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Nonvascular Plants: Mosses, Liverworts & Hornworts (Characters of…
Nonvascular Plants: Mosses, Liverworts & Hornworts
Characters of Nonvascular Plants
nonvascular plants are often called “bryophytes”
they have vascular tissue but not seeds
nonvascular plants are embryophytes that do not have vascular tissue
they have multicellular sporangia and gametangia
All mosses & many liverworts have leafy stems
that look remarkably like small versions of flowering plants
they are terrestrial & have a cuticle over much of their bodies
Classification of Nonvascular Plants
3 distinct divisions:
liverworts, division Hepatophyta
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mosses, division Bryophyta
All groups together are called nonvascular plants
hornworts, division Anthocerotophyta
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Division Bryophyta: Mosses
Metabolism & Ecology
The small size & lack of conducting tissues are 2 critical factors
the bulk of vascular plants protects them from
short-term fluctuations in air humidity & moisture availability
Leaves on moss gametophores,
have only a thin, incomplete cuticle
if exposed to dry air, the plants dry out
Many species grow in permanently moist microhabitats
or they grow in shallow depressions in rock or soil that retain moisture
Many moss species thrive at low temperatures near or even below 0°C.
The Sporophyte Generation
All nutrition for the zygote is supplied
by the grandparent generation, by way of endosperm formation
moss sporophyte is never a free-living plant
zygote of a moss undergoes a transverse division
& the basal cell develops into
a small, bulbous tissue called the foot
The foot is the interface w/ the gametophore
& it absorbs sugars, minerals & water
upper cell grows by cell division & expansion
into a simple apical sporangium called the capsule
capsule consists of:
an outer layer of sterile cells
an inner column of sterile cells
Between the foot & the sporangium is a narrow stalk, the seta
The Gametophyte Generation
Morphology
gametophores grow close together
tightly appressed and forming dense mounds
Moss gametophores grow from an apical meristem
that contains a prominent apical cell
Derivative cells subdivide
producing the tissues of stem & leaves in precise arrangements
Cuticle occurs only on the upper surface of most moss leaves
lack of a cuticle means mosses have little protection against desiccation
Water Transport
in some mosses, the innermost cortex is composed of hydroids
they conduct water & dissolved minerals
they are elongated cells that lose their cytoplasm when mature
end walls are partially digested
but not removed completely
Species that have hydroids typically also have leptoids
these are cells that resemble sieve cells
elongate & lack nuclei at maturity
majority of mosses lack hydroids & leptoids
water is conducted along the exterior of their stems by capillary action
Leaves & stems are so small they form spaces
narrow enough to act as capillary channels & transport water
At the base of the stem are rhizoids
small, multicellular trichome-like structures
penetrate the surface of the substrate
Development
Growth of the gametophore begins when a spore germinates
& sends out a long, slender chlorophyllous cell
cell undergoes mitosis & produces
branched system of similar cells
entire network is a protonema
protonema
can be distinguished by its many small chloroplasts in each cell
superficially resembles a filamentous green alga
are perennial and can grow extensively
Reproduction
All mosses are oogamous
every species has small biflagellate sperm cells
& large nonmotile egg cells
Sperms are produced in microgametangia called antheridia
antheridia consists of:
a short stalk
outermost layer of sterile cells
inner mass of cells that differentiate into sperm cells
Eggs occur in megagametangia called archegonia
Division Hepatophyta: Liverworts
The Gametophyte Generation
liverworts are small plants that have an alternation of heteromorphic generations
Hepatic gametophytes are divided into 2 basic groups:
leafy liverworts
gametophore greatly resembles the moss
(thin leaves on slender stem)
leaves have 2 rounded lobes w/ no midrib & no conducting tissue
arranged in 3 clearly defined rows
thallose liverworts
show less resemblance to mosses
not leafy at all but rather flat & ribbon like
or heart shaped & bilaterally symmetrical
the body is sometimes referred to as the thallus
In both groups, gametophyte phase is initiated when spores
germinate and establish a small, temporary protonematal phase
Male gametophores of Marchantia produce
an umbrella-shaped outgrowth called an antheridiophore
It has a stalk several millimeters tall
& dozens of antheridia grow from its upper surface
Archegoniophores also are stalked
their apex is a set of radiating fingers that project out & droop down
The Sporophyte Generation
Most liverwort sporophytes have a foot, seta & calyptra-covered sporangium
the seta is extremely delicate
The liverwort sporangium lacks a columella
the central mass of sterile cells found in mosses
Within the sporangium, some cells do not undergo meiosis
but rather differentiate into elaters
single, elongate cells with spring-shaped walls
simple sporophytes are produced in Riccia & Ricciocarpus
The zygote grows into a spherical mass
within the archegonium of the gametophore body
Division Anthocerotophyta: Hornworts
The Gametophyte Generation
Hornworts are a group of small, inconspicuous thalloid plants that grow on moist soil
Gametophores are always thin, at least along the edges
Only in the center do they become more than 4 or 5 cells thick
may be shaped like a ribbon or a heart
may grow outward irregularly, forming a disk
It does not tolerate drying
typically live for less than 1 year in temperate climates
hornwort gametophytes have numerous chambers
The Sporophyte Generation
Hornworts have a foot embedded in gametophore tissue,
but there is no seta or discrete sporangium
just above the foot is a meristem that
continuously produces new sporangium tissues
the sporangium is a long, horn-like cylinder,
typically 1 or 2 cm longin Anthoceros & Phaeoceros
At the tip, the sporangium is mature & open
as a result of dehiscence along two linear apertures
The outer layer of sterile cells is thick,
up to six cells deep & chlorophyllous
The basal meristem is active over a long period
and large numbers of spores can be produced by each sporophyte
