1. Evolution by Seed Plants
Plant Diversity II
Evolution by Seed Plants

2. cyanobacteria on land – 1.2 billion years ago
500 MYA – colonization by plants
closest relatives of land plants = charophyceans
molecular comparison of both nuclear and chloroplast genes confirms morphological and biochemical conclusions that the charophyceans are ancestors of plants
plant share characteristics with other more primitive organisms
1. multicellular, eukaryotic
2. photosynthetic autotrophs – brown, red, green algae
3. cell walls made of cellulose – green algae, dinoflagellates, brown algae
4. chloroplasts with chlorophyll a and b – green algae, euglenids and a few dinoflagellates

3. A reminder: several unique traits seen in plants and charophyceans
1. rose-shaped complexes for cellulose synthesis –called rosettes
synthesize cellulose microfibrils for the cell walls
2. peroxisome enzymes –enzymes to help minimize the loss of organic products as a result of photorespiration
3. flagellated sperm – some species of land plants have flagellated sperm
4. formation of a phragmoplast – involved in the synthesis of new cell walls during mitosis - via the formation of new cross walls called cell plates
Charophyceans

4. Defining the Plant Kingdom
land plants and green algae form a single clade = Viridiplantae
1.2 billion years ago – split into 2 clades: Chlorophyta (algae) & Streptophyta (land plants & charophyceans)
475 million years ago – the land plants began to evolve from the charophyceans into non-vascular plants
420 million years ago – evolution of vascular plants
traditional botanists call land plants embryophytes
embryo is nourished inside the archegonium/female gametophyte

5. Defining the Plant Kingdom
vascular plants now form a single clade – 93% of all plant species
categorized into three smaller clades
1. lycophytes – club mosses and relatives
2. pterophytes – ferns and relatives
3. seed vascular plants
A. gymnosperms - “naked seed” plants
B. angiosperms – flowering plants

6. 2. alternation of generations & multicellular, dependent embryos
ANOTHER REMINDER: four key traits that define land plants – absent in charophyceans
1. apical meristems
2. alternation of generations & multicellular, dependent embryos
3. walled spores in sporangia
two kinds of sporangia possible
4. multicellular gametangia
male and female

7. Heterospory the rule among seed bearing vascular plants
nearly all non-vascular plants are homosporous – produce one kind of spore which gives rise to a bisexual/monoecious gametophyte
with the evolution of seed plants – development of heterospory
megasporangium located on modified leaves called megasporophylls produce megaspores  female gametophytes
microsporangium located on modified leaves called microsporophylls –produce microspores  male gametophytes
both are found on specialized reproductive structures
e.g cones, flowers

8. Heterospory
these sporangia can either be located on the same plant = monoeicous
meaning “one house”
i.e. bisexual
or they can be located on “male” and “female” plants = dioecious
reproduction requires a male and female plant

9. Seed plants three key reproductive adaptations evolved in seed plants:
1. increasing dominance of the sporophyte generation – reduced gametophyte
2. advent of the seed – ovules and eggs
3. evolution of pollen as an airborne agent

10. 1. Reduced Gametophytes
gametophytes of mosses and ferns are the dominant stage
gametophytes of seed plants are mostly microscopic
miniaturization allows for the development of their gametophytes within the sporangium of the parental sporophyte
protects the delicate egg-forming gametophyte from environmental stress
also allows the growing gametophyte to derive nourishment directly from the sporophyte

11. Bryophytes Seedless Vascular Seed Vascular Sporophyte (2n) Gametophyte
Sporophyte dependent on gametophyte (mosses and other bryophytes)
Large sporophyte and small, independent game-tophyte (ferns and other seedless vascular plants)
Microscopic female
gametophytes (n) in
ovulate cones
(dependent)
Microscopic male
inside these parts
of flowers
Sporophyte (2n),
the flowering plant
(independent)
gametophytes (n)
in pollen cones
Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms)
Bryophytes
Seedless Vascular
Seed
Vascular

12. 2. Ovaries & Seeds
seed plants are unique in that the megasporangium is retained with the parent sporophyte
the megasporangium contains the developing megaspore
it is surrounded by layers of sporophyte tissue called integuments
in gymnosperms – the megaspore is surrounded by only one integument
angiosperms usually have two integuments

13. 2. Ovaries & Seeds
the megaspore (n) + megasporangium (2n) + integuments = ovule
inside each ovule is a future female gametophyte that develops from the megaspore
gametophyte can produce one or more egg cells within the ovule
these eggs are fertilized by the sperm that will develop from the pollen grain
an embryo results  found within a seed
Integument
Megasporangium
(2n)
Megaspore (n)
Unfertilized ovule
Fertilized ovule
Micropyle
Female
gametophyte (n)
Egg nucleus (n)
Discharged
sperm nucleus (n)
Pollen grain (n)
containing male
gametophyte
Seed coat
(derived from
integument)
Embryo (2n)
(new sporophyte)
Gymnosperm seed
Food supply
(female
tissue) (n)

