Lect16 Seed Plants `10 (2)

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The Seed Plants: Gymnosperms
& Angiosperms
Last day… evolution of plants adapted to land, from
‘bryophytes’ to ‘seedless vascular plants’
Today, the 2 groups that contain most living plants, the
Gymnosperms & Angiosperms
Success of seed plants
related to advantages
of producing seeds:
fertilized embryo &
its food supply,
surrounded by a
protective coat
Along with seeds, 4 associated evolutionary changes:
1) reduction of gametophyte stage
2) consistently heterosporous: microspores 
male gametophytes, megaspores  female gametophytes
- separate
gametophytes
will live different
lives
3) Ovules & production of eggs: sporophyte produces
protective layer of tissue (integument) enclosing
megasporangum & its megaspore
Ovule = integument + megasporangium + megaspore
Megaspore germinates within ovule, develops into tiny
female gametophyte
- produces egg which will eventually be fertilized
Seed contains tissue from three
generations of plants:
1) seed coat from mature
sporophyte
2) food supply from
female gametophyte
3) embryo is new
sporophyte
generation
Seed is protected, capable
of dispersal, can remain
dormant for very long
periods, & has food
supply to begin growth
4) Pollen grains: male gametophyte surrounded by
pollen wall (partly secreted by sporophyte)
- a separate, multicellular generation, even if reduced to an
extreme (2 cells, or 3 after germination)
Pollen grain capable of long dispersal, tough & resistant, &
sperm nuclei do not require external H2O for fertilization
Seed Plants divided
into 2 main groups:
Gymnosperms
(‘naked seeds’)
& Angiosperms
(‘container seeds’)
Some ancestors of seed plants found
in fossil record ~ 380 MYA,
& ‘seed ferns’ by 360 MYA
Carboniferous forests dominated by
lycophytes, horsetails & ferns but
early gymnosperms by 305 MYA
Drier conditions of Permian period
favored gymnosperms over seedless
vascular plants, & dominated through
whole Mesozoic era
Look at
life cycle
of pine to
illustrate
for
gymnosperms
Tree is sporophyte, has ovulate cones w. megasporangia
& pollen cones w. microsporangia
Integument does not entirely cover megasporangium (‘naked
seed’) so pollen lands on megasporangium
Megaspore produced by meiosis
Megaspore
develops into
female gametophyte,
which produces eggs
Fertilization
eventually
occurs,
one zygote
becomes
embryo
Ovule becomes
seed: embryo,
food supply &
seed coat
Four living phyla of gymnosperms: Phylum Cycadophyta
Cycads or ‘Sago Palms’ – only ~300? spp. living, but
Mesozoic was ‘Age of Cycads’
- warmer areas (including southeast USA)
Used as
ornamental
plants
Seeds or starch
from stem
consumed,
but neurotoxins
may remain!
Phylum Gingkophyta – the Gingko
- one species, ‘living fossil’, taxon common in Mesozoic,
rediscovered in central China
- delightful smelling fruits, leaves as traditional medicine
Phylum Gnetophyta – gnetophytes
- <100 species in 3 genera
Ephedra, 40 spp. in arid regions, ‘Mormon Tea’ in US
- ephedrine from some, medicine or formerly ‘supplement’
Gnetum, 35 spp. trees, shrubs, vines, in tropical Asia Africa
- some may be insect pollinated?
Welwitschia mirabilis – unique spp. of Namib Desert
- just 2 straplike leaves (get torn up) to 6.2 m
- may live > 1000 yrs.?
- absorbs dew for moisture
Phylum Coniferophyta (Pinophyta)– the conifers
Largest group of gymnosperms (~600 spp.)
- some of the largest & oldest organisms
- needle-leaved, usually ‘evergreen’
Dominant plants of boreal forests, & in some temperate areas
- very important for lumber, pulp, as well as ecologically
But by far the most important plants… the angiosperms
Phylum Anthophyta – ‘the flowering plants’
- > 260,000 spp., almost every habitat
Key innovations are flowers & fruits
Flower – structure specialized to facilitate transfer of pollen
between plants; specialized shoot w. 4 rings of modified
leaves
Sepals – basal, often green, enclose flower before opening
Petals – interior to sepals, often brightly colored
Stamens – filament supports anther where microspores
produced, develop into pollen grains
Carpels – sticky stigma for receiving pollen, style leads
down to ovary where 1+ ovule is, produces megaspores
which develop into female gametophytes
Fruit – mature ovary of a flower, thickens around seeds
- may include some additional tissues as well
- protects seeds & often enhances dispersal
- may be fleshy or dry
Flowering plants
are diploid
sporophytes,
produce microspores
and/or megaspores
- asexual reproduction,
no gametes or
fertilization…
- microspores develop
into male gametophytes
(inside pollen grain),
megaspores develop
into female gametophyte = embryo sac
Pollen transferred to
stigma, normally
cross-pollination
Female gametophyte
now has 8 nuclei
(7 cells)
- 2 sperm cells carry out
double fertilization:
one fertilizes egg, other
fuses w. 2 nuclei in central
cell
- central cell becomes triploid endosperm (food supply
for seed)
Embryo, w.
endosperm
& integuments,
develop into
seed
- surrounding
ovary tissue
forms fruit
Angiosperms probably split from gymnosperms about
305 MYA, but subsequent history not well known
- living lineages shared a common ancestor ~150 MYA
Earliest branch in phylogeny is Amborella, shrub found
only on New Caledonia
- lacks vessels in xylem
Other early branches include water lilies & the star anise
& relatives
- star anise family has female gametophyte w. only
4 nuclei, some spp. used as spices and medicines
Magnoliids are more speciose (~8,000 spp.), including
magnolias, laurels, & black pepper plant
The 2 big groups of angiosperms are the Monocots
(~70,000 spp.) & the Eudicots ( ~ 170,000? spp.)
Monocots include orchids, palms, lilies, grasses…
Dicots include… lots! (oaks, peas, roses, potatoes, etc.)
A number of distinctive characteristics generally make
Monocots & Eudicots fairly easy to distinguish
The value of seed plants to humans is… priceless
- six species (maize, rice, wheat, potatoes, cassava &
potatoes) provide 80% of calories consumed
- coffee, tea, spices & sugar also impt.
- source of building material, fuel, pulp, etc.
Many drugs from seed
plants, currently or
originally discovered
in plants
- most plants not
investigated yet for
potential uses
No less important to
organism around the
world, at least in
terrestrial habitats
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