Transcript document

Seeds and Fruits
Chapter 14
Fruits and Seeds
• Fruits
– Packaging structure for seeds of flowering
plants
• Seeds
– Mature ovules
– Contain embryonic plant
• Fruits and seeds
– Most important source of food for people and
animals
Seed – Mature Ovule
• Fertilization occurs
• Zygote develops into embryo
• Primary endosperm nucleus develops into
endosperm
– Suspensor supports embryo in endosperm
– Endosperm is nutrient-rich storage tissue
– Endosperm persists in many monocots and
only in a few dicots
Seed – Mature Ovule
• Integuments of ovule develop into seed
coat
– Seed coat acts as protective shell around
embryo
– Sometimes contains chemical substance that
inhibits seed from germinating until conditions
are right for germination
Common bean
Castor bean
Grasses
Onion
Monocot or
dicot
Dicot
Dicot
Monocot
Monocot
External
features of
seed
Hilum, micropyle, raphe
Caruncle – covers
hilum and micropyle,
raphe runs length of
seed
Micropyle
Micropyle
Endosperm
Not present
Massive amounts
Yes
Yes, small amount
Cotyledons
2 fleshy cotyledons
2 thin cotyledons
1 cotyledon
1 cotyledon
Embryo
Embryonic root (radicle) at one
end, shoot – epicotyl at other
end, hypocotyl – just below
cotyledons
Short hypocotyl, small
epicotyl, small radicle
Shoot apex and
several rudimentary
leaves ensheathed in
coleoptile, radicle
surrounded by
coleorhiza, scutellum –
secretes enzymes that
digest food stored in
endosperm
Simple embryo,
radicle, and simple
cotyledon are
prominent, shoot apex
close to midpoint of
axis and appears as
notch, embryo coiled,
radicle usually points
toward micropyle
Germination
Hypocotyl elongates, raises
cotyledons and shoot apex
toward light
Cotyledons first
function as absorbing
organs, cotyledons
emerge from seed
coat, become green,
photosyntesize, wither,
die
Primary root pushes
through coleorhiza,
adventitious roots
develop, coleoptile
elongates and
emerges
aboveground,
uppermost leaf pushes
through coleoptile and
becomes part of the
photosynthesizing
shoot
Slightly bent cotyledon
breaks soil surface,
straightens out, base
of cotyledon encloses
shoot apex, first leaf
emerges through
opening at base of
cotyledon
Seeds
• Key terms
– Hilum
• Large oval scar left when seed breaks away from
placental connection (funiculus)
– Micropyle
• Small opening in seed coat at one end of hilum
• Opening through which pollen tube enters ovule
Seeds
– Raphe
• Ridge at end of hilum opposite the micropyle
• At base of the funiculus
– Caruncle
• Spongy outgrowth of outer seed coat
• Absorbs water needed during germination
Germination
• 1st step in growth of embryo
• Begins with imbibition (uptake of water)
– Water activates enzymes that digest food
stored in cytoplasmic organelles called protein
bodies, lipid bodies, and amyloplasts
• 1st indication germination has begun
– Swelling of radicle
Germination
• Two types of germination
– Epigeal germination
• Straightening of hypocotyl raises cotyledons and
shoot apex toward light
– Hypogeal germination
• Cotyledons remain belowground
• Only apex and 1st leaf are raised upward
Dormancy of Seeds
• Seeds remain viable for long periods
• Many viable seeds will not germinate even
when conditions are right
– In state of dormancy
– Factors that break dormancy
• Light – some lettuce species
• Scarring or breaking through seed coat – legumes
• Exposure to temperatures close to freezing –
gooseberry
• Exposure to high temperature of fire – some pines
Fruits
• Ripened ovary
• Commonly refers to a juicy and edible
structure
• Functions
– Protect seeds
– Aid in dispersal of seeds
– May be factor in timing of germination of
seeds
Fruits
• Play important role in classification of
angiosperms
• Examples of fruits
– Apple, plum, peach, grapes, string beans,
eggplant, squash, tomato, cucumber, corn,
oats
Fruits
• Fruit wall (pericarp) has three layers
– Exocarp
– Mesocarp
– Endocarp
• Accessory
– Tissues other than ovary wall that form part of
