Botany session 5

© Copyright PCNM 2011
Botany and Pharmacognosy
Session 5

Review
Last week we covered:
Flowers & Fertilisation

This Session
During this session we will
cover:
Fruit & Seeds
Fruit Dispersal
Inhibitors

Fruit & Seeds
• Seed
• A seed is a ripened plant ovule containing an embryo and
normally capable of germinating to produce a new plant.
• Fruit
• A fruit is a ripened ovary with its seeds and any other structure
closely associated with it.
• Functions.
• The functions of a fruit are:
• To protect the developing seeds
• To disperse the mature seeds

Fruit & Seeds
• Many seeds we refer to are
actually fruit, eg corn, wheat,
oat seeds. Carrot, parsnip
and sunflower seeds are fruit
containing one or two seeds.
• Sometimes plants thought of
as vegetables are actually
fruit, eg broad beans,
cucumber, tomato. A
vegetable is the edible part of
a plant other than the fruit or
seed. Vegetables can be
leaves, stems or roots of a
plant.

Fruit formation
• When double fertilisation occurs the following happen:
• The stigma, style, stamens, petals and sepals wither and usually
drop off, although the dried and shrivelled sepals may persist.
• The ovule ripens into a seed. The zygote forms an embryo, with a
miniature root, a tiny seed and one or two baby or seed- leaves
called cotyledons. The endosperm cell forms the endosperm tissue
and the integuments (the outer two layers of the ovule) form the
seed coats.

Fruit formation
• The ovary develops into a fruit. The ovary
wall thickens to form the pericarp. The
pericarp has three layers, outer, middle and
inner, called the exocarp, mesocarp and
endocarp respectively. The pericarp may
become fleshy and succulent, or dry and
hard. Examples of each are: the tomato fruit
has a fleshy pericarp. The cherry has a thin,
tough exocarp, a fleshy mesocarp and a
hard, dry (stony) endocarp. The garden
green pea when mature has a dry, hard
pericarp.

Fruit formation
• In some plants other parts of the
plant may grow to form part of the
fruit. In an apple, the top part of the
flower stalk, the receptacle, is
swollen and fleshy and forms the
edible, succulent part of the fruit.
This is known as a false fruit.
• In general, fruit development occurs
after fertilisation, but in some
species fruit can form without
fertilisation. This gives rise to
seedless fruit, such as bananas,
navel oranges and pineapples.
These are parthenocarpic fruit,
(literally ëvirgin fruití).

Types of fruit
• There are three main types of fruit, simple, aggregate and multiple.
Simple
• A simple fruit is derived from the ovary of a flower with a single pistil,
such as legumes, tomatoes, cranberries, stone fruit and the herb
shepherds purse, Capsella bursa-pastoris.
Aggregate
• An aggregate fruit is derived from a single flower with many pistils
and ovaries. Each of these develops into a small fruit which is joined
tightly to other fruit from that flower to produce one large fruit, eg
raspberries and blackberries.

Types of fruit
Multiple
• A multiple fruit is derived from the ovaries of many closely
associated flowers, e.g. the mulberry, fig, pineapple.
Simple
Aggregate
Multiple

Accessory fruits
• Most fruits develop just from the ovary, but some fruits include
peripheral structures such as the receptacle and the peduncle (the
stalk of the inflorescence). These are accessory or false fruit.
There are three main types of accessory fruit:
• Simple-accessory.
• An example is the apple where the fruit includes the swollen
receptacle.
• Aggregate-accessory.
• An example is the strawberry, where the fruit is derived from
many pistils on the same flower as well as its receptacle.
• Multiple-accessory.
• An example is the pineapple where the fruit includes the
receptacles as well as the peduncle.

Simple fruits
• There are two main kinds of simple fruits, simple fleshy fruits and
simple dry fruits.
Simple Fleshy Fruits
• Simple fleshy fruits are fruits with fleshy, succulent and often juicy
pericarps.
• Simple fleshy fruits include drupes and berries. A citrus fruit is a
form of simple fleshy fruit called an hesperidium. It is a type of
berry with a tough, leathery rind.

Simple fruits
Simple Dry Fruits
• Simple dry fruits have woody or papery pericarps. Dry fruits which
split open to disperse several or many seeds are dehiscent.
• Dehiscent fruit include legumes such as beans and peas which
split open down two sides, and capsules such as daffodils,
portulaca and poppies.
• Other dehiscent fruit are follicles which split down one side only,
such as Hakea and Grevillea.
• Dry fruit which do not split open and usually have only one seed are
indehiscent.
• Example: grains and nuts.

Checkpoint!

