THE POLYSPORANGIATES

Plants in this phylum were among the simplest of the vascular plants. Indeed, they may include plants that did not have vascular steles, and, therefore, should be considered pre-vascular. What does seem to be true, though, is the polysporangiate nature of the persistent sporophyte. That is, plants in this group, whether fully vascular or not, had long-lived sporophytes that continued to grow forming multiple sporangia. This character alone separated them from their bryophyte precursors, which had determinant sporophytes, each with a single capsule.

HORNEOPHYTON

These were small plants (about 20 cm tall) with an endarch central strand (Figure 2). The xylem cells had a reticulate thickening around the cell wall. The base of the stem is swollen (corm-like) with rhizoids emerging from them. Terminal sporangia are cylindrical and branched. Also, they have a columella, sporangial structure seen only in certain of the bryophytes, specifically Sphagnopsida, Andreaeopsida, and Anthocerotophyta. The occurrence of the columella in the sporangium maybe a morphological character that connects the more primitive of the mosses (Bryophyta) and the hornworts (Anthocerotophyta) to the most primitive of the polysporangiates. Indeed, Qiu et al. (2006) indicate that of the living embryophytes, the hornworts are the sister group to all living tracheophytes.

The archegonial gametophyte has been identified (see Figure 2). It is upright (about 6cm high) terminating in an enlarged disk with archaegonia over the upper surface of the disk. Presumably, these archaegoniophores emerged from a corm-like base (Taylor et al. 2005).

These plants are well known because they were among the dominant taxa in the lower Devonian Rhynie Chert community of Scotland. Very likely, this line emerged in the middle to upper Silurian.

AGLAOPHYTON

The single genus in this class superficially resembles Rhynia. Indeed, it was formerly called R. major. It is part of the Rhynie Chert assemblage of plants, but it has characters that cause Kenrick and Crane (1997) to question its position as a vascular plant. That is, its water-conducting cells are smooth and tapered, more like the hydroids of Polytrichum. The central strand is endarch in its maturation pattern and the stems, which are about 15 cm high show a pattern of sparse dichotomous branching. The terminal sporangia are fusiform and dehisce obliquely. Notably, the capsules do not have columellas.

The gametophyte (the antheridial gametophyte) has been identified as a small aerial plant with cup-like structures on which the antheridia occur. Lower Devonian

VASCULAR PLANTS

All plants in this clade have vascular tissue that includes tracheids, water-conducting elements that have a double wall which is lignified. Friedman and Cook (2000) indicate that the land plants appeared in the Silurian, but fossils of tracheids appeared only in the upper Silurian and lower Devonian. Until the advent of tracheids, land plants were very small. Then, the tracheid appeared and plants began their rapid shift to the evolution of larger structures.

Tracheids are elongate cells that communicate by pores and slits with other tracheids thus making a continuous partitioned tube through which water can move. Though such cell exist in the bryophytes (e.g. hydroids), the tracheid innovation is two wall layers, with an outer wall that is impregnated with lignin, which is resistant to decay. Moreover, lignin strengthens the cellulose wall and makes it somewhat hydrophobic, both of which make water transport more efficient. The added strength of the wall also allows the transport of water by transpiration, a mechanism that maintains the column of water under negative pressure while stomates are open. Thus, plants with tracheids and other xylem elements can be the conduit for pulling water hundreds of feet above the ground. Such cells function to transport water only when they are dead.

The rhyniopsid xylem cells had annular thickenings typical of tracheids; however, the inner wall of the tracheid, although lignified, was very thin, and the outer wall was spongy. This distinctive rhyniopsid tracheid was termed S-type by Kenrick and Crane (1997) and found only in the Rhyniophyta. Later, xylem evolved larger pores and holes which made the movement of water more efficient by reducing the resistance to movement.

The movement of food materials is by specialized tissue is found throughout the embryophytes. Such vascular tissue, called phloem, operates while the cell is alive. The primary components are sieve tubes, which communicate cell-to-cell like the tracheids. They also have smaller associated cells called companion cells that help to load and unload materials from the sieve tubes.

Xylem and phloem are arranged in axes in particular ways as cylinders, usually with the xylem internal to the phloem. Such associations of vascular tissue are called steles. The earliest and simplest stele has the xylem in the center surrounded by phloem. When the xylem and the phloem are simple concentric cylinders, that association of vascular tissue is called a protostele.

When an axis develops and grows from the division of the apical meristem, the different tissues axis differentiate in particular ways. Xylem usually develops in one of two ways: exarch and endarch (also called centrarch) primary xylem. In an endarch stele, the earliest or primary xylem develops in the innermost (or central part) of the stelar axis. Exarch stele has primary xylem that appears first on the outer parts of the xylem followed by secondary development toward the inside.

RHYNIOPSIDA

These were the earliest true vascular (tracheid bearing) plants on earth. Most of the rhyniopsids were small, but some taxa were quite robust and grew to 50 cm high. They appeared in the fossil record during the middle Silurian and persisted until the middle Devonian.

All members of this group (except Nothia, Figure 5) had xylem cells with annular rings. Because Nothia seemed otherwise advanced, the absence of annulations would seem to be a loss rather than a primitive condition. All rhyniopsids had an endarch pattern of development. The upright stems, which branched dichotomously or with slight unequal branching, emerged from branched prostrate axes. Some of the upright axes terminated in sporangia that ranged from fusiform to globose to reniform.

Rhynia (Figure 6) was a dominant plant in the Rhynie Chert community in the lower Devonian of what is now Scotland. The plants had a branching horizontal rhizome from which dichotomizing upright photosynthetic axes arose. Many terminated in sporangia. The upright axes stood about 18 cm high and grew in a wetland area in association with a silica-rich hot spring, which provided the exceptionally detailed preservation of the fossils. A section of a Rhynia stem (Figure 7) illustrates the nature of the early stele. The xylem, composed of a few S-type tracheids, was in the center. That was surrounded by a thin-walled tissue presumed to be phloem. The bulk of the axis was made of cortex in which photosynthesis occurred and photosynthate was stored.

Cooksonia (Figure 8) appeared in the middle Silurian and was one of the earliest land plants known. Many of its stems (horizontal axes are unknown) had small dark-staining strands that have been interpreted as xylem. Its growth and physiology may have been very different from that of Rhynia because, as Boyce (2008) argues, Cooksonia had axes that were much too small (.05-3mm in diameter) to support photosynthesis. Thus, he argues that the sporophyte of Cooksonia must have been dependent upon a photosynthetic gametophyte that did not preserve.