1. How Plants Colonized Land
Chapter 29

2. Overview: Origins of Mulitcellularity
Quick look at over all diversity in Plant Kingdom
Characteristics of Plants
Special features, structures
Origin from green algae
Link to Charophyceans
Evidence to support
Dates
Alternation of generations
Origin of embryophytes
Bryophyte (moss) lifecycle – in Lab
Pteridophyte (fern) lifecycle – in Lab

3. Multicellularity Arose independently several times in Eukarya
Caused another new wave in evolution
Origins in simpler colonial forms – Volvox
Cellular specialization and Division of labor
Escape cell size limitations
Membrane area to cytoplasm volume ratio

4. Multicellularity solves ratio limits Fig 6.7

5. The Plant Kingdom Origins over 475 MYA 10 Divisions 4 Basic lifecycles
Green algae that evolved onto land
Evolved becoming more terrestrial, independent from water
Then coevolved with pollinators, dispersal

6. Seedless vascular plants
An overview of land plant evolution
Land plants
Vascular plants
Bryophytes
(nonvascular plants)
Seedless vascular plants
Seed plants
Mosses
Liverworts
Hornworts
Charophyceans
Gymnosperms
Angiosperms
Pterophyte
(ferns, horsetails, whisk fern)
Origin of seed plants
(about 360 mya)
Lycophytes
(club mosses, spike mosses, quillworts)
Origin of vascular
plants (about 420 mya)
Origin of land plants
(about 475 mya)
Ancestral
green alga
Figure 29.7

7. Major events: Living on land, spores Vascular tissues Pollen, Seeds
Flowers & Fruits

8. The Plant Kingdom? Who is in / out?
Plantae
Streptophyta
Viridiplantae
Red algae
Chlorophytes
Charophyceans
Embryophytes
Ancestral alga

9. We’ll see all but the hornworts in lab
Table 29.1
We’ll see all but the hornworts in lab

10. Evolutionary Sequence
Green Algae (Charophyceans) gave rise to:
Mosses (475 MYA) and liverworts, which gave rise to:
Ferns (420 MYA) and related plant groups which gave rise to:
Gymnosperms (360 MYA) Conifers and related plant groups which gave rise to:
Angiosperms (140 MYA) (flowering plants) which have form two groups:
Monocots
Eudicots

11. Charophyceans- plant’s green algae ancestor
Chara Coleochaete
Modern examples of charophyceans

12. Original Traits Derived Traits
Found in ancestral species and new species
Not found in ancestral species, new to daughter species
Older terms: primitive and advanced traits

13. What’s new in Plants: Some Derived Characteristics of Plants
Growth by divisions in Apical Meristems
Multicellular dependent embryos
Alternation of generations
Spores
Multicellular gametangia
Cuticle
Transport tissues
Secondary compounds

14. Growth by cell divisions in Apical Meristems
Localized regions of cell division
In shoot tips, roots

15. Multicellular dependent diploid embryos
Land plants are called embryophytes
Haploid charophyceans

16. Spores Walls of resistant sporopollenin Dry out and travel in the wind
Zygotes of Charophyceans protected by sporopollenin – precursor to spore walls?
Disperse then grow into gametophyte plants

17. Multicellular gametangia
Archegonia Antheridia
Make egg at base of produce many sperm
Vase like column cells that swim to egg
All haploid tissues, gametes form by mitosis

18. Cuticle Waxy covering layer prevents water loss, and microbial attack
Stomata allow for gas exchange
Thicker layers in plants adapted to arid conditions

19. Transport tissues Xylem carries water up from the roots to the leaves
Phloem carries a sugary solution through out the plant

20. Secondary Compounds Metabolic side branches off common pathways
Provide benefit to plant ( defense, etc.)
Can be used by People as Flavorings, drugs insecticides, etc.

21. APICAL MERISTEMS ALTERNATION OF GENERATIONS
of shoot
Developing
leaves
100 µm
Apical meristems of plant shoots
and roots. The light micrographs
are longitudinal sections at the tips
of a shoot and root.
Apical meristem
of root
Root
Shoot
Haploid multicellular
organism (gametophyte)
Mitosis
Gametes
Zygote
Diploid multicellular
organism (sporophyte)
Alternation of generations: a generalized scheme
MEIOSIS
FERTILIZATION 2n n
Spores
ALTERNATION OF GENERATIONS

22. WALLED SPORES PRODUCED IN SPORANGIA Sporangium
Sporophyte and sporangium of Sphagnum (a moss)
Longitudinal section of
Sphagnum sporangium (LM)
Sporophyte
Gametophyte
MULTICELLULAR GAMETANGIA
Female gametophyte
Archegonium
with egg
Antheridium
with sperm
Archegonia and antheridia of Marchantia (a liverwort)
Male
gametophyte
MULTICELLULAR, DEPENDENT EMBRYOS
Embryo
Maternal tissue
2 µm
Embryo and placental transfer cell of Marchantia
10 µm
Wall ingrowths
Placental transfer cell

23. Origin of plants How did the sporophyte generation come about?
Preadaptations may have lead the charyophyceans onto land

24. Hypothesis for origin of alternation of generations
Delayed meiosis maximizes output of sexual reproduction.
More spores can be produced per fertilization event
Adapting to drying conditions with fewer surviving spores
Lineages separate before sporophytes evolved much

25. Preadaptation
Evolutionary adaptation that was selected for under one set of conditions
This trait then gives an advantage for a new situation with different conditions.

