FLOWERS AND FRUITS: an Edible Lab!
Objectives:
1) Describe the purpose of a flower, identify the four floral whorls and describe their function
in individual flowers.
2) Define pollination and explain why plants require pollinators.
3) Characterize pollination syndromes and use floral morphology to predict potential pollinators.
4) Describe the purpose of a fruit and its importance in seed dispersal.
5) Characterize different types of fruits and use fruit morphology to predict potential
dispersal agents.
INTRODUCTION
The angiosperms (Phylum Anthophyta) are unique from the rest of the plant phyla in that they
produce flowers and fruits as part of their sexual reproduction. Since plants are not mobile,
they are not able to travel in order to seek mates, copulate, and disseminate their offspring. As
a result, plants have evolved a number of novel strategies in order to accomplish these tasks.
Flowers and fruits are modified in a variety of ways in order to facilitate pollination and seed
dispersal, both by biotic and abiotic means. In this lab, you will explore a variety of flower and
fruit types that are adapted for a wide range of strategies designed to accomplish pollination
and seed dispersal. When moving between flowers and fruits, try to gain a sense of the
relationship between the flower and fruit as the fruit represents merely a later developmental
stage of the flower. AND, don't forget to sample the edible fruits and flowers! Enjoy!!
FLOWER STRUCTURE
A "flower" is really nothing more than a shoot (stem and leaves) modified for reproduction.
Flowers can arise singly or in clusters called inflorescences. A stem-like structure called a
peduncle supports an inflorescence or a solitary flower. Pedicels are the structures which
support individual flowers of an inflorescence. The end of the peduncle is often expanded to
form a receptacle to which the actual floral parts attach. Flowers can have up to 4 whorls of
flower parts. Working from the outside to the inside, the parts that make up those whorls
include:
1) Sepals - often leaflike and green, the sepals protect the flower during the bud stage. Some
sepals are modified to look nearly identical to the petals, but they are always located to the
outside of the actual petals. The collective whorl of sepals is referred to as a calyx.
2) Petals - petals are found to the inside of the calyx and are often pigmented and showy in
order to visually attract pollinators. Petals may be separate or fused; together, they are
collectively referred to as a corolla.
3) Stamens - stamens consist of a stalk-like filament supporting a pollen-producing anther. The
collective arrangement of stamens represents the male part of the plant and is referred to as
the androecium ("house of men").
4) Carpels - carpels are the female floral parts and lie to the inside of the androecium. A carpel
consists of 3 parts: a) ovary - the broadened base of the carpel which contains the ovules, b)
style - an elongated structure extending from the top of the ovary and through which the pollen
tubes will grow in order to deliver sperm to the egg, and c) stigma - the often sticky terminal
end of the style which receives and adheres pollen grains. The carpels are referred to
collectively as the gynoecium ("house of women").
The corolla and calyx together are
known as the perianth and
represent the infertile parts of
the flower while the androecium
and gynoecium represent the
fertile (sexual) parts. A flower is
considered "complete" if all 4
whorls are present and
"incomplete" if 1 or more whorls
are absent. "Perfect" flowers have
both male and female parts
present; "imperfect" flowers are
either male or female.
When looking at a whole flower, if
the floral parts can be arranged so
that any cross section of the
flower has a mirror image, it is
called radially symmetric. If the
flower only has only one cross
section which produces a mirror
image, the flower is described as
having bilateral symmetry.
FLOWER FUNCTION
In angiosperms, pollination refers to the transfer of pollen from an anther to a stigma. Because
plants are not mobile, they are unable to seek out and physically interact with a mate to carry
out sexual reproduction. As a result, they must rely on other means to bring the male and
female gametes together. In some cases, abiotic (non-living) agents such as wind and water are
responsible for pollen transfer. However, the great diversity in angiosperms is thought to be
largely a result of coevolution with biotic (living) agents such as animals – insects in particular.
About 50 million years ago there was a rapid increase in the diversity of flower-visiting insects.
This correlates with a rapid increase in angiosperm diversity and is hypothesized to be the
primary factor driving angiosperm evolution.
Pollination by abiotic agents requires different modifications of the flower to facilitate pollen
transfer. This can be very different from pollination by biotic agents. In order to attract
animals, a plant must reward the animal for its effort to ensure that it keeps visiting the plant.
The more attractive a plant is to a pollinator, the more likely the pollinator will visit the plant.
