Download Seed Plants

Last day… got started on
Kingdom Plantae,
general characteristics,
& looked at ‘non-vascular’
groups (Hepatophyta,
Anthocerophyta, &
Bryophyta)
Then, started looking
at characteristics of
the vascular plants
Vascular plants evolved specialized roots to absorb water &
minerals from the soil
- and specialized leaves, microphylls (small, single vein)
in lycophytes & megaphylls (large, branched veins) in
most vascular plants
Another innovation was
sporophylls, modified
leaves bearing sporangia
Some seedless vascular
plants (and all seed
plants) became
heterosporous:
megaspores for female
gametophytes, males
from microspores
Seedless vascular plants placed in 2 phyla:
Phylum Lycophyta – Club mosses, spike mosses,
quillworts – 1200 spp.
- club mosses & spike mosses produce clusters of
sporophylls called strobili
- some are epiphytes (on other plants), quillworts are aquatic
Phylum Pterophyta – ferns, horsetails & whisk ferns
- 12,000 species (15 horsetails, 7 ‘whisk ferns’)
Ferns widespread, but primarily in moist areas, may reach
tree size
- horsetails may be abundant in temperate areas, hollow
stems with silica (‘scouring rush’)
- whisk ferns may be ferns that lost leaves, roots?
Success of seedless vascular plants may have caused an
ice age… and global warming
Ferns & other seedless vascular plants not of huge economic
importance but…
Only 2 main groups of plants
left : Gymnosperms
& Angiosperms…
But, those 2 groups contain most living plants, the
Gymnosperms & Angiosperms
- seed plants seem to be monophyletic
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
Living gymnosperms in 4 phyla, some not very diverse…
Phylum Cycadophyta – Cycads (Sago palms)
- only about 130 spp. now but abundant in Mesozoic
Male and female cones on separate plants
Phylum Ginkgophyta – Ginkgos
- well known as fossils (80 MYA), but only one surviving sp.
- fan-like leaves, fruit with interesting odor
- turns yellow, & drops leaves abruptly
Phylum Gnetophyta – Gnetophytes
- only 3 genera…
Welwitschia – one sp.
in SW Africa
Ephedra – one genus
w. about 40 spp. in
arid areas
…and Gnetum, one genus of trees, vines or shrubs, about
35 spp. in warm areas of Old World
Final phylum of gymnosperms: Phylum Coniferophyta
- the conifers (including pines, spruces, cedars…) with
about 600 spp. (largest group of gymnosperms)
- often large & long-lived, dominate large areas of northern
forest, & grow elsewhere in rocky or sandy soils
Next big group: Phylum Anthophyta - Angiosperms or
‘flowering plants’
- extremely diverse group with about 170,000 spp.
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,
develops 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…
Eudicots 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 &
sweet 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
organisms around the
world, at least in
terrestrial habitats