Fruit Function and Anatomy

W HAT DO tomatoes, soybean pods,

acorns, and rice have in common?
They are all fruits. While we benefit greatly
from the fruit of plants, fruit are really an
evolutionary adaptation of flowering plants
that helps in the reproduction of the
species. This unit examines fruit
structures, types, and development.

Objective:

þ Describe the structures and

functions of fruit, the types of
fruit, and the processes of fruit
development.

Key Terms:

Ñ achene
aggregate fruits
follicle
fruit
pepo
pericarp
berry hesperidium pome
capsule indehiscent fruits pyxis
caryopsis legume samara
dehiscent fruits mesocarp simple fruits
disseminated multiple fruits stenospermocarpy
drupe nut uricle
endocarp nutlet
exocarp parthenocarpy

Fruit
After the fertilization of flowering plants, the ovule develops into a seed. The surrounding
ovary wall enlarges and forms a fruit around the seeds. Technically, a fruit is a mature, ripened

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 ovary. The two main functions of fruit are to prevent the seeds from drying and to disperse the
seed. The fruit may be either fleshy or dry.
Fleshy fruits, like the tomato or apple, hold juices that prevent the seeds from drying until
they are mature. Fleshy fruits also serve to help disperse the seeds. For example, some animals
are attracted to the nutritious fruit and eat the seeds along with the fleshy fruit. The seeds pass
through their digestive tract and are dispersed or disseminated away from the parent plant.
This dissemination of seed is an important evolutionary trait for the survival of the plant spe-
cies.
Although dry fruits are not fat and juicy like the tomato, they do help prevent the seed from
drying. Dry fruits have other means of dissemination. For instance, the dandelion has evolved
a dry, feathery fruit to take advantage of the wind for dissemination.
There is great diversity of fruits. Three major divisions include simple fruits, aggregate
fruits, and multiple fruits.

SIMPLE FRUITS
Simple fruits are defined as having developed from a single ovary of a single pistil. Simple
fruits are often classified as being fleshy or dry.

Fleshy Fruit
Fleshy fruits are juicy. Berries, hesperidium, pepo, drupes, and pomes are categories of
fleshy fruits.
A berry has an entirely fleshy ovary. Tomato, date, blueberry, banana, pepper, and cran-
berry are examples of berries.
A hesperidium fruit has a leathery rind. Examples include oranges, grapefruits, lemons,
and limes.
A pepo is a type of fruit defined by a hard rind and a fleshy inner matrix. Watermelons,
cantaloupe, squash, and pumpkins are pepos.

Peach (drupe)

Tomato (berry)

Watermelon (pepo)
Orange (hesperidium) Apple (pome)
FIGURE 1. Examples of various types of fleshy fruits.

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 A drupe is a fruit with a fleshy exterior and a single hard, stony pit surrounding the seed.
Cherry, peach, olive, and plum are examples of plants with drupes.
A pome has a fleshy exterior and a center with papery carpels. Apples and pears are pomes.

Dry Fruit
Dry fruits may be indehiscent or dehiscent.
Indehiscent fruits or those that do not split open at maturity and usually contain one or
two seeds. Some types of indehiscent fruits are achene, caryopsis, samara, nut, uricle, and
nutlet.
An achene is one-sided fruit with a seed attached at only one place to the pericarp. Sun-
flowers and buckwheat have achene type fruit.
A caryopsis is similar to an achene. However, the pericarp sticks or clings to the seed.
Corn, rice, barley, rye, amaranth, sorghum, oat, and wheat have caryopsis fruit.
A samara is usually single seeded with a membranous wing. Examples are maple, elm, and
ash.
A nut is a hard, one-seeded fruit.
Oak, walnut, filbert, and hickory pro-
duce nuts.
A uricle is like an achene, but the
ovary wall fits loosely around the seed.
Examples are finger millet and pigweed.
A nutlet is a small version of a nut.
Birch and hornbeam are examples.
Dehiscent fruits are fruits that split
open upon maturation. Dehiscent fruit
types are legume, follicle, capsule, and
pyxis.
A legume (pod) is composed of a
single carpel and has two longitudinal
FIGURE 2. Walnuts are categorized as dry fruits.
sutures. Soybeans, green beans, and peas
are legumes.
A follicle is composed of a single car-
pel and splits open along one suture.
Milkweed fruit is a follicle.
A capsule is composed of more than
one carpel that are united and form
many-seeded fruits. The fruit of okra and
cotton are capsules. Plants in the mustard
family have a specialized form of capsule
called a silique.
A pyxis is a type of capsule with a lid
that falls from the fruit. An example is
FIGURE 3. Soybean pods are also called legumes.
purslane.

