Chapter 28 Seed plants

Seed plants are well adapted to life on land :
a) have pollen, carried by wind or animals, instead of sperm.
b) do not have a separate gametophyte plant, instead the female gametophyte is protected inside the seed.
c) seeds are much larger than spores, with big food reserves so they can live much longer before germinating.

All are heterosporous ( pollen and egg cell ). They first evolved about 400 million years ago.

Gymnosperms

Gymnosperms have naked seeds ( not surrounded by the ovary ). Table 28-1.They are the earliest seed plants that evolved.
They do not have any flowers, instead the seeds are produced in cones. Gymnosperms (such as pine trees) are usually wind pollinated.

Present day gymnosperms are divided into four phyla:

1) Phylum Gnetophyta

Close relatives of flowering plants ( angiosperms ). Two main genera survive today : Ephedra and Welwitschia.

Ephedra is a desert plant ( western US ) with tiny leaves. Fig 28-8 b). It produces nectar to attract insects for pollination. Common name: "Mormon tea", it is used medicinally by native Americans. Ephedra contains ephedrine, used to treat asthma, hay fever. Dietary supplements containing Ephedra were banned in 2004, after they were linked to over 100 deaths.

Pseudoephedrine is used in cold medication (also used in making methamphetamine). DEA

Welwitschia is a desert plant ( Africa ) with only two leaves, and a long tap root. Fig 28-8 c). It gets most of its water from fog and dew that condense on the leaves at night. Fossils of Welwitschia are almost 300 million years old.

2) Phylum Cycadophyta (cycads)

Mainly tropical / subtropical. Cycads are often planted ornamentally ( Common name "sago palms" no, they are not related to palm trees, which are flowering plants!). Fig 28-6. Cycads are dioecious, each plant is either male or female. Wind pollinated ; pollen tube grows close to the egg, then releases sperm.
Examples : genus Cycas (Sago palm) Sago palm. Commonly planted in Florida, it is native to Asia.
Zamia (Coontie, or Arrowroot) Coontie. This is native to Florida. The Timucuan tribe of North Florida ground up the roots of this plant to make bread.

3) Phylum Ginkgophyta

Only 1 species survives : Ginkgo biloba. Native to China, it has remained almost unchanged for almost 300 million years. It is dioecious and has triangular leaves are deciduous. Fig 28-7. Leaves Sperm are released from the end of the pollen tube (as in cycads).
Ginkgo is resistant to air pollution, so is planted in many cities. The seeds smell very bad (like rotting meat), so typically only the male trees are planted.

Ginkgo is claimed to be useful in treating Alzheimer's disease, although more testing is needed.

4) Phylum Coniferophyta (conifers)

Over 700 species still exist.Usually evergreen, drought tolerant: they have needle shaped leaves, thick cuticle, stomata sunk in pits.

Conifers are often the dominant trees in cold climates eg Canada. Fig 28-3. The stem is strengthened with lignin so they can grow very tall. The stem gets wider ( secondary growth ) due to the cambium which produces xylem on the inside of the stem and phloem the outside.

Wind pollinated ; pollen tube grows all the way to the egg. Fig 28-4. Usually bisexual ( or monoecious ): each tree produces small male cones and large, long lived female cones. Fig 28-5.

Conifers are important commercially for wood, paper etc. Slash pine In Florida almost half the state is forested, and in northeast Florida the paper and lumber industries have over $7 billion a year in output. Example : genus Pinus.

Dendrochronology

 Dendrochronology involves dating past events by studying tree ring growth. Trees typically grow fast in spring, and growth stops in the fall, producing annual rings in the xylem. A sample ("core") of the wood is removed using a stem borer. Dendrochronology

Conifers are often used, eg Pinus longaeva (Bristle cone pine). The oldest living bristle cone was 5,000 years old. Current records (using trees that died centuries ago) in Europe and US go back over 9,000 years.

Dendrochronology is used in:
a) dating archaeological sites.
b) studying past climatic changes.
c) showing fire and insect damage in the past.

Phylum Anthophyta (Flowering plants)

This is the only phylum of seed plants that is not included in gymnosperms. Anthophyta evolved recently ( 150 million years ago ). There are roughly 350,000 species (much more than all the gymnosperms).
Anthophyta have efficient vessels in the xylem to carry water (gymnosperms have tracheids instead of vessels).

Flowering plants are typically insect pollinated. Flowers (Fig 28-10) consist of :
Anthophyta have double fertilization : one male gamete fertilizes the egg ( making an embryo ) and the other male gamete fuses with 2 polar nuclei (making a triploid endosperm ). Fig 28-13. The seed is released surrounded by the ovary, as a fruit.

Plants have several defenses against herbivores:
a) many plants produce toxic chemicals. Toxins

b) plants may have mutualistic relationships with animals such as ants, which keep herbivores away. Ant/acacia mutualism

c) plants may release chemicals to attract predators that feed on herbivores. Scientific American

Most wild plants are flowering plants. Duval county plants. Anthophyta are vital as crops, including GM (genetically modified) plants.

Classes of Anthophyta (See Table 28-2.)

Monocotyledons                    Dicotyledons
One cotyledon (seed leaf)         Two cotyledons
Parallel leaf veins               Net-like veins in the leaf
Scattered vascular bundles        Vascular bundles in a ring
No cambium                        May have cambium
Flower parts in multiples of 3    Flower parts in 2, 4 or 5		
Fibrous roots                     Tap roots
eg wheat, grasses, palms          eg rose, oak, tomato

Pollination of Flowers (See Figs 36-4, 36-5 and 36-6.)

Color                    Pollinator             Example

Yellow or blue           Insects eg bees        Vanilla
White (opens at night)   Bats                   Musa (banana)
White( opens in day )    Beetles                Magnolia
Red                      Bird                   Hibiscus
Green                    Wind                   Zea  ( corn )

Seed dispersal (Fig 36-16).

Wind - light seeds with hairs or wings eg Acer.
- tumbleweeds : the whole plant blows around eg Salsola. Wind dispersal

Water - floating seeds eg Cocos ( coconut ).

Animals - hooks or barbs catch on fur eg Bidens.
- ants collect seeds that have an elaiosome Fig 35-14 eg Acacia.
- larger fruits have hard seeds that can survive through the animals digestive system eg Malus ( apple ). Botanical records
Hot pepper seeds are spread by birds, not mammals: Peppers

Some cultivated plants, like bananas, produce fruit without seeds, so reproduce asexually. Seedless tomatoes and grapes are often sprayed with plant hormones to artificially produce parthenocarpy (fruits without seeds). Finally seedless watermelons are produced by crossing two varieties that have different numbers of chromsomes, so the offspring are sterile (just like a mule, produced by crossing a horse and donkey, is always sterile).

 Last edited October 2009, David Byres, dbyres@fscj.edu