Earth Science: Seedless Plants – Mosses
Mosses are small plants that can be found nearly everywhere on Earth. They are especially common in damp, shady environments.
Based on differences in their anatomy, mosses can be divided into two large groups: liverworts and leafy mosses. The liverworts (Hepaticae) include some 10,000 species, which can again be divided into two groups: the thallose liverworts, in which the vegetative body has a lobed shape, and the leafy liverworts, which have small vertical or horizontal stems and small leaves without a central vein.
The leafy mosses (Bryopsida) include approximately 15,000 species, which always possess a vegetative body divided into small stems and leaves, usually with a central vein.
The highly complicated sexual reproduction cycle of mosses depends on the production of spores. The cycle begins when a spore germinates, producing a tiny chain of cells called a protonema. Through budding, the protonema develops into a green moss plant (gametophyte), which attaches to the ground using fine cell threads called rhizoids.
Rhizoids are not true roots, but are more comparable to the root hairs of higher plants. The gametophyte then develops female and male sex organs, called archegonia and antheridia, respectively. These may be found on different plants or on different branches of the same individual plant.
The archegonia are bottle-shaped vessels that hold the developing egg cells after fertilization by sperm cells from an antheridium. This can only occur in the presence of water (for instance, from rain), because the tailed sperm cells must actively swim to the egg. The fertilized egg cell develops into a sporophyte, which grows on top of the gametophyte, depending on it for water and nutrients.
The sporophyte takes the form of a capsule on a stem, and the spores develop within the capsule, beginning a new cycle. Thus an alternation of generations between the sexual (gametophyte) and spore-producing (sporophyte) phases is characteristic of this reproductive process.
PEAT MOSSES
Peat mosses {Sphagnum sp) play a key role in the development of peat bogs. Unlike most other plants, they are able to thrive in environments with stagnant water and low nutrient levels Peat mosses can store large amounts of water, and their tips are constantly growing while their lower parts die off, leading to the accumulation of a thickening deposit of dead plant material.
The upper layer is further thickened by the living peat mosses, which can often reach 20 times their dry weight by absorbing large amounts of water. Because the dead plant material decays very slowly in the resulting acidic and low-oxygen environment, a massive peat deposit can form over time.
BASICS
VEGETATIVE REPRODUCTION Many mosses reproduce not only sexually, but also asexually.
In the species Marchantia polymorpha, for example, buds develop from individual surface cells in small reproductive vessels.