Seeds
• A seed develops from an ovule after double fertilisation
Function
• The function of a seed is:
• To reproduce the parent species whilst allowing for genetic
variation and change
• To survive adverse seasons
• To disperse the parent species to a new location
• To protect the embryo until the correct time for germination
• To provide food and nutrients for the embryo during dormancy
and germination

Seed formation
Step 1
• The zygote forms an embryo. The embryo consists of a miniature shoot,
the plumule, a tiny root, the radicle and one or two seed leaves called
cotyledons. The cotyledons supply food to the embryo. Monocotyledon
embryos have one seed leaf whilst dicotyledon embryos have two
cotyledons.

Seed formation
Step 2
• The endosperm cell forms the endosperm tissue. In most seeds the
embryo develops using food from the endosperm. When the seed is
mature the endosperm disappears as the food reserve has either been
used up, or what has not been used has been transferred to the
cotyledons.
• These seeds are called non-endospermic seeds.
• Example: Peas and beans
• In endospermic seeds the embryo remains surrounded by the
endosperm, which contains nutrients and starch. The bulk of the volume
of the seed is the starchy endosperm.
• Example: Monocotyledon cereal grains such as wheat, rice and
sweet corn

Seed formation
Step 3
• The integuments form the seed coat,
the testa.
Step 4
• In most seeds the nucellus
disintegrates.

Seed formation
• Thus a mature seed is an embryo with a food reserve. The food is
stored in either the cotyledon/s or the endosperm. This reserve
enables the embryo to grow rapidly when the seed germinates until
the seedling is able to produce its own food by photosynthesis.
• Mature seeds are dry. When the seeds have fully developed the
parent plant shuts off the water supply to the seeds so that they dry
out and become dormant. This enables the embryos to survive until
conditions become favourable for germination. This may be for one
season or for many years. Remember the old saying: One yearís
seeds equal seven yearsí weeds!

Fruit and seed dispersal
• After flowering the ovary changes into a fruit and the fertilised ovules
develop into seeds. These provide the next generation of that plant.
• If these seeds were to germinate where the parent plant is growing
there would be overcrowding, with too much competition for light
and water.
• This would result in the death of too many young plants. In order to
avoid this plants have developed strategies which enable the
dispersal of fruits and seeds far from the parent plant.
• Fruits and seeds may be dispersed actively or passively.

Active dispersal
• The fruits are adapted to forcefully
expel the seeds so that they are sent
some distance from the parent plant.
• The ripe fruit opens with sharp
movements which scatter the seeds.
• Many legumes disperse their seeds in
this manner. The pod splits into halves
down two sides, the two halves curl
back suddenly and flick out the seeds.
• Sometimes this is so forceful that it can
be heard. Many dry dehiscent plants
scatter their seeds in similar fashion.

Passive dispersal
• External agents such as wind, water and animals disperse seeds
away from the plants

Wind dispersal
• Seeds dispersed by wind are typically small, light and produced in
large quantities.
• Many have wings or feathers to increase their buoyancy. They can
be carried by the wind for long distances.
• Cotton seeds have hairs so they can be blown in the wind.
These hairs are used to produce cotton.
• Dandelion seeds have a little parachute attached to them to aid
in dispersal by the wind.
• Some plants disperse their seeds with the aid of samaras which are
winged fruit. A samara is a flattened wing of fibrous, papery tissue
developed from the ovary wall.
• Maples, elms and the Tipuana tipu tree produce samara.
• Spinifex produces clusters of dried fruit which roll along the ground
when the wind blows and the seeds are dispersed

Water dispersal
• Some fruit and seeds float on water. They are then carried away
from the parent plant by the water currents. When they reach
suitable land the seeds can germinate and the new plants grow.
• Coconuts are dispersed in this manner.
• Water lily fruit have membranous envelopes containing air to
give them buoyancy.
• Seeds from plants growing in flood prone areas can usually survive
short term immersion and the seeds are spread at flood times.

Animal dispersal
• Many fruit and seeds are adapted for
distribution by animals, including
humans.
• Many fruit are fleshy, brightly
coloured juicy and scented. Animals
are attracted to them for food.
• The seeds may be discarded and
may thus be dispersed some
distance from the parent plant.
• If seeds are eaten they are usually
not digested but excreted with the
faeces, again often quite some
distance from the parent plant.

Animal dispersal
• Some seeds contain a sugary,
nectar-like liquid which attracts ants.
The ants carry the seeds away,
extract the liquid and leave the seed.
• Dry fruits may also be attractive to
animals and birds.
• Nuts are an example.
• Many Australian native plants
produce small, dry seed.
• Ants collect them and store them
underground. The seeds are thus
protected from fire and inclement
weather and may still be viable after
several years.

