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

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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.

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