Dawsonia is a classification of mosses. This genus is a member of the Polytrichaceae family. Dawsonia plants are commonly known to grow taller and have thicker leaves than other common mosses.
Mosses are known to grow when hydrated, however most common mosses lack cuticles and internal transport tissues. This makes it difficult for them to maintain substantial amounts of water and they typically do not grow tall.
Dawsonia are an exception to this common rule. Dawsonia mosses are acrocarpous, meaning they have vertical stems with terminal reproductive structures at the end of the axis allowing them to grow significantly taller. The tallest known moss in the world is the Dawsonia superba species. Its stem may reach up to 19.75 inches tall.
Dawsonia have similar conducting systems to that of vascular plants. Stems are made up of water conducting tissue and elongated cells. Species of Dawsonia will have a more complex cell structure on its leaves than a common moss. Lamella or chains of small cells increase the surface area of cell walls available for carbon dioxide uptake. In addition, layers of moist air are kept between its margins, which helps greatly with water loss.
Image Caption: Dawsonia superba in Abel Tasman National Park, New Zealand. Credit: Velela/Wikipedia (CC BY-SA 2.5)