Mosses are an ancient lineage of plants. They are representatives of the earliest attempts plants made at life on land. Like their algal precursors, mosses have motile sperm that must literally swim their way to a female. Of course, this process requires water. For some mosses, living on land makes reproduction difficult, even at the scale of a few centimeters. Distance is not the friend of diminutive, sexually reproducing mosses.
There are some groups of mosses that have evolved an interesting way around the issue of distance. Though it occurs in plenty of other genera, I would like to focus attention on one genus in particular, the Dicranum mosses. You can find these hairy-looking mosses growing in tufts or mats in forests throughout North America. Like all bryophytes, they exhibit an alternation of generations. The green gametophytes house the sexual organs and, after fertilization, give rise to the stalked sporophytes that produce and disseminate their spores.
An inspection of Dicranum patches in the wild may reveal that all of the gametophytes seem to be female. Despite this observation, there would seem to be no shortage of sporophyte stalks poking above the mat. How is this possible? How does sperm make it from some undisclosed male population to fertilize the eggs of these entirely female mats? The answer will come only after you observe the females under a microscope.
Under magnification, you may notice that many of the female gametophytes appear to have hairy little outgrowths scattered around their tiny leaves. Under a higher powered lens you may then notice that these hairy outgrowths contain antheridia, the sperm producing organs of males. What is going on here? Are these mosses hermaphroditic? Nope! What you are seeing are indeed the males of this species.
Spores of Dicranum don't start out as either sex. Instead, their fate in the environment determines what they eventually develop into. If a spore makes it to new terrain, it will become a female. Females are larger and can handle the rigors of establishing new territory. If a spore lands on another clump of moss, something different happens. The female gametophytes emit hormones which direct the development of that spore into one of these dwarfed males. Settled in among a forest of females, this tiny male individual is now primed and ready to release sperm. As author Robin Wall Kimmerer puts it in her book 'Gathering Moss', these males become "a captive mate that will become the father of the next generation in the matriarchy."
For this genus, it doesn't make sense fore males to grow into full blown adults in such situations. The bigger a male gets, the more distance separates his sperm from the eggs of females. A reduction in size allows the males to insert themselves into colonies made entirely of females to serve as the reproductive agent for that grouping. Quite a fascinating life history trait if you ask me. Mosses have also been at the survival game much longer than pretty much all other forms of life we encounter on land. I think it goes without saying that they certainly deserve a greater recognition.
Photo Credit: Wolfram Sondermann (http://bit.ly/1GUR0A6)