Lecture 03 Pre

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Evolution of Land Plants

Biol-111 (2013) Lecture 3

Rajinder Dhindsa

What types of new challenges and opportunities the life formswould as it moves (in evolutionary time) from living in water toliving on land?

How can these new challenges be met and opportunities

exploited?


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Some of the new terms introduced inthis lecture

Alternation of generationsGametophyte and sporophyteHomospory and hererosporyIsogametes and heterogametes.Waxy cuticle

Microphyll and megaphyllArchegoniumAntheridiumMosses, hornworts, horsetails, club-mossesXylem vessels and tracheids

Phloem sieve tubes

But first, a clicker question to check if myreceiver is working!


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Some Important necessitiesof plant life

1. Optimum hydration: Avoid dehydration of the body.

2. Protection of gametes from dehydration and successfulfertilization.

3. Maintenance of physical structure and posture of the body.

4. to obtain sufficient water and nutrients from surroundingmedium.

5. To carry out maximum photosynthesis.

6. Maximum body size possible resulting from optimal ntrionand growth.


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Plants living on land

1. Water availability frequent problem, thus evolution of speccialorgans for absorption of water and minerals through roots.

2. Water conservation through controlled loss through stomata.

3. Special pathways evolved for distribution of water and mineralsthrough long-distance transport (Xylem, phloem)

4. Gametes are protected. Water is made available only in preparationfor fertilization.

5. Evolution of seed habit resistant to dehydration.

6. Cell wall thickening in tissues to provide mechanical strength.

7. To benefit from higher light intensity, leaves with large surfaceevolved for maximum photosynthesis.

8. Large sporophytic (diploid genome)body evolved.


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All adaptations needed for flourishing life on land did notarise at the same time during the course of evolution. Theirsequential acquisition can be seen various land plants.

1. Protected embryo alreadythere in the ancestor.

2. Stomata3. Large, green self-sustaining

sporophyte4. Small primitive leaves5. Vascular elements for

distribution systems.6. Large complex leaves with

network vasculature.7. Seeds with protected embryo

inside adapted for prolongeddehydration and storage.

8. Naked-seeded plants.Ovules/seeds not protected.

9. Flowering plants withovules/seeds protected insidethe ovary/fruit wall.


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Alternation of Generations

In sexually reproducing organisms,there is a phenomenon calledAlternation of generations. Ahaploid form called gametophyte(each cell has one set of chromo-somes) alternates with a diploid

form called sporophyte (each cellhas two sets of chromosomes).

Gametophyte produces gametesthat fuse to form zygote thatdevelops into sporophyte. Thelatter forms haploid spores through

meiosis. Each spore develops into agametophyte (male or female).


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Advanced green algae are ancestors of plants(Gametes already protected in these ancestors)

All green algae have

Photosynthesis withchlorophyll a,b

Cellulose cell walls

Haploid dominant

Advanced groups of greenalgae sucha s Charales orcoleochetales jointly have:

1. Multicellular thallus growthform.

2. Gametes are produced and

protected inside gametangia:Antheridia producing spermsand archegonia produce egg.

Coleochaete

Chara up to 5cm long cells


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Advanced algae like Chara and Coleochaeteas ancestors of land plants

Coleochaete-like alga could serve as an ancestor of thallus-based land plants like liverworts and hornworts (see later)

Chara-like alga possesses linear upright plant body with

branching growth patter and could serve as an ancestor formost land plants with erect growth habit.


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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.

Isogamy: The twogametes are similar inappearance


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Alternation of generations inMarchantia 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 umbrella-

shaped structure and produces spores

through meiosis. Each spore produces

male or female thallus. Non-motile and

motile gamete. Sporophyte is

matrotrophic.

Heterogamy: The two gametesare dissimilar in appearnce andare protected in gametangia. Itshows homospory (all sporesare similar).

Waxy cuticle to prevent drying first appears in this group: mosses

and liverworts. Marchantia, shown here, is a liverwort.


