1. Plants and the Colonization of Land
AP Biology – Chapter 29

2. Evolution of Plants
There were NO PLANTS until about 500 million years ago.
Until that time all life had been essentially aquatic…
Terrestrial Earth – barren
It’s hard to Imagine…

3. Evolution of Plants WATER TO LAND!!!!!!!
The evolutionary history of plants is a story of ADAPTATION TO TERRESTRIAL CONDITIONS from water.
WATER TO LAND!!!!!!!
Water…
To Land

4. Since their beginnings, Plants have been very successful…
Plants have diversified into about 290,000 living species.

5. The Ancestors of Land Plants…
Were most likely some type of Green Algae

6. Relatives to ancient plants – Green Alage
Chara
Coleochaete
“Kingdom” Protists
Spirogyra
Volvox

7. The closest algal relatives of land plants today…
Charophyceans (Charophytes)
It is likely that plants and charophytes share a common ancestor.
In other words, charophytes did not give rise to plants, but share a common ancestor with them.
This makes charophytes the nearest living algal relative to plants.

8. Plant and Algae shared characteristics
These characteristics are shared with many algae.
Multicellular
Eukaryotic
Photosynthetic autotrophs
Cellulose cell walls
Chlorophylls a and b present
Store surplus carbs in form of STARCH
What then is it that links plants specifically to charophytes?
Evidence for the
green algae/plant
connection

9. What links the plants to the charophytes?
4 additional characteristics
Shape of the complexes that make cellulose (rose – shaped)
Enzymes contained in the peroxisomes
Sperm structure is similar
Certain details of cell division processes
Also…the DNA!

10. Ancient Algae and the Water to Land Transition
Some ancient algae (like modern charophytes) inhabited shallow waters at edges of ponds/lakes
Survived occasional drying
Had coatings on spores to protect from drying
Natural selection would favor individual algae that can survive periods OUT of water.
Led to ADAPTIVE RADIATION of plant ancestors onto LAND

11. Why move to Land? Land has some big assets!
Bright sunlight unfiltered by water
Abundance of CO2 in atmosphere
Soil rich in mineral nutrients
Few herbivores and pathogens (at least at first!)

12. Problems in taking advantage of Land’s assets…
However…to take advantage of these assets plants had to figure out how to do without water!! This means:
Prevent Bodies from drying out…
Prevent Gametes from drying out
Fertilization without water
Prevent Embryos from drying out
PLANTS have solutions to these problems. ALGAE do NOT.

13. So, to distinguish plants from Algae we use…
…the solutions to the problems of land living that plants developed but that water-living algae did NOT develop.
Adaptations to land

14. Problem 1 – accessing resources on land
Location of Resources in TWO Very Different Places
Above ground – light,
Below ground – water and minerals
How to access these two different environments simultaneously is a huge problem
Remember – algae do NOT have this problem
Continually bathed in uniform water environment
No need to protect tissues from drying out
All tissues have equal access to light, dissolved nutrients, etc.

15. Problem 1 – accessing resources on land: SOLUTION
Apical Meristems
Small regions of growth in tips of roots and shoots
increase access to resources in different directions: soil/light.
Cells produced in these meristems differentiate into tissues suited for these two different environments.
Example:
Protected epidermis for leaves exposed to air
Cuticle; stomata
Roots unprotected and designed for absorption
Apical Meristems

16. Problem 1 – accessing resources on land: SOLUTION
Access to light and soil, generally require a plant to gain length in both directions
But how to get substances transported across long distances within the plant’s body?
VASCULAR TISSUE
Tissue that can move LARGE QUANTITIES of material through a plant
Without it, osmosis is the only alternative
SLOW; limits size a land organism can attain.

17. Apical Meristems

18. Problem 2: Evaporation of water from tissues on Land -
Algae is constantly bathed in water
Evaporation from tissues is NOT a concern
Also, algae tissue freely accepts nutrients from water
thus no need for access to nutrients in soil
No need for support – water supports the algae and floats it near the surface to receive light
Algae on land would rapidly lose water from tissues with no way to replenish; also, access to light would be limited without support structures

19. Problem 2: Evaporation of water from tissues on Land Solution
Protective tissues
Epidermis and secretion of waxy cuticle
Prevents water loss
Stomata
Allows gas exchange even though cuticle is covering the plant
Cross-section through a leaf

20. Problem 3 – Drying out of sex cells and embryos
Aquatic organisms do NOT have to “worry” about
Gametes drying out during development
Gametes drying out when released
Embryos drying out during development
Embryos drying out when released
LAND organisms DO have to “worry” about these things.
Water > Land transition required solving these problems

21. Problem 3 – Drying out of sex cells and embryos: Solution
Protection of Gametes during
Production
Gamete production is protected inside structures called gametangia
Prevents drying out of sperm/egg
Dispersal
In more advanced plants, gametes are protected during dispersal in structures (pollen) that can travel through air.
NOT dependent on swimming sperm.

