Mosses and Ferns


First Land Plants
  • Algae that could live out of water at least part of the time evolved 500-600 million years ago
  • Phylum Bryophyta - includes mosses, liverworts, and hornworts
  • Phylum Tracheophyta - includes ferns and the rest of the higher plants

Demands of Life on Land
  • All cells need a constant supply of water
  • Plants must obtain water and deliver it to all of their cells. Once plants provide water to their tissues they must protect that water against loss by evaporation to the atmosphere
  • Plants must be exposed to as much sunlight as possible to make food for itself
  • Land plants takes up water and nutrients in roots but make food in leaves.
  • Land plants must transport water and nutrients upward and the products of photosynthesis downward
  • Land plants must exchange water and carbon dioxide with the environment without losing too much water in the process

Mosses, Liverworts, and Hornworts
  • Bryophytes have life cycles that involve an alternation of generations between a haploid gametophyte and a diploid sporophyte
  • Bryophytes need water for reproduction to occur
  • Bryophytes grow mostly in tropical areas and along the western coast of the United States
  • Some look like miniature evergreen trees, soft green carpet, and others look like leaves lying on the ground
  • Almost all bryophytes are less than a few centimeters tall
  • From the base of the shoot, of a moss plant, grow a number of thin branches called rhizoids that penetrate into the ground and act like roots to securely anchor the plant
  • Liverwort plants are scarcer than mosses and need to live in places that remain wet constantly
  • Liverworts look like flat green leaves growing along the ground, and when they mature, they look like tiny green umbrellas (the “umbrellas” carry the structures that produce eggs and sperm)
  • The gametophytes of hornworts look much like the gametophytes of liverworts
  • The hornwort sporophyte differs from the liverwort sporophyte
  • The hornwort sporophyte looks like a tiny horn

Physical Characteristics of Bryophytes
  • Bryophytes lack several critical adaptations to life in dry places
  • Lack the water conducting tubes that are found in higher plants
  • In bryophytes, water passes from cell to cell by osmosis and by means of surface tension around the stems
  • These methods of transporting water work well over short distances which is why bryophytes never grow tall
  • Lack a protective surface covering to keep water from evaporating from their cells
  • Lack true roots = true roots contain water-conducting tubes that enable a plant to absorb and transport water efficiently
  • Instead of roots, bryophytes have rhizoids that anchor them in the ground
  • Have sperm cells that must swim through water to fertilize the eggs, which is why they must live in areas that are wet for at least part of the year
  • Some can survive dry periods, but to do so they must stop growing

Alternation of Generations in Mosses
  • Antheridium - produces tiny flagellated sperm cells
  • Archegonium - produces eggs
  • The reproductive structures of mosses are designed to protect the gametes from drying out unlike the reproductive structures of algae
  • Which makes the eggs of mosses have a better chance of surviving during dry conditions
  • Some species of mosses have both male and female reproductive organs on one gametophyte, other species have male and female reproductive structures on separate gametophytes
  • Mosses can reproduce sexually only when standing water is present
  • Sperm can swim to the archegonium only when the gametophytes are covered with rainwater or dew
  • When sperm swims to an egg, syngamy(the fusing of gametes) occurs and a diploid zygote is produced
  • When the zygote germinates, it produces a diploid sporophyte
  • As it grows the sporophyte is supplied with water and nutrients by the gametophyte
  • Moss sporophytes cannot live independent of the gametophyte from which they grow(one way bryophytes differ from all other land plants)
  • The mature sporophyte is composed of a “foot” that remains stuck in the gametophyte--a long stalk-- and a capsule that looks like a salt shaker
  • Inside the capsule, haploid spores are produced by meiosis
  • When the capsule ripens, special pores opens and are shaken out to be carried off by wind and water
  • If a spore lands in a moist place, it germinates and grows into a mass of tangled green filaments called a protonema
  • As the protonema grows, it forms rhizoids that grow into the ground and shoots that grow into the air
  • The shoots develop into moss gametophytes, and the cycle continues

