose sporangia resemble those of C. pertoni are known from older sediments (414 million to 412 million years in age), but no vascular tissue has been found in their stems. The oldest fossil plant from the Western Hemisphere that resembles Cooksonia comes from Bathurst Island in the Canadian Arctic (420 million to 414 million years in age). No roots or rhizomes are known for any Cooksonia. If the older specimens of Cooksonia lacked vascular tissue, Cooksonia may not be a valid genus. These plants are best referred to collectively as cooksonioids, and plants showing these traits could be bryophytes, protracheophytes, or rhyniophytes.

Fossil Spores

Fossils spores are known from sediments that are about 40 million years older than those from which cooksonioids are recovered. The most abundant spores were not produced by the vascular land plants and are called cryptospores. Banded tubules and sheets bearing cell outlines (collectively called nematoclasts) are recovered with the cryptospores. Similar structures are produced by the sporangial epidermis of modern mosses (cellular sheets) and liverworts (banded tubes). The cryptospores had a worldwide distribution and showed limited diversity. These spores were probably produced by early bryophytes.

To identify the type of plant that produced a specific spore, researchers must find the spore within a sporangium. Although free spores are common, no spores are known from sporangia for the first 65 million years of land plant evolution. The presence of cryptospores and the occasional spore that might have been produced by a vascular land plant indicates that land plants first appeared about 500 million years ago. Most of the 65-million-year period from which sporangia are unknown was characterized by high sea levels, and little continental deposition occurred. The missing continental deposits are where the necessary sporangia would have been found. The oldest spores from continental deposits are about 435 million years old. Cryptospores are found in sporangia from England that are between 414 million and 412 million years old. Although they predominated early, cryptospores form a very minor component of the spore flora about by 414 million years ago. The decrease in the number of cryptospores reflects the growing importance (diversification) of the vascular land plants.

Ancestors of the Rhyniophytes

The first land plants, bryophytes, protracheophytes, and tracheophytes, evolved from green algal (charophycean) ancestors that lived in freshwater habitats whose appearance was unpredictable (not seasonal) and of short duration. Their algal ancestors were preadapted to life on land. They were resistant to microbial attack due to the structure of their cell walls. They had a genetic (heritable) basis for lowering or suspending metabolic activity during drought (dry conditions that could last for days or months) and desiccation (dry conditions that could last for decades). Their reproductive bodies did not dry out during air transport from one water body to another. The first to colonize the land successfully were the bryophytes. The protracheophytes and tracheophytes appeared after the bryophytes were well established on land.

Bibliography

Gensel, Patricia G., and Henry N. Andrews. Plant Life in the Devonian. New York: Praeger, 1984. Good descriptions of the rhyniophytes with valuable illustrations. Illustrations, bibliographical references, index.

Gensel, Patricia G., and Dianne Edwards. Plants Invade the Land: Evolutionary and Environmental Perspectives. New York: Columbia University Press, 2001. Provides the current information on the rhyniophytes in the context of the first land plants. Illustrations, bibliographical references, index.

Kenrick, Paul, and Peter R. Crane. The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D.C.: Smithsonian Institution Press, 1997. Detailed cladistical treatment of the rhyniophytes. Excellent line drawings, bibliographical references, index.