22. Problem 3 – Drying out of sex cells and embryos: Solution
Protect delicate Embryos from drying out, too!
Egg fertilization occurs inside gametangium
Zygote develops into embryo inside gametangium
Embryo is retained and nourished inside gametangium
Embryos dispersed in structures that also prevent drying out:
Spores
Seeds

23. However, solutions to all these problems were NOT achieved at once…
These solutions were generally achieved in STEPS
We can use these steps in evolution of land adaptations as a way to determine a “plant family tree” or PHYLOGENY

24. A new period of adaptive radiation followed
Green Algae
Moss
Ferns
Conifers
Flowering Plants
Each of these circled branch points indicates some new adaptation that led to even more success on land…
A new period of adaptive radiation followed
Algae/Plant
Common
Ancestor

25. Plant Classification Domain Eukarya
Kingdom Plantae (currently excludes the algae, but there is DISAGREEMENT about this!)
Some think green algae should be included within the plant kingdom!
10 major phyla (actually called divisions in plants)
3 of these are nonvascular (bryophytes)
7 are vascular and include the vast majority of plants (93%)

26. Generalizations Regarding Plant Groups
Plants can be loosely and informally placed into two major groups
Nonvascular (the bryophytes/mosses)
More ancient / primitive
Rely on cell to cell diffusion for transport
Small (like mosses)
No true roots, stems or shoots
Vascular (everything else)
More recent / advanced
Use sophisticated “plumbing” (vascular tissue) to move food/water
Can become quite large
True roots, stems and shoots (by definition, roots/stems/shoots are all structures that contain vascular tissue)

27. Generalizations Regarding Plant Groups
Vascular Plants can be informally grouped into
Seedless Vascular Plants
First to arise from nonvascular ancestors
Possess (obviously) vascular tissue, but that’s about all that sets them apart from nonvascular plants
Gymnosperms
Naked seed plants / Conifers
– 1st seeds, but seeds unprotected
Angiosperms
Flowering plants – seed development protected inside ovaries

28. FIRST Period of Plant Evolution: Origin of Plants from Aquatic Ancestors
Living groups from this first period of plant evolution:
Division Bryophyta – the mosses
10,000 species
Division Hepatophyta – Liverworts
6,500 species
Division Anthocerophyta – Hornworts
100 species

29. Typical Bryophyte (moss) life cycle
Plant Life Cycles called ALTERNATION OF GENERATIONS
Life cycle alternates between two different multicellular bodies, with each form producing the other.
One body form is haploid
The other body form is diploid
Do NOT confuse with haploid/diploid stages of animals, etc. The haploid form in plants is a multicellular body, not just a gamete.

30. Alternation of Generations – Why do we care?
ALL plants undergo this pattern called alternation of generations
Why do we care?
Differences in plant life cycles give us important clues about evolutionary advances of each major plant group

31. Haploid spores land in suitable environment
Sperm must SWIM to egg
DIPLOID Sporophyte grows out of haploid female gametophyte
Egg/ ovum
MITOSIS
Fertilization In female gametophyte creates DIPLOID ZYGOTE
MEIOSIS occurs in spore capsule
Male Gametophyte
Haploid generation
Female Gametophyte
Haploid generation
Spore capsule opens and releases HAPLOID spores
Haploid spores germinate and grow into haploid gametophytes
Haploid spores land in suitable environment

32. FIRST Period of Plant Evolution: Origin of Plants from Aquatic Ancestors
Adaptations to Terrestrial Life
Cuticle
Jacketed gametangia protect gametes and embryos
BUT, still tied to water because…
Still have swimming sperm
MUST have water for fertilization to occur
LACK vascular tissue
Means no real means of efficiently conducting food and water
Size is limited – very small
No seeds – spores used for embryo dispersal
Gametophyte is dominant generation
Diploid generation is present ONLY as ZYGOTE!!