14. 2. Ovaries & Seeds
seed = ovule after fertilization – contains the embryo
seed = embryo + food supply + seed coat (from the integuments)
allows for the developing embryo to resist harsh conditions
evolutionary advantage of seeds:
until seeds – the spore was the only protective stage in the life cycle
unlike spores – seeds carry their own food supply
unlike spores - a seed can remain dormant for years following its release
Seed coat
(derived from
integument)
Embryo (2n)
(new sporophyte)
Gymnosperm seed
Food supply
(female
gametophyte
tissue) (n)

15. 3. Evolution of Pollen the microsporangium produces microspores
microspores develop into pollen grains
a pollen grain contains the male gametophyte enclosed within a pollen wall
outer wall is made by the sporophyte, inner wall is made by the gametophyte within
outer wall = protected by a coating of sporopollenin
transfer of pollen to the ovule = pollination
pollen grains are carried away from the parent plant by wind, insects
or they can travel to the female reproductive structures within the same sporophyte

16. in order to fertilize the egg - the pollen grain must germinate (grow)
it produces a pollen tube
pollen tube allows for the discharge of two sperm (gametes) into the ovule
sperm unites with the egg developing within female gametophyte (within the ovule)
in non-vascular plants (bryophytes) and seedless vascular plants (ferns) – the sperm is flagellated and swims to the female gametophyte in order to fertilize the egg which is also free living
in vascular seed plants – the female gametophyte produces an egg which never leaves the sporophyte ovule
3. Evolution of Pollen

17. Gymnosperms Ponderosa pine
“naked seed” – seeds are not enclosed in ovaries
seeds are exposed on modified leaves (sporophylls) that form cones or strobili
sporophylls bear sporangia for spore production
380 MYA – development of heterosporous trees with woody stems – but did not bear seeds = progymnosperms
first seed plant in the fossil record – 360 MYA
earliest fossils of gymnosperms – 305 MYA
early Mesozoic era (250 MYA) - domination by gymnosperms
drier environment favored gymnosperms over the bryophytes and ferns
most common existing gymnosperms are the conifers – spruce, pin, fir and redwood
Ponderosa pine

18. Gymnosperms
Cycas revoluta
four gymnosperm phyla: Cycadophyta, Ginkgophyta, Gnetophyta and Coniferophyta
Phylum Cycadophyta – cycads
second largest group of gymnosperms
130 species survive
Phylum Ginkgophyta - ginkos
only one species left – Ginkgo biloba
Phylum Gnetophyta – three genera alive today
Gnetum – 35 species of tropical trees, shrubs and vines (Africa and Asia)
Welwitschia – one species, Welswitchia (Africa)
Ephedra – 40 species, desert shrubs
Phylum Coniferophyta – largest group
“cone-bearing”
600 species of conifers
many are large trees
most are evergreens – retain their leaves throughout the year
Ginko biloba
Welwitschia mirabilis.
Ephedra.

19. Phylum Coniferophyta Pine fascicle also called Division Coniferophyta
575 species
largest genus – Pinus
leaves of conifers are always simple needles or scales
pine leaves – needles or needle-like
arranged in clusters or bundles of two to five leaves each bundle
cluster = fascicle
Pine fascicle

20. Phylum Coniferophyta pine needle structure:
needle is comprised of a outer epidermis coated with a thick cuticle
below that is one to two layers of cells = hypodermis
stomata are recessed in sunken cavities
veins (vascular tissue) run down the center of the needle and are surrounded by an endodermis
also contain resin canals – occur in other parts of the pine
these canals are lined with special cells that secrete a resin – aromatic and antiseptic
prevents water loss and fungal attacks on the needles
deters insects

21. Life Cycle: The Pine pine tree is the sporophyte
sporangia are located on scale-like leaves (sporophylls) packed into cones – strobili (single = strobilus)
two types of cones produce two types of spores
small pollen cones produce microspores – pollen
larger ovulate cones produce megaspores – egg
ovulate cones also known as seed cones – most are woody
cones of the juniper can resemble a fruit (berry)
Life Cycle: The Pine

22. Life Cycle: The Pine Mature sporophyte (2n) Pollen cone
Microsporocytes
Longitudinal
section of
pollen cone
Ovulate
ovulate cone
Micropyle
Ovule
Key
Haploid (n)
Diploid (2n)
Megasporocyte (2n)
Integument
Megasporangium
Germinating
pollen grain
grains (n)
(containing male
gametophytes)
MEIOSIS
Microsporangium
Sporophyll
Female
gametophyte
Archegonium
Egg (n)
pollen grain (n)
Discharged
sperm nucleus (n)
tube
Egg nucleus (n)
FERTILIZATION
Embryo
(new sporophyte)
Food reserves
(gametophyte
tissue) (n)
Seed coat
(derived from
parent
sporophyte) (2n)
Seedling
Seeds on surface
of ovulate scale
Surviving
megaspore (n)

