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 H
2
O 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