33. Moss

34. 2nd Period of Plant Evolution – Diversification of Vascular Plants
Group known as SEEDLESS VASCULAR PLANTS
Adaptations
From Previous Group:
Cuticle and gametangia of previous plants
New to SEEDLESS VASCULAR PLANTS:
Sporophyte is dominant Generation
VASCULAR TISSUE added!!
Can efficiently move food and water throughout body
Can attain larger size
Still share these primitive characteristics:
Still depend on swimming sperm
Still no seeds – use spores for embryo dispersal

35. 2nd Period of Plant Evolution – Diversification of Vascular Plants
Living groups from this period of plant evolution
Division Psilophyta – whisk ferns
10-13 species
Division Lycophyta – club mosses
1,000 species
Division Sphenophyta – horsetails
15 species
Division Pterophyta – ferns
12,000 species

36. Ferns

37. Ferns

39. 3rd Period of Plant Evolution – Diversification of Vascular Plants
Group known as GYMNOSPERMS or “Naked Seed Plants”
Adaptations
From previous groups:
Cuticle, gametangia, dominant sporophyte and vascular tissue
NEW to GYMNOSPERMS:
SEEDS – to further protect embryos from dessication (and other hazards) and aid in dispersal.
Pollen – air transported sperm – no longer need water to transport swimming sperm
Still primitive in that…
NO Flowers
NO Fruit

40. 3rd Period of Plant Evolution – Diversification of Vascular Plants
“naked seeds” refers to seeds NOT being enclosed in any specialized chambers (fruit) Seeds are exposed to environment on highly modified leaf surfaces (ex. Cones)
A seed consists of
Embryo
Stored food (starch)
Protective covering (seed coat)
Pollen – airborne sperm (sort of)
Highly reduced male gametophyte
No longer dependent on water for fertilization

41. 3rd Period of Plant Evolution – Diversification of Vascular Plants
Living groups from this period of plant evolution
Division Coniferophyta (conifers)
Largest group
Needle-like leaves
550 species
Division Cycadophyta
100 species
Division Gingkophyta
1 species
Division Gnetophyta
70 species

42. 4th Period of Plant Evolution – Diversification of Vascular Plants
Emergence of Flowering Plants - Angiosperms
Adaptations
From previous groups:
Cuticle, gametangia, dominant sporophyte, vascular tissue, seeds and pollen of previous groups
NEW to ANGIOSPERMS:
FLOWERS
FRUITS
Double fertilization
Fert of egg to make embryo
Fert of 2 polar nuclei to make endosperm
TRIPLOID tissue (endosperm – stored food
2 polar nuclei + 1 sperm nuc. = 3n

44. 4th Period of Plant Evolution – Diversification of Vascular Plants
Advantages of flowers and fruits
Flowers draw pollinators which are very efficient at transferring pollen from flower to flower
Fruits aid in seed dispersal either by water, wind or animals
Other plants depend on water or wind for gamete and seed dispersal which is not near as precise.
Angiosperms may use wind also, but fruits allow for elaborate structures to carry seeds FURTHER by wind.

45. 4th Period of Plant Evolution – Diversification of Vascular Plants
Advantage of triploid tissue in seeds
Has greater ability to pull food resources from “mother” than embryo does. Thus nutrients can be stored in the seed for later use.
Also, fact that endosperm must be fertilized ensures that endosperm will not be made if fertilization has not occurred. Avoids waste.

46. 4th Period of Plant Evolution – Diversification of Vascular Plants
Living groups from this period of evolution
ONE Divison – Anthophyta
235,000 species
Two Important Classes
Monocot
Dicot

47. Double Fertilization!!

49. Alternation of Generations
Haploid gametophytes and diploid sporophytes take turns producing one another.
Gametophyte
Plant that makes GAMETES
HAPLOID PLANT
MAKES HAPLOID CELLS
Sporophyte
Diploid plant
Cells inside undergo MEIOSIS
Plant that makes SPORES – HAPLOID CELLS
Why make spores?
Efficient at dispersal of “babies”
Wind dispersed

50. Evolutionary Trends in Alternation of Generations
Initially in the evolution of land plants, gametophytes were the dominant generation.
Larger than sporophyte
Sporophyte dependent on gametophyte for everything – sporophyte NOT photosynthetic

51. Evolutionary Trends in Alternation of Generations
Trend in plant evolution was for SPOROPHYTE to become dominant and GAMETOPHYTE to become dependent
Why?
Diploid tissue is less subject to the negative effects of mutation than haploid tissue
In diploid tissue there are two copies of genes. If one gets damaged, the other may be able to function normally.

52. Why Study Plant Life Cycles?
Features of the life cycles are key evolutionary adaptations to terrestrial environments
Example: replacement of flagellated sperm by pollen

54. Egg/ ovum
________
____________ occurs in spore capsule