seed

P L A N T B I O L O G Y : Z O L J A R G A L C H I N B A T

THE SEED

2

SEEDS

• Seeds contain a young, developing plant embryo

• Seeds are covered with a protective seed coat.

• Inside is stored food or endosperm that the young plant uses as it begins to sprout or germinate.

• Seeds form from ripened ovules after fertilization

SEED STRUCTURE• External

• Seed coat (testa)

• Hilum

• Embryo

• Cotyledon

• Epicotyl / Hypocotyl

• Plumule

• Radical

SEED COAT• testa

• The seed coat protects the embryo

• Can be of varying thicknesses, depending on the seed

type.

HILUM• Scar from the seed being attached to the parent plant

EMBRYO• The embryo is what forms the new plant once the

opportune conditions are present.

COTYLEDON

• The cotyledon is the first

leaf that germinates.

• It is filled with stored

food that the plant uses

before it begins

photosynthesis.

• Some plants have 1

cotyledon (monocot)

and some have 2

cotyledons (dicot).

EPICOTYL /HYPOCOTYL

• The basis for the plant’s

stem.

• It is known as the epicotyl

above the cotyledon and a

hypocotyl below the

cotyledon.

• These grow upward in

response to light.

PLUMULEThe shoot tip with a pair of miniature leaves.

THE RADICLE• The part of the seed where the root develops.

THE STRUCTURES OF THREE DIFFERENT SEEDS ARE SHOWN HERE

12

Parts of a Seed

Seed

• Flowering plants have different adaptations to enable

their seeds to be dispersed as far away as possible.

• The different methods of seed dispersal are…

• by wind

• by animals

• by water

• by explosion

REPRODUCTION IN PLANTS — GETTING THE SEEDS

DISPERSED

Seed Dispersal

by Explosionby

wind

seed dispersal

by

water

Adaptations for reproduction

plants

by

animals

DISPERSAL BY WIND

• Some seeds are carried to a new

place by the wind. These seeds are

very light.

• The seeds of the orchid are almost

as fine as dust.

• Many have hairy growths which act

like little parachutes and carry the

seeds far away from the parent

plant.

• The seeds of the dandelion are

carried by the wind.

dandelion

Orchid

DISPERSAL BY WATER

• Water lily and the coconut palm are

carried by water.

• Coconuts can travel for thousands of

kilometres across seas and oceans.

• The original coconut palms on South

Sea islands grew from fruits which

were carried there from the mainland

by ocean currents.

DISPERSAL BY ANIMALS

• The animal eats the fruit but only the juicy

part is digested.

• The stones and pips pass through the

animal's digestive system and are excreted

to form new plants. This can be far away

from the parent plant.

• Blackberry, cherry and apple seeds are

dispersed in this way.

• Birds also like to eat fruit and they help to

disperse seeds to other areas through their

droppings.

• Squirrels collect nuts like acorns and bury

them for winter food, but they often forget

where they have buried them and these

grow into new trees.

• Some fruits like that of the burdock plant

have seeds with hooks.

• These catch on the fur of animals and are

carried away.

DISPERSAL BY EXPLOSION

• Some plants have pods that explode when ripe and shoot out the seeds.

• Lupins, gorse and broom scatter their seeds in this way.

• Pea and bean plants also keep their seeds in a pod. When the seeds are ripe and the pod has dried, the pod bursts open and the peas and beans are scattered.

SEED DORMANCY

What is seed dormancy?

Seed dormancy means to delay germination until

favorable environmental conditions are present for

seed germination and sprout growth.

• Types of dormancy

1. Primary dormancy delays initial seed germination

2. Secondary dormancy

PRIMARY DORMANCY

• Seed coat physical dormancy- is when the seed coat prevents the seed from absorbing water.

• Seed coat mechanical dormancy- is when the seed coat prevents the embryo from breaking through.

• Chemical dormancy- is when chemicals within the seed coat prevent the seed from germination.

• Morphological dormancy- is when the seed has not completed at the time the seed leaves the plant

• Physiological or intermediate dormancy- occurs because a physiological condition is not being met.

• Deep dormancy- is when plants stay dormant until favorable conditions are met, usually involving a period of cooling.

• Double dormancy- is the combination of any of the previously mentioned factors.

SECONDARY DORMANCY

• Secondary dormancy prevents the seed from

germinating after it absorbs water.

Is caused by temperature extremes, prolonged

darkness, prolonged light water stress, or oxygen

extremes.

SEED GERMINATION

• What is it?

Process that causes a seed to begin to grow

into a new plant.

• Factors influencing germination

1.Water

2.Oxygen

3.Temperature

4.Light

WATER AND OXYGEN

Water

• Mature seeds have relatively little moisture to

prevent decay.

• Seeds usually mature in fall.

• To germinate, seeds must take in water.

Oxygen

• Oxygen in the presence of enough moisture causes

respiration to start.

• This creates energy for the germination process.

TEMPERATURE AND LIGHT

Temperature

• Most seeds go through a cold period before

germination.

• Helps prevent seeds from sprouting as soon as they

mature.

• Proper temp. for most seeds= 68-85 F.

Light

• Amount of light needed varies.

• Depth of seed planting influences this.

• Too deep=not enough light oxygen.

• Too shallow may=too much light.