Mosses are nonvascular flowerless plants that belong to the scientific category Bryophyta . The parent group bryophytes, which includes liverworts, mosses, and hornworts, is sometimes known as Bryophyta .Mosses grow in dense green clumps or mats, usually in wet or shaded areas. Individual plants are often made up of basic leaves that are only one cell thick and are joined to a stem that can be branched or unbranched and plays only a minor function in water and nutrient transport. Although some species contain conducting tissues, they are typically underdeveloped and physically distinct from vascular plant tissue. Mosses lack seeds and form sporophytes with unbranched stalks topped by solitary capsules carrying spores following fertilisation.

Characteristics of Mosses

• The plant exists as a haploid gametophyte which is the dominant stage, and diploid sporophyte.
• The leaves are simple and small, spirally arranged and sometimes layered with only one row of cells and thick midribs.
• The roots are present in the form of thread-like structure called rhizoids which help them in attaching to the substrate.
• The stems are weak and free standing, and are usually greenish to brown colored.
• It reproduces by spores (since no seeds are found).
• The sporophytes are short lived and have long, unbranched stems. They are mostly dependent on the gametophyte for water and nutrition.

Life Cycle of Mosses

The life cycle of mosses alternates between the haploid gametophyte and the diploid sporophyte called alternation of generation. The male and female gametophyte produces haploid gametes, which fuse to form zygote and give rise to diploid sporophyte. The sporophyte then produces haploid spores which germinate to haploid gametophyte.

Gametophyte

• The gametophyte is structurally differentiated into stems and leaves, and forms the male and female sex organs at their tips.
• The female sex organ are known as arechegonia which are shaped like bottle containers and only one cell thick. They are protected by modified leaves known as perichaetium.
• The male sex organ is known as antheridia and is a tiny, stalked and club shaped structure. They are protected by modified leaves known as perigonium.
• The antheridium mature to release antherozoids, which are biflagellate in structure. They swim in water and fertilise with the egg of the archegonium.
• The formation of diploid zygote leads to the second life phase of mosses, that is the sporophyte.
• The archegonium later goes on to divide to form calyptra which acts as a protective structure for the capsule in sporophyte.

Sporophyte

• The diploid zygote develops into a diploid sporophyte. A sporophyte consists of a foot, a long stalk called seta, a capsule which is capped by the operculum.
• The sporophyte remains attached to the gametophyte much like a parasite, dependent for food and water on the gametophyte, and divides by mitosis.
• The capsule has spore producing cells which undergo meiosis to form haploid spores.
• The capsule has teeth like structures called peristomes which prevents the falling off of spores in wet conditions.
• When the conditions are favorable, that is, the spores are ready to be dispersed, the operculum and peristome fall off and the spores are dispersed in the environment.
• The spores on falling on a wet damp ground germinate to form protonema, which are threadlike, filamentous structures. Protonema acts as a transitional structure which is later developed into a gametophyte, completing the life cycle.

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By Team Learning Mantras