Portion of a stem of a modern tree fern (probably Cyatheaceae or Dicksoniaceae). Ferns cannot produce true wood. The M- and U-shaped structures are segments of the vascular cylinder, which is called a dictyostele. Each of the individual segments is known as a meristele. Specimen is from the Cornell University Paleobotanical Collection (CUPC), Ithaca, New York. Length of specimen is approximately 14.5 cm. Model by Emily Hauf.