Plantae -- Mosses

By Ben Yudysky

Mosses: Sporophyte and Gametophyte
Mosses: Sporophyte and Gametophyte

Diagnostic Characteristics of Mosses:

Mosses belong to one of the three phyla of bryophytes, one of the four main groups of land plants. Mosses are the most common form of bryophytes, and they are the phylum of bryophytes most closely related to vascular plants. However, bryophytes are nonvascular plants, meaning that they do not contain the water-conducting tubes that are present in the three other main groups of land plants (pteridophytes aka ferns, gymnosperms aka cone-bearing plants, and angiosperms aka flowering plants). It is thought that bryophytes were the first plants that inhabited the terrestrial communities of Earth (for about 100 million years, they were the only terrestrial plant forms!).

There are some "moss posers" out there, so be sure to differentiate between Plantae Bryophyta (the latinized name for mosses) and...
Irish moss (a red seaweed), reindeer moss (a lichen), club mosses (pteridophytes), or Spanish moss (a flowering plant).

The Evolution of Plants
The Evolution of Plants

Habitats and Environmental Adaptations:

Mosses are common in habitats that provide them with an adequate amount of water. For example, these types of plants prefer moist alpine, boreal (aka taiga), temperate, tropical forests, and wetlands. But mosses can be considered a hearty plant because they have the ability to live in climates that vary in temperature and moisture content. They can live on even after losing a substantial amount of their body water without dying - when the opportunity to rehydrate presents itself, mosses absorb water and can revitalize their cells. They are also world-wide in distribution and can be found at sea level as well as the highest altitudes occupied by plants. (JJF)

The location where a moss' spores land is crucial to the survival and growth of the plant. If the spores are dispersed to an area of moist soil or tree bark, they have a good chance of survival. Nonetheless, mosses can adapt to their environment (as is exemplified by their ability to control their water absorption). Mosses also have phenolic compounds in their cell walls, which absorb UV and short-wavelength radiation. This function is important to the plant because radiation (found in high concentrations in deserts and at high altitudes or latitudes) is harmful, so the phenolic compounds prevent decay.

Originally, mosses evolved from algae. The evolutionary adaptations occurred due to the demands of moss' new, terrestrial habitat. One example of an evolutionary adaptation involves moss' reproductive process. The mother plant is attached to the multicellular embryo, which allows the mother plant to protect and nourish the embryo. The reproductive process will be further explained in the section titled "Reproduction." Another specific environmental adaptation relates to spore dispersal, which is described in the section titled "Basic Anatomy and Transport of Materials." A part of the moss (the seta) elongates and the likelihood of spore dispersal via wind gusts increases.
Habitats of mosses (Ali Kirsch)
Basic Anatomy and Transport of Materials:

As previously mentioned, all bryophytes are nonvascular plants, which means they lack tubes present in vascular tissue that transport water and nutrients throughout the plant's body. (In some rare cases, bryophytes do have these tubes.) The stems of mosses have been know to show some different structures called hydroids and leptoids, which are like the xylem and phloem in plants. They do not, however, connect to leaves or stems that branch, they are very simple in structure (JL). So, mosses must compensate for the lack of vascular tissue in order to transport materials.

The gametophytes[a gametophyte is a multicellular structure, or phase, that is haploid, containing a single set of chromosomes. RS], which are haploid (one set of chromosomes), produce gametes (sperm & eggs). The gametophytes can also be described as "leafy," for they have stem-like structures and leaf-like appendages that are one cell thick. The one-cell or few-cell structure of moss' gametophytes allow for the transfer of water and nutrients because the cells are located very close to the materials the plant needs. The life cycle of mosses (the alternation of generations) involves gametophytes much more than sporophytes[ A sporophyte is the generation of plant or algae that has a double set of chromosomes. RS].

The Alternation of Generations
The Alternation of Generations

The sporophytes, which are diploid (two sets of chromosomes), perform meiosis (cell division performed by sexually reproducing organisms) and produce spores, reproductive cells that can develop into multicellular organisms through mitosis (nuclear division) without fusing with other cells. Sporophytes rely on the gametophytes to which they're attached for sugars, amino acids, minerals, and water.

The specific parts of the sporophytes include the foot, the seta, and the sporangium (aka the capsule) - each of which play an important role in the transfer of materials. The materials from the parent are gathered by the foot, conducted by the seta (which elongates) to the capsule, which uses the resources to produce spores and subsequently disperses them.
Here are the components of the sporophyte that play major roles in material transport. (KNS)
Compared with other plants, mosses have the smallest and simplest sporophytes that become tan or brownish-red as they prepare to release their spores. Spores that are dispersed and begin to germinate develop into protenema, many one-cell-thick filaments with the ability to absorb water and nutrients efficiently due to its large surface area. From there, buds form that contain meristems, which create the gametophyte. This process is described in more detail in the section titled "Reproduction."

