Plants - Mosses

Omer Zaidi


star_moss.jpg
star moss (www.highlandguides.com/

Diagnostic characteristics of the group
There are known to be approximately 14,500 species of mosses. Mosses are plants characterized mainly by a lack of vascular tissue, tissues used for transporting water and nutrients through vascular plants and also helping keep their structure. Mosses also spend most of their lives in the hapliod stage (only has one copy of its DNA) known as the gametophyte. The fuzzy green plants we know as moss are the gametophytes of the plants. The sporophytes are much smaller and more difficult to see; the sporophytes grow out of the tip of the stocks of what we percieve as moss(1). (SV)
Mosses are closely related to lichens (another phylum in the plant kingdom) structurally. Both require a damp environment to support their transport system, and both have developed special ways of adapting to a change in their environment. (9 HS)




Habitats
Mosses can be found basically all over the world, from alpine forests, tropical forests, temperate forests, and wetland areas to environments with extreme weather such as deserts and the arctic tundra. This is because mosses were the earliest plants and existed when Earth was experiencing lots of change in regards to climate, temperature, and landscape. They have the capability of surviving in all types of habitats (1). One source states that while mosses have the capability to live in many different types of habitats, there are no cited moss species that have the ability to survive and thrive in a salt water habitat (2). Mosses tend to grow in damp, low lit areas. They are extremely common in woods and near the edges of streams. They're even in urban sidewalk cracks. Where ever they are, moisture is vital because of the thinness of its tissues, lack of a cuticle, and need for water to complete fertilization. It has been proposed that in northern latitudes, north sides of trees and rocks have more moss than other sides because of a lack of water on the sun-facing sides of the trees/rocks. South of the equator, is the opposite. (AR, 5).
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Moss on a tree (said to be in Brazil, and therefore on the south side of the tree). (VM)



Major types
The phylum Bryophyta is more commonly known as mosses. According to fossil evidence, mosses are the earliest forms of plants. Along with the phylum Bryophyta, there are two other phyla: phylum Hepatophyta (known as liverworts) and phylum Anthocerophyta (known as hornworts). Together, these three phyla make up the bryophytes. It's important not to confuse the term bryophytes with the phyla bryophyta, because not all bryophytes are in the phyla bryophyta. Some organisms such as club mosses (pteridophytes), Irish moss (a type of seaweed), and reindeer moss (lichen), are not true mosses. Mosses, liverworts, and hornworts all evolved before the arrival of vascular plants, but all diverged independently as time progressed (1).
According to one site, some of the earliest land plants resembled modern liverworts. There are two major types of liverwort, which are classified on the basis of their general structure; there are the leafy liverworts, and the thallose liverworts. In leafy liverworts, the leaves are generally only a single cell thick and have no protective surface layer, or cuticle. Approximately 80% of the world’s liverworts are leafy liverworts. In thallose liverworts, the plant body has no stem-leaf structure, but instead consists instead of a "thallus," which is a type of flattened tissue, one to several cells thick. (4) (SS)
The Anthocerophyta phylum of mosses is full of a diverse group of organisms. Some of the Antherocerophyta are small organisms, but others can grow to cover vast areas of ground or whole sides of trees. The phylum leaves behind a very undefined fossil, leading many scientists to speculate that early Antherocerophyta fossils are actually Hepatophyta fossils. (12) (RJS)

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http://farm1.static.flickr.com/28/103835444_03f7332204.jpg?v=0
In this picture are thallose liverworts, part of the phylum Hepatophyta, with tissue that are only several cells thick. "Thallus" means that something is like a sheet (7) [HZ].




