So the other day I stumbled upon the paper "Roles of bryophytes in stream ecosystems" by the Stream Bryophyte Group in the Journal of the North American Benthological Society (1999). I consider myself interested in anything weird, and yet I'd never really thought about bryophytes in streams.
And why should I? Small streams are often thought of as being primarily heterotrophic (that means more energy is being consumed within the stream than is being produced) because of large inputs from terrestrial systems and low productivity in-stream due to shading from riparian vegetation. What primary production that occurs is usually done by algae attached to rocks or logs. And I use the word usually here, because that's what you're usually going to see in text books and manuscripts on the subject. Vascular plants are just not as important in most streams as they are in terrestrial or even lake ecosystems (so the mantra goes).
There are lots of reasons for this: Shallow, frequently dessicated habitats, storm-related high flow events, and a lot of competition for light from riparian vegetation. The streams in which I have observed lots of vascular plants avoid all those conditions (open canopies, slowly rising and falling water levels). And of course, that is completely beside the point of this post.
You see, bryophytes are not vascular plants. That's really their only distinguishing characteristic. Bryophytes include things like mosses, liverworts, and hornworts. If you walk outside, right now, and look around, you're almost certainly going to see plants. And the plants you see: Probably all vascular. Vascular plants include Christmas trees, your lawn, those pretty flowers that are dead because its winter, the old oak in the backyard. Some comically huge percentage of all terrestrial plants in the world.
Vascular plants are so-named for the enclosed system they use to transport materials around within the plant (xylem and phloem). If you put a gun to my head and asked me to discuss the corresponding system in humans, I would probably point towards the blood vessels, although it isn't a perfect analogy. Regardless, bryophytes don't have that.
Bryophytes are a paraphyletic group. Quick, who remembers what that means? Oh, you, in the back. Very good. As we've mentioned previously in this space, a paraphyletic group is one that does not include all the descendents of a common ancestor. Evolutionarily speaking, the vascular plants emerged from a branch within what we call the bryophytes. Some debate exists about which bryophytes are most closely related to vascular plants, but let's just leave the taxonomy for someone else.
Among the interesting things about bryophytes is that they are gametophyte-structured (I assume there's a word for this, but I can't find it). Some of you may be asking...wha-? Ok, let's talk briefly about sexual reproduction.
Within any sexually reproducing organism there are two types of cells: Diploid and haploid. Diploid cells contain two copies of every gene, and haploid cells contain one.
Let's talk about how this works in humans. An adult human is composed mostly of diploid cells. The cells in your skin, your arms, your brain, virtually every tissue of your body is diploid. These are the cells that form the basis of your body's structure. These diploid cells contain two copies of every gene, one from your mother and one from your father. And you may ask: How did I get those genes?
As it turns out, there is one type of cell in your body that is haploid: Your gametes. In humans, males produce gametes we call sperm, and females produce gametes we call eggs (or ovum). Reproduction occurs when these two haploid cells come together to make a new diploid cell. If everything goes right, that new diploid cell grows up to be a new human.
Now, lets go back to byrophytes. Byrophytes operate in the opposite manner. The visually obvious 'structure' of mosses and liverworts are haploid cells. They only contain one gene copy. The gametes they produce, called antheridium (male) and archegonium (female), are diploid cells, containing two of each gene. The weirdness inheret in this system is fascinating, and could really fill its own post. Sufficed to say: This kind of things keeps biologists up until the wee hours of the morning.
What else should we mention on the subject of bryophytes? In the paper that got me started on this whole avenue of thought, the authors discuss the incredible hurdles that bryophytes face with reproduction in streams. Getting the gametes together is difficult. Free swimming antheridium have to get to the archegonium in order for sexual reproduction to occur, and when you're getting washed away downstream, that's not easy to achieve. Some species produce male and female plants (others have both on the same plant), and as you can imagine, when males and females are further apart they are much less likely to reproduce. You can also add in the problem of vegetative reproduction. Because the plant is much more likely to grow by just spreading from rock to rock vegetatively, in all likelihood any antheridium (male gamete) pushing out to find a female is going to just run into its own clone. As the authors of that paper point out:
"Given the obstacles to successful fertilization of bryophytes in streams, it is interesting that it can occur at all." - p 153
And indeed, they go on to document that essentially no one has seen in happen in a stream and its damned hard to cause to happen in a lab under controlled conditions.
I've really only scratched the surface of what is known about bryophytes here. To be honest, humankind has barely scratched the surface of what could be known. According to the same authors, during the 10 year period from 1989 to 1999 less than 1% of all papers indexed in Biological Abstracts addressed bryophyte ecology. Of course, its been almost ten years since this was written, so somebody needs to see if this paper spurred a boost in that research.
Somebody like me?
Paper referenced above is available on JSTOR (not open sourced though):
Stream Bryophyte Group. 1999. Roles of bryophtes in stream ecosystems. Journal of the North American Benthological Society 18:151-184.