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Biol-111 (2012) – Lecture 5
Rajinder Dhindsa
Evolution of Land Plants
What types of new challenges and opportunities the plant life
would face in living on land as opposed to living in water?
How can these new challenges be met and opportunities
exploited?
2
Some of the new terms
introduced in this lecture
Alternation of generations
Gametophyte and sporophyte
Homospory and hererospory
Waxy cuticle
Microphyll and megaphyll
Archegonium
Antheridium
Mosses, hornworts, horsetails, clubmosses
Xylem vessels and tracheids
Phloem sieve tubes
ETC.
But first, a clicker question to check if
my receiver is working!
3
Do you have a clicker?
(I need to check if my clicker receiver is working
1.
2.
3.
Yes.
What is a clicker?
I don’t like clickers
76%
13%
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During the evolution of land plants,
plants moved to land from water. What
challenges did they confront?
• Make a list of important necessities of life and
then compare how a plant fulfills them living in
water and a plant living on land.
• Think about the limitations and advantages of
living in water and living on land.
5
Some Important necessities
of plant life
1. Optimum hydration: Avoid dehydration of the body.
2. Protection of gametes from dehydration
3. Maintenance of physical structure and posture of the body.
4. to obtain sufficient water and nutrients from surrounding
medium.
5. To carry out maximum photosynthesis
6. Maximum body size possible.
6
Plants living in water
1. No deficiency of water.
2. No special protection of gametes needed. They can be
released into water.
3. No problem in maintaining physical structure and posture of
the body as water provides buoyancy. Organisms in water
don’t have to support their weight.
4. Water and mineral enter the plant body by diffusion through
simple diffusion through the body surface. No special organs
ot structures needed
5. Photosynthesis is limited by attenuation of light intensity
under water.
5. In general body size is limited
7
Plants living on land
1. Water availability frequent problem, thus evolution of organs
for absorption of water and minerals through roots and root
hair. Water conservation through controlled loss through
stomata.
2. Special pathways evolved for distribution of water and
minerals through long-distance transport (Xylem, phloem)
3. Gametes are protected. Water is made available only in
preparation for fertilization.
4. Evolution of seed habit resistant to dehydration.
5. Cell wall thickening in tissues to provide mechanical
strength.
6. To benefit from higher light intensity, leaves with large
surface evolved for maximum photosynthesis.
5. Large sporophytic body evolved.
8
If density of water is 1.0 and that of human body is
1.1. How much of his weight would a 100-kg man
would have to support in water?
1.
2.
3.
4.
39%
38%
Approximately 9 kg
Approximately 1 kg
Approximately 90 kg
I don’t know
21%
9
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kg
3%
20
Arriving at the answer:
• A body density of 1.1 implies that a 100 kg man has
an approximate volume of 91 liters.
• On complete submersion in water, his body will
displace 91 liters of water which weighs 91 kg.
• So, man’s weight will decrease by 91 kg and he will
have to support only 9 kg or 9%.
• In the air he will displace the same volume, 91 liters
which weigh only 0.114 kg (Air density is
1/800th of that of water). Therefore, in the air
the same man will have to support 100-0.114 =
99.9 Kg or 99.9 % of his weight. Hence, the
need for skeleton.
10
All adaptations needed for flourishing life on land did not
arise at the same time during the course of evolution. Their
sequential acquisition can be seen various land plants.
11
Alternation of Generations
In sexually reproducing
organisms, there is a
phenomenon called Alternation
of generations. A haploid form
called gametophyte with one
set of chromosomes alternates
with a diploid form called
sporophyte with two sets of
chromosomes.
Gametophyte produces
gametes that fuse to form
zygote that develops into
sporophyte. The latter forms
haploid spores through
meiosis. Each spore develops
into a gametophyte.
12
Advanced green algae are ancestors of plants
(Gametes already protected in these ancestors)

All green algae have




Chara – up to 5
cm long cells
Photosynthesis with
chlorophyll a,b
Cellulose cell walls
Haploid dominant
Coleochaete
Pair of advanced groups
jointly have
 Multicellular thallus
growth form
 Antheridium &
oogonium protect
gametes
13
Oedogonium with oogonium and
antheridium
Advanced algae like Chara and Coleochaete
as ancestors of land plants
Coleochaete-like alga could serve as an ancestor of thallusbased land plants like liverworts (see later)
Chara-like alga possesses branching growth patter and could
serve as an ancestor for most land plants with erect growth
habit.
14
Adaptive changes that evolved for life on land also
altered the pattern of alternation of generations
Let us follow the changes in relative predominance and
independence of sporophytic and gametophytic phases of lifecycle.
Remember:
gametophyte is haploid (every cell has one set of
chromosomes)
Sporophyte is diploid (every cell has two sets of
chromosomes).
Think about possible consequences of this all-important
difference.
15
Isogamy: The two
gametes are similar in
appearance
This figure shows the alternation
of generations in
Chlamydomonas, an unicellular
green alga. It has asexual as
well as sexual modes of
reproduction.
