Theme 3-Topic 14
Nonvascular Plants: Bryophytes
Introduction to Bryophytes
Definition and Classification:
Bryophytes are the earliest lineages to diverge from the common ancestor of plants.
They form a paraphyletic group composed of three phyla of small herbaceous (non-woody), nonvascular plants:
Liverworts (Phylum Hepatophyta)
Approximately 9,000 species.
Etymology: hepato = liver, phyt = plant, wort = Old English for herb.
Example: Lunularia cruciata (Liverwort).
Mosses (Phylum Bryophyta)
Approximately 12,000 species.
Etymology: bryo = moss.
Mosses are the most diverse and widely distributed bryophytes.
Note: "Bryophyta" refers only to the phylum of mosses, while "bryophytes" refers to all nonvascular plants.
Hornworts (Phylum Anthocerophyta)
Approximately 225 species.
Etymology: antho = flower, ceros = horn.
Characteristics of Bryophytes
Absence of Vascular Tissue:
Bryophytes lack true vascular tissue and lignin.
The absence of vascular transport systems for water and nutrients limits the size of bryophytes.
Most tissues in bryophytes are only one or a few cells thick, affecting structural support and growth.
Structure of Sporophytes:
Bryophyte sporophytes are unbranched and lack true roots and leaves.
In contrast to vascular plants which have branched sporophytes with leaves and roots.
Bryophytes do not form roots; they absorb water through their surfaces and are anchored by structures called rhizoids.
This limits bryophytes to habitats where water is abundant.
Stomata in Sporophytes:
Sporophytes of mosses and hornworts possess stomata for gas exchange; however, liverworts do not have stomata.
It is noteworthy that no extant gametophytes have stomata.
Life Cycle of Bryophytes
Dominance of Gametophytes:
In all three phyla of bryophytes, gametophytes are larger and longer-living than sporophytes.
Bryophytes have a free-living haploid (1n) gametophyte as the dominant phase of their life cycle.
Diploid (2n) sporophytes are smaller, present only part of the time, and are dependent on the gametophyte.
Development and Fertilization:
The life cycle of a moss begins when a spore germinates into a gametophyte (1n), which is composed of protonema and a gamete-producing gametophore.
Mosses often exhibit separate male and female gametophytes.
Fertilization requires water, as flagellated sperm swim through a film of water to fertilize eggs.
The male reproductive structure, antheridia, produces and releases the flagellated sperm, while the female reproductive structure, archegonia, produces an egg and serves as the site of fertilization (with the egg retained within the archegonia).
Gametes are produced by mitosis within multicellular organs called gametangia (female archegonia and male antheridia).
Asexual Reproduction:
Many bryophytes can reproduce asexually via structures such as brood bodies, which are small gametophyte plantlets that detach from the parent plant and develop into new, genetically identical organisms.
Structure of Sporophytes:
The sporophyte consists of a foot, a seta (stalk), and a capsule (sporangium).
The diploid (2n) zygote, embryo, and sporophyte are retained and nourished by the gametophyte.
Bryophyte sporophytes (2n) are noted to be the smallest and simplest of all plant groups, with their capsules (sporangium) producing thousands of haploid spores (1n) through the process of meiosis.
These spores are dispersed into the air and possess cell walls containing sporopollenin.
Water Requirements for Bryophyte Life Cycles
Fertilization and Dispersal:
Fertilization is contingent upon water presence as flagellated sperm must swim through a film of water to reach the eggs.
In contrast, spore dispersal occurs independently of water, allowing spores to be released into the air.
Ecology of Bryophytes
Habitat Preferences:
Bryophytes thrive primarily in moist forest and wetland environments, where mosses frequently dominate the ground cover.
Example: The moss Sphagnum (often referred to as "peat moss") plays a crucial role in regulating water flow, particularly in peat bogs (also called muskegs) situated in Arctic and boreal regions.
Remarkably, Sphagnum mosses can endure significant water loss, entering a dormant state before rehydrating and reviving cellular function when moisture becomes available.
Peat Formation and Ecological Impact:
Sphagnum bogs contribute to extensive deposits of undecomposed organic material known as peat.
Peatlands are characterized by low temperatures, acidic pH, and low oxygen levels, conditions which inhibit the decay of moss and other organisms.
Importantly, peatlands serve as critical carbon reservoirs, helping to stabilize atmospheric levels of CO2.
Noteworthy Ecosystem Role:
Peat-forming ecosystems, such as muskegs, have significant ecological implications within Arctic and boreal regions by supporting unique habitat dynamics and mitigating climate change effects through carbon storage.