Basic anatomy
All mosses, and bryophytes in general, are attached to the ground by rhizoids, long tuble-like cells which do not act like roots of vascular plants, as they neither collect water or minerals nor have any specialized conducting cells. Because they evolved before vascular plants, mosses lack all the transport tubes such as xylem and phloem and are thus relatively short, about a few centimeters tall. Mosses also lack cuticle, a waxy coating that helps vascular plants retain moisture. However, one type of moss, Polytrichum, actually has leaf-like appendages which help absorb more sunlight and are actually coated with cuticle (1). A sporophyte is the spore bearing structure of the moss. Typically, it is a capsule shaped structure called a seta. Since the sporophyte does not obtain or produce its own energy, it lives parasitically off the mother moss until the spores are released. The released spores will have the potential to form a new generation of gametophytes (3, NK). The process for how these independent gametophytes along with the sporophytes are created as haploid, having half the number of chromosomes, or diploid, having the normal double chromosome amount, is called alternation of generations (11 BL). As shown by the diagram below, the calyptra is a cap-like structure that is lost before the spores are about to be released.

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www.backyardnature.net/ mosses.htm
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The diagram above is of a moss gametophyte. As shown by the diagram, the calyptra is a cap-like structure that is lost before the spores are about to be released from the capsule, the site of meiosis and spore production. The peristome is specialized to realease speores gradually. The seta brings materials as sugars water, and minerals that are absorbed by the protonema up to the capsule. (1) (RK)




Transport of materials
Because mosses lack vascular tissue, they do not have xylem or phloem tubes used for transporting water. To remedy this problem, moss gametophytes are usually one or a few cells thick. This allows the cell more contact with water and other minerals. By having lots and lots of tiny protonema (for the definition, please see the "Reproduction" section) there is a large surface area which enhances water and mineral absorption (1). Also because of a lack of vascular tissue Mosses are more likely to be found in damp swampy environments so it can stay in close contact with water at all times(SJB)(1)
Therefore, transport of materials in mosses greatly depends on simple diffusion (when a substance can pass through the cell membrane without the help of a protein) and active transport (when a substance passes through the membrane against the concentration gradient). (YS)




Reproduction
Mosses undergo an alternation of generations, a cycle switching between haploid (one set of chromosomes) to diploid (two sets of chromosomes) stages.In order to reproduce, mosses have separate male and female gametophytes (respectively called antheridia and archegonia). The sperm leaves the antheridia through moisture and travels into the archegonia where it fertilizes the egg inside. The now-zygote ( a dipolid) divides by mitosis to form an embryonic sporophyte,the diploid stage during alternation of generations. Over time, the sporophyte begins to grow taller while remaining dependent on the nutrition of the archegonia. There are three parts to a moss sporophyte: a foot, a seta (a long stalk), and the sporangium, also known as a capsule. The foot is where nutrients such as sugars, water, and amino acids are gathered from the archegonia. The seta then transports these materials to the sporangium. Inside the sporangium, meiosis occurs and begins to develop spores, special reproductive cells that don't need another cell to develop into a new organism. Once the "lid" of the sporangium pops off, the spores are released. From here the paths of the spores diverge. Some grow into antheridia while others grow to become archegonia. Both spores germinate by dividing by mitosis and produce protonema, little, green threadlike filaments. Because protonema have a large surface area, they are excellent for absorbing water and other minerals. The protonema continue to grow and eventually become their respective mature gametophytes. And from there on, the cycle continues again (1).


29-23-FernLifeCyc-L.gif
<http://io.uwinnipeg.ca/~simmons/16cm05/1116/29-23-FernLifeCyc-L.gif> (Jesse Carmen)
As seen from the picture, the "mature" sporophyte is what humans see as a moss. Because of this, mosses' spend a dominant portion of their life in the sporophyte phase. A moss starts off as a "young" gametophyte. On the "young" gametophyte egg and sperm come together to form a zygote (egg.) Eventually a sporophyte is formed, and this sporophyte will mature. The process can repeat itself when the "mature" sporophyte releases spores, which eventually produce a gametophyte (where reproduction occurs and a new sporophyte or moss will develop.) (Jesse Carmen) (4).



