When in college decades ago, I took a botany course from a professor who lectured in a monotone and who believed the best way to learn about bryophytes—mosses and liverworts—was by rote. Bleak, colorless diagrams projected in the darkened room allowed some of us to doze or slip out the rear fire escape, returning unnoticed before the lights came back on. How, I wondered, could anyone be interested in moss?
A few weeks ago, while walking in a field near home in St. Johnsbury, Vt., I noticed small spots of red in an otherwise dark green carpet of moss covering a rocky outcrop. Their presence piqued my curiosity.
I already knew something about the reproductive cycle of mosses, which, like liverworts and ferns, produce alternating sexual and non-sexual generations. And now with a special camera I was able to record a stage of moss reproduction I had never noticed.
The reddish spots in the field turned out to be the moss’s perigonia, tiny cup-like rosettes of scaly leaflets that house the male reproductive organs on the sexual, or gametophyte, generation. The sperm-producing structures, called antheridia, were almost impossible to detect, hidden as they were between flattened scales.
The male mosses with their colorful perigonia appeared in clusters amidst the more abundant undecorated female gametophytes, which grew nearby, perhaps an inch away, which seemed extremely distant, when measured on a Lilliputian scale.
How do the microscopic sperm cells travel such a long way? They do it by swimming through dew or, after rainfall, a thin film of water, propelling themselves with whip-like flagella.
On a computer monitor my greatly enlarged photos revealed how sperm are launched in this particular kind of moss. Each reddish perigonium is a little “splash cup,” wide at the mouth and narrowing toward the bottom. This shape compresses the force of any raindrop hitting the cup head-on, a force that splashes out water carrying sperm cells. You can demonstrate this effect by placing dry peas in a tapered wineglass and dumping in a cup of water. The result? The peas shoot all over the place along with the splattered water.
Such splashing allows sperm cells to be catapulted an inch or so away from the male parent. This gives sperm a head start before they begin homing in on a female moss’s hormone-secreting, egg-holding reproductive organ, the archegonium.
What happens next? After fertilization, an entirely different generation develops, and this one is the plant’s sporophyte, or non-sexual generation.
The sporophyte emerges as a tall wiry stalk growing vertically, with its base fixed in the leafy female. At first green and photosynthetic like its parent, it soon turns brown as its tip swells into an urn-like capsule with a fitted lid, or operculum, hidden under a hood-like covering, the calyptra. It is in this tight little capsule that vast numbers of asexual spores are produced.
(With your own sharp eyes, you too can view a sporophyte. Select one with a capsule the size of a grain of rice—a brown, dry mature one—and gently pinch the top of the pointed hood with your fingernails and pull it off. Now revealed, the lid appears with a little knob in the center. Using the tip of a needle or pin, pry off the lid and look inside of the capsule at the closely packed spores.)
When the sporophyte’s capsule is fully mature, the dry, shrinking hood shreds and floats off in the wind, exposing the lid, which, in turn, becomes dry and loose enough to pop off. The capsule is now open to the air, and the exposed, dried spores are free to go their way. The slender supporting stalk, vibrating in the breeze, shakes the capsule, throwing out its spores. These spores may land nearby, or they may rise into the sky and catch prevailing winds to a far-off continent.
A chance encounter with tiny red perigonia, and good luck in photographing them, allowed me to examine their highly magnified images on a computer monitor. This, coupled with close study of the wiry little sporophytes that we notice every year, revealed details of a process I had only partially learned about many years before. Too bad my professor didn’t have colorful computer images to flash on the screen, or didn’t keep us awake with demonstrations of peas shooting from a wine glass. If he had, we might have stayed off the fire escape.