The greater the reward for the visit, the more loyal the animal pollinator is likely to be. Flowers
have evolved many different ways to attract pollinators and most importantly, facilitate pollen
transfer. Some clues are visual, like flower color and pattern, flower size, and showy petals and
sepals. Some flowers use scents to attract pollinators and foods like nectar and protein-rich
pollen to reward them for the visit. Other flowers use pure trickery to attract pollinators
under false pretenses.
Many different animals pollinate plants. The table below shows some of the common biotic
pollinators and characteristics of the flowers to which they are often attracted. The suite of
characters exhibited by a plant that is associated with a specific pollination vector is called a
Pollination Syndrome. Often you can infer the pollinator of a plant based on its floral
morphology. Do any of the pollination syndromes seem familiar to you? Look at the flowers in
class and see if you can determine their pollinators.
Pollinator Color
Bee
Blue, yellow,
purple
Scent
Flowering
Corolla
time
Fresh and
Day
strong
Brightly
Fresh and
Butterfly colored; often
Day
weak
red
Moth
White or pale
Bilateral; with a
landing platform
Reward
Examples
Nectar
and
pollen
Marsh Marigold,
Foxglove, and
Snapdragon
Landing platform;
sometimes nectar Nectar
spurs
Butterfly Bush,
Milkweed,
Composites
Sweet and Night or
strong
dusk
Dissected;
sometimes nectar Nectar
spurs
Daisy, Evening
Primrose,
Tobacco
Nectar
Radial symmetry;
and
shallow flower
pollen
Wild Radish
Fly
(reward)
Light
Faint
Fly
(carrion)
Brownish,
purplish –
looks like
rotting flesh
Rotten
Day and
and strong night
Enclosed or open
None –
Carrion Flower,
often lay
Dutchman’s Pipe
eggs
Beetle
Various
Day and
Green or white but often
night
strong
Enclosed or open
Nectar
and
pollen
Magnolia and
Spice Bush
Bird
Brightly
No scent –
colored; often birds can’t Day
red
smell
Tubular or
pendant; ovary
often protected
Nectar
Red Columbine,
Fuchsia,
Hibiscus
Bat
Whitish
Very showy
flower or
inflorescence
Nectar
and
pollen
Saguaros
Cactus, Bananas
and Mangoes
Day
Musky and
Night
strong
FRUIT STRUCTURE
Recall that ovules are contained in the ovaries of flowers. After pollination and fertilization of
the egg inside the ovule, the ovule develops into a seed and the surrounding ovary develops into
a fruit. In some cases, ovaries develop into fruits without fertilization of ovules. This kind of
fruit development which does not require fertilization is called parthenocarpy and the resulting
fruits are called parthenocarpic fruits. Because fertilization is required in order to produce a
viable seed, parthenocarpic fruits do not have seeds. Many of the "seedless" varieties of fruits
found in the supermarket (e.g., watermelons, grapes, bananas, cucumbers, etc. . .) are the result
of parthenocarpy.
Fruits are classified according to the arrangement of the carpels from which the fruit
develops:
1) Multiple fruits consist of gynoecia of more than one flower. Pineapple and mulberry are good
examples of multiple fruits.
2) Aggregate fruits are formed from separate carpels of a single gynoecium. Individual parts of
aggregate fruits are known as fruitlets. Examples include raspberry, strawberry, and magnolias.
3) A simple fruit develops from one carpel or several united carpels. This is the most common
type of fruit, of which several categories are discussed below.
Categories of Simple Fruits:
1) Dry fruits are simple fruits that are dry, woody, or papery at maturity.
Dehiscent fruits are dry fruits that break open at maturity to release the seeds. Dehiscent
fruits are classified by the way the ovary wall breaks apart:
 Follicles - the fruit wall breaks open along 1 seam (milkweeds)
 Legumes - the fruit wall breaks open along 2 seams (beans, peas, lentils)
 Siliques - the ovary wall breaks open with seeds intact on the central portion of the
fruit (canola, members of the Brassicaceae (cabbage family))
 Capsules - split open longitudinally or have holes through which the seeds are released
(okra, cotton and poppy)
Indehiscent fruits are dry fruits in which the seeds remain within the fruit and are dispersed
with the fruit wall intact.
 Achenes - small single-seeded fruits with the seed attached to ovary wall only at one
point (buttercup and buckwheat)
 Samaras - achenes with "wings" modified for wind dispersal (ashes and elms)
 Caryopsis - seed and fruit wall are totally fused (rice, wheat and other cereals)
 Cypsela - the fruit of Asteraceae (dandelion and marigold) family that has an achenelike fruit but is attached to pappus, which helps in its dispersal.