26. Preadaptation
Evolutionary adaptation that was selected for under one set of conditions
This trait then gives an advantage for a new situation with different conditions.
Feathers arose first for insulation, then helped with flight

27. Adaptation to shallow waters preadapted Charophyceans to life on land
Adapted to periodic drying during low tides, droughts
Leads to cuticle?
Adapted to higher light intensities
Leads to common chloroplast structure?
Zygote protected from drought within archegonia with a layer of sporopollenin
Leads to spore wall?

28. Alternation of Generations
Separate multicellular haploid and diploid phases
(2n) Sporophyte make spores by meiosis
(n) Gametophyte makes gametes by mitosis
Sperm and egg (moss & fern)
Pollen and Ovule (gymnosperm & angiosperm)
The sporophyte and gametophyte are very different in morphology
Vascular tissues only appear in sporophyte phase
Sporophyte becomes more dominant in new plant groups
Charophyceans lack sporophyte phase

30. Charophycean life cycle

31. Characteristics that Plants share with the green algae group Charophyceans
Autotrophic Multicellular Eukaryote
Have cell walls made of cellulose
Made by rosette cellulose-synthesizing complexes
20-26% of wall material, closest match in algae
Chloroplast similarities
have chlorophyll a & b, use β-carotene as accessory
Thylakoids stacked in grana
Chloroplast DNA comparisons
Peroxisome enzymes
Cell plate formation by phragmoplast
Nuclear membrane breaks down during mitosis
Sperm ultrastructure - biflagellate
Gene sequences – rRNA, Cytoskeleton proteins

32. Switch to sporophyte dominance

33. Figure 29.13 Hypotheses for the evolution of leaves
Vascular tissue
Microphylls, such as those of lycophytes, may have
originated as small stem outgrowths supported by
single, unbranched strands of vascular tissue.
(a)
Megaphylls, which have branched vascular
systems, may have evolved by the fusion of
branched stems.
(b)

34. What’s new in Mosses? Spores / sporangia Sporophyte phase
Upright growth on land
Cuticle
Multicellular gametangia

35. The Bryophytes Bryophytes are represented by three divisions:
Division Hepatophyta - liverworts
Division Anthocerophyta - hornworts
Division Bryophyta – mosses
Liverworts and hornworts are believed to be more similar to what early plants were like.

36. Bryophyte lifecycle: moss
Haploid dominant
No vascular tissues
Filamentous protonema stage
Swimming sperm
Disperse by spores
Dependent sporophyte
Dioecious gametophytes
No true leaves
Rhizoids, not roots

37. What’s new in Ferns? Vascular tissue True roots and stems and leaves
Stomata
Sporophyte (2n) dominate

38. Pteridophytes evolved over 400 MYA
Seedless, Vascular plants (having Xylem & Phloem). Today represented by two divisions:
Pterophyta: Ferns, Horsetails (Equisetum)
Lycophyta: Club moss
Cooksonia, an extinct plant over 400 million years old, is the earliest known vascular plant.
The branched sporophytes
were up to 50cm tall with
small lignified cells, much
like the xylem cells of
modern pteridophytes.

39. Vascular tissue Allows plants to grow taller
More support by lignified xylem tracheids
Can pull water up from soil
Can tolerate soil that is drier on the surface
Form parts of true leaves and roots.
Only found in diploid tissue
Lead to sporophyte dominance?

40. Fern Lifecycle Diploid dominate
Gametophyte still independent, short lived,
monoecious in fern (Pteridophyta)
dioecious in club moss (Lycophyta)
Sporophyte
in Lycophyta is Monecious
in Pteridiophyta is Dioecious
Spores disperse plant
Sporophyte perennial, monoecious

41. Figure 29.23 The life cycle of a fern

42. LYCOPHYTES (PHYLUM LYCOPHYTA)
PTEROPHYTES (PHYLUM PTEROPHYTA)
WHISK FERNS AND RELATIVES
HORSETAILS
FERNS
Isoetes
gunnii,
a quillwort
Selaginella apoda,
a spike moss
Diphasiastrum tristachyum, a club moss
Strobili
(clusters of
sporophylls)
Psilotum
nudum,
a whisk
fern
Equisetum
arvense,
field
horsetail
Vegetative stem
Strobilus on
fertile stem
Athyrium
filix-femina,
lady fern

43. Carboniferous forest based on fossil evidence