Seed morphology
Seed - dispersal units
In seed plants, the smallest unit of
dispersal for the next generation is
called a diaspore.
The diaspore is typically referred
to as a “seed” and in its basic form
it is a mature ovule that contains an
embryo bounded by a nutrient
tissue and the seed coat.
However, the diaspore can be a
diverse tissue set additionally
including parts of the ovary (fruit)
and flower parts in the final
dispersal unit.
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Seed morphology
Seed - dispersal units
A true seed is a ripened ovule.
At the time of separation from
the parent plant, it consists of an
embryo and stored food supply,
both encased in a protective
covering.
Seed coat
The activation of the seed’s
metabolic machinery leading to the
emergence of a new seedling plant
is known as germination.
Endosperm
Radicle
Cotyledons
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Seed morphology
Seed - dispersal units
The seed unit (diaspore) is botanically
diverse and can be differentiated by
the tissue that becomes the
protective covering structures.
These include the:
1. Integuments (seed coat)
2. Ovary tissue (pericarp)
3. Flower tissue (anthrocarp)
Golden raintree (Koelreuteria)
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Seed morphology
Seed - dispersal units
The protective covering
structures for “true seeds”
are derived from the
integuments that form the
layers of the seed coat.
“True seeds” in angiosperms
are produced within a
covering ovary (fruit).
Tomato seeds are an example of “true seeds”.
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Seed morphology
Covering structures – Ovary tissue (pericarp)
In some cases, the seed units (diaspores)
are not simple “true seeds” but also
contain fruit parts that cover the seed.
Fruits dispersed as the seed unit include:
Drupes
Achenes
Nuts and nutlets
Mericarps
Samaras
Caryopsis
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Oak acorn (Quercus)
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Seed morphology
Covering structures – Ovary tissue (pericarp)
In a drupe, there are three fruit tissue layers that make up the pericarp –
the outer exocarp, the fleshy mesocarp, and hard inner endocarp. The inner
“stone” (endocarp) and the true seed are difficult to separate and usually
constitute the seed unit.
Exocarp
Endocarp
Seed
Mesocarp
Endocarp
Seed
Seed
coat
Plum - Prunus
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Seed morphology
Covering structures – Ovary tissue (pericarp)
A coconut is a large drupe. The covering layers are not fleshy and the
mesocarp is composed of a stringy coir layers. These layers and the
large liquid endosperm facilitate a water dispersal strategy.
Exocarp
Mesocarp
Endocarp
Seed
Seed
coat
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Seed morphology
Covering structures – Ovary tissue (pericarp)
The palm seed that develops in the coco de mer (Lodoicea maldivica) drupe
represents the largest seed in the plant kingdom.
Seeds can reach 1.5 feet in diameter and over 60 pounds.
Seed with fruit tissue removed.
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Seed morphology
Covering structures – Ovary tissue (pericarp)
An achene is a single-seeded
dry, indehiscent fruit that is
usually dispersed as the seed
unit.
Pericarp
Seed coat
The pericarp covers the true
seed.
The seed coat is usually
papery and the protective
function of the seed
coverings is provided by the
hard pericarp.
Pericarp partially removed
Sunflower- Helianthus
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Seed morphology
Covering structures – Ovary tissue (pericarp)
A nut is a large, dry, indehiscent fruit usually containing only a single seed with a
hard, outer wall. An acorn is a familiar nut that is dispersed as the seed unit.
Cap
Pericarp
Cotyledons
Embryo radicle
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Oak - Quercus
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Seed morphology
Covering structures – Ovary tissue (pericarp)
Small nuts are called a nutlets. They are usually single-seeded fruits.
Zelkova (Zelkova)
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Seed morphology
Covering structures – Ovary tissue (pericarp)
A schizocarp is a fruit that develops from a compound ovary where the carpels
break apart into separate single-seeded, achene-like, seed units called mericarps.
Queen Anne’s Lace (Daucus)
The simplest schizocarps have two mericarps.
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Seed morphology
Covering structures – Ovary tissue (pericarp)
Sycamore is a more complex schizocarp with hundreds of mericarps (achenes)
united in a ball-like fruit.
