Moss is something we often overlook. To the untrained eye, moss is nothing but a furry, blurry, juvenile mess plaguing the trees and ground around us. Neglected due to the lack of personality and forgotten due to our own callowness. We completely disregard the wonders of this amazing species. Would the sense of normalcy go unaffected if the image of moss were to disappear?
Moss, this primitive species has kept itself alive, even with the decaying air quality, rising environmental issues, and lack of human acknowledgement or care, through some pretty impressive means. Being so unique in structure has enabled them to develop different designs for themselves. Although, these designs are all created to achieve that one crowning goal- survival, and the most efficient survival to say the least.
Evolved to fit whatever said habitat; Moss have included necessary components into their makeup. They’ve had to ensure that the damage from desiccation and /or rehydration was limited to a minimum, maintained cellular integrity in desiccated state, and also activate or mobilize repair mechanisms upon rehydration. As non-vascular plants, they don’t have specialized conductive tissues so they take moisture in directly through their cell walls. The method or the structure of the said moss type weigh heavy on how much moisture the moss can hold, but also for how long they can hold onto it.
“The drying rate, length of desiccation, intensity of desiccation, prior dehydration and temperature, all have an effect on the ability of desiccation”.
Acclimation means the physical change that adjusts to new conditions. Moss are very susceptible to changing along with the atmosphere around them. “Depending on the relative humidity of air this can be a slow or rapid progression to equilibrium and dryness; the higher the humidity the slower the drying rate”, prior dehydration and temperature, all have an effort on the ability of desiccation”.
Although moss isn’t what most people recognize as aesthetically pleasing, there is a beauty hidden within this furry, blurry, mossy moss. The trick is looking close enough to see it.
In this article, we are going to be talking about the physics of how exactly mosses are like sponges. Mind-blowing, right?
Mosses are a part of a plant group called bryophytes that include other plants like liverworts and hornworts. Bryophytes are an early plant group on the evolutionary tree of plants and they work a bit differently than the complex flowering plants that dominate the plant world today.
Plants are believed to have come from a freshwater origin, and when they first colonized land their first concern would have been developing mechanisms that would help them survive without having a water constantly surrounding them.1 Bryophytes give us a peek into how plants first started this evolutionary process.
So moving onto land wasn’t as easy as it sounds. There was an extremely high risk of plants dying out due to drying out or what’s called desiccation. Many modern day plants especially flowering plants developed a nifty system to help transport water throughout their entire body but plants didn’t just have this feature from the start.2 Bryophytes don’t have this feature but fear not, they had their own trick up their sleeve.
TOLERANCE VS AVOIDANCE! DESICCATION SURVIVAL SMACKDOWN
Ok, so it’s not actually as extreme as it sounds. Evolution isn’t really a contest to see who’s better or not. Just because mosses are considered to have “primitive” features doesn’t mean that vascular plants, which have a more complex structure, are better at surviving. So really we’re just going to examine the differences between desiccation tolerance and desiccation avoidance.
So most modern day plants use desiccation avoidance which means that they have mechanisms that help them avoid desiccation.3 These plants wouldn’t be able to survive through desiccation so they just evolved to make sure that they don’t go through desiccation. They are essentially dodging a bullet.
Bryophytes ,on the other hand, are desiccation tolerant which means that they have mechanisms that help them survive through desiccation.3 These plants run headlong into desiccation and endure it. Moss are boss!
So desiccation tolerance is cool and all but how do they do it? Well, I’m pretty sure, I mentioned a sponge analogy earlier and that seems like a pretty good place to start. When you leave a sponge out, it gets really dry and hard. But once you add some water it becomes larger, squishy and wet. If you leave the sponge out again, it will again return to a dry and hard state. That’s how moss works. Generally. But to understand how this works mechanically, we gotta use physics.
BUT WHAT DOES PHYSICS HAVE TO DO WITH ANYTHING?
A whole lot. And to understand how mosses work, we need to understand how water works. Water has physical properties such as cohesion and adhesion. Cohesion is when the water molecules are attracted to other water molecules and adhesion is when water molecules are attracted to other molecules like the surface of glass. Another important phenomenon that happens that relates to cohesive and adhesive forces is capillary action. Capillary action is the tendency of a fluid to be raised or suppressed in a narrow tube. Both the cohesive/adhesive properties and capillary action make up the physics of water uptake in mosses.4
If this is new knowledge to you, here’s a quick crash course if you want further information on the physical properties of water.
Instead of through the roots, water uptake happens across the moss surface which is where the adhesive property of water comes into play. When the water is absorbed through the surface, the actual movement of the water is facilitated by capillary action. The water is then taken from the surface and into the plant via this method. Once inside, the water is transported through the cells via diffusion.3
Because of its ability to quickly re-hydrate after a period of desiccation, moss has been referred to as a type of “resurrection plant”. There has even been talk of scientists using this unique feature in order to create crops that can withstand dry periods.
MOSS BUDDIES: THE COOL CRITTERS THAT LIVE IN MOSS
By the way, did you know that mosses and other bryophytes aren’t the only ones that are desiccation tolerant! It just so happens that some cute microscopic critters that live in moss are also in on the fun. And one of these critters is called a rotifer! Rotifers collect food from the water that that is being uptaken by the moss and even more interesting is that when the moss go through dry periods, the rotifers are inactive! This is known as cryptobiosis which refers to the state of an organism when it shows no visible sign of life. This means the rotifer’s productivity correlates with the productivity of the moss!5 For information on this, there’s a link below.
