The moss sporophyte is composed of a long stalk or stem-like structure called a seta with a capsule at the tip. The capsule contains plant spores that are released into their surrounding environment when mature.
Spores are typically dispersed by wind. Should the spores settle in an area that has adequate moisture and light, they will germinate. The developing moss initially appears as thin masses of green hairs that eventually mature into the leaf-like plant body or gametophore. The gametophore represents the mature gametophyte as it produces male and female sex organs and gametes. The male sex organs produce sperm and are called antheridia , while the female sex organs produce eggs and are called archegonia.
Water is a 'must-have' for fertilization to occur. Sperm must swim to archegonia in order to fertilize the eggs. Fertilized eggs become diploid sporophytes, which develop and grow out of the archegonia. Within the capsule of the sporophyte, haploid spores are produced by meiosis. Once mature, the capsules open releasing spores and the cycle repeats again. Mosses spend the majority of their time in the dominant gametophyte phase of the life cycle.
Mosses are also capable of asexual reproduction. When conditions become harsh or the environment is unstable, asexual reproduction allows mosses to propagate faster. Asexual reproduction is accomplished in mosses by fragmentation and gemmae development.
In fragmentation, a piece of the plant body breaks off and eventually develops into another plant. Reproduction through gemmae formation is another form of fragmentation. Gemmae are cells that are contained within cup-like discs cupules formed by plant tissue in the plant body. Gemmae are dispersed when raindrops splash into the cupules and wash gemmae away from the parent plant. Gemmae that settle in suitable areas for growth develop rhizoids and mature into new moss plants.
Liverworts are non-vascular plants that are classified in the division Marchantiophyta. Their name is derived from the lobe-like appearance of their green plant body thallus that looks like the lobes of a liver. There are two main types of liverworts. Leafy liverworts closely resemble mosses with leaf-like structures that protrude upward from the plant base.
Thallose liverworts appear as mats of green vegetation with flat, ribbon-like structures growing close to the ground. Liverwort species are less numerous than mosses but can be found in almost every land biome. Though more commonly found in tropical habitats, some species live in aquatic environments, deserts , and tundra biomes. Liverworts populate areas with dim light and damp soil. Like all bryophytes, liverworts do not have vascular tissue and acquire nutrients and water by absorption and diffusion.
Liverworts also have rhizoids hair-like filaments that function similarly to roots in that they hold the plant in place. Liverworts are autotrophs that require light to make food by photosynthesis. Unlike mosses and hornworts, liverworts do not possess stomata that open and close to obtain carbon dioxide needed for photosynthesis.
They often grow in colonies that carpet the ground. Hornworts are minute nonvascular plants, similar in size to liverworts. They also have very fine rhizoids and lack stems.
Their sporophytes are long and pointed, like tiny horns. They rise several centimeters above the gametophytes of the plant.
Mosses are larger nonvascular plants that have coarser, multicellular rhizoids that are more like roots. They also have tiny, photosynthetic structures similar to leaves that encircle a central stem-like structure. Mosses grow in dense clumps, which help them retain moisture.
Summary Nonvascular plants are called bryophytes. Nonvascular plants include liverworts, hornworts, and mosses. They lack roots, stems, and leaves. Nonvascular plants are low-growing, reproduce with spores, and need a moist habitat.
With no vascular tissue, the bryophytes cannot retain water for long periods of time. Consequently, water must be absorbed directly from the surrounding air or another nearby source. This explains the presence of mosses in moist areas, such as swamps and bogs, and on the shaded sides of trees.
The life cycle of the moss is typical of the bryophytes. Flask-shaped archegonia, located among the top leaves of the female gametophytes, produce one egg cell each. The body or thallus of a hornwort resembles that of a liverwort, but the two can be distinguished by their spore cases. Hornworts produce green spikes horns that mature and slowly split lengthwise from the tip down, making and releasing their spores slowly. The major difference between this group and the others is the life cycle.
In all three groups of non-vascular plants the gametophyte dominates the life cycle. Note that in this group, structures that produce sperm are known as antheridia and the structures that produce eggs are known as archegonia.
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