5.3: Mosses

Mosses, Phylum Bryophyta

Gametophyte Generation

Mosses produce only leafy gametophytes. You can differentiate them from leafy liverworts because the leaves are arranged in a spiral and usually have a midrib-like struture called a costa. Like the other two groups of bryophytes, simple pores on the gametophyte allow for gas exchange (no guard cells, meaning pores are permanently open).

Figure $\PageIndex{1}$: A cushion of moss gametophytes in the genus Dicranum. Moss sporophytes tend to look more "fuzzy" than leafy liverworts due to their spirally arranged leaves. Photo by Maria Morrow, CC-BY-NC.

Figure $\PageIndex{2}$: A closer look at spiral leaf arrangement, this time with Polytrichum commune, the haircap moss. Note that leaves emerge from all directions of each gametophyte. Photo by Mizuki Maeda, CC-BY-NC.

Figure $\PageIndex{3}$: Most mosses have leaves with a costa, a thickened line of cells that traverses the center of the leaf, much like a midrib. In this image, we are looking at a moss leaf under the microscope. The darker region that runs through the center of the leaf is the costa. Photo by Shane Hanofee, CC-BY-NC.

Figure $\PageIndex{4}$: These moss gametophytes show spiral leaf arrangement and a distinct, tan costa running down the center of each leaf. Two of these are indicated by white arrows. In most mosses, the costae do not extend beyond the length of the leaf. Photo by Geoffrey Cox, CC-BY-NC.

Figure $\PageIndex{5}$: On the left is a labeled cross section of the antheridial head of a Mnium male gametophyte. On the right is an actual image of the splash cup (antheridial head) from a bird's eye view. The cross section is labeled as follows: A) An antheridium, B) paraphyses, C) sterile jacket of the antheridium, D) sporogenous tissue, E) male gametophyte. First photo by Maria Morrow, CC-BY-NC. Second photo by Bosmon, CC-BY-NC.

Figure $\PageIndex{6}$: On the left is a labeled cross section of the archegonial head of a Mnium female gametophyte. On the right is an actual image of a female gametophyte. The cross section is labeled as follows: A) An archegonium, B) neck of the archegonium, C) venter, D) egg, E) paraphyses, F) female gametophyte. First photo by Maria Morrow, CC-BY-NC. Second photo by Tisrel, CC-BY-NC.

Sporophyte Generation

Moss sporophytes have a complex set of structures at the top of a seta. When the sporophyte emerges, it tears off a piece of the female gametophyte's archegonium, leaving a coating called the calyptra. Mosses have a capsule, where the sporangia are housed. This capsule has a lid-like structure called an operculum, which pops off when spores have matured. Depending on the moss, the sporophyte may have a peristome (teeth-like sheets of cells that aid in spore dispersal). True stomata are present for gas exchange.

Figure $\PageIndex{7}$: Moss sporophytes are composed of a seta and capsule, much like liverworts. When the sporophyte emerges from the archegonium, it brings a piece of that archegonium with it. This gametophyte remnant sits on the capsule like a cap, which is why it is called a calyptra. Photo by Maria Morrow, CC-BY-NC.

Figure $\PageIndex{8}$: When the calyptra falls off, another feature of the sporophyte is visible: the operculum. This is a lid-like structure on the capsule that pops off when the spores are mature. All of the sporophytes visible in the two images above have a distinct operculum visible on the end of the capsule opposite the seta. Photos by Maria Morrow, CC-BY-NC.

Figure $\PageIndex{9}$: The image on the left shows a moss sporophyte with a hairy calyptra covering the capsule. The sporophytes in the image on the right are from the same moss but are at a later stage in development. The calpytra and operculum have been shed, exposing a structure present in many mosses: the peristome. The peristome is a series of flaps that line the edge of the capsule opening and aids in spore dispersal. Photos by Maria Morrow, CC-BY-NC.

Mnium Life Cycle

Figure $\PageIndex{10}$: The Mnium life cycle. Starting with meiosis (left, center): Meiosis produces haploid spores which will grow by mitosis into either male or female gametophytes. Moss spores germinate and begin to grow into a chain of cells called a protonema. Male gametophytes are flat topped, called splash cups. Many antheridia are produced within the splash cup, surrounded by sterile cells called paraphyses. Antheridia produce biflagellate sperm by mitosis. Female gametophytes produce multiple archegonia at the top of the gametophyte. These are also surrounded by paraphyses. Each archegonium produces a single egg by mitosis. When water hits the splash cup, it can splash sperm onto a female gametophyte. If the sperm are able to swim through the water to an egg, fertilization occurs producing a diploid zygote. The zygote grows into a sporophyte from within the archegonium. The remaining archegonial tissue is called the calyptra and will remain on top of the sporophyte as a cap-like structure. A mature sporophyte will have a sterile stalk called a seta. Underneath the calyptra, there is a capsule containing cells that will undergo meiosis to make haploid spores. The capsule has a lid-like operculum that will pop off when the spores have matured. Peristome teeth along the opening of the capsule aid in spore dispersal. Drawing by Nikki Harris CC-BY-NC with colors and labels added by Maria Morrow.