 Nuts - have a stony fruit wall (hazelnuts, cashews, and acorns)
 Schizocarps - fruits which break into one-seeded bits at maturity (maples)
2) Fleshy fruits are simple fruits that are soft when ripe. Most of the fruits and vegetables we
eat belong to this group. Different terms are applied to different types of fleshy fruits based
on the structure and texture of the different fruit layers and where the seeds are placed in
the fruit. Some of the types of fleshy fruits are listed below along with a few examples of
each:
 Berries have a soft and fleshy inner wall. (tomatoes, blueberries, peppers)
 Pepos have a papery outer wall and fleshy inner wall to which the seeds are attached.
(melons, cucumbers, squashes)
 Hesperidia (singular: hesperidium) can be divided into segments (citrus fruits, such as
oranges, lemons, and grapefruits)
 Pomes have a thin papery inner wall. (apples, other fruits of Rosaceae (rose) family)
 Drupes have a stony inner wall with a single seed. (peaches, plums, cherries and olives)
FRUIT FUNCTION
Beyond differences in structure, different categories of fruits also suggest differences in
modes of dispersal. Most fleshy fruits are attractive and brightly colored at maturity. They
tend to have soft fruit walls and high concentrations of sugars. This attracts a variety of
animals that will eat the sweet flesh and subsequently disperse the seeds. As seeds pass
through the digestive systems of some animals, their seed coats are weakened by the animal's
digestive acids which aids in germination. In some species of plants, the seeds will not germinate
until they’ve passed through the gut of their animal disperser.
Some fruits are modified to be dispersed by attaching to the fur and feathers of animals.
These fruits are generally small and dry. They might also have sticky substances, barbs, hooks,
or spines to aid in attachment to their dispersal agents. Fruits adapted for dispersal by wind
often have appendages like wings or modified floral whorls attached to the seeds. In plants
such as orchids, appendages for flight are not necessary as the seeds are sufficiently small
that they are easily dispersed by wind. In plants like tumbleweed, seeds are dispersed as the
whole intact plant is blown across the landscape. Fruits dispersed by water, such as coconuts,
often have air trapped inside their tissues that help them float and get carried to different
places.
In most cases, fruits are passive and different agents actively disperse them. However, in some
plants, the fruits are adapted so that the plant itself can actively disperse the seeds. This is
usually accomplished by a fruit wall modified to shatter as the fruit flesh dehydrates, throwing
the seeds a distance from the parent plant. For example, in the parasitic eudicot mistletoe, a
very high hydrostatic pressure builds up in the fruit and the seeds are ejected as far as 15
meters from the parent!
ACTIVITIES:
1) Examine and dissect the flowers available in the classroom. Make sure you can identify the
four floral whorls. Are the flowers complete or incomplete? Does the flower have radial or
bilateral symmetry? See if you can determine the potential pollinators using the table provided.
2) Several different kinds of fruits are on available on display. Try to classify the fruits as
fleshy, indehiscent, dehiscent, etc. . . following the rules given in the handout. Once you've
identified them by classification, EAT THEM!! Try something new you've never tasted!
POSTLAB QUESTIONS:
1. Draw a flower from lab labeling the 4 floral whorls. Is this flower perfect/imperfect?
Complete or incomplete? Describe its symmetry. What is its likely pollinator and why can you
make that assumption?
2. The table in the lab describes the properties of several flower types pollinated by biotic
means. Create an entry for the table for the abiotic pollinator "wind". Include all the
categories: color, scent, flowering time, corolla, reward, and examples.
3. Immature fruits are generally green in color but change to bright, attractive colors when
ripe. What is the potential evolutionary advantage of fruits changing color with respect to their
potential for dispersal? (i.e., why aren’t immature strawberries red?).
4. In seedless varieties of fruits (e.g., grapes, watermelons, bananas) fruits develop even though
there has been no fertilization. Why are there no seeds in those fruits? How are those plants
propagated if there are no seeds?
5. Pick an example of a fruit type (e.g., a pome, a berry, an achene). Draw and label an example
of that fruit type represented by the materials available in lab. What structures did you
observe that allowed you to classify it as the fruit type you indicated?
6. What were the best/weirdest fruits you tried today? Will you try them again?