Environmental adaptations
In order to survive during the harsh conditions of an earlier Earth, mosses have made rather important and unique environmental adaptations. Mosses are able to lose a lot of water without dying and then later be able to rehydrate and to restart the cells in the presence of water. In addition, mosses are able to survive in dessicating environments with their "clump growth pattern" (6, DJ). By growing in clumps, mosses reduce air movement across most of the plants, and therefore increases the humidity in the clump (6, DJ). In effect, the rate of evaporation is lowered (6, DJ). Many vascular plants would have died in a similar environment without this adaptation. Also, mosses are able to absorb high levels of UV and other forms of short-length, high frequency radiation with phenolic compounds; this can be a problem in deserts and high altitudes for vascular plants (1). Mosses live low to the moist ground in order to maximize each cell's intake of water (ER). Additionally, their high surface area to volume ratio facilitates each cell's access to moisture (ER) (1). Also, mosses require the use of water to let the sperm travel to the egg (1) (JAC).

By clumping, this Venezuela Moss is able to retain more moisture than it would normally be able to if the moss was more spread out. (DP) http://www.dartfrog.co.uk/plants/images/venezuela-moss.jpg
By clumping, this Venezuela Moss is able to retain more moisture than it would normally be able to if the moss was more spread out. (DP) http://www.dartfrog.co.uk/plants/images/venezuela-moss.jpg



Edited by:
Sarah Vlach

Grace Rehnquist
NK
Jesse Carmen
SS
Arielle Reiter

Sam Blatchford
Daisy joo
Ethan Richman
Hanna Zhu
Yasheka Sharma
Rachel Kornetsky
Kevin Nayer
Hilary Stepansky
Josh Czik
Vonai Moyo

Becca Levenson
Sarah Schwarzschild
DP
Meru Nangia
Brittany Marcus-Blank

Review Questions:
1) Describe the process by which Mosses are able to reproduce. (KN)
2) Since mosses lack vascular tissue, what methods or structures do they use to transport their nutrients? In what way has the lack of vascular tissue been beneficial?
3) Descibe the basic anatomy of a moss. (Nangia)
4) Explain where mosses can live and why they can live there/ like to live there. (BMB)

Source:
(1) Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. Boston: Benjamin-Cummings Company, 2002.
(2) http://www.daviddarling.info/images/circulatory_system.jpg
(3) Kornfeld, Ari. "Mosses." Perspective.com. 3 Dec. 2008 <http://www.perspective.com/nature/plantae/bryophytes.html>.
(4) Hale, Alan. "Liverworts." Mosses and Liverworts in Wales. 7 Dec. 2008 <http://home.clara.net/adhale/bryos/livworts.htm>.
(5) "Moss." Wikipedia.com. 7 Dec 2008. <http://en.wikipedia.org/wiki/Moss>.
(6) Simmons, Kent. "INTRODUCTION TO THE PLANT KINGDOM." Campus Manitoba Evolution, Ecology and Biodiversity. University of Winnepeg. 7 Dec. 2008 <http://io.uwinnipeg.ca/~simmons/1116/16moss.htm>.
(7) "Hornworts and Thallose Liverworts." 13 Dec. 2008 <http://www.botany.hawaii.edu/faculty/webb/BOT311/BOT311-00/CellTissOrgan/ThalloseLiverworts.htm>.
(8).
Ogden R, James and M. Fogiel. High School Biology Tutor. Research & Education Assoc., 1992.
(9) Kornfeld, Ari. "The Plant Kingdom: Mosses and Allies." Natural Perspective. 29 Sept. 1997. 14 Dec. 2008 <http://www.perspective.com/nature/plantae/bryophytes.html>.
(10) "Moss". 15 Dec 2008 <http://farm1.static.flickr.com/68/196265398_48bbc86814.jpg>
(11) "bryophyte." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 17 Dec. 2008 <http://www.britannica.com/EBchecked/topic/82573/bryophyte>.
(12) Arens, Nan. "Introduction to the Anthocerotophyta." 18 Dec 2008 <http://www.ucmp.berkeley.edu/plants/anthocerotophyta.html>.