But you’ve got to be tiny
I remember that when I got back to Scott Base from a stroll up Observation Hill and showed our botanist Keith Thompson the photo of a cluster of moss plants he exclaimed: “Wow, you found a forest in Antarctica!” And now I wish to get a little deeper into the topic of a moss being called a forest.
Mosses are Bryophytes, usually rather small plants with thin leaves of just a single cell layer and no proper root system. Even the tallest moss, Dawsonia from New Zealand, with a spore-bearing stalk up to 50 cm tall, lacks like all of the other smaller Bryophyte species, the typical vascular system and flowers of the higher plants. These tough little plants, with an ancestry that goes back 400 million years or more, were planet Earth’s terrestrial pioneers. With the exception of some freshwater species like Fontinalis antipyretica (there are no mosses in the sea) and those of swamps and bogs like Sphagnum, whose dead layers form the peat, Bryophytes are known to grow on almost anything that at least occasionally becomes soaked in water: stones, tree trunks, bottles, roofs, monuments, and even other living plants and animals. The weevil Symbiopholus of Papua Niugini may support the growth of moss on its back and sloths, too, can have mosses in their fur. What is sometimes referred to as “moss caterpillars”, however, are not caterpillars with moss on their backs, but the larvae of nymphalid butterflies that possess moss-resembling protuberances on their bodies.
As a student in the Botany course many, many years ago, the plants I loved most were the true mosses (not including liverworts). The stage in their life cycle that has leaves and we recognize as a moss, has sexual organs and represents the gametophytes. Yes, mosses possess so-called antheridia (which produce bi-flagellated sperm) and archegonia (which produce the egg cells). The reproductive organs, depending on the species, may be on the same or two different plants. The plants and thus their sperm and egg cells are haploid. To produce a fertilized egg, a sperm needs to reach archegonia where it can find an egg cell. For that ‘journey’ the sperm needs to swim and must wait until there is sufficient moisture. Once an egg cell has been fertilized, a new structure (attached to and growing out of the moss’ leafy gametophyte) develops and becomes noticeable as the thin and sometimes several cm tall, unbranched sporophyte. This structure possesses diploid cells and develops at its tip a spore-containing capsule. The spore-producing cells undergo meiosis, so that the spores are all haploid, some with male and some with female traits. When the spore capsule opens to release the spores, the wind carries them to various places or in the rare cases of species that grow on carrion or dung little flies may carry away spores. Should the spores land in a suitable spot, they grow into a thin threadlike protonema, which resembles a green alga, before changing into the more familiar moss with its little stems and green leaves. Being able to soak up water (but also surviving months without it), these moth gametophytes form a habitat for a multitude of invertebrates.
Most famous of the latter are the cute tardigrades, champion survivors, just like the ever present rotifers and some tiny roundworms. Using a magnifying glass and even with the naked eye one would almost certainly encounter springtails, various mites, tiny eyeless arthropods known as Protura and Diplura, and probably very small flies, minute beetles, book lice (Psocoptera) and thrips. If one is really lucky, one may come across some pseudo-scorpion, a top predator in this micro-world. Easily recognizable as relatives of centipedes, but much smaller and thinner, are the multilegged and pale Symphyla and Pauropoda. Yellow Geophilus centipedes, ants and tiny micryphantid spiders would be the giants in this ‘forest’ and visible with the naked eye, but to spot the many ciliates and bacteria that are present, you’d need a microscope. Organisms like larval craneflies, caterpillars of micropterygiid mini-moths, and stigmaeid mites, feed on the mosses’ green parts and some nematodes are even known to induce the formation of tiny moss plant galls. Unfortunately, such a variety of different organisms would not have been present in my Antarctic “moss forest”, but take a clump of green moss sometime and go exploring: take a walk on the wild side (of the mini world).
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