Arborescent Lycopods

Living lycopods (Lycopodium is shown here) are small, inconspicuous plants up to 10-20 cm in height. The plants are characterized by small, spirally-arranged leaves that cover the stem and branches. Spore-bearing cones, consisting of spirally-arranged cone scales (modified leaves) are located in a terminal position.

A magnificent reconstruction of a Carboniferous swamp forest from the Field Museum of Natural History in Chicago. The very large trunks represent arborescent (tree-sized) lycopods such as Lepidodendron, Lepidophloios, and Sigillaria.

Mature arborescent lycopods such as this Lepidodendron (Stewart, 1983) reached a height of over 30 meters. This particular reconstruction shows leaves (Lepidophylloides) confined to the tips of the ultimate branches and leaf scars disappearing on the older areas of the trunk. Specimens from southern Indiana indicate that the typical leaf scars or cushions can be found all the way down to the base of the plant and it is possible that the entire trunk bore leaves as well. Note the dichotomous branching of both the upper branches and the root-like axes.
 

A Stigmaria root cast (Frenkenberg and Eggert, 1969) from Yorkshire England, that measures almost 6 meters across. On the right side is a close-up of a stigmarian axis with the typical spiral arrangement of scars. This regular arrangement suggests that the root-like organs were actually stems and the ribbon-like "rootlets" were actually modified leaves.
 

Impressions of the bark-like periderm are extremely common and characterized by densely-packed leaf scars or leaf cushions. This specimen (DeMichele, 1979) represents Lepidiphloios, since the leaf scars are diamond-shaped in outline, but wider than they are tall.

A reconstruction of Sigillaria (Stewart, 1983), showing the basic stigmarian base, but upper branches less-profusely-branched than a typical Lepidodendron. Sigillaria periderm has circular leaf scars/cushions that appear to be arranged in vertical rows (although their actual arrangement is spiral). As shown here, most arborescent lycopod leaves were long and grass-like and spores were produced by cones in the crown of the plant.

Arborescent lycopods dominated Carboniferous swamp forests until the very end of the period, where they abruptly declined, possibly as a result of moisture stress. Most became extinct in the Permian, although progressively smaller forms survive through the Mesozoic. The modern quillwort (Isoetes) is the last surviving member of the group.