Photo by Doug Beckers licensed under CC BY-SA 2.0

For all the attributes we apply to the world of bryophytes, height is usually not one of them. That is, unless you are talking about the genus Dawsonia. Within this taxonomic grouping exists the tallest mosses in the world. Topping out around 60 cm (24 inches), Dawsonia superba enjoys heights normally reserved for vascular plants. Although this may not seem like much to those who are more familiar with robust forbs and towering trees, height is not a trait that comes easy to mosses. To find out why, we must take a look at the interior workings of bryophytes.

Mosses as a whole are considered non-vasular. In other words, they do not have the internal plumbing that can carry water to various tissues. Coupled with the lack of a cuticle, this means that mosses can be sensitive to water loss. For many mosses, this anatomical feature relegates them to humid environments and/or a small stature. This is not the situation for the genus Dawsonia. Thanks to a curious case of convergent evolution, this genus breaks the physiological glass ceiling and reaches for the sky.

Photo by Salsero35 licensed under CC BY-SA 4.0

Unlike other mosses, Dawsonia have a conduction system analogous to xylem and phloem. Being convergent, however, it isn't the same thing. Instead, the xylem-like tissue of these mosses is called the "hydrome" and is made up of cells called "hydroids." The phloem-like tissue is called the "leptome" and is made up of cells called "leptoids." These structures differ from xylem and phloem in that they are not lignified. Mosses never evolved the ability to produce this organic polymer. Regardless of their chemical makeup, Dawsonia vascular tissue allows water to move greater distances within the plant.

Another major adaption found in Dawsonia has to do with the structure of the leaves. Whereas the leaves of most mosses are only a few cells thick, the leaves of Dawsonia produce special cells on their surface called "lamella." These cells are analogous to the mesophyll cells in the leaves of vascular plants. They not only function to increase surface area and CO2 uptake, they also serve to maintain a humid layer of air within the leaf, further reducing water loss.

All of this equates to a genus of moss that has reached considerable proportions. Sure, they are easily over-topped by most vascular plant species but that is missing the point. Through convergent evolution, mosses in the genus Dawsonia have independently evolved an anatomical strategy that has allowed them to do what no other extant groups of moss have done - grow tall.

Photo by Jon Sullivan licensed under CC BY-NC 2.0

Photo Credits: Wikimedia Commons, Doug Beckers, and Jon Sullivan