1. Plant Diversity I How Plants Colonized Land
Chapter 29
Plant Diversity I How Plants Colonized Land

2. Overview: The Greening of Earth Looking at a lush landscape
It is difficult to imagine the land without any plants or other organisms
Figure 29.1

3. For more than the first 3 billion years of Earth’s history
The terrestrial surface was lifeless
Since colonizing land
Plants have diversified into roughly 290,000 living species

4. Concept 29.1: Land plants evolved from green algae
Researchers have identified green algae called charophyceans as the closest relatives of land plants

5. Morphological and Biochemical Evidence
Many characteristics of land plants
Also appear in a variety of algal clades

6. There are four key traits that land plants share only with charophyceans
Rose-shaped complexes for cellulose synthesis
30 nm
Figure 29.2

7. Peroxisome enzymes
Structure of flagellated sperm
Formation of a phragmoplast

8. Comparisons of both nuclear and chloroplast genes
Genetic Evidence
Comparisons of both nuclear and chloroplast genes
Point to charophyceans as the closest living relatives of land plants
Chara,
a pond
organism
(a)
10 mm
Coleochaete orbicularis, a disk-
shaped charophycean (LM)
(b)
40 µm
Figure 29.3a, b

9. Adaptations Enabling the Move to Land
In charophyceans
A layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out
The accumulation of traits that facilitated survival on land
May have opened the way to its colonization by plants

10. Concept 29.2: Land plants possess a set of derived terrestrial adaptations
Many adaptations
Emerged after land plants diverged from their charophycean relatives

11. Defining the Plant Kingdom
Systematists
Are currently debating the boundaries of the plant kingdom
Plantae
Streptophyta
Viridiplantae
Red algae
Chlorophytes
Charophyceans
Embryophytes
Ancestral alga
Figure 29.4

12. Some biologists think that the plant kingdom
Should be expanded to include some or all green algae
Until this debate is resolved
This textbook retains the embryophyte definition of kingdom Plantae

13. Derived Traits of Plants
Five key traits appear in nearly all land plants but are absent in the charophyceans
Apical meristems
Alternation of generations
Walled spores produced in sporangia
Multicellular gametangia
Multicellular dependent embryos

14. Apical meristems and 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
Figure 29.5
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
Figure 29.5

15. Walled spores; multicellular gametangia; and multicellular, dependent embryos
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
Archegonia and antheridia of Marchantia (a liverwort)
Antheridium
with sperm
Male
gametophyte
MULTICELLULAR, DEPENDENT EMBRYOS
Embryo
Maternal tissue
2 µm
Embryo and placental transfer cell of Marchantia
10 µm
Figure 29.5
Wall ingrowths
Placental transfer cell

16. Additional derived units
Such as a cuticle and secondary compounds, evolved in many plant species

17. The Origin and Diversification of Plants
Fossil evidence
Indicates that plants were on land at least 475 million years ago

18. Fossilized spores and tissues
Have been extracted from 475-million-year-old rocks
Fossilized spores.
Unlike the spores of
most living plants,
which are single
grains, these spores
found in Oman are
in groups of four
(left; one hidden)
and two (right).
(a)
Fossilized
sporophyte tissue.
The spores were
embedded in tissue
that appears to be
from plants.
(b)
Figure 29.6 a, b

19. Whatever the age of the first land plants
Those ancestral species gave rise to a vast diversity of modern plants
Table 29.1

20. Land plants can be informally grouped
Based on the presence or absence of vascular tissue

21. 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

22. Concept 29.3: The life cycles of mosses and other bryophytes are dominated by the gametophyte stage
Bryophytes are represented today by three phyla of small herbaceous (nonwoody) plants
Liverworts, phylum Hepatophyta
Hornworts, phylum Anthocerophyta
Mosses, phylum Bryophyta

23. Debate continues over the sequence of bryophyte evolution
Mosses are most closely related to vascular plants

24. Bryophyte Gametophytes
In all three bryophyte phyla
Gametophytes are larger and longer-living than sporophytes

25. The life cycle of a moss Figure 29.8 4 3 8 6 5 7 1 2
Mature sporophytes
Young sporophyte
Male
gametophyte
Raindrop
Sperm
Key
Haploid (n)
Diploid (2n)
Antheridia
Female gametophyte
Egg
Archegonia
FERTILIZATION
(within archegonium)
Zygote
Archegonium
Embryo
Female gametophytes
Gametophore
Foot
Capsule (sporangium)
Seta
Peristome
Spores
Protonemata
“Bud”
MEIOSIS
Sporangium
Calyptra
Capsule with peristome (LM)
Rhizoid
Mature
sporophytes
Spores develop into
threadlike protonemata. 1
The haploid
protonemata
produce “buds”
that grow into
gametophytes. 2
Most mosses have separate
male and female gametophytes,
with antheridia and archegonia,
respectively. 3
A sperm swims
through a film of
moisture to an
archegonium and
fertilizes the egg. 4
Meiosis occurs and haploid
spores develop in the sporangium
of the sporophyte. When the
sporangium lid pops off, the
peristome “teeth” regulate
gradual release of the spores. 8
The sporophyte grows a
long stalk, or seta, that emerges
from the archegonium. 6
The diploid zygote
develops into a
sporophyte embryo within
the archegonium. 5
Attached by its foot, the
sporophyte remains nutritionally
dependent on the gametophyte. 7
Figure 29.8

26. Bryophyte gametophytes
Produce flagellated sperm in antheridia
Produce ova in archegonia
Generally form ground-hugging carpets and are at most only a few cells thick
Some mosses
Have conducting tissues in the center of their “stems” and may grow vertically

