Bryophytes

Bryophytes are found in terrestrial environments with humidity, although some present adaptations that allow them to occupy a greater variety of environments, both by resisting immersion in totally aquatic environments and by surviving dehydration, acting as primary colonizers of, for example, rocks or bare soil, and even in regions of polar freezing. They remain, however, dependent on water, at least for the displacement of the flagellated anterozoid to the oosphere. This division has no representatives in marine environments.
They are eukaryotic, multicellular organisms, except for their reproductive cells, which are unicellular.

Bryophytes are found in terrestrial environments with humidity, although some present adaptations that allow them to occupy a greater variety of environments, both by resisting immersion in totally aquatic environments and by surviving dehydration, acting as primary colonizers of, for example, rocks or bare soil, and even in regions of polar freezing. They remain, however, dependent on water, at least for the displacement of the flagellated anterozoid to the oosphere. These organisms have no marine representatives.

Life Cycle

Bryophytes are diplobiontic, presenting an alternation of generations between a heteromorphic, branched, photosynthetic and independent sporophyte, and a branched and at least partially dependent gametophyte.

The structures of the sporophyte, which occur in specialized environments, develop and produce spores that germinate into gametophytes. In some cases, spores can give rise directly to the reproductive structures of the plant, but normally they first produce a filamentous phase, a uniseriate, branched filament with oblique transverse walls and longitudinal axes (protonema), which will give rise to the leafy gametophyte.

Gametophyte

The gametophyte may be divided into rhizoids, phyllids and caulids. The simplest forms do not present a clear differentiation between phyllids and caulids; they are generally prostrate and called thalli, while those in which these structures are clearly distinguished are normally erect and are called leafy.

At the apex of the gametophyte, reproductive structures called archegonia, which contain the female gamete (oosphere), and antheridia, which contain the male gametes (anterozoids), are formed.

In bryophytes, the zygote germinates while still attached to the mother plant, and the resulting sporophyte remains attached to it throughout its life, showing total or partial dependence.

Sporophyte

Sporophytes are never branched and present different degrees of complexity depending on the group to which they belong. They may be divided into foot, seta and capsule. The immersed foot consists of gametophyte tissue and is responsible for absorbing substances. The seta supports the terminal sporangium, called the capsule, which has an outer wall with a protective function, while the spores are differentiated from the inner layers (sporogenous tissue). In certain cases, when the capsule dehisces, an operculum is formed that opens to allow the spores to pass. The capsule may also be totally or partially covered by the calyptra, formed from tissue of the archegonium that is carried up during development of the sporophyte, providing additional protection. Although the sporophyte is always dependent on the gametophyte, in certain classes of Bryophyta (Musci and Anthocerotae), it can perform photosynthesis, at least at the beginning of its development.

Reproduction

Bryophytes can reproduce in three ways:

1. Gametes: Under suitable humidity conditions, the small biflagellate anterozoids are released from the wall of the antheridium, while the cells of the archegonium open a channel that releases a fluid that guides the anterozoids to the oosphere, thus enabling fertilization;

2. Spores: Release of the spores occurs through hygroscopic movements of the peristome teeth. These movements are caused by variations in air humidity;

3. Vegetative reproduction - four forms:

• Fragmentation: development of a new individual from fragments of the thallus.

• Gemmae (or propagules): distinct, defined structures that will give rise to a new individual. In some species, the gemmae are produced in cup-shaped structures called conceptacles.

• Apospory: development of the gametophyte from the sporophyte without spore formation. Normally, a regeneration fragment of the seta gives rise to a gametophyte. This can result in the formation of polyploid organisms.

• Apogamy: development of the sporophyte from the gametophyte without fertilization. It may occur not only from gametes, but also from phyllids or from the protonema itself.