Veena Kuchibhotla

external image moss.jpg

Transport of materials: In all three bryophyte phyla, gametophytes are the most conspicuous, dominant phase of life. The sporophytes are smaller and present only part of the time. If bryophyte spores are dispersed to a favorable habitat such as moist soil or tree bark, they germinate and grow into gametophytes by mitosis. Germinating moss spores typically produce a mass of green, branched, one-cell-thick filaments called protomena. Protonemata have a large surface area enhancing absorption of water and minerals. When sufficient resources are available a protonema produces buds having tissue-producing meristems. The meristems generate the mature, gamete-producing structure called gametophores. Together the protonema and gametophores make up the gametophyte body of a moss. Bryophyte gametophytes are generally only one or a few cells thick, which place all cells close to water and dissolved minerals. Most bryophytes lack conducting tissues that can distribute water and organic compounds within thick tissues. Though some bryophytes do have conducting that function in water and solute conduction, the cell walls in these tissues do not have the lignin coating that is characteristic of the vascular plants, resulting on a limitation of the height of bryophytes. However, Sphagnum can transport materials along its stem through plasmodemata, small perforations between the cells (DPOD 3). Most bryophytes grow close to the ground, only a few centimeters tall anchored by delicate colorless rhizoids. Rhizoids are filaments of cells in mosses. Moss gametophytes are described as being “leafy” because they have stem-like structures that bear many leaf-like appendages. Mosses “leaves” are usually only one-cell thick and often lack a cuticle, a feature that enhances water and mineral absorption from the environment.

Reproduction: Unlike most kinds of plants, mosses are haploid. This means that they have only one homologous pair of chromasomes rather than two. (EK7) When gametophores of bryophytes are mature, they produce gametes in gametangia. Gametangia are the gamete producing structures on the gametophyte plant, with male and female gametangium called the antheridia (male) and the archegonia (female). (6 AL) Eggs are produced singly in vase-shaped archegonia and large numbers of sperm are produced in in elongate antheridia. Jackets of protective tissue enclose both types of gametangia. Bryophyte gametophytes can produce many archegonia and antheridia and in mosses these are usually on separate male and female plants. Flagellate sperm are released into water films in which they swim toward eggs, passing down the openings of archegonia in response to chemical attractants. The eggs are not released but remain within the basses of archegonia. Then gamete fusion results in the formation of zygotes also retained by archegonia during their development into young sporophytes. The layers of placental nutritive cells help transport materials from parent gametophyte to embryos. The nutrients help support the development of embryos into mature sporophytes.
It is important to note that moss reproduction can only occur when there is enough water present. This restricts mosses to wet habitats, and habitats close to rivers and streams etc. (JE).

(CH 2)

Basic anatomy: The cells of bryophyte sporophytes contain plastids that are usually green and photosynthetic when the sporophytes are young. However, bryophyte sporophytes are not capable of living apart from their maternal gametophytes. A bryophyte sporophyte remains attached to its paternal gametophyte throughout the sporophyte’s lifetime and is dependent on the gametophyte for supplies of sugars, amino acids, minerals, and water. Bryophytes have the smallest and simplest sporophytes of all modern plant groups. Moss sporophytes are large and complex and posses epidermal stomata similar to those of vascular plants. The moss sporophytes are turn tan or brownish redwhen they are ready to release their spores and often remain visible for months after spore discharge has occurred. Moss sporophytes consist of a foot, an elongated stalk known as a seta, and a spore-producing organ, the sporangium, or capsule. The foot gathers sugars, amino acids, water, and minerals from the parent gametophyte via transfer cells. The set conducts these minerals to the capsule, which uses the resources to produce spores. In most mosses, the seta becomes elongated, which elevates the capsule and enhances spore disposal. The moss capsule (sporangium) is the site of meiosis and spore production. When the capsule is immature, it is covered with a protective cap of gametophyte tissue, the calyptra, but this cap is lost when the capsule is ready to release spores. The upper part of the capsule known as a peristome, is often specialized for gradual spore discharge. As a result, the capsule disperses spores gradually rather than all at once allowing mosses to take advantage of periodic wind gusts that can carry spores long distances. The leaves of mosses are generally composed on a single cell layer with a midrib (see image below) in the center (EG).
This diagram illustrates the anatomy of mosses described above. (AW)
midrib (EG)
midrib (EG)

Habitats: Wind dispersal has distributed bryophytes around the world. Mosses are generally found in areas of dampness and low light. (17 DC) These plants are particularly common and diverse in moist alpine, boreal, temperate, tropical forests, as well as wetlands where they form essential habitats for a variety of tiny animals. Some mosses even inhabit such extreme environments as mountaintops, arctic and antarctic tundra, and deserts. The mosses' ability to endure extreme environments explains why in the stages of succession, i.e. in the wake of a volcanic eruption, mosses are one of the first species to inhabit the area. (1 AS) Also in terrestrial ecosystems, mosses are important because they prevent soil erosion (18 SC).

