Bryophytes

The term bryophyte is an informal collective term for three groups of green plants, mosses, liverworts and hornworts. These plants are generally small, do not produce flowers, and lack a vascular system and roots. As with all land plants, bryophytes possess a regular alternation between two morphologically distinct generations, the haploid gametophyte (n) and the diploid sporophyte (2n). However, in bryophytes, the gametophyte, rather than the sporophyte, is the larger, long-lived generation. Bryophytes inhabit many ecosystems from tundra to tropical regions and are mainly found in moist, shaded environments (on rotten trunks, on rocks, forming carpets over soil, or as epiphytes), but also inhabit deserts, live under water, and a few even grow under saline conditions.

Gametophyte of Polytrichium commune

Detailed morphology of the sporophyte of Leucolepis acanthoneuron

Mosses

Mosses are the most diverse of the bryophytes; the group is comprised of approximately 14,300 species grouped in about 700 genera. A moss consists of two parts, the gametophyte and the sporophyte. The gametophyte is the part that you would mainly associate with a moss; it is the free-living, green, leafy stage characterized by leaf-like structures called phyllidia or phyllids (but commonly referred to as leaves) that are located around a stem-like structure termed the caulid. Mosses stay attached to the substratum by root-like structures called rhizoids. The sporophyte is diploid (two copies of the chromosome) and is the equivalent to the main body that you observe in vascular plants such as trees. The sporophyte grows partly as a parasite on the gametophyte. In most of the mosses, the sporophyte consists of an unbranched stalk termed the seta with a sporangium inside the capsule at the apex. The sporangium is usually covered by a cap known as the calyptra and often has a swollen base, the apophysis, and a lid known as the operculum. If you are curious, and you take off the calyptra and separate the operculum from the capsule, you will see the peristome, which is made of a ring of peristome teeth surrounding the mouth of the capsule. These teeth respond to changes of moisture in the environment and are involved in spore dispersal.

Sporophyte and gametophyte of Rhizomnium glabrescens

Life cycle

Mosses can reproduce sexually or asexually. After sexual reproduction, spores are produced. These are generally dispersed by wind or water, and if they are deposited on an appropriate substratum, they germinate to produce a microscopic filamentous structure, the protonema, that branches and creeps along the substrate. This structure will later produce masses of cells that differentiate and grow into the leafy gametophyte. The gametophyte produces the sex organs. The microscopic female organ, the archegonium, has an egg cell inside. Many of these female structures are produced on either a terminal or lateral branch. Each female sexual branch, or perichaetium, has a mass of protective leaves, the perichaetial leaves, which protect the tiny archegonia. Male sexual branches, or perigonia, produce the male sex organs called antheridia. Antheridia produce many sperm cells that are released into the water. Sperm are microscopic and their propulsion depends on two hair-like structures called flagella. Once in the water, sperm are attracted to chemicals produced by the archegonium and they swim toward it. When a haploid sperm cell unites with a haploid egg cell, a new diploid cell called a zygote is formed. This new cell is the first stage of the sporophyte. Once fully grown, the sporophyte will produce the spores that start the cycle again.

Life cycle of a moss

Line drawings modified from Richardson (1981). Protonema and germinating spore modified from Bogdanoff-Lord (2003) at www.csupomona.edu/~jcclark/classes/bot125/resource/graphics/bry_protonema.html Antheridium modified from CMSC (2004) at www.scitec.uwichill.edu.bb/bcs/bl14apl/lab2.html

However, mosses not only depend on sexual reproduction for propagation. Almost any part of a moss, such as a piece of leaf, a small specialized branch, a whole shoot, or a specialized structure is capable of generating a new plant. Thus, if you divide a moss into several pieces, each piece that has at least some whole living cells can generate a protonema that under suitable environmental conditions will produce buds that will eventually produce a new moss plant.

Liverworts

Liverworts are small, moss-like plants. They are generally small in size and are an inconspicuous part of the vegetation. Similar to mosses, the dominant generation is the gametophyte that is usually a prostrate thallose or a leafy form. A thallose gametophyte is flat and has rounded lobes while a leafy gametophyte looks similar to mosses, and is differentiated into stems and leaves. Leafy liverworts make up about 85% of the world’s liverwort species. The gametophytes, whether thallose or leafy, are anchored to the substrate by rhizoids. There is a great diversity in forms of the gametophyte. The sporophyte consists of a terminal globose to cylindrical spore case called a capsule, a stalk called the seta, and a basal mass of cells called a foot which is embedded in the gametophyte. In liverworts, the capsule remains embedded in the gametophyte tissue until it is mature, when the seta elongates and the capsule is free to release its spores. Seta elongation in liverworts is due to water uptake which causes cell expansion, while in mosses the setae grow by cell division before the capsule is mature.

All photo taken by Yan Zhuang
Diagrams designed by Isidro Ojeda

Morphology of a leafy liverwort: gametophyte of Scapania undulata

Morphology of a leafy liverwort: Bazzania denudata

Classification of Bryophytes

Mosses were previously classified with the liverworts and hornworts in the Division Bryophyta. Under this grouping, the class Musci contained the mosses. However, the current taxonomy splits up these three groups, and now the Division Bryophyta refers specifically to mosses. Liverworts and hornworts are classified under their own Divisions: Hepatophyta (with 320 genera and 8000 species) and Anthocerophyta (5 genera and 100 species), respectively. This reflects the current taxonomic view that the liverworts are the most primitive, followed by mosses, and hornworts are more closely related to the vascular plants.

The Division Bryophyta is split into three main Classes:

• Class Bryopsida: this includes the "true mosses" and makes up 95% of the mosses in the world. There is a great variety of shapes and colours for mosses in the Bryopsida. Class Bryopsida is split into several Orders.
• Class Andreaeopsida: Mosses in this Class are called "lantern mosses", and they grow exclusively on the surfaces of rocks.
• Class Sphagnopsida: These mosses, also known as "Peat Mosses", are used as fuel and soil conditioners.

There are approximately 700 genera of mosses and more than 14,300 species worldwide. In Canada there are about 940 species of mosses (including Sphagnum and Takakia). In British Columbia approximately 720-730 species have been reported and about 120 species of mosses inhabit Stanley Park. There are 20 endemic genera of mosses and three endemic genera of liverworts in North America (including north of Mexico); all have only one species (monotypic) except Thelia that includes three species.

References and Further Reading

De Luna, E., Newton, Angela E., and Mishler, B.D. 2003. Bryophyta. Mosses. http://tolweb.org/Bryophyta/20599/2003.03.25 in The Tree of Life Web Project, http://tolweb.org

Hicks, M.L. 1992. Guide to the liverworts of North Carolina. Duke University Press, USA.

Cagel, R.F., Bandoni, R.J., Maze, J.R, Rouse, G.E., Schofield, W.B. and Stein J.R. 1982. Non vascular plants. An evolutionary survey. Wadsworth Publishing Company, USA.

Richardson, D.H.S. 1981. The biology of mosses. Hasted Press, USA.

Schofield, W.B. 1992. Some common mosses of British Columbia, 2nd ed. Royal British Columbia Museum, Victoria, Canada.

Schofield, W.B. 2002. Field Guide to Liverwort Genera of Pacific North America. University of Washington Press, Seattle.

Schofield, W.B. 2004. Endemic genera of bryophytes of North America (north of Mexico). Preslia, Praha 76: 255–277.

By Isidro Ojeda

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