Nonvascular Plants: Mosses, Liverworts, & Hornworts
Characteristics of nonvascular plants
embryophytes with no vascular tissue
multicellular sporangia and gametangia
reproductive cells surrounded by one or several layers of sterile cells
bodies composed of true parenchyma derived from apical meristems
terrestrial and have a cuticle over much of their bodies and many have stomata
life cycle
alternation of heteromorphic generations
sporophyte
gametophyte
in flowering plants they are large with leaf and roots
tiny and occur inside pollen grains and ovules
however they are the larger more prominent generation in nonvascular
in nonvascular they are smaller, more temporary and very inconspicuous
green moss
absorb minerals from gametophytes and remain attached to them
never independent of of the gametophyte, never branch, and never have leaves
visible if you examine moss closely, and look like brown or green hairs
wont grow to be very large, but being small provides certain advantages
selective advantage in habitats
Classification of nonvascular plants
not known how closely related mosses, liverworts, and hornworts are yet
they have features in common, but also differ in certain aspects
treated as three distinct divisions
mosses= Bryophyta
liverworts= Hepatophyta
hornworts= Anthocerotophyta
Division of Bryophya: mosses
Gametophyte generation
mosses= ubiquitous, occurring in all parts of the world and in almost every environment
Morphology
leafy stems = gametophores which grow closely together, tightly appressed, and form dense mounds
cool wet environments produces plants more open and loose
all moss stems have leaves
did not evolve from same structures as vascular plants stems and leaves did
gametophores grow from an apical meristem that contains a apical cell
tissues and leaves in precise arrangements
cuticle occurs only on top part of most moss leaves making the underside capable of absorbing water directly
lack of cuticle= no protection against desiccation
no stomata on moss leaves
moss stems are always slender and have little tissue differentiation
surface not to different from underlying so no epidermis
stem tissues all called cortex, may be uniform in all parts
outer cells more narrow with thickened walls
inner cells are larger and more parenchymatous and chlorophyllous
Water Transport
in some mosses, primarily the family Polytrichaceae, the inner most cortex has cells composed of hydroids
conduct water and dissolved minerals
elongated cells that lose cytoplasm when mature and end walls are partially digested away
aligned with those above and below
species with hydroids usually have leptoids then
resemble sieve cells
no nuclei, prominent gap junctions between cells, elongated and retain nuclei
majority of mosses lack hydroids and leptoids
water conducted along the exterior of stems via capillary action
leaves and stems so small they form spaces narrow enough to act as capillary channels
at the base of the stem is rhizoids
no leptoids means water transported between parenchyma cells
small, multicellular thrichome like structures
anchor stems by penetrating substrate
Development
gametophore growth begins when a spore germinates and sends out long slender, chlorophyllous cell
cell undergoes mitosis and produces a branched system= protonema
Reproduction
gametophore produces gametangia
all mosses are oogamous= every species has small bifilagellate sperm cells and large nonmotile egg cells
sperm produced in microgametangia called antheridia
eggs occur in megagametangia called archegonia
both occur on same gametophore in bisexual species
other species have male and female gametophores
special mechanisms used to get sperm to the egg
Sporophyte generation
moss gametophytes are both large and photosynthetic and they support the sporophyte its entire life
zygote undergoes a transverse division and develops into a small bulbous tissue called the foot
foot of gametophore used to absorb sugars, minerals, and water
cells of foot are transfer cells in m any species
upper cells grow by cell division and become simple apical sporangium= capsule
between the foot and the stalk is the seta
sporophyte is structurally simple
true epidermis with stomata
apex of sporangium is a cap like lid called operculum
breakage of cell from operculum separating from sporangium= peristome teeth
teeth trap spores when humid, and opening to release when air is dry
sporangium covered by calyptra
Metabolism and ecology
mechanisms used to retain water due to inability of mosses to retain water
many species grow in permanently moist environments
some mosses tolerant of desiccation and drying out does not damage them
as long as 30% of their weight is water, then they remain dormant but alive
dessicated mosses resistant to high or low temps and UV light
grow on hard or impervious surfaces
important in later establishment of other species
Division of Hepatophyta: Liverworts
small plants that have alternation of heteromorphic generations
few species ever become large
some species are leafy and greatly resemble mosses
others form small, solid, ribbon like gametophytes with a less conspicuous sporophyte then mosses
Gametophyte generation
divided into two basic groups
leafy liverworts
thallose liverworts
in both the gametophyte generation initiated when spores germinate and establish a protonematal phase
gametophore resembles that of moss, slender stem with leaves
leaves differ from moss though however
show less resemblance to mosses
not leafy at all but rather flat and ribbon like or heart shaped and bilaterally symmetrical
body referred as a thallus which has no roots, stems, or leaves
much thicker then leafy
many cells contain oil droplets next to substrate and those on the side away from substrate have more chlorophyll
cells arranged in a aerenchyma with large air chambers that open to exterior via large air pores
not stomata
both may either be bisexual, or unisexual
Sporophyte generation
little variability and basic morphology is like that of mosses
differ in fundamental structure though
liverwort lack a columella
dehiscence occurs via 4 longitudinal slits, not by an operculum
no peristome teeth
cells in sporangium do not undergo meiosis, rather they differentiate into elaters
Division Anthocerotophyta: Hornworts
small, inconspicuous thalloid plants that grow on moist soil, hidden by grasses and other herbs
resemble thalloid liverworts
alternation of heteromorphic generations occurs
large, single chloroplast per cell
chloroplasts have a pyrenoid
Gametophyte generation
gametophores are very thin
gametophyte is parenchymous and does not tolerate drying
internally gametophytes have numerous chambers
chambers start off as mucilage chambers, but become invaded by bacteria to form a symbiotic relationship
special mucilage chamber used to develop antheridia
archegonia formed via superficial cells
Sporophyte generation
similarities hard to find between mosses and liverworts
hornworts have a foot embedded in gametophore tissue, but no seta or discrete sporangium
sporangium is a long, horn like cylinder
some species have stomata
spores are green, yellow, brown, or black
have a columella as in mosses, and have elaters like liverworts, however these are multicellular unlike liverworts