Life Cycle of Mosses

Ferns and the First Vascular Plants
  • The members of the phylum Tracheophyta are “true” land plants because they have evolved ways of freeing themselves from dependence on wet environments
  • Vascular tissues - specialized tissues of tracheophytes
  • Vascular tissues transport water and the products of photosynthesis throughout the plant
  • Two types of vascular tissue: xylem and phloem
  • Xylem tissue - associated with the movement of water from the roots to all parts of the plant
  • Phloem tissue - responsible for the transport of nutrients and the products of photosynthesis
  • Tracheid cells - carry water from roots in the soil to leaves in the air (most important type of cells in xylem tissue)
  • Tracheid cells have thick, strong cell walls that strengthen stems and help plants stand up against the pull of gravity
  • All the plants in the phylum Tracheophyta have tracheids
  • The other kind of vascular tissue, phloem tissue, carries important nutrients and the products of photosynthesis from place to place within a plant
  • Vascular cylinder - vascular tissues gathered in a central area of the root
  • True leaves are photosynthetic organs that contain one or more bundles of vascular tissue gathered into veins
  • Cuticle - waxy covering on leaves which help prevent water loss by evaporation

The First Vascular Plants
  • Fossils of phsilophytes were the first vascular plants found early in this century
  • These small creeping plants had primitive xylen and phloem tissues but lacked true roots and true leaves
  • Most botanists think the psilophytes are extinct, some believe that two species of living plants are actually living psilophytes

Club Mosses and Horsetails
  • Only living descendants of large and ancient groups of land plants
  • First appeared more than 400 million years ago
  • Over the next 100 - 200 million years, many more spices evolved
  • Some ancient lycophytes(mosses) and sphenophytes(horsetails) grew into huge trees up to 40 meters tall
  • The earth’s very first forests were made up of vast numbers of these plants
  • Over time the climate of the Earth changed and these primitive plants could not compete with new types of plants that have evolved with the changing climate
  • Forests of lycophytes and sphenophytes were replaced by forests of extirely new plants
  • Few species of lycophytes an sphenophytes alive today are relatively small plants that live in moist woodlands and near stream beds and marshes
  • Lycopodium, the common club moss, looks like a miniature pine tree about 9 centimeters tall
  • Equisetum, a living genus of sphenophytes, grows about 1 meter tall and is commonly called horsetail or scouring rush because its stems contain crystals of silica which are quite abrasive

Physical Characteristics of Ferns
  • Evolved about 400 million years ago
  • Important part of the lycophyte forests that covered the ancient Earth
  • More successful at competing with other plants that have appeared during the Earth’s long history
  • Today more than 11,00 species of ferns are still alive
  • Ferns are well-developed tracheophytes
  • Have true vascular tissues, strong roots, creeping or underground stems called rhizomes, and large leaves called fronds
  • Commonly grow in the United States, range in height from a few centimeters to about a meter
  • Grow best in the rain forests or the Pacific Northwest and in tropical areas
  • Some species of ferns grow as large as small trees

Alternation of Generation in Ferns
  • Because of their well-developed vascular tissues, these sporophytes can grow in drier places than bryophyte sporophytes
  • Sexual reproduction in ferns depends upon the presence of standing water for sperm to swim to eggs
  • Fern sporophytes produce haploid spores on the underside of their fronds
  • Sporangia - tiny containers produced by spores
  • Sori - sporangia grouped into large clusters
  • When spores are ripe, they are released from the sporangia and may be carried by wind and water over long distances
  • If environmental conditions are right for the spores to germinate, they develop into haploid gametophytes
  • The gametophytes first grows a set of rootlike rhizoids and then flattens out into a thin heart-shaped green structure called a prothallium
  • When the antheridia are mature, sperm is released
  • Fertilization can take place when the ground and the prothallia are covered with a thin film of water
  • In bryophytes, fern sperm have to swim to the archegonia to fertilize the eggs
  • As the sporophyte grows, the gametophyte withers away
  • Fern sporophytes often live for many years

Life Cycle of Ferns

Where Mosses and Ferns Fit into the World
  • Mosses and ferns are well adapted to certain types of environments
  • Ferns are often found living in shadows of forest trees
  • Both mosses and ferns are important plants to gardeners
  • Several kinds of mosses are grown in graders for decorative purposes
  • Gardeners add peat moss to the soil because it improves the soil’s ability to retain water
  • Mosses were grounded up and used by Native Americans to treat burns and bruises
  • Sphagnum moss is also used to add flavor to Scotch whisky
  • Certain species of moss forms peat after the mosses die and are subjected to enormous pressure for long periods of time. Peat is a kind of coal that is cut from the ground and burned as a fuel
  • Few types of ferns are eaten by humans