Describe the life cycle of moss plant and explain in the phenomenon of alternation of generation

Gametophyte of Moss:

Moss plant is a gametophyte. It consists of a unbrnched ort branched shoot with spirally arranged small green leaves. At the base of shoot, numerous multicellular rhizoids are developed. These rhizoids functions as roots. Funaria, a moss is normally monoecious and bears male and female sex organs at the apices of two different shoots of the same plant. The male sex organ is known as antheridium and female sex organ is known as archegonium.

Antheridium:

Antheridia are borne in clusters in the apex of the male shoot is called as antheridial head. Antheridial head is surrounded by large number of leaves called perichaetium. There are also large numbers of uniseriate multicellualr hair like structures with swollen terminal cells present among the antheridia called paraphyses. Each antheridium is short stalked and club-shaped structure. It is covered by single layer of jacketd contains numerous male gametes called antherozoids. Each antherozoid is a small spirally coiled and biflagellated structure. When antheeridium matures, jacket layer bursts releasing antherozoids in mucilage covering which is soon dissolved to release antherozoids into water.

Archegonium:

Archegonia are borne in clusters at the apex of the female shoot called Archegonial head. Normally female shoot arises laterally from the base of male shoot. Archegonial head is also surrounded by leaves and Archegonia are also interspersed with paraphyses. Each Archegonium is a short stalked and flask-shaped structure. It consists of a basal swollen Venter and a long neck. The Venter contains an egg cell at the base and a ventral canal cell. The neck contains six vertical rows of cells surrounding a number of neck canal cell.a Venter is surrounded by a wall of 2-3 layers of cells.

Fertilization:

When Archegonium matures, the neck canal cells and ventral canal cell are dissolved to form a passage. Archegonium secrets mucilage and sugar, which attract the antherozoids. Antherozoids swim towards the opening of the neck and enter into the Venter. Only one Antherozoid fuses with an egg resulting in the formation of zygote or an oospore. Zygote is the first cell of sporophyte.

Sporophyte:

The diploid zygote grows inside the Venter of the Archegonium into a very complex structure called sporogonium. Sporogonium grows on the gametophyte as semiparasite. It draws its nourishment from the gametophyte as semi parasite. It draws its nourishment from the gametophyte and also synthesizes its food to some extent. It consists of foot, seta and capsule.

Foot:

It is a basal part of sporogonium and is buried in the apex of archegonial head.

Seta:

It is a pear-shaped structure covered at its distal end and helps in conduction of food.

Capsule:

It is a pear-shaped structure covered with calyptras at its apex which soon falls off. The capsule is differentiated into operculum, middle theca and basal apophysis. The operculum is a cup-shaped structure covering the apex of the capsule. The central tissue of the capsule is compact and sterile and is called as colomella. Spore sacs contain spore mother cells, which undergo meiosis and form haploid spores. Surrounding the spore sacs, there are large numbers of air cavities, which are separated from one another by trabuculae. The outermostlayer of capsule is the epidermal wall, which contains few stomata at lower region. The apophysis consists of compactly arranged, elongated and thin-walled cells at the center surrounded by spongy parenchyma containing choloroplasts.

Germination of Spores:

When spores are maturated, operculum is thrown off and these are scattered by the wind. Under favourable condition, each spore germinates into a green and branched structure called protonema. Lateral buds are developed in protonema. These buds later on are detached and grow into a new moss gametpphyte.

Alternation of generation:

Every plant completes its life cycle passing through two generations, the gametophytic generation and sporophytic generation. These two generations may be independent or gametophytic generation may be independent and sporophytic generation depends on it. When gametophyte and saprophyte are structurally similar, such alternation of generation is called isomorphic type. When saprophyte is very much elaborate and complex and gametophyte represents only few cells or one cell, such alternation of generation is called cyclosporae type.

In moss, the gametophyte is haploid in nature and reproduces sexually. It produces male and female sex organs called antheridia and archegonia. These sex organs produce male and female haploid gametes by mitosis, which undergoes fusion to start the saprophyte. Thus, saprophyte becomes diploid in nature and reproduces asexually by spores. Spores are produced after meiosis and are haploid in nature. They germinate to form the gametophyte. In this manner, the two generations alternate with each other.

Moss plant is an independent gametophyte, which alternates with dependent saprophyte. It exhibits a true alternation of generation.