Ralph E. Taggart, Professor
Department of Plant Biology
Department of Geological Sciences
Michigan State University
All the more complex land plants possess a vascular system made up of two tissue types - xylem and phloem. For this reason, the term vascular plants is virtually synonomous with complex land plants.
Xylem cells are dead at maturity and consist of three main cell types - tracheids (the most primitive water-conducting cells), vessels, and fibers (used primarily for support). The xylem transports water (and dissolved mineral nutrients from the soil) by means of passive cohesion of polar water molecules, driven by the evaporation of water (transpiration) from the plant. Thus, water transport, transport of mineral nutrients, and cooling of the plant are all driven by solar energy.
Phloem cells are living cells that transport the products of photosynthesis using energy derived from metabolism. Phloem cells are very delicate, which makes understanding how they function very difficult. It is known that phloem tissues can transport different materials in different directions, and at different rates simultaneously.
Because all higher land plants possess this distinctive
vascular system, they were thought to be monophyletic and were placed
in the Division Tracheophyta. Much later, Harlan Banks, a paleobotanist
from Cornell, noted that the earliest land plants seem to be of two distinctly
different types, suggesting that the Tracheophyta were polyphyletic
- unacceptable in terms of a phylogenetic system of classification.
The Rhynia Type
Early in this century, Kidston and Lang described two species in the genus Rhynia, named for the village of Rhynie, in Scotland, where the fossils were discovered.
The Rhynia plants consisted of an underground stem (rhizome) with simple, erect dichotomous branches bearing terminal, ovoid sporangia. The stems must have been photosynthetic, since the plants lack any trace of leaves. The rhizome was equipped with simple rhizoids that may have helped anchor the plants, but true roots were lacking. The sporangia split at the tip, releasing the airborne spores. The life cycle was essentially fern-like and fossils, presumably of the gametophyte stage of the life cycle, have recently been described. In terms of age, Rhynia is Middle Devonian and the fossils (petrified in a chert probably derived from a marsh) were found along with somewhat more complex vascular plants. Initially, two species of Rhynia were described, but in recent years the larger of the two has been moved to its own genus - Aglaophyton.
Later, in rocks as old as Late Silurian, an even simpler Rhynia-type plant, called Cooksonia, was found:
While the largest species of the Rhynia-type (Aglaophyton major) reached as much as a meter in height, Cooksonia was a very small plant only a few inches tall. However, the pattern of growth is the same as Rhynia/Aglaophyton, with terminal sporangia (globose in this case) that open terminally, no roots or leaves, but with a rhizome and erect, dichotomous branches.
The Rhynia-type plant lineage was ancestral
to most groups of land plants, with the exception of the lycopods.
A reconstruction of a late, mid-Devonian landscape from eastern North America (Knight, Field Museum of Natural History). The large, skeletal-looking plants in the foreground (A) are Protolepidodendron, an ancestor of the tree-sized lycopods of the Carboniferous. The smaller rush-like plants in the foreground (B) are Calamophyton, an early example of a horsetail. The palm-like plants in the background (C) are Eospermatopteris, a plant that resembled a primitive tree fern.
The Rhynia and Zosterophyllum-type plants of the earliest Devonian evolved very rapidly, as there were many potential niches in the new terrestrial habitat. Since these simple plants tended to be crowded into the moist habitats surrounding bodies of water (some may actually have been aquatic, with their rhizomes in shallow water), competition for light was a major factor in the evolution of all groups. The plants generally increased in size and the complexity of branching:
Psilophyton, a Middle Devonian plant first described by Dawson from the Gaspe' Peninsula of Quebec in the mid-1800's. Clearly linked to the Rhynia-type lineage, with its terminal sporangia and the dichotomous branching in the smaller branches, this plant was larger and the main axis shows a tendency toward monopodial branching. This is the type of branching seen in most advanced vascular plants, with a larger primary branch and smaller secondary branches. Not shown here are small flaps of photosynthetic tissue (enations) that covered the branches. These were not true leaves (they had no vascular tissue) but would have served to increase the total area of photosynthetic tissue. Psilophyton was the first really early land plant to be described and primitive land plants were, for a time, referred to as psilophytes.
By the end of the mid-Devonian, several lineages were developing true leaves, the first of these being primitive lycopods that evolved leaves by vascularization of simple enations. Leaf evolution was slower in other lineages in the Rhynia-type line, involving flattening of terminal branch systems and infilling by photosynthetic tissues. The early evolution of leaves by lycopods may have given them a distinct competitive advantage. Leaves, with the increased loss of water they produced, created selective pressure to improve water absorption, leading to the appearance of true roots. While most Devonian plants were free-sporing, with fern-like life cycles, the first seed plants appear by the end of the period.
The potential for growth in height was initially limited
by the fact that the early land plants had only primary growth -
the stems could grow in length but not in diameter. By late
mid-Devonian, secondary growth, where new vascular tissue could
be added, increasing the diameter of the stem, had evolved in some
lineages. This permitted to evolution of tree-sized growth forms.
The first of these consists of large logs, up to 100 ft. long and several feet in diameter. These petrified logs are called Callixylon. As shown above, their xylem or wood (resulting from secondary growth) is very much like that of advanced gymnosperms such as pine. Distinctive patterns of pits on the walls of the xylem tracheids (above, left) make the wood easy to recognize, even with a hand lens. Callixylon was thought to represent a pine-like tree that grew to quite large size.
The second widespread plant fossil consisted of large, fern-like leaves widely distributed in offshore black shale deposits. These fossils were named Archaeopteris and were thought to represent a large tree fern.
The long-held idea of two large Upper Devonian plants - one a pine-like tree and the other a tree-fern, was demolished when Charles Beck, a paleobotanist at the University of Michigan, described fossils where the leaves of Archaeopteris were attached to Callixylon stems! Here was a plant with advanced stem anatomy yet relatively primitive leaves! Beck created a new group, the Progymnosperms, for plants of the Archaeopteris/Callixylon type.
The Devonian was a period of very rapid evolution in land
plants, fostered by competition for light and water. By the end of the
period, most major lineages of spore bearing plants, the Progymnosperms
(possibly ancestral to more advanced tree-like gymnosperms), and the first
seed plants had appeared. Plants ranged in size from small, herbaceous
growth forms to large trees, setting the stage for the Carboniferous period,
where we would see the first true forests.
Ralph E. Taggart (email@example.com)