DK Science: Seedless Plants
Ferns, horsetails, mosses, and liverworts do not produce flowers or grow from seeds. The life cycles of these plants have two distinct stages – one in which SPORES are produced, and one in which sex cells (sperm and eggs) are produced. Most seedless plants live in damp and shady habitats. Certain types of mosses, called PEAT MOSSES, grow in vast expanses of wetlands in the northern parts of the world.
Bryophytes do not have true roots. They have hairy, rootlike growths called rhizoids that anchor the plants to the soil, but do not draw up water. Instead, their leaves absorb moisture in the air. Because they need little or no soil in which to root, bryophytes are often the first plants to colonize thin soil. Like the liverwort in this picture, bryophytes can also grow on bare rocks.
The horsetails alive today are very similar to the types of horsetails that lived hundreds of millions of years ago, before there were any flowering plants. At this time, seedless plants dominated the land, and giant horsetails made up some of the earliest tall forests. Fossils of these prehistoric horsetails have been preserved in rocks from this period.
Adult ferns produce spores in capsules inside chambers on the underside of their leaves. In dry conditions, the capsules release the spores into the air. When a spore lands on moist ground, it develops into a tiny, heart-shaped structure called a prothallus. This produces the sex cells. Fertilized by male sperm, the female egg of the prothallus develops into a new adult plant.
The leaf of a fern is known as a frond. At first, a young frond is curled up into a structure called a fiddlehead. The fiddlehead has this shape because its lower surface grows faster than the upper surface. As the plant matures, the frond unfurls. Fiddleheads of certain kinds of ferns have been used as a source of food, but some contain poisons.
Mosses and liverworts are known as bryophytes. Adult bryophytes produce the sex cells. Fertilized female eggs then grow into a stalked sporophyte, or spore capsule. Once they are released, the spores develop into the next generation of moss.
Spores are minute, independent cells. Unlike sex cells, spores can divide on their own to make many-celled bodies. They have a simple structure, which consists of genetic material encased in a protective coat that can survive dry conditions. When spores land on damp ground, they grow into a plant that produces sex cells.
Spores are dispersed in vast numbers by wind or water. Fern spore capsules crack apart when they dry out. Most moss capsules have a mouth covered with a lid. When the spores ripen, the capsule lid falls off, revealing inward-turning teeth that block the mouth of the capsule. In dry weather, the teeth open outward, and the spores disperse.
Peat mosses, which are also called sphagnum mosses, grow in wetland areas known as peat bogs. These mosses have a spongy texture and can absorb large amounts of water. To get all the minerals they need, peat mosses use special chemical reactions that release acid by-products into the surrounding soil.
Peat is dead peat moss and plant matter that has collected in peat bogs over hundreds of years. Peat forms in layers, which are compressed by the weight of water and living moss on the surface. Over time, the living layer dies down and is replaced by new moss. Peat is harvested and dried for use as fuel and fertilizer. The overuse of peat threatens peat bog habitats.