The gametophytic phase is
predominant.
The sporophytic phase is limited
to just the zygote.
Asexual reproduction is just the
multiplication of the haploid
gametophytic phase.
16
Waxy cuticle appears to prevent drying
Heterogamy: The two
gametes are dissimilar in
appearnce and are
protected in gametangia.
It shows homospory.
Alternation of generations in
Marchantia – a liverwort. The
gametophyte is predominant and
its body is called thallus – a flat
mat-like structure. A thallus
produces male or female
umbrella-shaped structures
which produce antheridia
(containing motile sperms) or
archegonia (containing egg).
After fertilization, the zygote
grows right on the umbrellashaped structure and produces
spores through meiosis. Each
spore produces male or female
thallus. Non-motile and motile
gamete. Sporophyte is
matrotrophic.
17
Life-Cycle of a moss – Stomata appear for
gaseous exchange
Each archegonium
produces a single egg.
Each antheridium
produces millions of
sperms (the travelling
gamete)
Stomata appear for
the first time!
Sporophyte is
matrotrophic.
18
Anthoceros is a hornwort. The cylindrical sporophyte can
be nearly 8” tall and is matrotrophic. Hornworts and
vascular plants are believed to share common ancestors.
Again,
Sporophyte is
matrotrophic.
Clicker Question Next !
19
Have you ever been matrotrophic?
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15
Heterospory first appears in the earliest vascular
plants – the Pteridophytes – club-mosses or lycopods,
ferns and horsetails.
21
Evolution of microphylls (simple leaves with a single vascular
projection) and megaphylls (expanded leaves with many
vascular veins) to maximize size and light interception
Microphylls
Megaphylls
22
Selaginella – a
lycopod, shows both
heterospory as well
as heterogamy.
Microphylls appear
Selaginella is a heterosporous pteridophyte (an
early vascular plant related
to ferns). It produces
microspores giving rise to
male gametophyte , and
megaspores giving rise to
female gametophyte.
After fertilization, in some
species, the zygote
undergoes dormancy before
germinating (as in seeds).
23
Life cycle of a common fern
Ferns include both homosporous
and heterosporous species
Megaphylls appear
Xylem and phloem appear
24
Have you ever a seen fern stem?
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No, it doesn’t exist
No, It is microscopic
What is a fern?
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25
20
Equisetum – a horsetail, grows abundantly in
McGill University Arboretum
Up to here, sperms have
flagella and are motile
26
Gymnosperm life cycle
• Sperms are not
motile
• But still no flowers
• No xylem vessels
• Gametophyte
matrotrophic
• 300 ft tall sporophyte
• Same tree bears male
strobili and female
cones
• Naked ovule develops into a
naked seed
• Polyembryony: several
embryos develop
within one seed.
27
Naked ovule borne on the scale
Angiosperm Life Cycle: Flowers appear,
xylem vessels appear, sperms not motile
Gametophyte
matrotrophic
Male gametophyte
represented by mature
pollen grain consisting
of two cells.
Female gametophyte
represented by 8-celled
embryo sac. One of
those cells is the egg.
Double fertilization
Only one embryo per
seed
28
Female
gametophyte
Higher Plant vasculature: Tracheids, Vessels, Seive
tubes and Companion cells
Phloem seive tubes and
Xylem vessels and tracheids
29
companion cells
Evolution of Alternation of Generations
Gametophyte
predominant and
nourishes smaller
sporophyte
30
Gametophyte much
smaller than
sporophyte but both
live independently
Sporophyte predominant and
nourishes the hidden and
inconspicuous gametophyte
Sprophyte/Gametophyte Size Ratio
Group
Sporophyte Gametophyte Sporophyte/Gametophyte
Dependence
Algae: Variable
Bryophytes
Mosses
2 cm
Hornworts upto 20 cm
5 cm
0.4
thallus 0.4
20
Pteridophytes: 50-200 cm upto 5 mm
400
Gymnosperms
>10,000 cm
and Angiosperms
<1 mm
100,000
Sporophyte matrotrophic
Sporophyte matrotrophic
Both live independently
Microscopic gametophyte
matrotrophic
The approximate sporophyte/gametophyte ration is less than 1.0 in mosses, 20 in
hornworts, 400 in pteridophytes and greater than 100,000 in gymnosperms and
angiosperms.
31
So, how did the plants meet special
challenges associated with living on land?
1. Control of water loss through stomata (first appear in
mosses) and surface cuticle (all land plants.
2. Protection of gametes and evolution of seed habit.
3. Expansion of photosynthetic surface (leaves) to take
advantage of higher light intensity by evolution from
microphylls to megaphylls in pteridophytes.
4. Special tissues with thickened cell walls for mechanical
strength to support the plant body and to constitute
vascular system for transport of materials.
5. Evolution from homospory to heterospory for better
differentiation of sexes.
6. Seed habit – Dehydrated seed capable of being stored and
making agriculture possible for us.
7. Evolution of the diploid sporophyte as the predominant
body form to provide cushion against mutations as well as
higher gene dosage to increase body size.
32