Chapter 20 Nonvascular Plants: Mosses, Liverworts, and Hornworts
Concepts
plants traditionally divided into
nonvascular plants
have neither
vascular tissue
seeds
vascular cryptogams
have vascular tissue
but not seeds
spermatophytes
have both
vascular tissue
seeds
modifications in plants
low, compact, multi-cellular body
low surface-to-volume ratio
better at retaining water
water-proofing cuticle
reproduction modifications
coordinate gamete reproduction
with periods of moisture
sperms need to swim
height
nonvascular plants
use turgid parenchyma cells
cannot grow very tall
vascular plants
xylem evolved
good at conducting water
Phloem then evolved
shaded basal part
could be nourished
allowed plants to grow taller
made evolution feasible for
roots
meristems
organ primordia
allows for mobilization of
sugars
minerals
hormones
Characteristics of Nonvascular Plants
examples
moss
liverworts
hornworts
characteristics
embryophytes that do not
have vascular tissue
multicellular sporangia and gametangia
reproductive cells are always surrounded
by one or several
layers of sterile cells
bodies not composed of
filaments but rather
true parenchyma derived by
three-dimensional growth
usually from apical meristems
all mosses and many liverworts
have leafy stems
almost exclusively terrestrial
have cuticle over
much of their bodies
many have stoma
nonvascular plants have
life cycle with
alternation of heteromorphic generations
sporophyte and gametophyte
differ from each other structurally
gametophyte
larger, more prominent generation
sporophyte
much smaller
more temporary
often very inconspicuous
never independent of gametophyte
never branch
never have leaves
stay attached
absorb minerals from them
collects minerals
can spread rapidly
via asexual reproduction
thrive in microhabitats
stone walls
fences
bare rock
Division Bryophyta: Mosses
The Gametophyte Generation
Morphology
derivative cells subdivide
producing tissues of
stems
stem tissues
all called cortex
outer cells may be
slightly narrower
walls somewhat thickened
OR
may be uniform in
all parts
little tissue differentiation
always slender
leaves
no stoma occurs
would be useless
because leaves are unistratose
underside
little protection against
desiccation
capable of absorbing water
directly from
fog
rain
dew
uncutinized
cuticle occurs only
on upper surface
of most moss leaves
thickness
most are one cell thick
except at
along the margin
midrib
most have midrib (costa)
aligned in three rows
at least as young
gametophores grow from
an apical meristem
contains a prominent apical cell
have leaves but
not homologous to
vascular plants
gametophores
some plants
grow loose and open
particularly in cool wet areas
leafy stems
many grow close together
form dense mounds
tightly appressed
Water Transport
some mosses
primarily the family Polytrichaceae
innermost cortex composed of cells
called hydroids
conduct
water
dissolved minerals
elongated cells that
lose cytoplasm when mature
species with hydroids
typically have leptoids
resemble sieve tube cells
elongated
have relatively prominent interconnections
with adjacent cells
lack nuclei at maturity
majority of mosses
lack hydroids and leptoids
water is conducted along
exterior of their stems
by capillary action
leaves and stems
are so small they
form spaces narrow enough to
act as capillary channels
and transport water
Development
growth of the gametophore
begins when a spore germinates
sends out long, slender
chlorophyllous cell
undergoes mitosis
produces a branched system
of similar cells
entire network is called
a protonema
resembles a filamentous green algae
but can be distinguished by
numerous small chloroplasts in each cell
then nodules of
small cytoplasmic cells form on it
organize an apical cell
then grow upright as
a stem with leaves
the gametophore
are
perennial
can grow extensively
producing many buds
Reproduction
gametophore produce
gametangia
all mosses are
oogamous
every species has
small biflagellate sperm cells
large non motile egg cells
sperms produce in microgametangia
called antheridia
consist of
short stalk
outermost layer of sterile cells
inner mass of cells that
differentiate into sperm cells
eggs occur in megagametangia
called archegonia
shaped like
vase with long neck
neck is hollow at maturity
single egg located at base
The Sporophyte Generation
supported by gametophores
zygote of moss undergoes
transverse division
and basal cell
develops into
small, bulbous tissue
called foot
interface with the gametophore
from which it
absorbs
water
sugars
minerals
cells are
transfer cells
upper cells grow
by cell division and expansion
into a
simple apical sporangium
called capsule
consists of
outer layer of sterile cells
inner column of sterile cells
simple morphologically
but relatively complex structurally
has a
true epidermis
with a stomata
at least on base
of sporangium
apex differentiates as caplike lid
called operculum
cells break apart creating
peristome teeth
respond to humidity
bending outward and
opening the sporangium
when air is dry
trapping spores when humid
Division Hepatophyta: Liverworts
The Gametophyte Generation
Hepatic Gametophytes divided into
two groups
leafy liverworts
thallose liverworts
gametophyte phase initiated by
spores geminate and
establish small, temporary protonematal phase
never as extensive
or ramified
or long lived
as mosses
only a few cells
are produced
apical cell establishes
gametophyte growth begins
gametophyte greatly resembles
that of moss
thin leaves on
a slender stem
however
they typically have
two rounded lobes with
no midrib
no conducting tissue
leaves arranged in
three clearly defined rows
two rows bigger
with one small
gametophore stem
grows by apical cell
with
3,4,or 5 sides
one uppermost
others embedded in stem
tissue is simple parenchyma
no thickened walls
no conducting cells
show less resemblance to mosses
not leafy at all but
flat and
ribbon like or
heart shaped and
bilaterally symmetrical
body
referred to as a thallus
a body without
roots
stems
leaves
stratified
tend to be thicker
than those of
leafy liverworts
mosses
side next to substrate
bears unicellular rhizoids
many cells contain large oil drops
SIde away from substrate
cells
have no oil but
rich in chlorophyll
sexuality
bisexual
producing
antheridia and archegonia
unisexual
male gametophytes
produce antheridiophore
umbrella-shaped outgrowth
The Sporophyte Generation
most liverwort sporophytes have
foot
seta
calyptra-covered
extremely delicate
composed of
clear, thin-walled cells
collapse quickly
sporangium
is globulose
while young
is bright green
is chlorophyllous
lacks a columella
the central mass of sterile cells
Division Anthocerotophyta: Hornworts
The Gametophyte Generation
as few as
three or four
protonema cells are produced
before the gametophore phase
is established in most species
gametophores
always thin
at least along edges
shaped like ribbon
or a heart
or may grow outward irregularly
gametophyte
parenchymatous
succulent but brittle
does not tolerate drying
typically live less than a year
in temperate climates
act as winter annuals
appear in
cool, moist autumns
internally
have numerous chambers
young plants
have mucilage chambers
formed by
as cells break down
altered chemically
The Sporophyte Generation
similarities with mosses or liverworts
hard to find
have foot embedded in
gametophore tissue
but no seta
or discrete sporangium
just above foot
is a meristem
that continously
produces new sporangium tissue
sporangium
long, horn-like cylinder
at tip
is mature and open
as result of
dehiscence along two linear apertures
outer layer of sterile cells
is thick and chlorophyllous
click to edit
have a columella
Cross-link Description: Sporophytes live shorter than gametophytes because they are smaller, dependent on others, and cannot tolerate temperatures well.
Cross-Link Description: The gametophytes of hornworts and thallose liverworts can both shape into ribbon-like plants.
Cross-link Description: Mosses can thrive in habitats, such as rock walls, with little nutrients. One reason moss can do this is because of their ability to absorb water in a variety of ways.