Life Cycle of Bryophytes

Classification

• Kingdom – Plantae
• Sub-kingdom – Embryophyta
• Division – Bryophyta
• Class – Bryopsida
• Order – Funariales
• Family – Funariaceae
• Genus – Funaria

In plants, two types of plant bodies are produced during their life cycle, i.e. the gametophyte and the sporophyte.

• Gametophyte – It is the haploid plant body and is responsible for the formation of gametes. The fusion of gametes results in the development of the diploid plant body, the sporophyte.
• Sporophyte – It is the diploid plant body and is responsible for the formation of haploid spores. These spores germinate and result in the development of the haploid plant body, the gametophyte.

The development of the sporophyte from the gametophyte and vice versa is called alternation of generations.

General characters

Habitat – moist, damp and shady places like river beds, rock crevices, etc.

Body structure

Gametophyte

• It is the main (dominant) plant body.
• In its juvenile form, the gametophyte is green, photosynthetic and filamentous, and is called the protonema.
• In its mature form, the gametophyte is erect and leafy, having root-like, stem-like and leaf-like structures called rhizoids, cauloid and phylloids respectively.

Sporophyte

• It grows as a parasite on the gametophyte body.
• The sporophyte is photosynthetic but obtains its water and nutrients from the gametophyte body.

Bryophytes lack vascular tissues, i.e. xylem and phloem.

Reproduction – both asexual (budding, fragmentation) and sexual modes are present.

Life cycle of mosses

Haplo-diplontic life cycle

The life cycle of mosses starts in the form of single-celled structures called spores (n). These spores are haploid cells, produced by meiotic divisions in the parent plant. The bryophytes (mosses) produce only one type of sexual spore, and hence are called homosporous.

These spores are dispersed by special spore dispersal mechanisms present in the sporophyte (2n) plant body (discussed later) and are carried by the wind.

Gametophyte (n)

On finding a suitable site, these spores start to germinate and divide by mitotic divisions. This results in the development of the juvenile gametophyte called the protonema (n). The development of this filamentous stage helps in rapid growth, as the protonema (n) divides asexually by fragmentation.

The juvenile gametophyte now starts producing buds, which grow vertically to form the leafy gametophyte (n), having slight differentiation of body parts. Each leafy gametophyte develops two branches: one bearing male sex organs (antheridia) at its tip, called the male branch, and the other bearing female sex organs (archegonia) at its tip, called the female branch.

Male branch

The tip of the male branch is surrounded by a group of leaves called perigonial leaves (n). Numerous male sex organs called antheridia are also present at the tip. These antheridia are surrounded by a layer of sterile cells (n) called the jacket, and contain androcytes (n), which are fertile haploid cells that will mature into male gametes later. Long, thread-like structures called paraphyses (n) arise from the tip, which help in the accumulation of water and in the release and transfer of male gametes. During the rainy season, water accumulates at the tip due to the paraphyses. The antheridium (n) releases the androcytes (n) into the accumulated water, and these androcytes (n) now transform into male gametes called antherozoids (n). Each antherozoid is a long, spirally coiled and biflagellate male gamete.

Female branch

The tip of the female branch is surrounded by perichaetial leaves (n) and also bears paraphyses (n). The female sex organs, the archegonia (n), are surrounded by a sterile layer of cells forming the jacket. Each archegonium has a long and narrow upper part called the neck (n) and a swollen base called the venter (n). The neck contains 6–10 cells called neck canal cells (n), and the venter contains a single venter canal cell (n) and a single large egg cell (n). During the rainy season, water accumulates at the tip of the female branch too.

Gamete transfer and fertilization

The antherozoids (n) may be transferred from a male branch to a female branch along with splashes of water drops. The NCC (n) and VCC (n) of the archegonium dissolve to release chemicals. This clears the path for the entry of male gametes into the archegonium and also directs the chemotactic movement of male gametes.

The antherozoid (n) fuses with the egg (n), which results in the formation of a diploid cell called the zygote (2n). The zygote represents the first cell of the sporophytic generation. The zygote now divides mitotically to produce the embryo (2n), and as the developing embryo is retained in the gametophyte permanently, the sporophyte plant body develops over the gametophyte plant body as a partial parasite (as it obtains water and nutrients from the gametophyte).

Sporophyte (2n)

The sporophyte plant body is divided into 3 parts:

• Foot (2n) – penetrates the gametophyte to provide anchorage and absorb water and nutrients.
• Seta (2n) – long part which helps to expose the capsule for spore dispersal.
• Capsule (2n) – swollen tip of the sporophyte, which is protected by a haploid structure called the calyptra (n) during the initial stages of development. The capsule contains diploid cells called spore mother cells (2n), which divide by meiosis, resulting in the formation of haploid spores (n). When spore formation is complete, the tip of the capsule, called the operculum, ruptures and exposes tooth-like structures called peristome teeth. Each capsule has 2 rings of teeth with 16 teeth in each ring: an outer ring called the exostome and an inner ring called the endostome. The dispersal of spores occurs due to the hygroscopic nature of the outer ring of peristome teeth. These spores are now dispersed by the wind and germinate to develop a new gametophyte.

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