Environmental Adaptations: Mosses are able to exist in very cold or dry habitats because many are able to lose most of their body water without dying, then rehydrate and reactivate their cells when moisture again becomes available. Also, phenolic compounds in moss cell walls absorb damaging levels of UV and other short wavelength radiation present in deserts or at high altitudes and latitudes.

In addition, the spores of moss are so lightweight that they can be carried by wind for immense distances. However, because they are so small, they have little food for survival. So, they must germinate in only the most ideal environment to gain a foothold (17T2).

Evolution of land plants: Mosses diverged before the evolution of vascular plants (AR 20)
Evolution of land plants: Mosses diverged before the evolution of vascular plants (AR 20)

Major Types: Some of the major types of mosses are the Polytrichum, Sphagnum, the Polytrichum Commune, and the Brachythecium.

Brachythecium: A type of moss in which the sporangium has a tip filled with two rings of “teeth” which regulate the gradual release of spores from the capsule. When conditions are moist the two rings of teeth interlock and keep the spores inside the capsule. Drying causes the teeth to separate, allowing spores to disperse. They can also be used for insect repellent and they are able to resist rot. The leaves are loosely erect and is considered one of the green silk mosses. (EK)
An Example of Brachythecium, Which Can be Found in Dry, Shady Areas (CS 18)

Polytrichum Commune: A type of moss informally known as the star moss in which the gametophytes have produced the stalked sporophytes, which have caplike sporangia at their tips. Polytichum Commune usually grow to heights of about 5- 10cm. However, they are known for reaching heights up to 30- 70cm. (7 HL)
Like other mosses, the Common Hair Cap Moss, also known as Polytrichum commune, is a dark green color and doesn't grow very tall (1 MB)
Like other mosses, the Common Hair Cap Moss, also known as Polytrichum commune, is a dark green color and doesn't grow very tall (1 MB)

Polytrichum: A type of moss informally known as the “hairy cap” mosses “leaves” with ridges that enhance absorption of light and are coated with cuticle. Polytrichum and other mosses also possess conducting tissues in the center of their “stems” and can grow as tall as 2m as a result. Polytrichum usually form large mats in habitats such as peat bogs, old fields as well as other areas of high soil acidity. Of the ten species found in North America, the most widely distributed species is P. Commune (2 KA).

Sphagnum: A type of moss genus informally known as the “peat moss” that is widespread and abundant. It is a wetland moss that forms extensive deposits of undecayed organic material known as peat. Boggy regions dominated by this moss are known as peat bogs and extensive high altitude boreal regions occupied by Sphagnum are among the wetlands known as peatlands. Sphagnum and the peat formed from it do not decay readily because of the resistant phenolic compounds embedded in the moss’s cell walls. Sphagnum moss also secretes acidic and phenolic compounds which may reduce bacterial activity.Sphagnum moss can potentially harbour the chronic fungal disease, **sporotrichosis**. Sporothrix schenckii spores enter the skin via abrasions, scratches, and small puncture wounds as a result of unprotected contact exposure to Sphagnum moss. (4DO).


Diagnostic characteristics
: Mosses are part of the plant kingdom and are the most familiar type of bryophytes. Bryophyta is the taxonomic name for the phylum of mosses. The terms bryophyte and bryophyta are not synonymus. The term Bryophyte refers informally to all nonvascular plants-the phyla Hepatophyta, Anthocerophyta, and Bryophyta. These diverse bryophytes are not monophyletic. Although gene sequence data, comparative cell structure, and other evidence indicate that species from these catagories of phyla diverge independently in plant evolution before the origin of vascular plants, most plant systematists now regard mosses as the bryophyte group most closely related to vascular plants (pteridophytes, gymnosperms, and angiosperms). All bryophytes are not very large and grow close to the ground because their lack of vascular tissue makes it impossible to transport water or other vital materials to any significant height. (19RM)

Review Questions:
1. In the two-phase life cycle of mosses, is the sporophyte or gametophyte dominant? What is unique, chromosome-wise, about this form? (SW 10)
2. What do bryophyte sporophytes need from their maternal gametophytes? (19 AN)
3. What are protomena and what is important about their function? (J. Stein)
4. Habitats of mosses are resistricted to damp areas with easy access to water. Why do mosses need these environments to survive? ( J. Sun 17)

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Biology, Sixth Edition//. 6th ed. San Francisco: Benjamin Cummings, 2001. Print.

1. http://www.backyardnature.net/mosses.htm

2. http://farm4.static.flickr.com/3013/3316760662_70e2883040.jpg