Download - Lecture 03 Pre
-
7/27/2019 Lecture 03 Pre
1/26
1
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?
-
7/27/2019 Lecture 03 Pre
2/26
2
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!
-
7/27/2019 Lecture 03 Pre
3/26
3
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.
-
7/27/2019 Lecture 03 Pre
4/26
-
7/27/2019 Lecture 03 Pre
5/26
5
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.
-
7/27/2019 Lecture 03 Pre
6/26
6
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.
-
7/27/2019 Lecture 03 Pre
7/267
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).
-
7/27/2019 Lecture 03 Pre
8/268
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
-
7/27/2019 Lecture 03 Pre
9/269
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.
-
7/27/2019 Lecture 03 Pre
10/26
-
7/27/2019 Lecture 03 Pre
11/2611
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
-
7/27/2019 Lecture 03 Pre
12/2612
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.
-
7/27/2019 Lecture 03 Pre
13/2613
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.
-
7/27/2019 Lecture 03 Pre
14/2614
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.
-
7/27/2019 Lecture 03 Pre
15/2615
Heterospory first appears in the earliest vascularplants the Pteridophytes club-mosses or lycopods,ferns and horsetails.
-
7/27/2019 Lecture 03 Pre
16/2616
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
-
7/27/2019 Lecture 03 Pre
17/2617
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.
-
7/27/2019 Lecture 03 Pre
18/2618
Evolution of megaphylls (expanded leaves with many vascularveins) to maximize size and light interception
Megaphylls
-
7/27/2019 Lecture 03 Pre
19/26
19
Life cycle of a common fern
Ferns include both homosporousand heterosporous species.
Megaphylls (Expanded, multi-veined leaves) appear.
Xylem and phloem appear
-
7/27/2019 Lecture 03 Pre
20/26
20
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.
-
7/27/2019 Lecture 03 Pre
21/26
21
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.
-
7/27/2019 Lecture 03 Pre
22/26
22
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
-
7/27/2019 Lecture 03 Pre
23/26
23
Higher Plant vasculature: Tracheids, Vessels, Seivetubes and Companion cells
Phloem seive tubes and
Xylem vessels and tracheids companion cells
-
7/27/2019 Lecture 03 Pre
24/26
24
Evolution of Alternation of Generations
Gametophytepredominant andnourishes smaller
sporophyte
Gametophyte muchsmaller thansporophyte but bothlive independently
Sporophyte predominant andnourishes the hidden andinconspicuous gametophyte
-
7/27/2019 Lecture 03 Pre
25/26
25
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
-
7/27/2019 Lecture 03 Pre
26/26
26
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.