Mosses, Liverworts, Hornworts
Mosses reproduce by dispersing spores from a capsule called a sporangium. The capsule is ringed with tiny teeth that open and close with changes in humidity, so the spores don't drop all at once.
Illustration by Amadeo Bachar.
In the distant mists of the past, around 425 million years ago, green algae made the transition from water to land. This momentous event marked the beginning of life on land for a wide variety of organisms and a major change in evolution. Plants had to find a solution to the problem of desiccation, and reproduction on land posed a new challenge as well.
New DNA research is helping us unravel just how evolution proceeded. With land plants, those first green algae evidently diverged in two different directions. One line led to what we consider the familiar plants around us—the vascular plants, which come equipped with their own plumbing system that efficiently moves water and food. The other line, the bryophytes, took a simpler route: They remained small and dependent on a moist environment for growth and reproduction. Today there are thousands of species of bryophytes worldwide, including what we call mosses, liverworts, and hornworts.
The Bay Area is home to more than a hundred species of bryophytes, ranging from the obscure flattened ribbons of a liverwort like Asterella to the conspicuous “fir-tree” moss Dendroalsia, whose firlike branches decorate the tree bark of madrones and oaks.
Despite their small size, bryophytes hold many surprises, including a gift for adapting to environments that range from desert to rain forest. How is this possible? Moss plants form cushions that consist of dozens of tiny stems anchored to soil, rock, or bark by hairlike rhizoids. They bear tiny, spirally arranged leaves. These leaves are nothing like the leaves of flowering plants, and the rhizoids are not comparable to real roots—neither structure has complex tissues to move water and food or provide a backbone to support the plants, and neither is capable of increased height or girth. The rhizoids are like wicks that directly absorb water and minerals from the soil, and the leaves are tissue-thin, chlorophyll-bearing blades that make photosynthesis possible. There are no veins and hence no pathways to move the food manufactured in the leaves.
- Thalloid liverworts like this Asterella grow as flattened green ribbons directly on the soil surface. Photo by John Game.
But there is a secret here: When they dry out, mosses can go into a state of dormancy that can last several months or even a few years. Then when it rains or the fog drips, moss leaves immediately absorb water through their skins and plump up. If the moisture persists long enough, the cushion will start to grow again. For this reason, mosses and other bryophytes can live where it’s dry most of the year yet still manage to survive. Although they don’t compete well for light and other resources, bryophytes can live in the most unlikely microhabitats such as on tree bark, on rock faces, in shallow soil, or in desert sands. The only true competitors are lichens and small ferns. Mosses often provide a spongelike substrate in which the seeds of vascular plants germinate and get a roothold.
If the main body of the plant seems different from “ordinary” plants, bryophytes’ life cycles also differ. Bryophytes and some more primitive vascular plants such as ferns are called cryptogams (“hidden marriage” in Greek) because their reproductive parts are tiny and thus seem secretive. To illustrate the life cycle, let’s look at moss reproduction.
The humble moss cushions we find are haploid; that is, each cell has only a single set of chromosomes. Unlike with many vascular plants and multicellular animals, a moss’s main mode of existence is this sexual generation, which has only half the organism’s complement of chromosomes. Tiny globe-shaped bodies called antheridia produce sperm, while equally tiny flasks called archegonia hold a single egg each. These structures are not obvious without a microscope—unless, as occurs in some mosses, they’re borne inside what look like tiny green flowers at stem tips.
Fertilization is tied to water just as growth is; only in wet conditions are sperm released to swim to an archegonium. So in the Bay Area, most moss growth and fertilization occurs in the winter and early spring. This journey is more arduous than you might suspect; many mosses produce their antheridia and archegonia on separate plants and the sperm are able to find their way only because the archegonia release hormones.
The fertilized egg now contains two sets of chromosomes and is diploid. At this stage, a new kind of plant—the sporophyte—develops a slender stalk a few inches long, topped by a tiny bulbous sac called the capsule. The stalk grows out of the end of the branch that bore the archegonium with its fertilized egg, so this new generation is dependent on the parent moss cushion for its sustenance. The capsule contains hundreds of cells that divide in a special way to make haploid spores. (Spores are microscopic single cells.) The capsule, called a sporangium, is covered by a miniature cap (calyptra) that turns brown and pops off when it’s time for the spores to be released.
At the far end of each sporangium is a mechanical marvel called the peristome. (You need a very strong hand lens to see this clearly.) Each kind of moss has its own signature peristome but the general mechanism is a series of interlacing “teeth” that shrink when the air is dry and swell when the air is humid. (Think of the way a camera lens opens and closes.) Each time the peristome dries and the teeth open, more spores are shaken out, so the moss doesn’t shed all its spores at one time. The slightest breeze whisks the spores away.
- This Leucolepis moss is found on soil or old logs in damp, shadyplaces. The stems visible at the center are topped by spore-bearingcapsules. Photo by John Game.
When a spore meets a suitable habitat, it sends out a tiny filamentous thread that forms a green mat. Soon, this mat forms buds that grow into a new moss cushion, and the life cycle is complete. The moss cushion itself lives for many years and will expand over time by adding new stems, but the tiny sporophyte plant only lasts a few weeks at best—a bit player that allows dissemination to potential new homes.
Besides mosses, there are liverworts and hornworts. Liverworts are a bit harder to recognize because they fall into two different groups. The thalloid liverworts look like flattened green ribbons that live directly over the surface of the soil. Under a hand lens, you can see a regular, often polygonal pattern of “cells” on the top surface; each of these has a tiny hole that connects inside to a minute air chamber. Leafy liverworts consist of prostrate stems with four rows of very precisely aligned, flattened leaves. (Here, too, you need a hand lens to see the details.) From a distance, many “leafies” look as though they had been spray-painted on tree bark because they’re so flat. Leafy liverworts are most numerous in the fog zone near the coast.
Hornworts—the smallest group by far—consist of tiny tufts of leaf rosettes that look a bit like fanciful miniature lettuces. Because of their size and scattered populations, hornworts would hardly get a second glance were it not for the sporophyte plant, which grows directly out of the center of each rosette. This sporophyte is a spindle-shaped green horn, several inches long, that unravels at the top to discharge spores while the bottom part is constantly growing to replace the tattered ends. Most hornworts live in damp, shady places on the edge of woods and forests.
This winter, when you’re out on a walk, take some time to notice these unappreciated plants growing on rocks, bark, and bare soil. And have fun; the idea is to see a new miniature, hidden, and ancient world.