a fruit
Main Categories of Fruits
• Simple
– Derived from single ovary
– Dry or fleshy
– Dehiscent (splits open) or indehiscent
• Compound
– Composed of more than one fruit
Main Categories of Fruits
– Two types of compound fruits
• Aggregate
– Derived from many separate ovaries of a single flower
– Example: strawberry
• Multiple
– Enlarged ovaries of several flowers grown more or less
together into a single mass
– Example: pineapple
Criteria for Classifying Fruits
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Structure of flower from which fruit develops
Number of ovaries involved in fruit formation
Number of carpels in each ovary
Nature of mature pericarp (dry or fleshy)
Whether pericarp splits (dehisces) at maturity
If pericarp dehisces, manner of its splitting
Role accessory tissues play in formation of
mature fruit
Simple Fruits – Dry and Dehiscent
• Legume or pod
– Arises from single carpel
– At maturity usually dehisces along two sides
– Example: pea
• Shell – pericarp
• Pea - seed
Simple Fruits – Dry and Dehiscent
• Follicle
– Develops from a single carpel
– Opens only along one side
– Example: magnolia
• Capsules
– Simple fruits derived from compound ovaries
– Dehisces in various ways along top surface
– Example: poppy
Simple Fruits – Dry and Dehiscent
• Silique
– Dry fruit derived from superior ovary
consisting of two locules
– Dry pericarp separates into 3 portions
• Seed attached to central, persistent portion
– Example: members of mustard family
Simple Fruits – Dry and
Indehiscent
• Achene
– Dry, one seeded fruit
– Pericarp easily separated from seed coat
– Example: sunflower
• Caryopsis or grain
– Fruit of grass family
– Dry, one seeded indehiscent fruit
– Pericarp and seed coat firmly united all
around embryo
Simple Fruits – Dry and
Indehiscent
• Samara
– Outgrowths of ovary wall form wing-like
structure that aids in seed dispersal
• One seeded simple fruit
– Example: elm
• Two seeded simple fruit
– Example: maple
Simple Fruits – Dry and
Indehiscent
• Schizocarp
– Two carpels that split when mature along
midline into two one-seeded indehiscent
halves
– Example: celery
• Nut
– One seeded, indehiscent dry fruit with hard or
stony pericarp (shell)
– Example: walnut
Fleshy Pericarp
• Popular for food
• Fleshy fruit wall
– Attractive to animals
– Seeds tend to have hard seed coat not
broken down as it passes through animal
Fleshy Pericarp
• Drupes
– One seeded
– Derived from single carpel
– Hard endocarp
– Thin exocarp
– Fleshy mesocarp
– Examples: cherry, almond, peach, apricot
Fleshy Pericarp
• Berry
– Derived from compound ovary
– Many seeds embedded in flesh
– Types of berries
• Hesperidium
– Exocarp and mesocarp – rind with numerous oil cavities
– Endocarp – thick, juicy pulp segments composed of
wedge-shaped locules
– Juice forms in juice sacs or vesicles
» Outgrowths of endocarp wall
– Examples: lemons, oranges, limes, grapefruit
Fleshy Pericarp
• Pepo
– Rind consists mainly of receptacle tissue that surrounds
it and is fused with exocarp
– Flesh of fruit
» Mainly mesocarp and endocarp
– Examples: watermelon, cucumber, squash
Fleshy Pericarp
• Pomes
– Fruit derived from flower with inferior ovary
– Flesh
• Enlarged hypanthium (fleshy floral tube)
– Core
• From ovary
– Example: apple
Compound Fruits
• Aggregate fruits
– Formed from numerous carpels of one
individual flower
– Many simple fruits attached to a fleshy
receptacle
– Example: blackberry
Compound Fruits
• Multiple fruit
– Formed from individual ovaries of several
flowers all grouped together
– Fruit
• Enlarged fleshy receptacle
– Example: fig (drupes)
– Example: pineapple (berries)
Partheocarpy
• Parthenocarpic fruits
– Develop without fertilization
– Seedless fruits
– Regularly produced in cultivated plants
• Eggplant, navel orange, banana, pineapple
– In orchids
• Placing dead pollen or water extract of pollen on
stigma may start fruit development