Sycamore (Platanus)
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Three united mericarps
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Seed morphology
Covering structures – Ovary tissue (pericarp)
Hollyhock fruit is composed of many mericarps surrounding a central axis.
When the fruit dries, each mericarp separates as a single seed unit.
Schizocarp
Mericarps
Hollyhock (Althea)
Developing schizocarp
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Seed morphology
Covering structures – Ovary tissue (pericarp)
A samara is a one-seeded, dry, indehiscent fruit with a specialized wing for wind
dispersal. Maple and ash fruit are easily recognized as samaras and are examples
where the wing is on one side of the fruit.
Seed
Wing
Ash (Fraxinus)
Maple (Acer)
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Seed morphology
Covering structures – Ovary tissue (pericarp)
In elm and birch, the wings of the samara completely surround the seed.
Wing
Birch (Betula)
Elm (Ulmus)
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Seed morphology
Covering structures – Ovary tissue (pericarp)
A caryopsis is a dry, indehiscent fruit with the pericarp and seed coat
fused to form a single seed unit.
Pericarp
Embryo
Corn (Zea)
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Seed morphology
Covering structures – Flower tissue (anthrocarp)
Stigma
In addition to pericarp enclosed
structures, flower tissue can be
associated with the “seed unit”
at the time of dispersal.
Style
Seed with
anthrocarp removed
Pericarp
This can be described as an
anthrocarp.
Globe amaranth (Gomphrena)
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Seed morphology
Covering structures – Flower tissue (anthrocarp)
In some composites (Asteraceae), the sepals (calyx) forms a tuft of
hairs (bristles) called the pappas to aid in dispersal.
Last remaining fruit
Pappas
Achene
Pappas
Holes in the receptacle where
achenes were attached.
Achene
Bigleaf ligularia (Ligularia)
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Seed morphology
Covering structures – Flower tissue (anthrocarp)
The pappus can act like a parachute for
wind dispersal of the achene.
This arrangement of specialized sepals
(pappas) and achene is called a cypsella.
Cypsella
Individual
floret
Flower head
Pappus
Petals
Sepals
Achene
Seed
head
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Ovary
Dandelion (Taraxacum )
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Seed morphology
Covering structures – Flower tissue (anthrocarp)
The top of the modified sepals can also develop into scales as in purple coneflower.
Scales
Scales
Pericarp
Pericarp
Seed
Seed
coat
Coneflower - Echinacea
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Seed morphology
Covering structures – Flower tissue (anthrocarp)
In Dock (Rumex), the achene is surrounded by an elongated sepal
that is called a valve. The midrib of the valve swells into a structure
called a grain that covers the achene.
Developing
fruit
Mature fruit
Grain
Adhering sepal
called a valve
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Grain covering
the achene
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Seed morphology
Covering structures – Flower tissue (anthrocarp)
In some members of the buttercup
family (Ranunculaceae), the feathery
style remains attached to the achene
to aid in wind dispersal.
Modified
style
Anemone
Clematis
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Seed morphology
Covering structures – Flower tissue (anthrocarp)
In some members of the geranium family (Geraniaceae), the style is modified
into a structure that is coiled when dry and expanded when wet. During wet
and dry cycles, these seeds will drill themselves into the surrounding soil.
Stylar beak
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Seed morphology
Covering structures – Flower tissue (anthrocarp)
In grasses, several of the subtending bracts (glumes, lemma and palea)
can be parts of the seed unit.
The basic tissue arrangement in
a grass floret
Awn
Anther
Stigma
Palea
Ovary
Lemma
Rachilla
Second
Glume
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First Glume
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Seed morphology
Covering structures – Flower tissue (anthrocarp)
The seed dispersal unit in the grass Elymus.
With glumes removed
Awns
Glume
Glume
With glumes, lemma
and palea removed
Lemma
Seed
(caryopsis)
Palea
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Seed morphology
Covering structures – Flower tissue (anthrocarp)
Gama grass (Tripsacum ) produces a seed unit that is a little different from
other grasses. It produces a cupule that consists of hardened rachis and glume
tissue surrounding a papery lemma and palea directly covering the caryopsis.
Developing
fruits
Intact seed unit
Glume partially removed
Glumes
Glume
Lemma
Rachis
Lemma
and palea
Palea
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Rachis
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