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 AGGREGATE FRUITS
Aggregate fruits develop from a
single flower that has many pistils. Mul-
tiple, usually fleshy, fruitlets are attached
to one receptacle. Raspberries are an
aggregate of drupes. Strawberries are an
aggregate of achenes.

MULTIPLE FRUITS
Multiple fruits consist of a number
FIGURE 4. The raspberry is an aggregate fruit.
of flowers that fused to form a mass.
Pineapples are considered a multiple
fruit.

PARTS OF A FRUIT
A plant fruit has parts with different
functions. The tissue that surrounds the
seeds is called the pericarp, or fruit
wall. Three major parts of the pericarp
are the exocarp, the mesocarp, and the
endocarp. The exocarp is the outer wall
of the fruit. The exocarp, sometimes
called the epicarp, forms the tough outer
skin of the fruit. It can be thick and
tough, as in the case of oranges, or thin
and soft, like a grape. The mesocarp is FIGURE 5. Because pineapples are made of many flowers, they are
categorized as a multiple fruit.
the middle layer of the pericarp. It often
makes up the bulk of the fruit and is fleshy. The endocarp is the inner part of the pericarp. It
surrounds the seed or seeds. It may be hard like a peach or soft like a grape.

STAGES OF FRUIT DEVELOPMENT

Many things happen between the time of fertilization and the ripening of the fruit. The
processes associated with fruit development are dictated by plant hormones.
As seeds develop inside the ovary wall, they produce cytokinins that migrate from the seed
and promote cell division in the ovary wall. This results in added thickness to the fruit. The
seeds follow up by producing gibberellins. Next, it is exported to the wall of the ovary and

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 causes rapid expansion of each of the cells. The combination of more cells and expanding cells
leads to a tremendous increase in the size of the ovary.
Meanwhile, the plant produces abscisic acid, which causes the embryo in the developing
seeds to become dormant. This is significant because it prevents the seed from sprouting inside
the moist, unripened fruit.
Fruits that lack seeds can develop if a solution of gibberellic acid is applied to them. An
example is Thompson seedless grapes. Thompson seedless grapes are treated about three times
in the growing season with a dilute solution of gibberellic acid.
The developing ovules produce cytokinins that cause nutrients to be stored in the endo-
sperm tissues of the developing seed. In many species, these nutrients are later translocated to
the cotyledons.
As the ovary wall thickens, the developing seeds begin to produce either gibberellins or
auxins, depending on the species. These hormones cause cells to enlarge and the ovary wall to
expand. The combination of cytokinins increasing the number of cells and gibberellins
increasing the size of those cells leads to spectacular enlargement of the fruit.
At about this stage, the enlarged ovary can be called a fruit, and the ovules have become
mature seeds. The seeds have a drying seed coat (the former integument of the ovule) and
contain a mature embryo. Abscisic acid causes the seed embryos to remain dormant. The seed
embryos are prevented from growing until the seeds have been removed from the fruit or the
abscisic acid in the seed breaks down.
Eventually, the fruit reaches full size. However, fruit at this stage tends to be sour (acid),
mealy (starchy), green, hard, and lack fruity odor. It needs to be ripened before consumption.
The ripening process could take a few days after picking or it could depend on an environmen-
tal cue.
Most species must produce ethylene in order for the
fruit to ripen. Ethylene diffuses throughout the fruit tis-
sue and into the atmosphere around the fruit.
An increase in the rate of cellular respiration in the
fruit cells and synthesis of new enzymes usually accom-
panies the ripening process. Warm temperatures also
speed the process. The ethylene released by one ripen-
ing fruit can cause neighboring fruits to also ripen.
The manufactured enzymes break down complex cell
compounds. Acidic materials are broken down by an
enzyme called kinase, so the fruit is no longer sour.
Amylase converts starches to sugars and, in the process,
the fruit becomes juicier. Hydrolase breaks down chlo-
rophyll and large organic chemicals. With the chloro-
phyll gone, yellow pigments become visible, and red
pigments may develop. Some of the large organic com-
pounds become smaller molecules that give ripe fruit its
odor. Pectinase depolymerizes pectin, which is the glue
FIGURE 6. By placing a ripe apple in a bag next
that holds cells together. Without pectin, the fruit to an unripe fruit, the ripening process will be
becomes soft. sped up.