Animal dispersal
• Other fruits and seeds are adapted for attachment to birds and
animals for dispersal.
• These adaptations can be hooks, barbs or sticky seed coats.
• They attach to the fur or hair of animals, the clothing of humans or to
birds and may be carried some distance before their removal.
• Other seeds are carried in soil attached to animals feet or humans
shoes.
• Humans play a large role in dispersing fruit and seeds. Some of this
dispersal is unintended but much is deliberate

Seed Germination
• Seeds are usually very dry when they are
shed by the parent plant, with less than
ten percent water content.
• These seeds are alive but in a state of a
very slow rate of metabolism.
• Seeds cannot remain alive indefinitely but
can remain viable for quite some time,
depending on the species of plant.
• Viability can be extended by keeping
seeds dry and cool, away from light and
pests.

Dormancy
• When seeds encounter favourable environmental conditions most of
them germinate.
• However, some do not. Those that are not non-viable are dormant.
• Dormancy is an inability to germinate under favourable conditions. It
is considered a survival tactic, and is an internal characteristic.
• Many Western Australian plants produce seed which remains
dormant, especially over the long, dry summer. When the winter
rains commence these seeds germinate.

There are two main mechanisms of
dormancy:
• Hard, impermeable seed. The hard seed coats prevent entry of
water or oxygen necessary for germination. These hard seeds
remain dormant until the seed coat is weakened or broken.
• Fire can crack seed coats, which allows dormancy to break.
• The native plants Kennedia, Chorizema and Acacia seeds react
in this way.
• Other plants, such as Banksia and hakea, require the heat of fire
to release the seeds from the mature fruit.
• In desert-type conditions seeds are abraded against sand, which
weakens the hard coat so that water can enter. Seeds lying on the
soil surface may be subjected to large daily temperature
fluctuations. This cracks the seed coat, allowing water to enter

Chemical inhibitors
• Chemical inhibitors may be present within or just outside the seed
which prevent germination.
• The chemicals must be removed before germination can take place.
• The seeds of many desert plants will only germinate after there has
been sufficient rain to wash away the chemical inhibitor. This means
the seeds will only germinate after sufficient rain has fallen to enable
the plant to grow to maturity.
• Western Australian everlasting flowers, Schoenia and
Rhodanthe species, are examples of plants which germinate in
this manner. The chemical inhibitor adds survival value,
preventing germination when conditions will not be able to
support plant growth.

Chemical inhibitors
• Many fleshy fruits contain chemical inhibitors which prevent the
seeds from germinating within the fruit. The seeds need to be
removed from the fruit before they can germinate, ensuring their
dispersal before germination.
• Tomatoes are an example of this type of chemical inhibition.
• Many Western Australian native plants have seeds which are very
difficult to germinate. Fire and heat damage them, water and
chemicals do not induce germination. It was discovered in 1994 that
these seeds would germinate after they had been exposed to the
smoke, but not the heat, of burning native plants.
• Examples of this include the feather flowers, Verticordia sp, and
the native buttercup, Hibbertia.

General conditions for seed germination
• Dormant seeds need special conditions to break their dormancy and
begin germination. Seeds of different species have different
requirements for germination depending on how those plants survive
in nature.
• Seeds that normally survive through winter require stratification
before they can germinate. Stratification is exposure to lengthy
periods of cold. Other seeds need conditions as listed above.
• Other factors required to break dormancy are general to all seeds.
These are:
• Viable seeds
• Sufficient water
• Sufficient oxygen
• Favourable temperatures

General conditions for seed germination
• The correct amount of light; some seeds will only germinate in the
dark, others require light for germination. Strelitzia will only
germinate in the dark, Datura only in light.
• Growing media suitable for the species

Types of germination
• There are two main types of germination depending on whether the
cotyledons are carried up above the soil or remain underground.
Epigeal Germination.
• In epigeal germination the hypocotyle grows rapidly when the seed
germinates. The hypocotyl is the growing shoot between the seed
and the cotyledons.
• The cotyledons are thus carried above the soil surface and turn
green.
• Examples: The French bean, Phaseolus vulgaris

Hypogeal Germination.
• In hypogeal germination the region of shoot between the cotyledons
and the first true leaves, the epicotyl, grows rapidly, leaving the
cotyledons under the soil surface.
• Examples: The green pea, Pisum sativum

Summary
Today we have covered;
• Fruit & Seeds
• Fruit Dispersal
• Inhibitors

Next Session
We will cover:
Ethnobotony

Preparation
Brief Notes
• Don’t forget to log on to the LMS and download and print off your
brief notes and handouts for the next session.

Botany session 5

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