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.
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.
