How do mosses survive?
That is kind of a large, broad question. There are several factors that contribute to the survival of any living organism. I will keep this simple and focus three points that contribute to the success of mosses:
Mosses are poikilohydric. You rarely come across this word in biology. Basically it means that mosses do not have the ability to retain water. They do not have advanced tissues to help hold in water during periods of drought to keep photosynthesis going. Think of it as being "cold-blooded" in the plant kingdom: if it is wet, mosses are wet; if it is dry, mosses are dry.
Here's a more in-depth look: The average moss leaf is only 1 to 2 cell layers thick as opposed to several different, specialized layers found in vascular plant leaves. Moss leaves do not have an epidermis, vascular system or even stomata to regulate gas exchange and water retention like higher plants. Nutrients and water are absorbed directly into their cells from their environment. This lack of conductive tissues is also the reason why mosses are so small - they don't have this interior structure to hold them upright and allow them to grow higher like vascular plants. Of course those species that have the hydroids and leptoids (those nearly-vascular tissues) tend to be bigger in form.
Okay, so how is it possible mosses have survived hundreds of millions of years if they can't even master the simple task of holding in water? Amazingly they can lose up to 98% of their water content and still survive! Upon dessication they enter a state of suspended animation. Everything freezes, including respiration and photosynthesis (both of which require water to function), and the plant waits for the arrival of that ever-so-essential liquid. The phrase "patience is a virtue" resonates even within the micro-mossy world.
You have probably already witnessed this neat little adaptation when coming across dried up, crusty clumps of moss on rocks or other dry surfaces. That moss is not dead (unless its yellow or brownish - then maybe it is), but if it is still a shade of green or even blackish in appearance, then it is still very much alive - just not kicking. This ability to go dormant in the absence of water gives moss a special advantage over any other plant: they are incredibly drought tolerant. Need a candidate for your green roof project? Look no further!
Of course this does not mean mosses are immortal. All species have a different level of a dessication tolerance - some more than others, and this can be directly related to their morphological variability (more on that in the future). No moss can live forever in suspended animation, though some can wait weeks, months, years or decades before water comes around again, and they will happily reanimate and go about their mossy lives.
Here's something you could do to watch this reanimation. Next time you come across a shriveled up mass of moss, give it a little drink and watch the leaves thirstily soak up the water and begin to unfurl right before your eyes - some species will even dance for you :)
Mosses are adapted to live in the boundary layer. This is why mosses are happiest in rock crevices or a muddy footprint. I have explained this awesome evolutionary adaptation in my "Wind" post. Be sure to check that out if you have not already!
Finally, mosses have a haploid dominant generation; their dominant generation is the n condition (has 1 set of chromosomes); or to phrase it yet another way: only in bryophytes can you find that the gametophyte is the main generation you see (ergo, dominant) and the gametophyte is always haploid in any plant species. (Please see my What? post for a more detailed/simple explanation of this unique life-cycle).
What possible advantage does having only 1 set of chromosomes in their dominant generation give them? If you think about it, this n condition possesses “immediate fitness”. Any unfavorable environmental conditions would immediately select against unfit mosses; they would not have a chance to pass their genes to next generation. It gives them a "speedy evolution" so to speak. They don't have much of a chance to hold onto recessive genes until conditions become favorable enough to disperse their progeny like a vascular plant can.
Perhaps a story can better explain it. Note: You will need to believe that evolution is, in fact, a real phenomenon!
Once upon a time, on a dry rock face, far, far away, a brand new generation of moss was born. He was named Bryum. It just so happened that the genetic combination of Bryum's parents' gametes resulted in a genetic code that caused Bryum to express a particular trait; Bryum had especially sparse and narrow leaves compared to other nearby populations. Poor Bryum was picked on by the others because he couldn't hold as much water in his capillary spaces like everyone else, and living on this hot, dry rock face, water-holding capacity was pretty essential. Tried as he might, Bryum just could not continue living this way, and after a short while Bryum died; he never even got the chance to make little moss babies of his own...
I didn't say this story would end happily ever after... but here's the moral: you better be suited to your habitat if you want to pass on your genes and give your offspring a fighting chance. It's a good thing that Bryum died, because if he lived long enough to make moss babies he would have passed on his thin-leaf genes them and they would have equally bleak futures. So goes nature's mantra: Survival of the Fittest. Poorly formed mosses will die out a lot faster, killing off those undesired traits at a much faster rate than would happen if they were much bigger, bulkier and tougher; able to weather through the tough times - but that would slow their evolution. So that's why mosses have remained largely unchanged for 350 million years - they were evolving at warp-speed from the beginning!
Chew on this: In bryophytes the sporophyte (wiry stalk with capsule that holds spores) is dependent on the gametophyte (the leafy green part). This means that if the gametophyte dies, it couldn't produce a sporophyte and thus its genes die along with it - that's it, no second chances for that generation. But in vascular plants the gametophyte (pollen and embryo sacs) is dependent on the sporophyte (the tree, shrub, or herb). A tree can keep sending out the same genetic information over and over again, whether or not conditions are favorable. If its gametophytes die, it can just keep on making new ones, dispersing it's genetic information in perpetuity in hopes that some of its offspring will be successful.
Despite their small stature, mosses are quite tough on an evolutionary scale - not to mention innovative! How amazing is it that they can just freeze in time without water and after it returns just pick up right as they left off and go about their lives? (Though the process of rehydration is a little more complicated than that). It's a good thing mosses didn't lose this trait over time, because without these qualities mosses wouldn't have a fighting chance against higher plants. The ability to colonize and adapt to those little spaces otherwise neglected by larger life forms definitely adds another dimension to the natural world - search them out next time and ask yourself "how do they do that?".
Stay tuned for the next installment and learn just how many shapes, sizes and colors mosses come in!