Mosses are held close to the ground by rhizoids, long, tubular, root-like projections. Rhizoids differ from the roots of vascular plants because they lack the tissues that absorb water and minerals. They are primarily structural as opposed to functional. Mosses also have leaf-like structures, but lacking in a vascular means of transporting nutrients, these are not true leaves. (AZ)

Basic Anatomy of Mosses
Basic Anatomy of Mosses


So the gametophytes create the sporophytes, but the gametophytes still exist and transfer materials to the sporophytes? What sounds confusing can be explained by the chronological process of reproduction.

The life cycle of Polytrichum, a moss
The life cycle of Polytrichum, a moss

The gametophytes form gametes in the gametangia. There are female gametangia - known as archegonia - where the eggs form, and there are male gametangia - known as antheridia - where large numbers of sperm form. Mostly, the separate types of gametangia are located on separate plants (male and female), so the sperm must travel fro m one plant to another.

When sperm reaches a female plant, they swim through a water film to the archegonia. They are directed by an attraction to chemicals in the plant. A zygote forms in the archegonia when the gametes fuse. It is within the archegonia that the sporophyte develops, receiving materials from its parent through placental nutritive cells. By the time the sporophyte is ready to release spores, the capsule (described in the previous section) sheds the calyptra, a protective cap made of gametophyte tissue. As a result, the now uncovered, upper area of the capsule, known as the peristome, controls the dispersal of spores in a gradual fashion instead of all at once. This process takes advantage of the changing winds (another environmental adaptation).
[The peristome teeth are perhaps the most characteristic feature of the mosses; usually composed of cell wall remnants, they respond to changes in the humidity of the atmosphere. Under conditions of low humidity, the teeth dry out and splay away from the mouth of the capsule, thus, allowing the commonly more than 50,000 spores within to be gradually released. RS]

The reproductive system of moss means it can be blended (with nutrients) and reapplied in suitable environments as a paste. Moss Graffitti?! (DRM)

Examples of Mosses:

There are over 10,000 different species of mosses some of which include Sphagnum, Polytichum, Andreaeopsida, Tetraphidopsida, Bryopsida, just to name a few.
The Bryopsida is a mose that has a the structure of peristome, which is a ring of teeth made of cell walls, and 95 % of mose species are in this group.(CP)

The Polytrichum is a type of moss that has more complex "leaves" that allow for more sunlight absorption as well as a stem-like structure. Polytrichum is an outlier in the moss world, for it can grow up to 2 meters tall. The life cycle of this type of moss is depicted in Figure 29.16.

An example of an abundant moss genus is Sphagnum or peat moss. This moss forms peat, an undecayed organic matter. Sphagnum and peat does not readily decay due to the phenolic compounds in its cell walls (described in the section titled "Basic Anatomy and Transport of Materials.") The peat bog environment, which includes low temperatures, nutrient levels, and bacterial activity allow Sphagnum to thrive. Peat moss is used today as soil conditioner and packing plant roots for shipment due to its innate ability to absorb water. (Dry moss can absorb up to 20 times its weight in water.) [What makes this water absorbing ability possible is the large hyaline cells in the leaves of the moss. Hyaline cells have thickened bands of supporting material and also usually have pores. In some species, this ability can be attributed to dead porous cells of the stem: overlapping leaves that are arranged spirally. They hold water by capillarity (also called capillary action) and pendant branches (hanging branches) that are twisted around the stem to make a structure that functions similar to the way a wick functions. (DG)]

The Andreaeopsida is also known as the lantern moss and are most commonly found growing on rocky surfaces. They are able to stay attached to those surfaces because of rhizoids. They are normally redish-brown which decrease light intensity, which is already high due to the rocky location. (LD)


Review Questions:

1. What are the primary structures that transport materials throughout the plant? Describe these structures and explain how they transport materials. (SF)
2. What is the difference between the gametophytes and the sporophytes in mosses? How do their functions differ? (SW)
3) How are mosses different from ferns, gymnosperms, and angiosperms? (KTD)
4) In general plants have evolved to be out of water. Explain traits that mosses possess that charophyceans do not, and explain traits that pteridophytes have that bryophytes do not.(Walker K.)
5) How do mosses reproduce? What function do the gametophytes have in reproduction? (LS)
6.) What are Phenolic Compounds, where are they located, and why are they so important? (Jake S)
7) What is a typical habitat for mosses? Why is this habitat ideal for mosses to survive and flourish in? (JAF)
8.) Which moss has the structure of a "peristome" ? What does this mean? (AS)
9) Explain the importance of rhizoids in mosses. How are they different from the roots of vascular plants? (JM)

Plant Phyla. Online Photograph. 16 Oct. 2009. <>.
Webb, David. Bryophtye Reproductive Cycle. Online Photograph. 16 Oct. 2009. <>.
(APS) (Jesse Landy) (Alyssa Zisk) (KNS) (Ali Kirsch) (DG) (Liz Daley) (JJF); (RS)