Seeds and Fruits Chapter 14. Fruits and Seeds Fruits –Packaging structure for seeds of flowering...

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

• 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

• 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