Mosses


 * Kingdom Plantae-Mosses **
 * By Craig W. Cooper**

**Diagnostic Characteristics of Mosses:**
**Bryophytes,** classified as part of the Plant Kingdom, are nonvascular plants that inhabit land, but lack many of the terrestrial adaptations of vascular plants. Today, bryophytes are represented by three separate phyla: **phylum Hepatophyta**, commonly known as **liverworts**, **phylum Anthocerophyta**, commonly known as **hornworts**, and **phylum Bryophyta**, commonly known as **mosses**. Mosses are the most familiar of the bryophytes, but many people confuse them with Irish moss (a red seaweed), reindeer moss (a lichen), club mosses (pteridophytes), and Spanish moss (a flowering plant). Bryophytes are **nonvascular plants**, which means they lack xylem and phloem, the inner tubes that transport water and nutrients between various parts of a plant. Many scientists now believe that mosses are the bryophyte group most closely related to vascular plants, even more so than the liverworts and hornworts.

But beware! The terms Bryophata and bryophyte are not synonymous. Bryophata is the formal taxonomic name for a certain phylum, the mosses. The term bryophyte, however, refers to all the nonvascular plants, which also includes the aforementioned liverworts and hornworts. The above image is a hypothetical phylogeny of the four main groups of plant phyla. As you can see, the bryophytes are classified as land plants, but are not classified as having a vascular system. The three types of bryophytes in the image are the liverworts, hornworts, and mosses.

**Habitats:**
Mosses prefer habitats that are moist and wet. Because mosses lack **lignified vascular tissues**, the tissues that help transport water and nutrients throughout the plant, most mosses can only grow a few inches tall. Because of this height limitation, mosses tend to grow very close to the ground. Due to the wind dispersal of the lightweight moss **spores**, which are **haploid** cells (containing only one set of chromosomes) that eventually develop into a multicellular individual, mosses have been distributed around the world. They are particularly common in moist alpine, boreal, temperate, and tropical forests, as well as wetlands. Mosses are able to lose most of their body water without dying, and can thus live in more extreme environments such as mountaintops, the arctic and Antarctic tundra, and deserts.

"Mosses rely on diffusion so they need a moist environment to get water to their cells; they depend on diffusion because they lack vascular tissues. They do have water conducting tissue within them, but it is not nearly as elaborate as the system “tube plants” utilize. Mosses reproduce by allowing their sperm to swim to the female reproductive organs. In order to swim, there must be a layer of water or some other moisture. Mosses are reliant on living in moist environment because they would not be able to get water to their cells or reproduce without the moisture" (SD) (4).

**Major Types of Mosses:**
The **m ** ** ajor types of mosses are the **//Polytrichum//, //Sphagnum//, **the** //Polytrichum Commune//**, and the** //Brachythecium//. (SR) (11) One of the more abundant and widespread genera of mosses is the //Sphagnum//, a wetland moss that forms extensive deposits of undecayed organic matter known as **peat**. Boggy regions that are dominated by the //Sphagnum// are known as **peat bogs**. //Sphagnum// secrete acidic compounds that may reduce bacterial activity, which helps explain why peat does not decay quickly. The low temperatures of peat bogs might also explain why peat is slow to decay. These peatlands store 400 billion tons of organic carbon, and are thus extremely important in stabilizing the Earth's atmospheric carbon dioxide concentrations.

//Sphagnum// moss has been used in the past for diapers, and during wartime, as a naturally antiseptic packing material for wounds. Today, it is harvested for it use as a soil conditioner. The following are close-up images of the //Sphagnum//.



**Basic Anatomy of Mosses:**
[ Mosses are very small green plants. Typically they consist of a stem not much thicker than a thick hair, densely covered with leaves maybe 1/16th or 1/8th of an inch long. Often moss stems branch and rebranch. Usually many mosses grow together forming a thick green carpet. (MP) [|6] ] Mosses are nonvascular plants, which as already discussed, means they lack the inner tubes necessary to transport water and nutrients throughout the plant. Because most mosses are very short in height (usually only several centimeters tall), they are anchored to the ground by delicate **rhizoids**, which are filaments of cells. Rhizoids do not assist in the transport of water and minerals, and thus differ from the roots of a vascular plant. A **gametophyte** is a multicellular haploid structure of an organism that is undergoing alternation of generations. If bryophyte spores land in a favorable habitat, they may germinate and grow into gametophytes by mitosis. Gametophytes attach to a layer of rock or soil beneath the surface of the ground with the help of rhizoids, which help the gametophyte absorb minerals and water. (NI) [|5] Germinating moss spores generally produce a mass of green, branched, one-cell-thick filaments, known as **protonema**, which are sometimes mistaken for algae. Bryophyte gametophytes are typically only one or a few cells thick, which places all of the cells close to water and dissolved nutrients. **Moss gametophytes** are described as leafy because they have stemlike structures that bear many leaflike appendages. These are not true stems, as they lack lignin-coated vascular cells. Mosses contain **leaves** that are only one cell thick, and often lack a **cuticle**, which enhances nutrient absorption in a moist environment. A **sporophyte** is the generation of a plant that has a double set of chromosomes. Bryophyte sporophyte cells contain **plastids** that are usually green and photosynthetic when the sporophytes are young. [Plastids are organelles that manufacture and store chemical compounds important to the cellular function.(AC)(8) Bryophytes have the simplest sporophytes of all modern plant groups. These sporophytes have tiny bodies, consisting of a short stalk bearing a round sporangium with a protective epidermis. **Moss sporophytes** are green and photosynthetic when they are young, but turn towards a brownish-red when they have released their spores. Moss sporophytes consist of a **foot**, an elongated stalk called a **seta**, and a spore-producing organ, called the **sporangium**. The foot’s main job is to gather nutrients from the parent gametophyte, while the seta’s main job is to get these materials to the capsule. ==== [[image:http://www.palaeos.com/Plants/Images/MossSporophyteDevel.gif width="489" height="314" align="center"]]

