"Sigillaria" was a tree-like plant reaching a height up to 30 meters, with a tall, single or occasionally forked trunk that lacked wood. Support came from a layer of closely packed leaf bases just below the surface of the trunk, while the center was filled with pith. The long, thin grasslike leaves were attached directly to the stem and grew in a spiral along the trunk. The old leaf bases expanded as the trunk grew in width, and left a diamond-shaped pattern, which is evident in fossils. These leaf scars were arranged in vertical rows. The trunk had photosynthetic tissue on the surface, meaning that it was probably green.