27. Bryophyte Sporophytes
Grow out of archegonia
Are the smallest and simplest of all extant plant groups
Consist of a foot, a seta, and a sporangium
Hornwort and moss sporophytes
Have stomata

28. Bryophyte diversity Figure 29.9 LIVERWORTS (PHYLUM HEPATOPHYTA)
HORNWORTS (PHYLUM ANTHOCEROPHYTA)
MOSSES (PHYLUM BRYOPHYTA)
Gametophore of
female gametophyte
Marchantia polymorpha,
a “thalloid” liverwort
Foot
Sporangium
Seta
500 µm
Marchantia sporophyte (LM)
Plagiochila
deltoidea,
a “leafy”
liverwort
An Anthoceros
hornwort species
Sporophyte
Gametophyte
Polytrichum commune,
hairy-cap moss
Figure 29.9

29. Ecological and Economic Importance of Mosses
Sphagnum, or “peat moss”
Forms extensive deposits of partially decayed organic material known as peat
Plays an important role in the Earth’s carbon cycle
(a)
Peat being harvested from a peat bog
Sporangium at
tip of sporophyte
Gametophyte
(b)
Closeup of Sphagnum. Note the “leafy” gametophytes
and their offspring, the sporophytes.
Living
photo-
synthetic
cells
Dead water-
storing cells
100 µm
(c)
Sphagnum “leaf” (LM). The combination of living photosynthetic
cells and dead water-storing cells gives the moss its spongy quality.
(d)
“Tolland Man,” a bog mummy dating from 405–100 B.C.
The acidic, oxygen-poor conditions produced by Sphagnum canpreserve human or other animal bodies for thousands of years.
Figure a–d

30. Bryophytes and bryophyte-like plants
Concept 29.4: Ferns and other seedless vascular plants formed the first forests
Bryophytes and bryophyte-like plants
Were the prevalent vegetation during the first 100 million years of plant evolution
Vascular plants
Began to evolve during the Carboniferous period

31. Origins and Traits of Vascular Plants
Fossils of the forerunners of vascular plants
Date back about 420 million years

32. These early tiny plants
Had independent, branching sporophytes
Lacked other derived traits of vascular plants
Figure 29.11

33. Life Cycles with Dominant Sporophytes
In contrast with bryophytes
Sporophytes of seedless vascular plants are the larger generation, as in the familiar leafy fern
The gametophytes are tiny plants that grow on or below the soil surface

34. The life cycle of a fern Figure 29.12 Sporangia release spores.
Most fern species produce a single
type of spore that gives rise to a
bisexual gametophyte. 1
The fern spore
develops into a small,
photosynthetic gametophyte. 2
Although this illustration
shows an egg and sperm
from the same gametophyte,
a variety of mechanisms
promote cross-fertilization
between gametophytes. 3
Key
Haploid (n)
Diploid (2n)
Antheridium
Spore
Young
gametophyte
MEIOSIS
Sporangium
Archegonium
Sperm
Mature
sporophyte
Egg
New
sporophyte
Zygote
Sporangium
FERTILIZATION
Sorus
On the underside
of the sporophyte‘s
reproductive leaves
are spots called sori.
Each sorus is a
cluster of sporangia. 6
Fern sperm use flagella
to swim from the antheridia
to eggs in the archegonia. 4
Gametophyte
Fiddlehead
A zygote develops into a new
sporophyte, and the young plant
grows out from an archegonium
of its parent, the gametophyte. 5
Figure 29.12

35. Transport in Xylem and Phloem
Vascular plants have two types of vascular tissue
Xylem and phloem

36. Xylem Phloem Conducts most of the water and minerals
Includes dead cells called tracheids
Phloem
Distributes sugars, amino acids, and other organic products
Consists of living cells

37. Evolution of Roots Roots Are organs that anchor vascular plants
Enable vascular plants to absorb water and nutrients from the soil
May have evolved from subterranean stems

38. Evolution of Leaves Leaves
Are organs that increase the surface area of vascular plants, thereby capturing more solar energy for photosynthesis

39. Leaves are categorized by two types
Microphylls, leaves with a single vein
Megaphylls, leaves with a highly branched vascular system

40. According to one model of evolution
Microphylls evolved first, as outgrowths of stems
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)
Figure 29.13a, b

41. Sporophylls and Spore Variations
Are modified leaves with sporangia
Most seedless vascular plants
Are homosporous, producing one type of spore that develops into a bisexual gametophyte

42. All seed plants and some seedless vascular plants
Are heterosporous, having two types of spores that give rise to male and female gametophytes

43. Classification of Seedless Vascular Plants
Seedless vascular plants form two phyla
Lycophyta, including club mosses, spike mosses, and quillworts
Pterophyta, including ferns, horsetails, and whisk ferns and their relatives

44. The general groups of seedless vascular plants
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
Figure 29.14

45. Phylum Lycophyta: Club Mosses, Spike Mosses, and Quillworts
Modern species of lycophytes
Are relics from a far more eminent past
Are small herbaceous plants

46. Phylum Pterophyta: Ferns, Horsetails, and Whisk Ferns and Relatives
Are the most diverse seedless vascular plants

47. The Significance of Seedless Vascular Plants
The ancestors of modern lycophytes, horsetails, and ferns
Grew to great heights during the Carboniferous, forming the first forests
Figure 29.15

48. The growth of these early forests
May have helped produce the major global cooling that characterized the end of the Carboniferous period
Decayed and eventually became coal