What are bryophytes?
Bryophytes informally called ‘moss’ refers to three morphologically diverse plant phyla: mosses (Bryophyta), liverworts (Marchantiophyta) and hornworts (Anthocerotophyta). Bryophytes are generally small, but some aquatic and epiphytic species can reach 1 m or more in length. They occur in almost all terrestrial habitats, from littoral to montane zones and from the polar regions to the tropics.
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Like the flowering plants and ferns the majority of bryophytes make their own food via photosynthesis and because they contain chlorophyll, the majority are green. However, They lack lignified vascular tissue to move water and dissolved substances around efficiently, and do not possess roots but have threadlike rhizoids that attach them to their substrate.
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The mosses, liverworts and hornworts are believed to have evolved from ancestral green algae some 500 Mya and are thought to comprise the earliest lineages of plants. Because of their nature and small size, they are often overlooked by the scientific community and virtually unknown to the general public other than as the green they may have seen growing on walls, trees and other moist areas. But their feature and complexity under the microscope easily puts them on a par with their higher plant relatives.
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1. Moss vs Liverwort vs hornwort
This is a commonly asked question and, unfortunately, not a straightforward one to answer. Certain structures are found only in leafy liverworts – complicate-lobed leaves, underleaves, oil bodies and cell wall corner thickenings (trigones) for example. Whether they are present or not depends on the species though. Conversely, leaf nerves are only found in mosses, but not all moss species have nerved leaves.
Moss: This is perhaps the one that most resembles tracheophytes when in the field. They are composed of leaves arranged spirally around a central stem and can be branched or not. Their morphology can be divided into two types: pleurocarpous and acrocarpous. Pleurocarpous mosses are branched, sporophytes develop on the stem/branch and they tend to grow horizontally along the substrate. Acrocarpous mosses are unbranched, sporophytes develop at the end of the stem and they tend to grow upright.
Acrocarpous moss
Pleurocarpous moss
Liverwort: Liverworts are broadly divided into two types based on morphology: leafy and thalloid liverworts. Leafy liverworts have stems with well differentiated leaves and thalloid liverworts are composed of thalli loosely differentiated fleshy lobes, which can be arranged in rosettes or be spreading. Thalloid liverworts are further sub-divided into simple and complex thalloid. Simple thalloid usually have a midrib and two unistratose lateral wings but no specialized tissues. In contrast, complex thalloid have storage cells, air pores and air chambers
Leafy liverwort
Thalloid liverwort
Hornwort: Hornworts are a much smaller group of plants, only around 150 species wordwide. They resemble thallose liverworts, but have horn-like spore-producing structures unlike the capsules of either mosses or liverworts. All hornworts have symbiotic internal colonies of Nostoc, a cyanobacterium, which reveals itself as dark spots within the thallus.
2. Life cycle
All bryophytes share a fundamentally similar life-cycle that involves distinct relatively long-lived haploid phase (or gametophyte) and short-lived spore-producing diploid phase (sporophyte).
The gametophyte traps light energy and converts it into food for both generations. The sporophyte, on the other hand are parasitising and never detaches from the gametophytes, and is in fact useless for photosynthesis.
Multicellular haploid gametophytes develop from spores and produce antheridia and archegonia. Antheridia produce many biflagellate sperm by mitosis, and each archegonium produces a single egg. The sperm need a film of water to reach the archegonium. The zygote develops into the diploid sporophyte, consisting of a stalk and a single sporangium, which emerges from the archegonium.
Asexual propagules
Bryophytes, especially mosses, are clonal organisms. All bryophytes are able to spread vegetatively through fragments and propagules.
Clones have the advantage of maintaining moisture, but have the disadvantage of being genetically identical.
3. Life strategy
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Why don’t bryophytes sporophytes branch?
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Branching is suppressed to save energy (sporophytes are useless for photosynthesis)
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Why are there no tall bryophytes?
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Because of their reproductive system
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Sperms need to swim from antheridia to the archegonia
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Lack of structure to access deep soil water to provide anchorage
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They lack lignified water conduction
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How do bryophytes deal with water challenges?
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Low growth form, simple plants
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Abundance in damp habitats (however most diverse in dry situation)
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Reading on Bryophyte life history
4. Growth form
Pendent are mostly epiphytes, occur especially in tropical cloud forests. They have long main stem with short side branches. In humid, montane tropical forests, pendant form provides the most surface area for interception of the limited light without sacrificing moisture in this humid climate. Furthermore, they are able to trap water from mist and clouds. However, the great exposure makes them vulnerable to air pollution.
Dendroid are tree-like moss with their main stem with tuft of branches at top. Dendroid mosses would seem to be particularly vulnerable to desiccation, with only a single stem in contact with the substrate and many exposed branches.
Weft: are loosely interwoven, often ascending growth form. Weft mosses are exhibiting well-defined annual branching, they hold considerable capillary water; grow loosely and they are easy to remove from substrate. New layer grows each year
Mat: densely woven, horizontal growth form. Weft and mat are quiet similar
Turf are growth form with stems erect, parallel and close together; they often cover extensive areas.
Cushions are growth form with stems more or less erect, tightly clustered and somewhat radiating at edges. Cushion mosses are formed as exposed shoots are broken off by force of wind, abrasion, and desiccation
5. Habitat
Bryophytes are one of the most successful groups of plant as they are found on every continent and all terrestrial habitats. Despite their small size, bryophytes can occupy many different king of habitat, making them conspicuous in many habitats.