23. The Male Life cycle
pollen cones: bear modified leaves or sporophylls (microsporophylls) each containing two microsporangia
the microsporangium is comprised of diploid cells called microsporocytes (2n)

24. The Male Life cycle
microsporocytes divide by meiosis to form pollen grains which are haploid
pollen grains contain the male gametophyte – for the production of sperm

25. The Male Life cycle
grains travel to the ovulate cone where they begin to germinate and form a pollen tube through which the sperm will travel
pollen tube “digests” its way into female ovule through an opening called a micropyle
pollen development results in the production of 2 sperm cells within the pollen tube
two sperm travel into the ovule to fertilizes the eggs

26. The Female Life Cycle
ovulate cones: made up of scales or megasporophylls
base of the megasporophylls are the ovules
Mature
sporophyte
(2n)
Pollen
cone
Ovulate
Longitudinal
section of
ovulate cone
Micropyle
Ovule
Key
Haploid (n)
Diploid (2n)
Megasporocyte (2n)
Integument
Megasporangium
Germinating
pollen grain
grains (n)
(containing male
gametophytes)
MEIOSIS
Female
gametophyte
Archegonium
Egg (n)
pollen grain (n)
Discharged
sperm nucleus (n)
tube
Egg nucleus (n)
FERTILIZATION
Embryo
(new sporophyte)
Food reserves
(gametophyte
tissue) (n)
Seed coat
(derived from
parent
sporophyte) (2n)
Seedling
Seeds on surface
of ovulate scale
Surviving
megaspore (n)

27. The Female Life Cycle each ovule contains one megasporangium
the megasporangium contains a cell called a megasporocyte or a megaspore mother cell (2n)
the megaspore mother cell undergoes meiosis to form 4 haploid cells
only one survives as the megaspore (n)
the remaining degenerate
Longitudinal
section of
ovulate cone
Micropyle
Ovule
Key
Haploid (n)
Diploid (2n)
Megasporocyte (2n)
Integument
Megasporangium
Germinating
pollen grain
Pollen
grains (n)
(containing male
gametophytes)
MEIOSIS

28. The Female Life Cycle
the surviving megaspore develops into the female gametophyte (archegonia + eggs)
the female gametophyte contains two (or three) archegonia - each will form an egg inside (two or three eggs form by mitosis)
the archegonium with egg + surrounding tissue = female gametophyte
Female
gametophyte
Archegonium
Egg (n)
Germinating
pollen grain (n)
Discharged
sperm nucleus (n)
Pollen
tube
Egg nucleus (n)
archegonium
egg
female gametophyte
tissue fills the ovule

29. The Female Life Cycle
the germinating pollen grain develops its pollen tube and two sperm enter into the ovule through the micropyle
fusion of egg nucleus and sperm nucleus  Zygote (2n)
both eggs in the female gametophyte may be fertilized
Germinating
pollen grain
Female
gametophyte
Archegonium
Egg (n)
pollen grain (n)
Integument
Discharged
sperm nucleus (n)
Pollen
tube
Egg nucleus (n)
FERTILIZATION
Embryo
(new sporophyte)
(2n)
Food reserves
(gametophyte
tissue) (n)
Seed coat
(derived from
parent
sporophyte) (2n)
Seedling
Seeds on surface
of ovulate scale
Surviving
megaspore (n)

30. The Female Life Cycle the ovule with the zygote is now called the seed
the developing embryo is retained within the female gametophyte (within the ovule)
Germinating
pollen grain
Female
gametophyte
Archegonium
Egg (n)
pollen grain (n)
Integument
Discharged
sperm nucleus (n)
Pollen
tube
Egg nucleus (n)
FERTILIZATION
Embryo
(new sporophyte)
(2n)
Food reserves
(gametophyte
tissue) (n)
Seed coat
(derived from
parent
sporophyte) (2n)
Seedling
Seeds on surface
of ovulate scale
Surviving
megaspore (n)

31. A simplified pine cycle

32. Pollination conifer pollen arrives before the egg is mature
more than a year may pass between pollination & fertilization!!!

33. Pollination
MITOSIS within the pollen grain produces three cells – 2 small cells (which degenerate) and one large cell
this large cell divides to form: a generative cell and a tube cell
the tube cell elongates to form the pollen tube
the generative cell forms 2 sperm
germinating pollen grains
with pollen tube
as the eggs mature – the pollen tube is developing its two sperm cells
so the eggs and sperm mature at the same time
air bladder
generative cell
tube
cell

34. The Pine Embryo
following fertilization – the zygote does not immediately form
the first cells to form elongate as a suspensor
serves to connect the developing embryo to the food source of the seed
embryonic development is similar to angiosperms
the zygote splits into a basal cell and a terminal cell
terminal cell  embryo
basal cell  suspensor
the embryo combined with the integuments (derived from the ovule) and its food source – known as the seed