Angiosperms II

Fruits, Seeds, and Embryos

FRUITS• A “fruit” is derived from the ripened

ovary (ovaries) and sometimes other associated floral parts– to end the debate then, a tomato is a fruit

since it is derived from a flower• The fruit (usually a ripened ovary) is

surrounded by the fruit wall or pericarp– This wall is composed of three layers

(exocarp, mesocarp, and endocarp)– These layers may be distinct or fused

Fruit Structures (Pericarp)

Endocarp

Mesocarp

Exocarp

Fruit Types

• Multiple Fruits– derived from

MANY flowers– pineapple,

mulberry

For an interactive key to FRUIT TYPES, go to http://arnica.csustan.edu/key/key2.html

Fruit Types (cont.)

• Aggregate Fruits– derived from

SEVERAL separate carpels (pistils) of ONE flower

– strawberry, raspberry, blackberry

Fruit Types (cont.)

• Simple Fruits– derived from ONE carpel or pistil of ONE flower– are either fleshy or dry at maturity

Simple Fruits

• Fleshy Fruits (a few types)– with a fleshy hypanthium and/or receptacle =

pome (apple)– a single seed with stony endocarp = drupe

(plum, peach)– many seeded, endocarp fleshy = berry

(grape, tomato) or pepo (pumpkin, watermelon)

– Outer layer with a separable rind = hesperidium (orange, lemon, grapefruit)

Fleshy Simple Fruits

BERRY DRUPE

Fleshy Simple Fruits (cont.)

HESPERIDIUM

POME

PEPO

Simple Fruits (cont.)

• DRY FRUITS– Dehiscent Types

• seeds released through one seam = FOLLICLE• seeds released through 2 seams = LEGUME• seeds released though pores or multiple seams =

CAPSULE– Indehiscent Types

• pericarp hard and thick with a basal cup = NUT• Pericarp soft and thin, no cup = ACHENE,

CARYOPSIS etc...

Dehiscent Dry Fruits

FOLLICLE

LEGUME

CAPSULE

Indehiscent Dry Fruits

NUTS ACHENE

Why the variation in fruit types?

• Fruits are units of dispersal for the seeds

• Certain fruits are adapted for dispersal by wind (small, light, winged)

Fruit Dispersal (cont.)

• Others are dispersed by animals (fleshy, colored, sweet, or high in energy like nuts; or with spines, hooks)

Bidens – tickseed fuits

Fruit Dispersal (cont.)• Some for water dispersal (coconut)

Seeds

• A seed is surrounded by the seed coat derived from the integuments

• The embryo may have large cotyledons (as in lima beans) and little endosperm

• Some seeds have lots of endosperm and thin cotyledons (castor beans)

Pinto Bean vs.Castor Bean

The Grass Seed (Fruit)

• Outer pericarp is fused to the seed coat• Single massive cotyledon is called the

scutellum• Protective sheaths cover the early shoot

(coleoptile) and the root (coleorhiza)• Endosperm is surrounded by a special

layer of cells called the aleurone layer

CORN GRAIN

endosperm

scutellum

coleoptile

coleorhiza

embryonic leaves

Seed Germination

• Normal germination requires proper temperature, water, oxygen and sometimes light

• If, given proper conditions, a seed does not germinate, we say it is DORMANT

• Dormancy is annoying to us, but it is evolutionarily adaptive for the plant

Reasons for Seed Dormancy• Seed coat impervious to water and/or

oxygen (imbibition of water is often the first step in germination)– scarification required (physical or

chemical) to allow entry of water and/or oxygen

– many commercial applications for seed production

Seed Scarification

SeedDormancy

• Seed has an immature embryo

• Seed has chemical inhibitors in the seed coat that must be leached out

Desert in Bloom

Seed Longevity• Seeds may be

dormant for only a few weeks to thousands of years– record is over

10,000 years for Arctic lupine seeds from lemming burrows

Seed Longevity (cont.)

–Lotus seeds have been germinated after storage for more than 2,000 years

Seed Banks• Seed Banks help protect angiosperm

genetic diversity around the world

Kew Gardens, England. Home of the Millennium Seed Bank Project

Embryology

• Early embryo development in plants progresses through specific stages:

– much of the work done on Capsella bursa-pastoris (“shepherd’s purse”)

CapsellaEmbryology

–“ball” stage with basal cell, suspensor and the embryo proper

CapsellaEmbryology

(cont.)

– “heart-shaped” stage where the embryo’s two cotyledons become obvious and the differentiation of tissues become evident

CapsellaEmbryology

(cont.)

• “bending cotyledons” stage where the embryo moves toward its final form

CapsellaEmbryology

(cont.)

• “Mature embryo” stage

Embryonic Tissue Layers

• Thee distinct tissue layers differentiate early in embryo development:– PROTODERM which will give rise to the

plant’s epidermis and all associate structures (guard cells, trichomes, epidermal cells)

– PROCAMBIUM which will become the primary xylem and phloem tissues (vascular tissues)

Embryonic Tissue Layers (cont.)

– GROUND MERISTEM from which will be derived the pith, cortex, and associated structures in the stem and root

• These tissue layers correspond in a way to early tissues in animal embryos, namely, the ectoderm, endoderm, and mesoderm

• Development of most plant embryos has not been investigated