Seeds – specialized structures
Aril
An aril is a seed appendage
that is used to attract animal
or insect seed dispersers.
Arils usually have a bright color
and are often fleshy.
Arils contain a food reward for
the dispersal agent.
The yellow aril contrasts with the shiny black
seeds emerging from the red-brown fruit pod.
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Seeds – specialized structures
Aril
Arils usually develop from an
outgrowth of the funiculus
or integument (seed coat).
Funicular aril forming in Lab Lab bean
(Dolichos) in the legume family.
Some seed biologists
restrict the term aril for
only seed appendages that
arise from the funiculus or a
portion of the developing
seed coat.
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Seeds – specialized structures
Aril
Other seed biologists use the term
aril in a broader sense to include
seed attachments that function as
a dispersal attractant regardless
of the originating tissue.
These usually include appendages
that might form from a portion of
the ovary (fruit).
In cashew (Anacardium
occidentale), the aril
forms from the upper part
of the flower pedicel.
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Seeds – specialized structures
Aril
Arils can cover portions of the seed or completely enclose the seed.
In cases where the aril covers only part of the seed, the aril color
usually contrasts with the seed color.
Aril partially
covering the
seed.
Aril completely
covering the seed.
Orange red aril on a black seed in
glory bower (Clerodendrum).
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Orange-red aril in creeping euonymus
(Euonymus).
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Seeds – specialized structures
Aril
Some arils are specialized to attract ants as seed dispersers. In these seeds, the
aril is called an elaiosome. Elaiosomes contain oils and are nutrient rich.
They are part of a strategy for seed dispersal by ants called myrmecochory.
Wild ginger (Asarum)
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Chelondine poppy (Stylophorum)
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Seeds – specialized structures
Aril
The aril can also be composed of thread-like hairs. In twin leaf (Jeffersonia),
the hairs are oily and function as an elioasome. In bird-of-paradise (Strelitzia)
the orange colored hairs attract birds that disperse the seeds.
Strelitzia
Jeffersonia
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Seeds – specialized structures
Aril
In several Acacia species, the long funicular
aril suspends the seeds away from the pods
(fruit) exposing them to bird predation.
Acacia
auriculiformis
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Seeds – specialized structures
Aril
A type of aril that forms in members
of the euphorbia family
(Euphorbiaceae) is called a caruncle.
A caruncle is a hard aril that forms
at the micropylar end of the seed
and is derived from the integuments
(seed coat).
Castor bean (Ricinus)
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Caruncle
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Seeds – specialized structures
Aril
In passion fruit, the dark seeds can
be seen through the aril that covers
each seed.
Passiflora
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Seeds – specialized structures
Aril
Although the fleshy covering on
pomegranate seeds look very
similar to the arils covering
passion flower seeds, they differ
because of the origin of the
covering tissue.
In passion flower, the aril is
derived from the funiculus while
in pomegranate the covering is
actually the outer layer of the
seed coat that does not harden
and remains fleshy. This is
technically called a sarcotesta.
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Pomegranate
(Punica granatum)
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Seeds – specialized structures
Aril
Depending on how strictly one
adheres to the definition of an
aril, seeds with a sarcotesta
may or may not be considered an
arillate seed.
Regardless, to the casual
observer seeds with a covering
aril often look indistinguishable
from one with a sarcotesta.
Both types of seed structures
have similar dispersal functions.
Pomegranate (Punica granatum)
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Seeds – specialized structures
Aril
In euonymus, the orange-red seed
covering is a funiculus-derived aril.
Aril
Eastern wahoo
(Euonymus atropurpureus)
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Seeds – specialized structures
Aril
The orange-red seed covering in
magnolia looks very similar to the
aril covering in euonymus, but it is an
example of a seed with a sarcotesta.
Sarcotesta
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Seeds – specialized structures
Aril
In cacao (Theobroma cacao), the white
edible pulp that surrounds the chocolate
seed is a sarcotesta. To make chocolate
from the seeds, the pulp is removed by
fermentation, which flavors the seeds
prior to drying and roasting.
Fermenting
seeds
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Seeds – specialized structures
Aril
Ice cream bean (Inga), also has
an edible aril that forms as a
white pulp that completely
surrounds the seed.
However, rather than being a
sarcotesta it is derived from
inner layer (endocarp) of the
fruit pod.
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Endocarp
derived aril
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Seeds – specialized structures
Aril
An aril also forms in the seeds of the conifer yew (Taxus).
In this case, the aril is derived from the base of the ovule and grows
to partially enclose the hard ovulate cone (seed).
Aril
Seed
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Seeds – specialized structures
Aril
In nutmeg (Myristica fragrans), the seed is the spice called
nutmeg and the fascinating and attractive red aril that forms
around the seed is used as the spice called mace.
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Seeds – specialized structures
Aril
In akee (Blighia sapida), the large aril
around the seeds is used in Jamaican
cooking, but using the immature aril
before the seed turns black is poisonous.
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Seeds – specialized structures
Aril
Some seeds take on a seed coat
coloration that mimics having an aril.
Such seeds are called mimetic seeds
and trick seed dispersers into moving
the seeds without expending plant
resources in making an arillate reward.
Horse eye bean (Ormosia arborea)
Crab’s eye vine (Arbrus)
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