Homosporous and Heterosporous Plants
Green Algae and Land Plants
Homosporous: produce a single type of spore that develops
into a bisexual gametophyte that produces both gametes
Homosporous and Heterosporous Plants
Evolution of Pollen
• when pollen evolved, heterosporous plants lost
their dependence on water for fertilization
• pollen grains
• tiny enough to be carried by wind or animals
Heterosporous: the production of two distinct types of
spore-producing structures and two distinct types of spores,
male and female
Seeds
• allow embryos to be dispersed to a new habitat, away from the
parent plant
Seed Plants
-gymnosperms and
angiosperms
• are often dispersed by wind, water, or animals
• the evolution of heterospory, pollen, and seeds triggered a
dramatic radiation of seed plants (about 290 mya)
- production of seeds and
pollen grains
- embryo develops within
seed coat
Heterospory in Gymnosperms
ME IOS
PO
L
Microspore (n)
forms pollen
grain
Cones with
microsporangia
Pollen grains
disperse via wind
MITOSIS
IS
Pollen grain
(male gametophyte
LIN
AT
IO
N
Four meiotic
products; one is
large and forms
the megaspore (n)
Megasporangium
ME IOS
Ovulate cone
Ovules (contain
megasporangia)
Female
gametophyte (n)
Eggs (n)
ION
ILIZAT
FERT
MITOSIS
Pollen produces
sperm
Mature
sporophyte (2n)
Developing
sporophyte
Pollen
grain
Mother
cell (2n)
Archegonia
Embryo
(2n)
Three meiotic
products die
IS
Megaspore divides to form
female gametophyte (n), which
produces archegonia and eggs
by mitosis. (Only one egg is
fertilized and develops.) Note
that the red dots here and
elsewhere represent nuclei
Seed (disperses
via wind or animals)
Flowering plants - Angiosperms
Flowers
~250,000 spp.
• Flowers contain two key reproductive structures: stamens and
carpels
• The stamen contains the anther: microsporangia
• The carpel contains the ovary in which the ovules are found:
megasporangia
• The evolution of the flower is an elaboration of heterospory,
with the key innovation being the evolution of the ovary
success of angiosperms in terms of geographical distribution,
number of individuals, and number of species revolves around
a reproductive organ: the flower
Heterospory in Angiosperms
MITOSIS
Microspore (n)
forms pollen grain
Carpel
Ovule
Ovary
MITOSIS
MEIOSIS
Egg
Female gametophyte
(n: retained in ovary)
Nutritive tissue
Endosperm (3n)
forms nutritive
tissue in seed
FE
R
Mature
sporophyte
flower (2n)
TIL
IZ
Megasporangium
Megaspore
(n: retained
in ovary)
N
Anther
Stamen
Flower
Pollen grains disperse via wind
or animals (the red dots here
Pollen lands
and elsewhere are nuclei)
near female
gametophyte;
produces
PO
pollen tube and
LL
Pollen grain
sperm
IN
AT
(male gametophyte)
IO
N
Sperm travel
down growing
pollen tube to
reach egg
AT
IO
MEIOS IS
Flowers
Embryo (2n)
MITOSIS
Developing
sporophyte
Seed (disperses
via wind or animals)
Zygote
(2n)
Flowers:
• vary in size, structure, scent, and color in order to attract different
pollinators
• evolution of the flower made efficient pollination possible
Fruits
The Angiosperm Radiation
Fruit:
• a structure that is derived from the ovary and encloses one or more seeds
• evolution of fruit made efficient seed dispersal possible
• Angiosperms represent one of the great adaptive radiations in
the history of life
• The diversification of angiosperms is associated with three key
adaptations: (1) vessel elements, (2) flowers (modified leaves),
and (3) fruits
• These adaptations allow angiosperms to transport water,
pollen, and seeds efficiently
Innovations That Allow Plants to Reproduce Efficiently on
Land
Land plants
Vascular plants
Seed plants
C
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Gymnosperms
Key Lineages of Green Plants
Fruit
Flowers
Heterospory
Seeds
Pollen
Heterospory
Sporophyte-dominated life cycle
Green algae
Nonvascular plants
Seedless vascular plants
Alternation of generations
Retention of embryo on parent
Complex gametangia
Thick-walled spores
Retention of egg on parent
Simple gametangia
Green Algae
Ulvophyceae
Green algae
es
e tes aet rts
ga hy ch wo
l
a p o e
n
ed vo le o
R Ul Co St
Land plants
- unicellular and multicellular
- freshwater and marine
-important primary producers
in aquatic habitats
- alternation of generation in
multicellular species
Two groups most similar to land
plants
Bryophytes: Non-Vascular Plants
Land plants
Coleochaetophyceae
- most grow as flat sheets of cells
(attached to water lilies, cattails)
- strictly fresh water algae
- multicellular
-no alternation of generation
- only zygote is diploid
Non-vascular plants
Green algae
ts ts
or or s
rw rnw sse
e
v o
o
Li H M
Charaphyceae
(Stoneworts)
-strictly fresh water
- multicellular
-no alternation of generation
- only zygote is diploid
Nonvascular Plants
Hepaticophyta: Liverworts
• most basal lineages of land
plants
• 3 lineages with living
representatives: liverworts,
hornworts, and mosses
Anthocerophyta: Hornworts
• stomata*
• rhizoids
• cuticle*
• pores*
• rhizoids
Bryophyta: Mosses
• simple water conducting
tissues*
• rhizoids
The Seedless Vascular Plants
Land plants
Lycophyta:
Lycophytes or Club Mosses
Seedless vascular plants
es ns s
yt fer ail
ph sk set ns
o
c
hi or r
Ly W H Fe
- most ancient plants with roots *
- tree-sized lycophytes (dominated
the coal-forming forests of the
Carboniferous period)
Psilotophyta: Whisk Ferns
• no fossil record
• lack roots
• rhizomes *
Pteridophyta: Ferns
Sphenophyta (or Equisetophyta):
Horsetails
- Horsetail stems are
hollow, allowing oxygen
to diffuse down their
hollow stems to roots
Seed Plants
Land plants
• only seedless vascular plants to have large, welldeveloped leaves (fronds)*
Seed plants
Gymnosperms
.
al
s
et s
s yte al. erm
d
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ad gk w to s
yc in d e ne ng
C G Re Gn Pi
A
Gymnosperms
Cycadophyta (Cycads)
• “naked seed”
• no flowers
• lack motile cells – pollen*
• tracheids* (lack vessel elements found in
angiosperms)
• woody*
• compound
leaves (unique)
• many are treesized
• symbiotic,
nitrogen-fixing
cyanobacteria
Ginkgophyta (Ginkgoes)
Cycas revoluta
Gnetophyta (Gnetophytes)
- diverse form
• 1 species is
fossil
living day
- drug ephedrine has been isolated
from this group
alive today
• deciduous*
Pinophyta (pines, spruce, firs)
• named for its
reproductive structure,
the cone
• needle-like leaves*
Other Conifers (redwoods, junipers,
yews)
• all trees and shrubs
• narrow or scale-like
leaves
• wind-pollinated
Anthophyta (Angiosperms)
• flower: defining
adaptation of
angiosperms
• supply the food that
supports virtually
every species