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Mosses Stomata for gaseous exchange appear for the 1st time

Each archegoniumproduces a single egg.

Each antheridiumproduces millions of

sperms (the travellinggamete)

Stomata appear forthe first time inmosses

Sporophyte ismatrotrophic.


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A n t h o c e r o s is a hornwort. The cylindrical sporophyte canbe nearly 8 tall and is matrotrophic. Hornworts andvascular plants are believed to share common ancestors.

Again,

Sporophyte ismatrotrophic.


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Heterospory first appears in the earliest vascularplants the Pteridophytes club-mosses or lycopods,ferns and horsetails.


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Evolution of microphylls (small and simple leaves each with asingle vascular projection). Size is limited due to limitation ofvascular supply.

Microphylls

Examples are lycopodium and other club-mosses


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Selaginella is a hetero-sporous

pteridophyte. It produces

microspores giving rise to male

gametophyte , and megasporesgiving r ise to female

gametophyte.

After fertil ization, in some

species, the zygote undergoes

dormancy before germinating

(beginnings of seed habit).

Selaginella a lycopod,shows both heterosporyas well as heterogamy.

Leaf-like structures areMicrophylls.


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Evolution of megaphylls (expanded leaves with many vascularveins) to maximize size and light interception

Megaphylls


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Life cycle of a common fern

Ferns include both homosporousand heterosporous species.

Megaphylls (Expanded, multi-veined leaves) appear.

Xylem and phloem appear


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Equisetum a horsetail, grows abundantly inMcGill University Arboretum

All plants up to here(including horsetails),have motile sperms withflagella.

Sperm in equisetum isthe largest known inplants. It can be seenwith naked eye.


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Gymnosperm life cycle

Naked ovule borne on the scaleSperms are not motile.

Ovules not inside theovary but sit naked on theupper surface of bract in acone.

But still no flowers. Scales ofmale cone have nakedchambers producing pollengrains.

No xylem vessels. Xylemtracheids present. (see later)

Gametophyte matrotrophicon the sporophyte.

Some of the tallest trees.


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Angiosperm Life Cycle: Flowers appear,xylem vessels appear, sperms not motile

Female

gametophyte

Male gametophyterepresented by mature pollengrain consisting of two cellsat the time of release.

Female gametophyterepresented by 7-celledembryo sac. One of thosecells is the egg.

Double fertilization. Only oneembryo per seed.

Xylem tracheids and vesselspresent. (See later)

Gametophyte matrotrophic


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Higher Plant vasculature: Tracheids, Vessels, Seivetubes and Companion cells

Phloem seive tubes and

Xylem vessels and tracheids companion cells


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Evolution of Alternation of Generations

Gametophytepredominant andnourishes smaller

sporophyte

Gametophyte muchsmaller thansporophyte but bothlive independently

Sporophyte predominant andnourishes the hidden andinconspicuous gametophyte


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Sprophyte/Gametophyte Size Ratio

Group Sporophyte Gametophyte Sporophyte/Gametophyte Dependence

Algae: Variable

Bryophytes

Mosses 2 cm 5 cm 0.4 Sporophyte matrotrophic

Hornworts upto 20 cm thallus 0.4 20 Sporophyte matrotrophic

Pteridophytes: 50-200 cm upto 5 mm 400 Both live independently

Gymnosperms >10,000 cm


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So, how did the plants meet special challengesassociated with living on land?

1. Control of water loss through stomata (first appear in mosses) andsurface 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.4. Special tissues with thickened cell walls for mechanical strength to

support the plant body and to constitute vascular system fortransport of materials.

5. Evolution from homospory to heterospory for better differentiationof sexes.

6. Seed habit Dehydrated seed capable of being stored and makingagriculture possible for us.

7. Evolution of the diploid sporophyte as the predominant body-formto provide cushion against mutations as well as higher genedosage to increase body size.

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