Bryophytes and Mosses

4.1.1. Bryophytes

Almost all bryophytes are small plants, ranging height from 1 to 20 cm (0.3 to 8 in.). Because gravity restricts the processes of osmosis and diffusion, nonvascular plants grow close to the ground. Bryophytes also lack the rigid tissues that vascular plants have to support vertical growth. Some bryophytes, however, grow to a large size. Most are aquatic species that live in rivers and streams. Supported by the buoyancy of water, these aquatic bryophytes can grow larger than terrestrial species,

Because they do not have vascular tissues, bryophytes do not possess true roots, leaves, or stems. What appear to be "roots" and "leaves" in bryophytes are not specialized structures like those of vascular plants, but mere elongations of the "stem". The roots of vascular plants anchor the plant body and absorb water from the soil. Rootlike rhizoids in bryophytes perform the same functions but do not channel water to other parts of the plant. The upper parts of bryophytes obtain moisture through their leaves, which absorb water through pores. Bryophytes leaves are usually only two cells thick.

The life cycles of bryophytes exhibit alternation of generations. The gametophyte (n) produces gametes (n) by mitosis. During fertilization, the gametes fuse. The resulting zygote grows into the sporophyte (2n), which produces spores (n) by meiosis. When these spores germinate, they develop into the new gametophyte (n) generation. In all bryophytes, the gametophyte is the dominant form. In other words, the gametophyte is the green leafy plant that makes up the major portion of the organism’s life cycle. The phylum Bryophyta is grouped into three classes. Over 9,500 species are included in the class Muscopsida, the mosses. About 6,000 species of liverworts belong to the class Hepaticopsida. The smallest class, Antherocerotopsida, has about 100 species. Members of this class are commonly called hornworts.

Mosses

Mosses are small, soft plants that grow in clumps close together. They grow in a wide variety of moist, shaded habitats – on the sides of trees, in sidewalk cracks, on rocks and logs. Some mosses form a dense carpet on the floor of coniferous forests. The greatest member of mosses grows in areas of high humidity, such as the Olympic and Great Smoky Mountains, the rain forests of the tropics and in colder regions as well.

The body of a moss is composed of “leaves” arranged in a spiral around a central stem. Moss plants range in size from 1 or 2 cm (0.4 to 0.8 in.) to more than a meter (39 in.) long. Moss plants may stand erect or trail along the ground.

As in all bryophytes, the dominant generation in the moss life cycle is the haploid gametophyte. This form is the familiar green, leaf like moss plant. The sporophyte generation of mosses, which appears as a stalk tipped with a spore-bearing capsule, does not photosynthesize. Because it is dependent on the dominant generation for nutrition, the sporophyte remains physically attached to the gametophyte throughout its life.

Life cycle begins when the sporophyte (2n) releases spores. When the environment is suitably warm and moist, a spore (n) will germinate and produce a horizontal filament called the protonema. Protonema cells contain chloroplasts and carry out photosynthesis. As the protonema grows, it periodically produces buds that develop into additional gametophytes. Gametophytes produce gametes through mitosis. Sperm are produced in the male reproductive structure called antheridia. Each female reproductive structure or archegonia, contains an egg cell. In some species of mosses, both antheridia and archegonia are found on the same plant, but other species have separate male and female plants.

Because bryophytes transport materials by osmosis and diffusion, they need a large and constant supply of water to survive. Bryophytes also need water for sexual reproduction. Like algal sperm, bryophyte sperm must swim to the egg to fertilize it. For these reasons, most bryophytes grow in moist environments such as riverbeds, rain forests and low-tying areas where water tends to collect.

Because mosses are among the first plants to grow in otherwise barren areas, they are sometimes called pioneer plants.

They colonized tiny bits of rock. This process slowly creates new soil. Mosses rhizoids help prevent erosion by anchoring existing soil. When the bryophytes die, their bodies add organic matter to the soil. Over time, in extremely fertile mixture is formed called topsoil. Many vascular plants need topsoil in which to grow.

One kind of moss, sphagnum moss is commercially important. Sphagnum is the main component in peat moss, an organic fuel used in homes in Ireland, Canada, and other Northern countries. Peat is made up of the decomposing bodies of bryophytes.

Ferns

Approximately 12,000 species of ferns have been identified, more than any other group of seedless vascular plants. Most ferns prefer moist, fertile soil and live in the tropics but ferns have adapted to almost every climate. Certain types of ferns are even found in every cold areas north of the Arctic Circle or high atop mountains. Ferns have a wide range of sizes as well. Some are very small plants but others grow as tall as trees.

Date: 2014-12-22; view: 1026