XVI International Botanical Congess

Paleobotanical studies suggest that Silurian and Early Devonian tracheids have two layers of secondary wall, only one of which is lignified and survives in the fossil record. In contrast, tracheids of previously studied extant plants have secondary walls that are lignified throughout. We found that in Huperzia lucidula, one of the most primitive extant vascular plants, tracheids have two layers of secondary wall. These layers are clearly distinguished with the TEM and respond differently to treatment with wall degrading enzymes. This pattern of secondary wall layers was unknown in tracheids of extant plants, and matches Silurian and Devonian fossil tracheids. Our developmental study of Huperzia provides evidence that allows us to interpret the fossil record and form an explicit hypothesis of early tracheid evolution among vascular plants.