SO WHY DOES IT MATTER?
So mosses are like sponges. But what does this have to do with anything? The physics of how desiccation in moss works is useful because then we can understand the mechanics of what we are seeing and also have a better understanding of the physiology of mosses.
Also, it’s kind of a cool party trick.
 J. Oliver, J. Velten, B. D. Mishler, Desiccation Tolerance in Bryophytes: A Reflection of the Primitive Strategy for Plant Survival in Dehydrating Habitats? Integr. Comp. Biol. 45, 788–799 (2005).
 Proctor, M.C., Oliver, M.J., Wood, A.J., Alpert, P., Stark, L.R., Cleavitt, N.L. and Mishler, B.D., 2007. Desiccation- tolerance in bryophytes: a review. The Bryologist, 110(4), pp.595-621.
 Glime, Janice M. 2007 Bryophyte Ecology. Volume 1. Physiological Ecology. Ebook sponsored by Michigan Technological University and the International Association of Bryologists. accessed on _____ (provide date you accessed it) at http://www.bryoecol.mtu.edu/ .
 OpenStax College, College Physics. OpenStax College. 21 June 2012. <http://cnx.org/content/col11406/latest/>.
 Quirós, K. Science, These “Resurrection Plants” Spring Back to Life in Seconds. KQED Science, (available at http://ww2.kqed.org/science/2015/06/25/these-resurrection-plants-spring-back-to-life-in-seconds/).
Most organisms will evolve to use the least amount of energy necessary. In other words, if an organism doesn’t need to do something to survive, it probably won’t. (Same…)
Due to natural selection, most species fulfill some sort of niche in which they can thrive without competition (or at least with as minimal competition as possible – every species encounters competition, as resources like space, water, nutrients, and sunlight are constantly limiting factors in population growth).
All the factors that contribute to evolution has produced a vast and diverse pool of species that can be traced back to a common ancestor. The phylogeny of living organisms has been studied thoroughly, and the tree of life produced shows the relationship between them:
One of the ways in which the vastly broad categories of kingdoms (such as plants, animals, and fungi) are split up is by phylum. The kingdom of Plantae includes such phyla as Pteridophyta, Coniferophyta, Lycopodiophyta, etc. One of the most notable phyla is the Bryophyte phylum, which includes mosses, hornworts, and liverworts.1 Bryophytes are one of the most primitive phyla, being the first plants to colonize land. They are closely related to green algae (or charophytes), which is considered to be a common ancestor to all plants. In fact, the earliest moss fossil known to us is dated to be 320 million years old.
All Vascular Plants Have Mosses to Thank
Indirectly, of course: the phylum to which mosses belong bridges the gap between aquatic green algae and vascular land plants. Mosses are like distant cousins of the vascular phyla.
In general, byrophytes are low-growing, spongy, and prefer moist environments. There are a few distinguishing characteristics of bryophytes:
Unlike most land plants, bryophytes are gametophyte dominant.1 Most plant’s lifecycles are sporophyte dominant, where the green, fleshy parts of the plant are diploid – containing two sets of chromosomes. However, the green, fleshy parts of bryophytes are haploid: the cells only contain one set of chromosomes.
Mosses do not have vascular tissue. Plants which evolved post-bryophyte contain vascular tissue – xylem and phloem – which helps to transport nutrients and water into the furthest reaches of the plant. There are a few major consequences to bryophytes not containing vascular tissue:
Mosses prefer to grow in moist environments.* Food and water are essential in order to not die! The moss needs to grow in an environment with easy access to water, because they do not have xylem or phloem to move water to more remote areas of the plant.3
Mosses are generally low growing. Xylem and phloem are essential to move water and nutrients up long distances.
*Mosses don’t just like to grow in moist environments because they are thirsty. They also need it to reproduce. Water droplets spread the motile sperm on the antheridium to the egg inside the archegonia.3
Mosses in the wild often go overlooked when compared to more flashy plants. However, they are an important contributor to the stability of almost all the ecosystems in the world.2
Mosses are built to last. Their evolutionary traits allow them to thrive in almost any environment, so long as there is water that is easily accessible. One of the qualities they employ is their ability to survive arid conditions. If one were to go into the woods of the Pacific Northwest on a sunny day in the summer, the mosses would probably look shriveled up and dead. However, these mosses will likely bounce back when it inevitably rains again – this is called desiccation tolerance.
The Holy Trinity
As stated before, the Bryophyte phylum consists of liverworts, mosses, and hornworts. So how are these groups different?
Most of the differences are in morphology. All three groups have the same defining qualities of bryophytes.
 Jonathan Shaw, Peter Szovenyi, Blanka Shaw, Bryophyte diversity and evolution: windows into the early evolution of land plants. American Journal of Botany. 98, 352–369 (2011).
 E. Newton, C. J. Cox, J.G. Duckett, J. A. Wheeler, B. Goffinet, A.J. Terry, Evolution of the major moss lineages: phylogenetic analyses based on multiple gene sequences and morphology. American Bryological and Lichenological Society. 103, 187-211 (2000).