This is both a diagram of a sporophyte of moss. On the left is a drawn cross-section of the sporophyte, and on the right is a picture of the ourside of the sporophyte. (RG) [|(9)] ====

**Transport of Materials in Mosses:**
Mosses do not have the xylem and phloem that the vascular plants do. Instead, because their leaves are usually only one cell thick, they can exchange materials directly with the outside environment. However, some mosses, such as the //Polytrichum//, possess conducting tissues in the center of their “stems”, which help transport materials throughout the moss. "Mosses are nonvascular plants. Mosses absorb their water and nutrients directly into their bodies, not through their "roots". Instead of roots, they have rhizoids, which serve to stabilize the moss but do not have a primary function in water and nutrient absorption. They lack a vascular system both in their rhizoids and in their above-ground parts. They also lack lignin, which is a complex compound that stiffens the stems of higher land plants. This is why mosses are so soft. Their "leaves" are actually single-cell thick leaf-like appendages. True leaves have several layers of specialized tissues. The lack of vascular tissue forces mosses to be rather short, because they cannot transport water, nutrients, and metabolic products." [MS] [|2]

**Reproduction:**
When the gametophores of bryophytes are mature, they produce gametes in **gametangia**, the reproductive organ of bryophytes. Eggs are produced in the female version, the **archegonia**, while sperm are produced in the **elongate antheridia**, the male version. Both these types of gametangia are protected by protective tissues.

When the **sperm** are released into water films, they swim toward the eggs, passing down the openings of archegonia in response to chemical attractants. The eggs of the archegonia are not released, but rather stay within the bases of the archegonia. When the sperm and the egg fuse, they form a **zygote**, which is also retained by the archegonia. Layers of **placental nutritive cells** help transport materials from the parent gametophyte to these embryos, which help support their development into mature sporophytes.

When these moss sporophytes are ready to release their spores, the moss capsule (**sporangium**), the site of meiosis and spore production, releases its **calyptra**, the protective cap of gametophyte tissue. The upper part of this capsule, called the **peristome**, is often specialized for releasing the spores at a gradual rate. When the spores are released, they are carried great distances by air currents, and thus land in and find new habitats.

In terms of the life cycle of a moss, once these spores are released, they germinate by mitotic division, forming small, green **protonemata**. As these protonemata grow, they eventually form sexually mature gametophytes, which completes the cycle. A basic visual summary of this life cycle is displayed in the following diagram:



media type="youtube" key="jcWYAnmm-QE?fs=1" height="385" width="480" An animation and explanation of the moss life cycle. (JS) [|(7)]

(10)(SM)

**Environmental Adaptions:**
Bryophytes are believed to be Earth's only plants for the first 100 million years that terrestrial communities existed. Bryophytes, unlike angiosperms, produce no flowers at all. Because they have no tools to attract insects to pollinate them, they rely on the release of spores into the air to help increase dispersal. These spores are so lightweight that they can be carried long distances with very little amounts of air current.

Also, because some varieties of moss can live in very dry places, such as the desert, they have adapted mechanisms to help them survive with little water. For example, in the desert, some mosses can lose most of their body water without dying, and then rehydrate and reactivate their cells when moisture becomes available. Most vascular plants do not have the ability to do this.

"While mosses like moist environments, the Calliergon giganteum lives in the artic where it is not exposed to such an ideal habitat. However, it has adapted well to its cold climate. When it is not growing, it stores nutrients so new leaves can be made quickly next spring. The more leaves the more they can photosynthesize. It is adapted to the incredibly strong winds because it grows near to the ground. Because it can grow under water it is protected from the drying winds and cold, dry air of the frozen tundra. Its long life and slow growth are also adaptations to the short growing season and the cold." (AK) (3)

Review Questions : 1. How have the environmental adaptions of mosses affected their abilities to reproduce? Compare these adaptions to other organisms. What are the advatages and disadvatanges of the mosses and why do they matter?(CW) 2. Describe the reproductive cycle of mosses. Explain the life cycle. (MLK) 3. Do mosses have roots? If not, then what holds them in the ground and what are the other purposes of this structure? (AC) 4. Explain how the environmental adaptations allow mosses to thrive in the habitats they are found in. How does the structure of mosses affect their functions as plants lacking vascular tissue? (DB)

Sources Used: 1. Reece, Jane B., and Neil A. Campbell. // Campbell Biology. // San Francisco: Pearson Benjamin Cummings, 2009. Print. 2. [] 3. [] 4. []

[|5]. http://www.britannica.com/EBchecked/topic/501471/rhizoid 6. http://www.backyardnature.net/mosses.htm 7. [] 8. [] 9. [|www.palaeos.com/Plants/Images/MossSporophyteDevel.gif&imgrefurl] 10. @http://www.biology.clc.uc.edu/graphics/bio106/moss.jpg 11. http://www.hcs.ohio-state.edu/hcs300/liver2.htm