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Plant Diversity I
Level 1 Biological Diversity
Jim Provan
Campbell: Chapter 29
Main groups of land plants
Four main groups:
Bryophytes (mosses, liverworts and hornworts)
—
—
Differ from algae due to adaptation to terrestrial life
Mainly reproductive differences e.g. developing embryo
attached to mother plant
Pteridophytes (ferns, lycophytes, horsetails etc.)
—
—
Vascular system not found in bryophytes
Some bryophytes have a rudimentary water-conducting system
Gymnosperms (conifers etc.)
—
Seed plants: embryo, food supply and protective covering
Angiosperms
—
Flowering plants
Angiosperms
Gymnosperms
Ferns, horsetails, whisk ferns
Lycophytes
Hornworts
Liverworts
Mosses
Complex charophyceans
Simple charophyceans
Highlights of plant evolution
Ten phyla of extant plants
Common name
Approximate number
of extant species
Nonvascular plants (bryophytes)
Division Bryophyta
Division Hepatophyta
Division Anthocerophyta
Mosses
Liverworts
Hornworts
12,000
6,500
100
Lycophytes
Ferns
1,000
12,000
Vascular plants
Seedless vascular plants
Division Lycophyta
Division Pterophyta
Seed plants
Gymnosperms
Division Coniferophyta
Division Cycadophyta
Division Gingkophyta
Division Gnetophyta
Angiosperms
Division Anthophyta
Conifers
Cycads
Gingko
Gnetae
Flowering plants
550
100
1
70
250,000
Plants probably evolved from green
algae called charophytes
Evidence suggests plants
and charophytes evolved
from a common ancestor:
Homologous chloroplasts:
accessory pigments, stacked
thylakoids, DNA evidence
Biochemical: cellulose and
peroxisomes
Cell division: dispersed
nuclear envelope,
persistence of spindle etc.
Sperm ultrastructure
Phylogenetics
Differences between plants and
charophycean algae
Apical meristems
Light and CO2 found above ground
Water and nutrients mainly found below ground
Multicellular, dependent embryos
Alternation of generations:
Occurs in some algae but not charophyceans (no sporophyte
generation): evolved independently in land plants
Differs from standard haploid/diploid life cycles in that both
stages are represented by multicellular bodies
Walled spores produces in sporangia
Multicellular gametangia
Alternation of generations
Alternation of generations (continued)
Most plants reproduce sexually and most are also
capable of asexual reproduction: all plants have life
cycles with an alteration of generations:
Haploid gametophyte generation produces and alternates with
a diploid sporophyte generation  gametophyte
Life cycles are heteromorphic i.e. have different morphology
Sporophyte is larger and more noticeable, except in mosses
Comparison of life cycles among divisions is instructive:
Points to an important trend in plant evolution - reduction of
gametophyte generation and dominance of diploid sporophyte
Some features are adaptations to terrestrial environment replacement of flagellated sperm by pollen
Alternation of generations in plants may
have originated by delayed meiosis
Evolved independently in various groups of algae:
Does not occur among modern charophytes - occurrence in
plants had a separate origin from alternation of generations
in algae
Appearance in plants analogous to occurrence in algae
Coleochate may hold clues to evolution:
Parental thallus retains eggs - zygote also attached to parent
Thallus cells grow around zygote which enlarges, undergoes
meiosis and releases haploid swimming spores
Haploid spores develop into new individuals
Only diploid stage is zygote - no multicellular diploid
stage. This would arise if meiosis was delayed until
after mitotic division of zygote
Alternation of generations in plants may
have originated by delayed meiosis
Meiosis
delayed
Zygotic mitosis
produces multicellular
sporophyte
Adaptations to shallow water
preadapted plants for living on land
Many modern charophytes live in shallow water, as
did their ancestors
~440mya (Ordovician  Silurian), climatic changes
caused fluctuations in water levels
Natural selection favoured plants tolerant to periodic
drying - preadaptation to terrestrial life:
Waxy cuticles
Protection of gametes
Protection of developing embryos
Eventually, accumulated adaptations made it possible
for ancestral plants to live above water line
Bryophytes
The embryophyte adaptation evolved
in bryophytes
The embryophyte condition
was a pivotal adaptation - :
Antheridium (male gametangium)
produces flagellated sperm cells
Archegonium (female
gametangium) produces egg fertilisation/embryo development
occur within female organ
Bryophytes are not totally
independent of aquatic habitat:
Need water for sperm to swim in
Have no vascular tissue - diffusion
The gametophyte is the dominant
generation in the life cycle of bryophytes
Mosses (Division Bryophyta)
A tight pack of many moss
plants forms a spongy mat
that can absorb water
Each plant grips the
substratum with rhizoids
Photosynthesis occurs in
the upper parts of plants
Cover about 3% of land
surface and contain vast
amounts of organic carbon
Liverworts (Division Hepatophyta)
Less conspicuous than
mosses
Sometimes have bodies
divided into lobes
Live cycle similar to
mosses: sporangia have
elaters to aid dispersal
Can reproduce asexually
from gemmae
Most diverse in tropical
forests
Hornworts (Division Anthocerophyta)
Resemble liverworts but
sporophytes are hornshaped and grow from
mat-like gametophyte
Photosynthetic cells have
a single, large chloroplast
Most closely related to
vascular plants
Additional terrestrial adaptations in
vascular plants
Regional specialisation of the plant body: evolution of
roots to absorb water and stems/leaves to make food
Structural support: no buoyancy from water, so lignin
became embedded into cell walls
Vascular system:
Xylem: conducts water and minerals from roots. Composed
of dead, tube-shaped cells and provides structural support
Phloem: conducts food throughout the plant. Composed of
living cells organised into tubules and distributes sugars etc.
Pollen and seeds: to transport gametes
Increased dominance of the diploid sporophyte
Pteridophytes
The sporophyte-dominant life cycle
Exemplified by ferns:
Familiar leafy plant is the sporophyte
Gametophytes are small and grow on or below the soil surface
Vascular plants display two distinct reproductive
strategies:
Sporophyte of homosporous plants (e.g. ferns) produces a single
type of spore which develops into a bisexual gametophyte with
both male and female sex organs
Sporophyte of heterosporous plants produces two types of spore:
—
—
Megaspores develop into female gametophytes (archegonia)
Microspores develop into male gametophytes (antheridia)
A sporophyte-dominant life cycle
evolved in seedless vascular plants
MITOSIS
Lycophytes (Division Lycophyta)
Includes club mosses and
ground pines
Dominated land through the
Carboniferous Period (340280mya)
Some species of Lycopodium
are epiphytes:
Sporangia borne on sporophylls
Spores develop into
inconspicuous gametophytes
Mostly homosporous
Horsetails (Division Sphenophyta)
Survived through Devonian
and at peak during
Carboniferous
Only genus is Equisetum:
Lives in damp locations and
has flagellated sperm
Homosporous
Conspicuous sporophyte
generation
Has photosynthetic, freeliving gametophytes
Ferns (Division Pterophyta)
Co-existed with lycopods
and horsetails in
Carboniferous forests
Larger leaves with branched
system of veins - fronds are
compound leaves
Homosporous:
Specialised sporophylls
Gametophyte is small and
fragile
Embryo develops within
archegonium