Parthenocarpy
– Commercially induced in some plants
• Spray blossoms with dilute aqueous solution of
growth substance such as auxin
Role of Fruit
• Aid in dispersal of seeds inside
• Deter inappropriate seed-dispersing
animals from taking fruit or seed
• To protect seed from herbivores who
consume seeds but do not disperse them
Role of Fruit
• No nutritional relationship between fruit
and seeds within it
– Stored food in fruit cannot be used by
dormant seeds or by germinating seedlings
– Only stored food available to seedlings is in
endosperm and cotyledons within seed coat
Role of Fruit and Seeds
• Fruits and seed are rich in chemical
resources
– Sugar, starch, protein, lipid, amino acids,
variety of secondary compounds
– Caloric value is approximately 5,100
kcal/gram dry weight
Abiotic Mechanisms for Seed
Dispersal
• Wind
– Winged and plumed fruits common
adaptations for dispersal
– Seeds ballistically exploded by violent
dehiscence of pericarp
• Water
– Seeds float, germinate when washed ashore
– Flash floods spread seeds
Biotic Vectors for Seed Dispersal
• Ants, birds, bats, rodents, fish, ruminants,
primates
– Attracted to fruit by color, position, season
availability, odor, taste
Biotic Vectors for Seed Dispersal
• Biotic vector
– May eat fruit and discard seeds
• True of some primates
– Swallow seeds unchewed
• Seeds pass unharmed through gut
• Excreted some distance away
• Often case with birds
Biotic Vectors for Seed Dispersal
• May eat some seeds and cache others
– Seedlings later emerge from cached seeds
– Squirrels, jays
• May harvest seeds and deposit them in
granaries below ground
– Ants
Biotic Vectors for Seed Dispersal
• May eat elaiosomes (food bodies) at one
end of seed and then discard seed
– ants
Biotic Vectors for Seed Dispersal
• Sometimes animals transfer seeds in a
more parasitic fashion
– Seeds of some aquatic and marsh plants stick
to feet of birds in mud and are carried long
distances
– Birds carry sticky mistletoe seeds on their feet
to new host trees
– Seeds with beards, spines, hooks, or barbs
adhere to animal hair and human clothing and
are carried to new sites
Antiherbivore Mechanisms
• Mechanisms that discourage herbivores
include
– Reducing the time of fruit availability
– Making the fruit or seed coat physically hard
– Making the fruit or endosperm chemically
repellent
Antiherbivore Mechanisms
• Reducing the time of fruit availability
– Some species produce fruit and seed
abundantly only during mast years
– Low amount of seeds produced in off years
keeps number of seed eaters in check
– Seed-eating populations not large enough to
consume all seeds available during mast year
– Some seeds escape consumption and
germinate
Antiherbivore Mechanisms
• Making the fruit or seed coat physically
hard
– Prevents seed from being damaged by
grinding action in the crop of birds or the
mouths of chewing mammals
– Legume seed coats are hard and often pass
through animal guts unharmed
Antiherbivore Mechanisms
• Making fruit or endosperm chemically
repellent
– Effect is negative and often toxic
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Lectins – cause red blood cells to clump
Enzyme inhibitors
Cyanogens – release cyanide (potent nerve toxin)
Saponins - a detergent
Alkaloids – opium
Unusual amino acids
Distant Dispersal of Seeds
• Benefit of fruit and seed dispersal
– Spread species far from its parent
– Many fruits and seeds wasted because eaten
or deposited in places inappropriate for
germination
– In stressful habitats
– Advantageous to prevent or limit dispersal
away from parents
Distant Dispersal of Seeds
• Method of limiting dispersal
– Self-planting
• Grasses produce bent awns (slender bristles) that
drive grain into soil
– Peanut
• Fruits become buried as they mature
• Seeds never leave immediate proximity of parent
– Sea rocket
• Bipartie fruit
– Top half carried by ocean currents, bottom half attached
to parent