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Chapter 27
SEED PLANTS
SEED
SPORE
1. Multicellular embryo
Single cell
2. Food supplied by tissue
Food only in the cell
3. Multicellular seed coat
Covering not cellular
4. Diploid sporophyte
Haploid cell
5. Product of fertilization
Product of meiosis
There are two groups of seed producing plants, gymnosperms and angiosperms.

Produce seeds.

Vascular tissue: xylem for water and mineral transport and phloem for dissolved
sugars.
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Gametophyte is very reduced and totally dependent on the sporophyte.
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Heterosporous: microspores and megaspores.
GYMNOSPERMS
There are about 720 species found in all terrestrial habitats grouped into four divisions (phyla).
They have great economic importance: lumber, paper, chemicals.
I. CHARACTERISTICS OF GYMNOSPERMS
1. Woody trees and shrubs.
2. Xylem made of tracheids.
3. Seeds are borne, exposed, in cones (megastrobilus).
4. Pollinated by wind, seldom by insects.
5. Single fertilization: sperm + egg  embryo.
6. Most are monoecious: male and female organs on the same individual.
II. CONIPHEROPHYTA or conifers
There are about 550 species of conifers.
Many conifers produce resin, a complex mixture of organic compounds that protect the plant
from insect and fungal attack.

Resin is stored in resin ducts in the roots, stem, leaves and cones,
Cone bearing gymnosperms, e. g. pines, firs, cypresses, etc.
Leaves are needles or scale-like, rarely broad; venation is parallel and open.
A male cone is called microstrobilus, and contains the microsporangia that produce
microspores through meiosis that will develop into microgametophytes (pollen grains).
The female cone is called the megastrobilus, which produce megasporangia in each of which
a megaspore is produced through meiosis and will develop one megagametophyte. An egg
will form within the megagametophyte.
Pollination is by wind.
Note: Study the life cycle of pine and learn the terminology. Pages 572 - 574.
III. CYCADOPHYTA or cycads
Important in the Triassic (248-213 m .y. a), which is called sometimes the Age of Cycads .
Most species are extinct. There are about 140 living species in tropical and subtropical parts of
the world.
Cycads are palm or fern-like plants with compound leaves and simple seed cones.
The are dioecious: plants are either male or female.
They have a very large motile sperm within the pollen grain but do not need water for pollen
transport.
Pollination is by air and in some cases by ants.
IV. GINKGOPHYTA or ginkgoes
There is a single species of ginkgo alive today. It is native to China where it has been under
cultivation for centuries. It has been found in the wild in only two locations.
Ginkgoes are dioecious and have flagellated sperms.
Pollination is by air and seeds are borne exposed rather than in cones.
It is commonly planted in American cities because it is very resistant to pollution.
V. GNETOPHYTA
The gnetophytes consists of three genera and about 70 species.
A group of rare plants that share some traits with angiosperms.


Efficient water conducting cells in the xylem called vessel elements.
Reproductive structures resemble flowers.
ANGIOSPERMS
There are about 235,000 species of flowering plants.
This is the dominant group in terrestrial habitats. Our survival depends on them.

food, medicine, lumber, etc.
There are two groups of angiosperms: monocots and dicots.
First appeared in the Cretaceous about 130 million years ago.
I. CHARACTERISTICS
1. Woody or herbaceous.
2. Xylem elements have vessel elements and tracheids.
3. Produce flowers.
4. Pollinated by wind or animals.
5. Double fertilization: egg + sperm  embryo and 2 polar nuclei + sperm  endosperm.
6. Seeds enclosed in a fruit.
MONOCOTS have floral parts in multiples of three and the seed contains one cotyledon. The
endosperm provides the food for the embryo. Venation is usually parallel (there are exceptions).
Their vascular bundles are scattered throughout the ground tissue. The root system is fibrous.
DICOTS have floral parts in multiples of four or five, and their seeds contain two cotyledons.
The cotyledons usually absorb the food from the endosperm first, and then provide the food for
the embryo. Venation is netted. The vascular bundles in the stem cross-section are arranged in
circles (rings). They usually have a taproot system for at leas part of their life.
II. THE FLOWER
The flower is a reproductive shoot or branch.
It has four parts arranged in whorls or circles on a stalk or peduncle.
The parts of the flower are the sepals (calyx), petals (corolla), stamens and carpels.

Stamens consist of a filament and an anther.

Carpels are also referred to as pistils. They consist of an ovary, a style and a
stigma.
Flowers may be borne singly or in clusters called inflorescence.
Flower parts are considered modified leaves.
Flowers may be ...


Complete if has the four parts or incomplete if it lacks one of the parts.
Perfect if it has both stamens and carpels or imperfect if it lacks one of them.
See figure 27-10, page 579, for additional details of the floral structure.
III. DOUBLE FERTILIZATION.
It is characteristic of flowering plants.
Double fertilization results in the formation of a diploid zygote and a triploid endosperm.
The female gametophyte or embryo sac has an egg nucleus and two polar nuclei.
One sperm fertilizes the egg nucleus and forms the zygote, 2n.
Another sperm joins the two polar nuclei forming the triploid (3n) nutritive tissue called the
endosperm.
See figure 27-12, page 581, for details.
Seeds develop from the ovule following fertilization.
The ovary enlarges and forms the fruit.
In some instances other tissues also enlarge and become part of the fruit.
Fruits serve two purposes: protect the seed and aid in dispersal of the seeds.
EVOLUTION OF SEED PLANTS
Progymnosperms appeared about 375 million years ago in the Devonian.



they reproduced by spores.
had megaphylls.
woody tissue of secondary xylem similar to modern gymnosperms.
Their reproductive structures appear to be intermediate between those of spore producing
plants and seed plants.
 conifers
Progymnosperms
 seed ferns  cycads and possibly ginkgoes
Seed producing plants appeared in the Devonian, about 360 m. y. a.
By the end of Jurassic, 180 million years ago, several lines of gymnosperms existed with
features that resembled those of flowering plants.
Different groups of seed plants apparently appeared independently several times.
Angiosperms probably arose from ancient gymnosperms. They must have been dicots, which
then gave rise to monocots.
The oldest fossil record of angiosperms is pollen from the Cretaceous, about 130 m. y. a.
The oldest flower fossil is 120 m.y.a.
By late Cretaceous angiosperms had began to replace gymnosperms as the dominant group of
land plants.
Many angiosperm species apparently arose from changes in chromosome number.
COMPARISON OF GYMNOSPERMS AND ANGIOSPERMS.
GYMNOSPERMS
ANGIOSPERMS
GROWTH HABIT
woody trees and shrubs
woody or herbaceous
XYLEM CELLS
tracheids
vessel elements and
tracheids
REPRODUCTIVE
STRUCTURES
usually cones
flowers
POLLEN TRANSFER
wind
animals, wind, water.
FERTILIZATION
egg + sperm = zygote
double fertilization
SEEDS
exposed, borne on scales
enclosed in the ovary (fruit)