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 HOW SEEDLESS FRUIT ARE PRODUCED
Seedless fruits can develop by parthenocarpy, stenospermocarpy, and because the plants are
triploid.
In a process known as parthenocarpy, fruit may develop without fertilization. Seedless
pineapples and cucumbers result when pollination fails to occur. Pineapples are self infertile.
In other words, pineapples require cross-pollination for seeds to set. Cross-pollination does
not occur when a field is planted with all one variety. Many citrus fruits are seedless for the
same reason. Cucumbers may produce seedless fruit if not pollinated. If pollination takes
place, they produce seeds.
Technically, seedless grapes are not seedless. Normal pollination and fertilization occurs,
but the embryos abort when they are young. Often remnants of the seeds can be seen in the
fruit. This process is called stenospermocarpy.
Bananas and seedless watermelon are seedless because the plants are triploid. Because they
have three sets of chromosomes, meiosis fails to take place. The triploid banana varieties are
propagated asexually by removing and planting offshoots. Watermelons are produced from
seeds obtained by crossing diploid plants with tetraploid plants. The seeds from the cross are
triploid. The triploid plants grow and produce fruit after being pollinated but, because they are
sterile, fail to produce seed. Triploid plants must be grown near pollen-producing diploid
plants.

Summary:

2 A fruit is a mature, ripened ovary. The two main functions of fruit are to prevent
the seeds from drying and to disperse the seed.

Three major divisions include simple fruits, aggregate fruits, and multiple fruits.
Simple fruits are defined as having developed from a single ovary of a single pistil.
Simple fruits may be fleshy or dry. Fleshy fruit include berries, hesperidium, pepo,
drupes, and pomes. Dry fruits may be indehiscent or dehiscent. Some types of
indehiscent fruits are achene, caryopsis, samara, nut, uricle, and nutlet. Dehiscent
fruit types are legume, follicle, capsule, and pyxis. Aggregate fruits develop from a
single flower that has many pistils. Multiple fruits consist of a number of flowers
that fused to form a mass.

Three major parts of the pericarp are the exocarp, mesocarp, and endocarp.

The process of fruit development is dictated by plant hormones.

Seedless fruits can develop by parthenocarpy, stenospermocarpy, and because the

plants are triploid.

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 Checking Your Knowledge:

´ 1. What are the functions of fruit?

2. What are the three main divisions of fruit?
3. What are the major parts of a fruit?
4. How do fruit develop?
5. How are seedless fruit produced?

Expanding Your Knowledge:

L Obtain a variety of fruits from a store and from a local outside source. Dissect the
fruits and determine the different parts. Find out what type of fruit each is.

Web Links:

: Identification of Major Fruit Types

http://waynesword.palomar.edu/fruitid1.htm

Fruits
http://theseedsite.co.uk/fruits.html

Fruit Growth and Fruit Types

http://plantphys.info/Plants_human/fruittype.html

Agricultural Career Profiles

http://www.mycaert.com/career-profiles

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